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Upgrades (non TDI Engine related) The place of handling, lighting and other upgrades that do not relate to the performance or economy of the TDI engine. In other words upgrades to your TDI that don't fit into TDI Fuel Economy & TDI Engine Enhancements.Please note the Performance Disclaimer

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Old March 28th, 2006, 09:08   #16
cartog
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Thanks for this wonderful thread and all the research that lead up to it. I'm particularly appreciative of the background on why things are the way they are.

I've been busy sponging up your threads here and over at vwvortex. Even if I never tweak my suspension, at least I'm learning another dialect of jargon to impress my neighbors with
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Old March 28th, 2006, 11:50   #17
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Next is what is the Stock Suspension all about The following text belongs to Ceilidh from two years ago. Here the text unchanged:

4a.

Work is getting a little hectic right about now, and I'm finding less time for writing. So let me try something -- rather than write complete chapters (which can take a while to gather & organize thoughts), I'll try writing fragments whenever I have some time (that is, I'll try writing almost in free-form stream of consciousness, instead of working out what to write beforehand). The downside is that things will be harder to read (and much wordier), and the logical flow will be lessened; but the upside is that at least something will be get written down(!). Anyway, I'll try it this evening; if it doesn't work out, I'll go back to the planned out, full chapters.

So here goes for Section 4 sub-a: intro to the stock suspension:
As noted earlier, the stock suspension is (as you'd expect) the most forgiving of the suspension options, and it has the greatest and most persistent understeer. This understeer, however, doesn't quite work the way that many people expect, and if we're going to modify the Golf/Jetta IV suspension for better handling, it'd be useful to know just how and why the stock understeer exists:
Because tuning books and magazine articles spend so much time talking about lateral weight transfer in corners (e.g., stiffening the front of a car makes it understeer; stiffening the rear reduces understeer and leads to oversteer), one can very reasonably get the impression that weight transfer and roll stiffness are the primary determinants of handling balance. And this impression would be reasonably correct if the car in question is the sort of car that tuning books & articles like to talk about: race cars, or street cars that have been heavily tweaked for cornering grip. A hallmark of such cars is good camber control in corners: when a race car or race-modified car goes around a curve, its wheels (or at least the heavily loaded outside wheels) are more or less upright. And with upright wheels, weight transfer is the dominant factor in determining handling balance. But things change quite a bit if, as with the stock Golf/Jetta IV, the wheels are allowed to lean "the wrong way" (i.e., take on adverse camber) in a turn....

Now at this point, everybody's probably saying "Yes, yes, we know all about adverse camber and the stock suspension's stupid inability to keep the tires upright -- that's why we want to modify the car!" -- but here's the kicker: the adverse camber is not "stupid"; it's actually an integral part of what makes the stock suspension so forgiving, and we'll want to be careful about taking it out.

To understand this rather weird-sounding point, let's consider for a moment something very unlike the Golf/Jetta: let's consider a "perfectly balanced" (50:50 weight distribution, AWD, etc.) car with a "perfect" suspension that keeps the wheels perfectly upright in a corner. Such a car will respond "perfectly" to all the well-known suspension tweaks: put stiffer springs or a bigger anti-roll bar at one end of the car, and that end will breakaway first in a corner. Hence if we stiffen the front end a bit, we get mild understeer; stiffen it a lot, and we get heavy understeer, heavy enough to keep even the most ham-handed of drivers from spinning off the road. What could be simpler than that?

But now let's look in more detail at what's going on with the tires: When tires are loaded up in a corner, they initially respond in what's called a "linear" fashion: in colloquial English, they give back exactly what's asked of them -- if you load them a little harder, they corner a little harder; load them a lot harder, and they corner a lot harder; everything stays proportional. This :"linear regime" can persist for some time, but there comes a load after which there's a "transitional phase", where the tires give progressively less and less of what's expected of them, followed by a slip or sliding phase where they can't give any more at all (for the tire aficionados, I realize I'm mixing up concepts from slip angle curves and load curves, so please forgive me! - I'm just trying to get a general qualitative point across...). This passage from linear to transitional to slip is what permits chassis tuning via lateral weight transfer: When we stiffen up the front end of our "perfect" car, we induce understeer by having the front tires (when averaged left & right) hit the transitional and slip phases sooner than do the rears; when the fronts are in transition or slip, they're supplying less cornering grip than are the rears, and thus the front end breaks away first.

All this should sound pretty straightforward and familiar up to this point. But now here's the problem: the transitional and slip phases only set in when the tires are pretty heavily loaded in a turn, and most of the time (when you're on public roads) you're driving in the linear regime --- and in the linear regime, the vast majority of non-race drivers cannot feel whether the car is oversteering, understeering, or driving neutral. Instead, the car simply feels like it is cornering on rails. Moreover, a great majority of non-race drivers cannot discern a tire's entry into the transitional phase, either, and they will only notice something's changed when the tire is well into the slip phase of things. (This is not an indictment of the average driver: it's hard to detect something if you don't know what it feels like, and one of the many benefits of a good performance or race-driving school is that you get practice and instruction in how the tire limits really feel.) Thus for most drivers, our hypothetical "perfect" car will not seem to understeer until front traction is practically used up. Worse still, a tire that's in the slip phase is no longer listening to the steering wheel: once the front tires have entered this part of the cornering curve, the driver is more or less along for the ride. And that's not good.

The above scenario is the origin of the oft-quoted (and oft-misinterpreted) racing adage that "oversteer is where the passenger is scared; understeer is where the driver is scared". When a car with upright tires (e.g., a race car) begins to heavily understeer, the driver can no longer steer. That means that a car set up to moderately or heavily understeer on upright tires is -- for the vast majority of non-race drivers at least -- a very scary beast. Thrown into a curve at high speed, it will seem to corner absolutely neutrally for a very long time, but push too hard, and at some point the front end will "suddenly" and "savagely" break away, at which the steering wheel becomes absolutely useless. That is why race drivers detest heavy understeer; a little is ok, but a lot can be very deadly.

Now, we've just said that this racing adage is oft-misinterpreted, and the misinterpretation comes when we try to apply the adage to street and road cars. Race cars are designed to keep their tires upright in a corner, and understeer is scary. But if you allow a car to roll and to put its tires into adverse camber in a corner, you can transform understeer from a scary, savage beast into something intuitive and utterly benign. As to how that's done...well, I've run out of time, so it'll just have to wait for some other day.....
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Old March 28th, 2006, 11:55   #18
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And the following, his all the way to the end:

4b.

Right. Now, let's finish up the handling characteristics of the stock suspension.

To review thus far: (1) the stock suspension must understeer enough to keep neophytes from inadvertently spinning under all but the most extreme situations, but (2) creating such understeer via lateral weight transfer & upright tires is unacceptable, as the resulting car will seem (to the untrained driver) to understeer suddenly, savagely, and with almost complete loss of steering control. So what do we do?

We let the car roll, and we let the front tires take on adverse camber while keeping the rear wheels upright.

Let's look at this in detail:

The geometry of the McPherson strut front suspension is discussed fairly exhaustively elsewhere on this forum, so we we'll only briefly summarize the salient points here: when the Golf/Jetta IV rolls on the stock suspension, the outside front wheel (which takes most of the turning load under hard cornering) initially stays fairly upright, but then takes on increasing amounts of adverse camber (i.e., it leans towards the outside of the corner the car is negotiating). This leaning, or adverse cambering, is extremely progressive and continuous: corner lightly, and the tire's pretty upright; corner moderately, and the tire leans moderately; corner hard, and the tire leans a lot.

In contrast, the non-R32 rear suspension is a simple twist-beam with the beam mounted aft of the trailing arm pivot points. Geometrically this suspension behaves much like a semi-trailing arm setup, and the outside rear tire remains relatively upright as the car rolls (at least so long as both rear tires remain on the ground). Thus the geometry of the stock suspension ensures that as the car corners and rolls, the front tires lean (adverse camber) more than the rears, with the amount of leaning progressively increasing with cornering speed.

So why is this good? Ok, now here's the subtle part. When the front tires lean, two things happen: (1) the overall cornering grip at that end of the car goes down, causing understeer in the usual fashion (note that the rear tires stay much more upright); and (2) the tires contribute something called "camber thrust" (for devotees of Milliken and Milliken, yes radials thrust less than do bias plies, but the camber thrust is still there even with radial tires). This camber thrust does wonders for causing progressive understeer in the stock suspension.

Maybe this is a good time to switch to some plain English(!): if you take a tire, and tilt it, it wants to turn in the direction it's tilted; lean it left and the tire will turn left; lean it right and it turns right. This tendency to turn is called "camber thrust". Now, imagine we have a car that is turning to the left: the car rolls to the right, and if the outside front tire is leaning to the right, that tire will want to steer to the right as well. This steering to the right causes understeer (because the driver wants to turn to the left), but -- and here is the important point -- the understeer has nothing to do with the tire nearing its traction/grip limits. In fact, the understeer will occur quite vividly even when the tire is in its "linear" regime, where it still has all the traction in the world. This is a very, very useful phenomenon, because it means that when the car starts to understeer, the driver can still steer the car. In contrast to the upright-tire (race car) situation where significant understeer occurs only after the front tires have used up much of their traction (at which point they stop responding to steering inputs), the leaning-tire understeer occurs early enough that steering is still an option, and the amount of understeer progressively increases with cornering load.

Because the front tires on a stock Golf/Jetta lean, the car behaves in a very "natural" fashion as it corners: take a corner mildly, and the car simply goes around unfussed; go a little faster, and it understeers a little; go moderately faster and the understeer becomes moderate; go way too fast and the understeer becomes pretty heavy. But at no time does the front suddenly and savagely break away, leaving the driver helpless to do anything but stare at whatever he's about to hit. At all times, the driver can correct for the understeer simply by turning the steering wheel a little harder: because the front tires are still well below their traction limits (despite all the understeer), they will still steer the car back onto the cornering line. Thus whereas with a race car, "understeer is where the driver is scared", with the stock car understeer is not scary at all -- it's just slow and rather annoying.

Bottom line, the roll-induced moderate understeer on the stock Golf/Jetta IV provides some very nice characteristics for driving on public roads: if you go too fast, the front of the car gradually begins to drift wide of the intended line, and you can correct your path simply by slowing down or by turning the steering wheel a little or a lot harder. These handling characteristics are so "natural"-feeling that most drivers assume they're some sort of law of nature. They're not. They're there because of moderate understeer, induced by roll, and they're not present in a true race car.

(By the way, the stock car is set up so that the roll-induced understeer becomes intolerably unpleasant long before the tires hit their traction limits; on dry roads, it's very difficult for any but the truly masochistic to get to the point that the front tires no longer steer. If you do get to that point (it's more likely to occur on the slower corners), you'll feel the car just start "plowing forward" with a lot of tire scrubbing noises....)

Digression: a Pathological "Tuned" VW

There's still a bit more to be said about the stock suspension, but anyone wading through these last two "stock suspension" installments will probably be wondering why we're going through all this in a vortex thread that's ostensibly about improving handling. So I'll give a brief taster here, as an example of why an understanding of the stock suspension can help us in modifying our cars.

We'll consider a pathological case:

A couple of months ago, there was an interesting thread about anti-roll bars on this forum, and one of the individuals posting appeared to be a pretty nice, perceptive fellow who had been given some very bad advice. This advice had led him to install a fairly gargantuan front anti-roll bar on a car that was in other respects not far from stock. His report was that even with the huge front bar, understeer was significantly reduced and the handling was much sharper -- but in wet weather, the front would sometimes break away very suddenly and unexpectedly. Let's look at why this should happen:

First of all, remember that when upright tires are working in their linear regime, it's extremely difficult for untrained (non-race) drivers to discern whether a car is understeering, oversteering, or neutral -- the car simply feels like it's on rails. Secondly, remember that the stock suspension, with its leaning tires, is designed to give camber-thrust-induced understeer while the tires are still well within the linear regime. Put these two concepts together, and....

If you take a stock car and reduce the amount of roll, there is less tire lean, less camber-thrust, and less understeer -- almost irrespective of how you go about reducing the roll (at least so long as you're in the linear regime). Hence if you mount a gargantuan front bar on an otherwise stock car, it understeers less simply because it rolls less -- so long as you're going at moderate speeds. A trained driver, one who's used to driving racing-style cars, will of course detect almost immediately that the car is still understeering (because of the greatly increased front lateral weight transfer, caused by the massive bar), but an untrained driver will only note the cornering-on-rails sensation of upright tires in the linear regime, and he'll think that the understeer's gone. Hence the report that "Mounting a big front antiroll bar eliminated the stock understeer on my car!".

But now let's look at what happens in the rain: on dry roads, the cornering limits of a flat-cornering VW are high enough that most neophytes will not reach them; hence the driver of the big-front-bar car probably never left the linear regime, and so he never felt the car understeer. But in the wet, the tire limits are much lower, and they can be reached when the bar is still holding the car very flat (with the tires still upright). As a consequence, the big-front-bar car in the rain handles very much like an upright-tire race car with far, far too much weight-transfer-induced understeer: when corners are taken too fast, the fronts exceed their traction limits, and they break away in the regime where they no longer steer. Thus the supposedly understeer-free car in the dry takes on a sudden, frightening understeer in the wet. Not good.

Pathological Example #2

I wasn't going to do this (we're getting a little ahead of ourselves), but as long as we're talking about pathological suspension setups, let's also look at one of the very many reasons why a Golf/Jetta IV with a massive rear bar and stock everything else is also a very ill-handling beast (or, as Dick Shine has often warned people, a rear anti-roll bar cannot cure a fundamentally unsound suspension!).

We'll talk about the big-rear-bar/ soft front setups more in a later installment, but for now we'll just focus on the very common thread postings where someone (usually a novice) raves about the 28mm bar on the rear of his stock car, but then warns someone else (also usually a novice) to take the bar off in the winter, because otherwise there's sudden savage oversteer (usually these email exchanges have horrible misspellings, by the way, but that's a different topic...). Leaving aside the questionable merits of a suspension you have to remove on a seasonal basis, what's really going on here, and is this a good setup?

1) One possibility is that the novices never noticed any oversteer in dry weather because they never pushed hard enough to leave the linear regime. Again, when you make a car roll less than stock (here by installing a big rear bar), you make the front tires more upright, the roll-induced understeer decreases, and so long as you're in the linear regime, everything feels great. And also again, the actual performance limits of even the stock car (which invariably feels much closer to the limit than it actually is) are high enough that many drivers don't approach them in the dry. So one possibility is that the car was an oversteerer in both summer and winter, but only in winter was it noticeable. Perhaps.

2) But let's look at the dry, summer handling a little more closely. For reasons we'll perhaps get to later, someday, a big front bar can substantially reduce roll, but a big rear bar will only reduce the initial amount of roll. At some point, the inside rear wheel will lift (it'll lift on even a well-driven stock car), and when that happens, it really doesn't matter whether there's a big anti-roll bar in the back or not: as far as the car is concerned, there is one wheel on the ground in the back, and two wheels on stock springs and stock bar in the front. Hence at the dry-road 3-wheel cornering limit, a big-rear-bar car will understeer about as much as will a completely stock car (note: there's actually a bit of a difference in that a big rear bar will, by lifting the inside wheel higher, cause more leaning of the outside wheel, but there's so much understeer built into the stock suspension that the overall result is still understeer). Now, this is a fairly horrible handling set up (understeer sets in rapidly the moment the rear wheel leaves the ground), but there's no oversteer at the limit.

But what happens in snow? When the roads are truly slick with snow or ice, the tires reach their performance limit at a very low g-force -- so low that the car has hardly rolled at all. Thus in the snow, the big-rear-bar car (with stock front) behaves like an upright-tire race car with far, far too much lateral weight transfer at the rear: when the limits are reached, the rear breaks away, suddenly and savagely. Thus you have a car that seems neutral at low to moderate speeds in the dry, that understeers heavily at the dry limit, but which oversteers dramatically on slick roads.

And for this someone has paid $300?
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Old March 28th, 2006, 12:22   #19
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I think this is where we sprinkle in some of Peters famous suspension animation to help illustrate.
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Old March 28th, 2006, 16:05   #20
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My car has a bit of a darter feel to it, FWIW... then again, it IS a Mk2...

Thread WATCHED.
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Old March 29th, 2006, 06:26   #21
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I'm waiting with bated breath for the next installment! How do we make a MkIV better?
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Old March 29th, 2006, 08:06   #22
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4vdubs: What kind of handling are you targeting? What's your definition of better?

FWIW, if you're targeting "darter" handling, step one is to do an oil cap mod on your Mk4. Take the oil cap off, sell the Mk4, get a Mk2, put the oil cap on.
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Old March 29th, 2006, 12:42   #23
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Quote:
Originally Posted by bhtooefr
4vdubs: What kind of handling are you targeting? What's your definition of better?

FWIW, if you're targeting "darter" handling, step one is to do an oil cap mod on your Mk4. Take the oil cap off, sell the Mk4, get a Mk2, put the oil cap on.
Lol...I'm keeping mine thanks! I'm looking for the best street handling (grip) I can get out of my Mk4, with a little autocross thrown in to keep life interesting. I know this thing is far from perfect but I plan to make the best of it. I have a shine RSB and everything else is stock for now. With the Shine bar the car seems very neutral at least on dry roads as Peter says. I have explore the limits somewhat on some smooth dirt roads and experienced the oversteer but it seems easily controlable with a little counter steering. I await further advice from Professor Pyce.

Scott
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Old March 29th, 2006, 18:06   #24
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Quote:
Originally Posted by Ceilidh
Hello Everyone,
<understeer ...>
OK, color me confused. I like to think of it the following way (and where I'm wrong please correct)

Tires are not rigid objects, so in a corner the wheels and tires don't point in exactly the same direction. The angle formed by the crown line of the wheel and that of the tire at the contact patch is the slip angle, which is zero when the car is going in a straight line. In a corner, at any given instant, the crown lines of the front and rear tires will point in the same direction, but it doesn't follow that the crown lines of the wheels do. If they do, then the front and rear slip angles are equal, and it's a neutral steering condition. If the front SA > rear SA, that's understeer, and if the front SA < rear SA, that's oversteer.

On applying power in a front driver in a curve, you are asking the tire to accept a power input as well as a steering input, so the tire deforms more and the front SA increases, leading to understeer. You can induce a transient oversteer condition by lifting or braking, which unweights the back end and allows it to swing out, but you can't maintain oversteer in a front driver, not without pathological suspension geometry, anyway. On a rear-driver, OTOH, enough throttle input can cause the rear SA to increase to a value greater than the front, so you can maintain a power oversteer condition. You can, of course generate a transient oversteer in a rear-driver by lifting off, especially if you have swing axles (a la Triumph Spitfire) or semitrailing arms (especially^2 if you stick something heavy way out back, a la Porsche 911), although in the latter case, it's really an effective SA, since the geometry points the rear wheel outwards on roll loading. Other things being equal, a rear-driver will handle better than a front because 1) the rear wheels do real work, not just keep your hind end from dragging on the road, and the front only needs to deal with steering inputs; and 2) you've decoupled steering and power inputs, so you can use both ends of the car to adjust your attitude in the turn.

That's how I understand it. I do like the specific details re:suspension geometry, tire loading, etc -- please keep it up!
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Old March 29th, 2006, 21:29   #25
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Thanks, Peter & Ceilidh, for posting the fruits of your hard-won experience here in this more orderly venue... I go to the Vortex to look at the classifieds by those getting rid of stock pieces, but not much else. Don't have the patience.

I understand all you've said so far, and it jives with what I've learned in theory and simulation in my Vehicle Dynamics class (after May graduation I'll have time to delve deeper in that subject on my own... not now though)... we focused on a racier setup so we wouldn't have to factor in changing camber effects on tire performance... the modeling of such was beyond the allotted semester time. Anyhow - thanks for sharing... if people will stop and listen, you will make a lot more people informed if they wish to be. I like your categories, BTW.

In DFW, I should be after Slow Car Fast, but I used to live in the NM mountains, where GT would have been the sure ticket to wheeled nirvana... hope to get back there someday.

Keep preaching!
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Old March 30th, 2006, 14:08   #26
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I am sure he will come soon and address some of the questions, especially those regarding the Golf A2 and the Spitfire, as he actually used to own both of these cars, so stay tuned. Just give him time as the man is very busy with more important things in life.

Meanwhile, let's continue with his outstanding poetry on handling. Follows a copy-paste from the next chapter - the autocross setup. The following all his to the end:


5. Autocross Suspensions

There are a few more things to mention with the stock suspension, but we can cover those points when we discuss GT setups. For now, let's look at autocross.

This will be comparatively brief, as I (Ceilidh) have never autocrossed or set up a car for autocross. Still, a brief pass through the theory will make the GT setups (our ultimate destination) more intuitive.

So here we go: As discussed earlier, autocrossers drive at low speeds on smooth surfaces, and spend most of their time timing and transitioning between turns. Agility is at a premium, as is cornering grip, and (as we're talking track cars here) we don't care about ride quality or progressive, forgiving understeer, and we want the car to be neutral. Autocross cars make perhaps the most use of the various go-fast products on sale for VW use, and there's more room for variety here than anywhere else. Here, in terms of general theory, is why:

I. A Hypothetical, Perfectly Smooth Track

Imagine we're running on a perfectly smooth autocross track, one where there is not even the slightest suggestion of a bump, crack, or pavement ripple (this is the assumption that most tuning magazine articles seem to make, by the way, so what follows next will probably sound familiar). As we don't have to worry at all about road imperfections (in this hypothetical, completely unrealistic scenario), the basic setup theory is straightforward:

A) The more upright the tires are (especially the outside tires in a corner), the more cornering grip we can get.
B) The more we can get the car's weight evenly distributed among all four tires, the more ultimate grip we have.
C) The less the car rolls, and the stiffer the "roll rate", the less time the car spends taking a set for a corner, and the more rapidly we can transition from corner to corner.

To achieve these goals, we can "tune" our car exactly the way the tuner mags recommend:

1) We install massive springs and/or antiroll bars to cut roll to a minimum, and to increase the roll rate (the "springiness" of the car in roll). This modification keeps the front tires far more upright in a corner (which increases grip and cuts understeer) and speeds the car through transitions (because the car takes a set much more rapidly on corner entry).

2) We force the wheels into negative camber so as to counteract any residual roll, and to perhaps even get camber thrust to work with us by leaning the outside tires towards the inside of a corner.

3) We balance the springs/bars fore & aft so that the cornering balance is neutral (as the car is nose heavy, that means the rear should be a bit stiffer).

4) And we drop the car, front and back, as far as we possibly can, so as to lower the center of gravity (CG). Lowering the CG does not reduce roll -- it in fact increases it because of the oft-discussed rapid lowering of the front roll center, but we can counteract the increased rolling tendency simply by making the springs and bars still stiffer. The lowered CD does, however, reduce the total lateral weight transfer in a corner (the total transfer is simply a function of track width and CG height, and is independent of suspension design or spring/bar rates). This reduction in lateral weight transfer means the 4 tires are more evenly loaded in a corner, which increases overall grip and cornering speeds.

4-A) As a side note, dropping the car leads to a very low front roll center (it's actually below the ground now). For reasons we won't go into here (it's discussed in all the good textbooks), the roll center height controls lateral weight transfer at the initial instant of corner entry, and a lowered front roll center thus means better initial turn in.

5) We install big, stiff, quick-acting Bilstein or other monotube shocks to further snub down the chassis and thus speed up our speed in transitions.

6) And to further reduce play or compliance in the suspension (all of which increases the time it takes for the car to take a set, and/or reduces the effectiveness of our antiroll bars), we replace all the bushings with spherical joints, solid bushings, etc., etc.

The above really should sound extremely familiar, as it's what all the tuning shops and tuning magazines tell us to do.

So what's the problem? Well, for one thing, even an autocross track isn't perfectly smooth. And that has the potential to change everything:

II. The Problem With Bumps

An implicit assumption in the "perfect world" autocross setup is that we can make the springs and bars (and bushings, shocks, etc.) as stiff as we need to in order to control roll and body sway/heave/roll/pitch/etc. But in practice, a car's suspension exists for a reason, even in a race car where ride comfort is a non-issue: When there are bumps, a perfectly rigid car with a perfectly rigid suspension will spend most of its time with its tires flying from bump crest to bump crest, and during the time the tires are flying, the cornering grip is zero. The suspension on even a track car thus has the critical job of keeping the tires pressed to the pavement as evenly and as firmly as possible, and that requirement puts a limit on how stiff we can set the springs. Thus we have to add two new goals to A, B, C listed above:

D. The softer the springs are and the better tuned the shocks are to the spring rates, the better the "mechanical grip" between tire and track surface.

E. The more independent the suspension truly is, the better the mechanical grip.

Requirements "D" and "E" go at odds with many of the "standard" tuning tweaks advocated by the go-fast vendors. The complications are many, but here are some of the biggies:

1) If we stiffen the springs too hard, the tires lose mechanical grip. This puts a limit to how stiff we can go in an effort to control roll.

2) If we try to circumvent #1 by increasing the antiroll bar size, we eventually begin to lose grip by (a) non-independent suspension action (the bars tie the inside wheels to the outside, and start behaving like a solid axle suspension), and (b) an inability to tune the shock absorbers to match both the (softer) vertical spring rate and the (stiffer) roll rate.

3) Lowering the front suspension rapidly drops the front roll center, which increases the roll couple; the car therefore either rolls more, or requires stiffer springs to counteract the roll (which reduces mechanical grip). Both of these effects negate or partially negate the lateral weight-transfer benefits of lowering.

Thus the appearance of requirements "D" and "E" mean we can't simply slam the Golf/Jetta IV down to the pavement and stiffen the springs. From the autocross link that Alexb75 posted a couple of weeks ago, it appears that at least some autocrossers are actively experimenting with different avenues of optimization. In general theory, at least, one can try to:

1) go with a lowered car, and back off on the springs & bars (thereby rolling more, but maintaining mechanical grip and reduced lateral weight transfer)

2) keep the front end relatively high, and fit moderate springs & bars (which rolls less, keeping the tires upright and maintaining mechanical grip, but accepting more lateral weight transfer)

3) go moderate on the springs, but go stiff with the bars (which maintains mechanical grip on vertical bumps, but loses out on 1-wheel bumps, and in some cases loses out on transitions because of play in the antiroll bar linkage)

4) stay moderate on the bars, but go heavy with the springs (which loses general mechanical grip, but improves agility (non-Shine-style bars typically have a bit of play, whereas springs act right away in transitions)

5) or try various combinations of 1, 2, 3, or 4, and try different things on different ends of the car (e.g., soft rear springs and big Shine bar, plus stiff front springs and no bar, etc.).

What will actually work best? That's a question for the autocross experts, not me (Ceilidh)! But the main points here are that:

A) with Autocross setups, different combinations might work best on different tracks (e.g., a smooth tight track, vs. a rough, faster track, etc.) and with different drivers .

B) But far more importantly for the purposes of this thread, the need to absorb bumps is something that just can't be ignored in setting up a Golf/Jetta IV suspension -- even on something as comparatively smooth as an autocross track. If bumps didn't exist, we'd just mindlessly set up our cars the way the speed magazine breathlessly extoll (slam the car, stiffen the springs, mount big bars front & rear, etc.). But when reality sets in, in the form of actual track and road surfaces, the ideal speed-magazine setup gradually morphs into something a lot closer to the Shine -- and when we factor in ride comfort later on in this thread, the Shine will start to look even better.

Next up: road racing theory
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Old March 30th, 2006, 15:28   #27
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Let's continue with the copy-paste. Ceilidh on Road Racing and Shine all the way to the end....

6. Road Racing, and the Shine Real Street Suspension

Let’s take a brief look at the general characteristics of a decent road-race setup, after which we'll touch for a moment on a setup that gets a lot of press in these pages: the Shine SRSS.

Backdrop

If you can remember back to the Autocross installment, we started there by considering what the ideal setup would be if we ran on perfectly smooth roads (e.g., lower the car to drop the CG, stiffen the springs and bars hugely to reduce roll, go to negative camber to keep the outside tires upright, install ultrastiff shocks to control body movements during transitions, etc.); then we looked at how the unavoidable presence of bumps -- even on a smooth autocross track -- forces us to dial back on those "ideal" suspension mods; and finally we concluded by observing that, for autocross at least, the jury's still out on what's the ideal compromise....

If there's room for Golf/Jetta IV variety in Autocross, it'll be because of a tradeoff between the simplistic benefits of lowering & stiffening vs. the more subtle ability to maintain traction and control on bumps and rough surfaces. Because Autocross surfaces are in fact fairly smooth, and because the low vehicle speeds in Autocrossing make the bumps seem even smaller, it's conceivable that a low, very stiff setup might actually work (we'll leave it to experts to comment on whether such setups do or do not work in reality -- all we're saying here is that if such setups are to work at all, it'd be in Autocross...).

When we get to road racing, however, the much higher speeds and more varied terrain mean we can no longer even pretend to ignore the bumps, and that seems to pretty much rule out the ultrastiff, very low setups for the Golf/Jetta IV. In the Autocross installment, we touched on some of the reasons why that might be so, but let's now look at it with a little more detail:

Spring Rates and Mechanical / Rough Road Grip

As discussed earlier, for every combination of road surface, vehicle weight, unsprung weight, tire stiffness, and vehicle speed, there is a certain suspension spring rate (and shock damping) that will maximize tire grip. If the spring is too stiff, the vehicle is thrown into the air with every bump; if the spring is too soft, the vehicle might ride smoothly, but the inertia of the unsprung weight will carry the tires upwards and off the ground on each bump, and the springs will be too weak to force them back down. Both situations will lead to the tires unloading on the back side of bumps (in the too-stiff case, because the whole vehicle is rigidly leaping about; in the too-soft case, because the tires alone are hopping up and down), and traction is therefore lost.

(As an aside, the above is one of the several reasons why traditional sport utilities & trucks ride and handle so badly on rutted pavement: an old truck-based SUV has very heavy solid axles with a lot of inertia, and that causes the axles to leap about on rough roads. Soften the springs for a better ride, and the axles bounce around almost uncontrollably; stiffen the springs to force the tires back onto the pavement, and the whole truck gets jolted with every bump. It's partly for this reason that almost all the new generation SUVs are going over to independent suspension: one advantage of independent suspension (on a driven axle) is drastically reduced unsprung weight...)

So, in theory one has to carefully choose the spring rate on a car to maximize traction on a real road or track, as too-stiff and too-soft are both problematic. But in practice, at least with Golf/Jetta IV's, we don't have to worry about too-soft. Our independently-suspended cars are heavy, and the CG (even on a slammed car) is high relative to the track and wheelbase; hence if we start softening the springs, we run into problems with body roll, dive, squat, heave, etc. long before the unsprung weight gets out of control.

Or to put it another way: to control the body motions on our cars, we have to stiffen the springs enough that they are almost always too stiff for optimal mechanical grip. Hence, in practical terms, we have a more or less clear-cut tradeoff: stiffen the springs to control roll (and pitch, heave, etc.), or soften them to improve the tires' grip on the track.

Implications of Reduced Spring Rate -- Why Lowering Causes Problems

Thus because of bumps, we can't arbitrarily stiffen the springs to control roll. (And remember from the earliest installments: if the car rolls, the stock Golf/Jetta suspension is intentionally designed so as to progressively lean the outside front tire, which reduces grip and causes moderate to heavy understeer.) So once we leave the smooth, low speeds of the autocross track (and some would argue even *on* the smooth, slow autocross tracks), we have to soften the springs. And that softening causes a whole litany of problems for a lowered car, of which we'll highlight two of the biggest:

1) When we earlier discussed the "ideal" setup on a "perfect", smooth autocross track, we rather naively imagined that we could stiffen the springs to the point that the chassis no longer moved: our "perfect" setup wouldn't roll in corners, and neither would it pitch, dive, or squat under acceleration or braking. Whether or not that's a reasonable approximation on an autocross track is something we'll let the autocrossers debate, but once we're on a real road-race track, with real bumps and realistic spring rates, we can no longer pretend the body isn't moving around. It will move relative to the wheels as the car accelerates, decelerates, corners, and encounters bumps. And that movement has some big geometric implications, particularly at the front suspension:

For a variety of geometric and packaging reasons, it's almost impossible to design a production car front suspension that doesn't "bump steer": when the front wheels move up and down relative to the chassis, they don't stay pointing straight ahead -- instead, at some point they will begin to toe-in or toe-out.

Such toeing would be pretty undesirable for fairly obvious reasons: it means that the toe setting changes when the car pitches forward under braking, or when it pitches back under acceleration, or when it rises and falls over bumps. Even worse, if the car rolls, one tire might be toeing in while the other is toeing out (because one wheel is rising while the other is falling), which steers the car to one side (called "roll steer"); or if one wheel hits a bump but the other doesn't, the bumped wheel can toe in or out while the other keeps pointing straight, which again steers the car (called "bump steer"). Mix and match these various situations -- e.g., let's simultaneously decelerate and roll the car via trail-braking, and then hit a one-wheel bump on corner entry -- and the combined roll and bump steer effects can be extremely entertaining and ever-changing, thereby inspiring the driver to generously & politely compliment the race engineer for the wonderful setup (drivers really love race cars that dart about unpredictably in corners)....

As even production car suspension designers don't like to be shouted at, a good deal of engineering time is spent making sure that bump steer is rarely an issue in the normal life of a car. In practice that means specifying a geometry that crams all the toeing to the far limits of the suspension motions: so long as the wheels are moving up and down to positions reasonably close to the static load position, the toeing is negligible; only when the wheels move close to full jounce (all the way up) or full rebound (or all the way down) does the toeing become pronounced (if you look at a graph of toe vs. suspension movement, the graph is often a straight line (essentially zero toeing) for a good distance above and below the static load position, but then takes a pronounced hook as it approaches full jounce or full rebound). In this way bump & roll steer are ordinarily non-issues: under normal braking, accelerating, and cornering, the car handles fine, and bumpsteer only shows up on bumps so enormous that any steering effect is swamped out by all the other violent things that must be simultaneously going on.

So what happens when you drastically lower a production car? In severe cases, you move the static load position (the position where the suspension sits and works around) right into the region that contains all the bump steer. And so the car bump steers. It also roll steers. And the turn-in characteristics (which are greatly affected by toe in and toe out) will vary wildly depending on how hard you brake during corner entry, or whether you're hitting 2-wheel bumps. In short, your "race car" will adopt many of the delightful handling characteristics of a 1940's Buick, minus the comfortable ride and cool hubcaps.

(2 notes here: one is that the rear suspension will bump & roll steer as well, though the effect is usually less pronounced than in the front; the other is that it's standard practice to "bump steer" (meaning, to "reduce the bump steer effect on") a radically lowered car by changing the positions of the steering rack and steering arm pickup points; but I've never seen any discussion of such on the VW forums -- probably because the next point (below) makes the issue moot)

2) The second effect is one that's been discussed to death elsewhere on this forum, and which we'll only repeat here so as to put in the context of the earlier installments: Almost everyone following this thread will have already read somewhere that lowering a McPherson strut suspension will increase the tendency to roll. Some readers, however, might have wondered why that's an issue: if the car tends to roll more, why can't we just stiffen the springs to compensate?

The reason, as it should be clear by now, is that we can't arbitrarily stiffen the springs. If we stiffen everything so as to reduce roll, we lose mechanical grip over realistic bumps; if we soften the springs to increase mechanical grip, we roll the outside front tire into adverse camber. Catch-22. What we need, then, is a way to reduce roll without going crazy on the spring rates. And we can do that by keeping the roll center nice and high.

Such the reason for Dick Shine's oft-repeated assertion (oft-repeated not because he's been unclear, but because so many people seem unwilling to believe him) that lowering the front end of a Golf/Jetta IV will destroy the handling. If you don't keep the roll center fairly high, you lose grip either through increased roll & adverse cambering of the outside tire, or else through having to control the roll with overly stiffened springs. (Note: we'll not discuss drop spindles and major component swaps here).

Or to summarize it another way: there are three big means of increasing/ maintaining the cornering ability on our cars: (1) keeping the tires more upright by reducing roll; (2) retaining mechanical grip by not letting the spring rates get out of hand; and (3) reducing the total lateral weight transfer by lowering the CG. On a Golf/Jetta IV, #1 is the most important, and #3 is the least. Hence optimizing #3 while compromising #1 and/or #2 is not the way to go.

Continues below ……
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Old March 30th, 2006, 15:45   #28
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Continues from above on the Road Racing and Shine SRSS .... (Ceilidh):

Back again. =) In re-reading the earlier post, I see that somewhere along the way I morphed from true "road-race" to "streetable semi-track" setups; sorry about that. Most of what we're talking about now concerns cars that live somewhere between road and track, and our ultimate destination (given the original topic of this thread) is the GT suspension. for the cars we're talking about here, rough road performance is becoming increasingly important.

Right. Where were we? At the end of the last installment, we reviewed the oft-discussed reasons for why we have to keep the front end high. Now we'll clear up some additional points, and then go into just how high is high enough:


Caveats and Additional Notes

A) Before I forget: road racers tend to go for negative camber, which is beneficial on the track for a number of reasons (see Carroll Smith's "Tune to Win" for a discussion). As a personal caveat (from my experiences with "bigger" negative camber on RWD cars): if you overdo it, you can mess up your braking (the tires no longer sit flat even when the car's on the straight, and brake-dive makes the cambering even worse); you might wear your tires pretty unevenly (the inside edges go first, if you're not regularly cornering hard); you can lose on-center feel and a nice street-style progression on gentle curves (the steering becomes 2-step: first you're riding on the inside tire edges, then at some point in a corner the outside tire flops down flat and begins rolling onto the outside edge); and sometimes you get tramlining issues.

B) On a different note: some of you might be wondering "Why does Ceilidh keep going on about stiff springs? If the problem concerns roll vs. mechanical grip, why not use soft springs for grip, and reduce roll with antiroll bars?". That's a fine question, which we'll go more into later when we get to GT setups, but for now please just consider that a stiff bar (1) increases the 1-wheel bump rate; (2) reduces the independent action of a supposedly "independent suspension"; and (3) makes it hard to tune the shock damping to simultaneously suit 1-wheel and 2-wheel bumps. All these factors reduce mechanical grip, and thus there's a practical limit to how stiff the bar can be. In short, bars are nice, and they allow you to control roll with softer springs (compared to what you need if you didn't have bars at all), but ultimately you still have to worry about spring rate.

C) Last note: One thing I've completely ignored up to now is the degree of camber gain under roll (which determines how upright the tires stay as the car rolls). It's an important concept, but I've left it out because it's so straightforward, and because it's been so well explained on other threads. But for completeness: a McPherson strut suspension that's rolling will initially tend to keep the outside tire more or less upright; but with increasing roll, the outside tire will begin to lean (and thus incur camber thrust, reduced traction, etc.). When a strut suspension is dropped via lowering springs, the effect on the outside tire is curious: at small angles of vehicle roll, the tire is more upright than before; but at greater roll angles, the tire actually leans more than in the stock case. Hence if you try to lower the CG by installing lowering springs, and then try to maintain mechanical grip by keeping the springs relatively soft, not only do you get more roll, but the adverse camber on the critically-important outside front tire is even worse than you'd first expect.


How High Should the Front Suspension Ride?

Everything up to now has been qualitative: we've discussed why a lowish front roll center can be bad; how excessive lowering can cause bumpsteer problems; and how there are camber-gain issues if we lower too much and allow too much roll. But does that mean we can't lower at all, and should we be pushing things much higher than stock?

The second question is more easily dealt with: One might ask -- given that a stock front ride height is good -- whether a much-higher-than-stock front would be even better. The short answer is "No": aside from the bump steer problems you'd get from pushing the car towards the extremes of rebound (vs. jounce, with lowering springs), at some point the increased lateral weight transfer you get from a high CG (plus the positive static camber you get from a raised McPherson geometry) will hurt you more than you'll benefit from reducing roll and/or spring rate. So it's very possible to go too high.

The other question is less easy to answer: just where should the front end sit? The roll-center and camber-gain phenomena are not "step functions" -- it's not a case of, go 1mm too low and WHAM! the car is screwed up. And bumpsteer should not be an issue (on any modern production car) if you're within an inch or so of stock height, at the very least. So maybe we can go down an inch, or a half inch, or somewhere in between? In short:

"How low is too low?", and "How can one tell what's best?".

The answer to the first question is "It depends.", and the response to the second is that, unless you want to exhaustively test lots of different setups, "You'll just have to trust someone".

Let's explain:

For every car, there will be a magic combination of ride height and spring rate that will optimize the combination of (1) reduction in roll; (2) maximization of mechanical grip; (3) best use of camber gain in roll; and (4) minimization of total lateral weight transfer. That magic combination does not, a priori, have to be at stock height, and it will not be the same for all cars -- not even all cars that use a McPherson front suspension. For example, some people have suggested that BMW's seem to work just fine with moderate lowering; well, that's entirely possible. But as BMW's have a different CG height, track width, tire & rim size, camber gain, suspension travel, etc. from our VW's, their ability to work with lowering tells us nothing about what will work best with the Golf/Jetta IV.

Since this particular issue comes up so often on various threads, it might be worth repeating: just because race winning Beemers, or Hondas, or Toyotas, or etc. happen to use a particular ride height, doesn't mean that a VW should be set up the same way. Every car model is unique, and the optimum chassis setup is similarly unique.

So while I'm afraid it'll sound like a major cop-out, if we want to know what the ideal ride height is for a Golf/Jetta IV, and if we don't want to expend vast sums of $ and time to experiment with different combinations, we pretty much have to rely on the judgement of people who (1) have themselves performed lots of experiments; who (2) possess the experience and the know-how to assess what those experiments are telling them; and who (3) don't have an enormous amount to gain by telling us something that's not true.

To summarize, repeat, and stress once again: there's nothing inherent about a McPherson strut front suspension that requires the ideal performance variant to sit 0.5" above stock -- but neither is there any reason for it to be at any other height, whether higher or lower. What determines the ideal height is the particular combination of parameters (CG location, track & wheelbase, camber gain, etc.) designed into a particular car, and we as ordinary consumers have to trust somebody who has exhaustively tested different combinations to determine what is best. I (Ceilidh) would tend to trust Dick Shine, but you (dear reader) will have to decide for yourselves whom you will listen to! But regardless of whom you choose, please pick your "expert" on the basis of what he/she says concerning issues other than ride height (that is, how credible is this person on other issues?) -- regarding the Golf/Jetta IV front ride height, there is no a priori reason why it should be high, low, or anything in between.


The Shine Setup

And finally (and this will have to be brief, as I'm late for a meeting! ) -- if we take as a given that the Shine SRSS works well as a mild track setup, here's a possible theoretical explanation for it (Dick Shine, please do chime in and correct me if I go astray here!).

From the perspective of an outside armchair observer, the Shine philosophy seems to be to:

A) keep the front at or just above stock height to minimize roll with the use of reasonable spring rates;
B) increase front spring rates to control roll and pitch, but to keep the rates moderate, so as to retain maximum front mechanical grip;
C) refrain from increasing the front bar beyond stock size, again so as to maximize front mechanical grip (as briefly explained above);
D) increase rear spring rate to the limits of good mechanical grip and a tolerable ride while maintaining good balance for at-the-limit cornering;
E) drop the rear ride height so as to lower the overall CG (the rear roll center drops slightly less rapidly than does the CG, so the overall roll couple is also somewhat reduced); (this lowering of the rear also allows for more stiffening of the springs & bars, as it helps keep the car from getting too tail-happy)
F) provide the option of a rear bar to adjust handling balance, particularly in transitions. For reasons we'll discuss in a future installment, the rear bar does not significantly affect the ultimate grip of the Shine suspension (when the car is three-wheeling), but it sharpens turn-in by keeping the car flatter (on turn-in and transitions) and by increasing lateral weight transfer at the rear (again, primarily on turn-in and transitions). To have a noticeable effect on what is already a moderately stiffened suspension, the rear bar has to be fairly stiff; this stiff bar reduces mechanical grip in the rear for all the reasons alluded to earlier, but as the purpose of the bar is to reduce understeer in the appropriate situations, this loss of grip is acceptable.
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Old March 30th, 2006, 15:59   #29
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Hello Folks,

First of all, an apology for not replying to all the kind folks who have posted comments; life is a bit busy these days, but let me try to go through the posts, in reverse chronological order (newest first):

1. Nate, that sounds like a very interesting course that you're taking -- If you come up with any neat insights, do please tell everyone! (What software package are you using? Would you recommend it?). And where in NM did you live? I did the drive up from Los Alamos to Mesa Verde via Chama a few times: what heavenly roads!! Hope you get back out there sometime...

2. blacka5, your understanding is perfectly correct, and very well explained! The traction effects you discuss do in fact explain how you can get steady-state under- or over-steer in a theoretically neutral vehicle, and as you point out, it's pretty hard to induce continuous oversteer in a FWD car (I suppose you could do it via forward weight transfer on a continuous downhill curve, but that'd require you to drive down a spiral parking lot ramp, or something like that!).

To set blacka5's comments in a larger context: there's a teaching aid called the "friction circle" (Peter, can we have a picture?), which is used to show that a tire forced to exert acceleration or braking forces has less traction available for cornering (the idea is that when you're at 45 degrees on the circle, half of the traction is used for cornering, and half for braking, etc.). So long as the size of the circles (which represents the total traction available at each tire) remains fixed, spooling off some of the traction for acceleration will cause a RWD car to shift towards more oversteer (or less understeer), and a FWD car to shift towards more understeer.

(As an aside, this effect used to be much more vivid in the old days, when tires were skinny and slippery: I had an MGB GT that was just a joy to drive slowly on winding roads, as the engine torque needed just to maintain a constant speed was enough to drift the (RWD) back end outwards, and long sweepers could literally be driven on the throttle (a little more gas to tighten the turn, a little less to open it up -- essentially the opposite of how our front-drivers work in practice...)

To finish up the context: the points raised by blacka5 apply when the friction circles remain constant in size, but they're complicated when the circles change in size with different situations. With all cars, the rear circles grow and the front circles shrink with acceleration (because weight is shifted rearwards, increasing the rear traction while diminishing the front), whilst the opposite happens under braking. This growing & shrinking can sometimes overwhelm the "normal" friction circle effects (e.g., on that same MGB GT, the car was far more stable (less prone to spinning) in tight corners when under power, as the acceleration would increase the rear traction and foster some stabilizing understeer).

(Digressionary aside #2: one of the joys of those old British sports cars was how the handling balance would shift with circumstance. In some situations, more throttle would give you understeer, whilst in others it would give you oversteer -- sometimes the trend would even change in the course of a single corner! It sounds like a fault, but it wasn't! The cars simply gave you a really vivid idea of what was happening in terms of physics, and you could learn an awful lot without even breaking the speed limit; it was fun!)

(Digressionary aside #3: this has nothing to do with anything, but at one point I had my college roomate at the wheel of my car, and he kept scaring both of us by repeatedly slowing down at every corner -- each time he did, the car would get squirrely, which would scare him more, causing him to slow down even more on the next corner, which would cause yet more instability, and so on and so on in a vicious downward spiral; it took almost half an hour of my practically shouting at him (NO!!! DON'T BACK OFF!!! AAAGGGHH!) before he finally stepped on the gas at corner entry (by that point, we were crawling so slowly there was no way we could crash). The effect was astounding -- even my completely non-motorhead roomate (great guy, but a much better chemist than a driver) broke out into a wide grin as the previously "dangerous" car hunkered down and shot through the curve; after that he fed in lots of power on every tight turn, and we made great progress through the Berkshires...)

Where was I? Oh, yes -- great point, blacka5! When we combine your friction circle effects with how the circles themselves grown and shrink with forward/back weight transfer and camber changes (you can think of tire camber as making the circles long & skinny, or better yet of becoming laterally asymmetric), we'll have a much more complete picture of how a car's handling balance changes.
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Old March 30th, 2006, 16:06   #30
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Quote:
Originally Posted by 4vdubs
How do we make a MkIV better?
Replace it with a MkV...
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