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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 30th, 2006, 16:40   #31
peter pyce
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Few more and we are done with the copy-paste and then will extend with "modern" from 2006 thoughts..... Ceilidh all the way to the end:

7. Roll Centers and Weight Transfer

We're going to eventually get to the GT suspension, but there are still a few odds & ends to clean up. For this installment, you can decide whether you'll want to read it by first seeing how you do on a little quickie quiz:

Answer True or False:

1. Reducing the amount of body roll while cornering will improve performance by reducing the lateral weight transfer on the tires.

2. When a car rolls, it compresses the springs on the outside tires, which overloads the tires and reduces their effective traction. A car that rolls less will compress the outside springs less, which will load the tires less, which gives them better traction.

3. If you lower a car, it will roll less.

4. If you drop the front roll center and keep everything else the same, the car will understeer more.

5. Springs and antiroll bars are the primary factors that determine how lateral weight transfer is distributed fore & aft (which, together with camber effects, determines whether the car understeers or oversteers).

Ok, did you take the quiz? Honestly? With no peeking? Ok, then here are the answers:

(drum roll please....)......If you answered anything other than "False" for all of the questions, and if you care about suspension theory, you might want to read parts of this installment. =)


1 & 2 -- Why Reducing Roll Does Not Affect Weight Transfer

Ok, first thing is to get lateral weight transfer squared away: when a car goes around a corner, weight is transferred from the inside tires to the outside tires, and the amount of weight transferred is equal to:

(lateral g-force) * (car mass) * (CG height) / (track width)

Note that spring rate and antiroll bar rate have nothing to do with it. When a car goes around a corner, the weight transfer will occur regardless of whether its suspension is pillowy soft or absolutely locked solid. Making the springs stiffer will not reduce the weight transfer, and thus stiffer springs will not directly make the car perform better (if there's a performance benefit, it's an indirect gain from less tire camber, better control of unsprung weight, or less body movement over bumps, etc., as discussed in earlier installments).

Similarly, roll doesn't enter into the equation either (to be really accurate, it actually does to a very small extent, in that the CG shifts sideways when the car rolls -- but whilst this is a concern for big SUVs, buses, etc with a lot of roll and a very high CG relative to track width, for our cars it's small enough to basically ignore). Roll can do a lot of horrible things for handling (via tire cambers, body transients, etc.), but weight transfer is not the main issue. Hence any spring vendor that tells you that "stiff performance springs reduce weight transfer by reducing body roll" is either (A) clueless or (B) dishonest; either way you'll not want to trust him.

(Aside: So what does affect weight transfer? Track, and CG height. Here, for once, the advertising arguments do have a physical basis: if you drop the CG, you'll reduce the total lateral weight transfer, and if everything else stays the same, your handling will improve. Similarly, if you widen the track, you'll get less transfer, and if everything else stays the same, your cornering power will improve. That's why true race cars are built with the CG as low as possible and with the track width usually at the limits of regulation.

The hitch of course is the phrase "if everything else stays the same": Everything else never does stay the same. Dropping the CG by installing lowering springs causes a host of problems (discussed in this thread and more completely in the forum stickies). Widening the track by installing huge wheel spacers will destroy your bearings and screw up your steering geometry, which is also slow. All in all, "improving" your car by reducing weight transfer is not a good way to begin.)


3. Why Lowering Doesn't Necessarily Mean Less Roll

Here's another issue that has been amply discussed elsewhere, but on the off-chance there are novices who have been reading about roll centers, but don't really know what they are, here's a (very) abbreviated explanation:

a) Imagine a car with solid axles (meaning, there's literally a big beam running at hub-height from left wheel to right wheel, straight across each end of the car). Now imagine that you've drilled a horizontal hole straight through the midpoint of each axle, and that you've bolted the body of the car to the axles using those holes. If I've painted the picture correctly, you have a "suspension" where the wheels on each axle can't go up & down together, but they can pivot: if the left wheel goes up, the right wheel goes down, and vice versa. Set this strangely designed car on a level ground, steer it around a corner, and it will roll -- and the axis about which the car rolls will be the centrepoint of each axle (because that's where the body is bolted to the axles). In this situation, the axle centrepoint -- where the body is bolted to the axle -- is the "roll center".

b) In situation "a", the roll center is a physical thing -- you can see it and touch it, and it's really easy to see how the car rolls around it. There are some cars (well, there used to be...) that have such a physical roll center (chiefly some DeDion suspensions from WWII days), but unfortunately (for visualization purposes) that's not the norm. Nowadays we have suspensions where the roll center is not a physical pivot point, but a "virtual" point that moves around as the car shifts about on its suspension. But although the roll center is typically a virtual object, it controls the car's motions exactly the way that a physical pivot does. So if you have trouble imagining what the roll center does to a car, just imagine a solid, physical pivot sitting where the roll center is said to lie....

c) Now let's do some physics (shudder): suppose the car body in "a" has a center of gravity that lies 18" above the ground. How do we get it so that this car does not roll at all when it goes around corners? That's easy: we give it 36" wheels (overall diameter, wheel & tire together). In that situation, the physical roll center is sitting at 18" (axle height), which means the center of gravity (CG) is sitting directly on the roll axis (sorry -- the "roll axis" is the line connecting the two roll centers -- here it's a horizontal line running fore & aft 18" above the ground). Since the CG is sitting on the roll axis, there's no tendency to roll. So the car corners flat -- even though we don't have any springs or antiroll bars.

d) What happens if we make the wheels, say, 48" in diameter, so that the CG of the car body (at 18") is lower than the roll axis (at 24")? That's interesting. Now the body of the car is basically hanging down from the roll centers (from the physical pivots, in this example), and it'll actually bank into turns, the same way a bucket will "bank" into turns if you carry it by the handle.

e) Conversely, what happens if we drop the roll center way down, all the way to the ground? The car rolls, a lot, as now the CG is above the roll center.

f) In practice, automotive roll centers always like below the CG (situation "e"). Given that, a car's tendency to roll depends upon how far the CG is above the roll center. The greater that distance, the more the car wants to roll.

g) Now, if you go to the "Lowering" sticky at the top of the forum, you'll see that if you lower a McPherson strut suspension, the roll center drops faster than the CG -- which means that the distance between CG and roll center grows, which means that you have more tendency to roll This phenomenon is really well-discussed elsewhere, so we'll say no more about it.

h) What we will mention in passing, however, is what happens at the back of the Golf/Jetta IV: for everything other than the R32, we have a twist beam rear axle, and the roll center in such a setup lies right in the middle of the twist beam. If you go look at this axle on your car, you'll see that it doesn't behave like the McPherson front: if you lower the rear of the car, the CG drops (because you're lowering the car), but the twist beam doesn't drop quite as much (because of the way the trailing arm is pivoted: the wheel end doesn't drop at all, while the pivot end drops with the body....and the twist beam is in between). Hence when you drop the rear end of these cars, you simultaneously lower the CG while slightly reducing the tendency to roll. Thus you're free to lower the rear end of the car pretty much all you want -- theoretical roadholding will improve because of the (modest) reduction in roll and (modestly) lowered CG.

And so, if you've ever wondered why the Shine setup looks so, um, interesting, that's why: Shine keeps the front high, for all the roll center and camber gain issues discussed in the previous installments, while dropping the rear for less weight transfer and less roll.

(continued next post)
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Old March 30th, 2006, 16:42   #32
peter pyce
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(continued from above on Roll Centers and Weight Transfer)

4 and 5) Roll Centers and Understeer / Oversteer

First off, let's be hastily clear: if you blithely lower the front of you Golf/Jetta IV, it will understeer more -- that's because you'll drop the roll center faster than the CG falls, the car will roll more, and the front tires will go more into adverse camber. But the question in #4 was subtly different: it was asking what is the effect of roll center height on understeer/ oversteer?

If we had a way to drop the front roll center without changing any other parameter: the car somehow rolls exactly the same as before, the spring rates are the same, the CG lies at the same point, etc., then the car will understeer less than before. For the front of the car, this kind of a moot point -- we have no way of dropping the roll center without screwing up the roll, and there are many good reasons in any case for keeping the roll center where it is -- but it's of interest for the rear, and we'll get to it via our solid-axle thought example:

a) Remember our solid-axle car with the body physically bolted to the axles? Think back to the case where the wheels are 36" in diameter and the CG is at 18" -- this is the car where the CG lies on the roll axis, and the car won't roll at all in a turn, even if it has no springs.

b) Will there be lateral weight transfer when this car goes around a corner? Absolutely -- like we said before, there's going to be weight transfer regardless of whether the car rolls.

c) Ok, next question: if this car has no springs, but there's weight transfer....what exactly is causing the weight transfer to occur? We usually think of the car rolling, and compressing the outside spring, and thus putting more weight on the outside wheel, but here there's not spring -- and yet there's weight transfer. So what gives?

d) The answer is that the roll centers themselves cause a lot of the weight transfer to occur on a car.

e) If the roll centers are up at the CG height, all the weight transfer (more exactly, all the transfer for the sprung mass) goes through the roll centers, and none of it goes through the springs. If the roll centers are on the ground, then all of it goes through the springs, and none of it goes through the roll centers. If the roll centers are halfway between the ground and the CG height, then half of the weight transfer goes through the roll centers, and half of it goes through the springs. And so on. The higher the roll center, the more weight transfer that goes through it.

f) Hence, if you keep springing constant, and play with roll center heights, you can change the fore & aft distribution of weight transfer. Start with a neutral car with 50/50 weight distribution, uniformly high roll centers, equal springs front & rear, etc. Drop the roll center in the front and raise the roll center at the rear, and more weight is transferred at the back, so the car oversteers. Raise the front roll center while dropping the rear, and you get more front transfer, and hence you get understeer.

f) The reason most of you have probably never read about this phenomenon is that on most performance cars, it's not an issue. There are many good reasons to design a suspension such that the roll center is just above the ground. If you do this, not a whole lot of weight transfer goes through the roll centers, and the weight that does is pretty balanced (because both front and rear centers are uniformly low). Thus if you spend your time reading about performance car suspensions (and who wants to read about non-performance car suspensions?), you won't hear much about roll-center effects on lateral weight transfer.

g) Unfortunately, however, the Golf/Jetta IV has a twist beam rear suspension, about which Milliken & Milliken are only able to say (p. 661): "This family of rear suspensions is....used on front wheel drive cars. The only time one would be on a racing car is in a showroom stock-type class."

For this suspension, the roll center (up in the twist beam) is unusually high. On my GTI I've measured it at 8.5"; on a brand-new Golf it'd be at around 9.5". This height is not quite so extreme as what you'd find on a vintage British race car (on my MGB it was about 12"), but it's still high enough to cause a lot of weight transfer that has nothing to do with the springs. On my MGB (for which we knew enough to do a reasonable analysis), over half the weight transfer at the rear was due to the roll center height, and less than half was controlled by the springs. For the GTI (for which nobody seems to have reliable information on even basic things such as stock spring rates), I would guess that maybe a third of the weight transfer if roll-center-controlled.

What does this mean? We'll use this tidbit in later discussions, but for now it's just another reason why if we want to appreciably improve the Golf/Jetta IV suspension (for handling), we should be focusing on the front. Simply put, there's a lot of lateral weight transfer already occurring at the back of a Golf/Jetta IV: in addition to the springs, the twist-beam, and the stock antiroll bar inside the twist-beam, we also have a really high roll center that's throwing an awful lot of weight onto the outside wheel -- and with all that, the car still understeers a ton under hard cornering. When you read a tuning guide urging a stiffer rear to curb understeer, the guide is generally assuming you're starting with only a moderate amount of rear weight transfer in stock form, in which case a small change can have a big effect; with our cars, however, we start with a lot, and there's only so much stiffening we can do back there before we start getting into pathological cases (note: we're not saying that stiff rear bars aren't good -- but if you want to use a big rear bar, you ought to stiffen things at the front first).

Or to put it a different way: apparently this thread really got going when Peter Pyce went to stiff fronts and a soft rear, and many people told him he was crazy, that he would understeer off the road, that there was no way this suspension could work, etc. These people assumed (quite reasonably, given everything that's written out there) that such a setup would cause much more weight transfer in the front than in the rear, which would cause massive understeer. But in fact, there's a heck of a lot of weight transfer occurring at the rear for purely geometric reasons, and it's possible that in stock form there's already more transfer at the rear than at the front (no, I haven't done an analysis, so I'm not sure -- it's not an uncommon situation, however, for passenger cars with a high rear roll center and an overall abundance of stock roll); if so, the only thing keeping the stock car from oversteering off the road is the extreme amount of tire leaning that takes place at the front while cornering. By putting on stiff front springs, Peter has probably brought the F/R lateral weight transfer distribution closer to a more traditional balance (with the front maybe 10% more than the rear), while keeping the front tires more upright through reduced roll.

Anyway, all the above is just a conjecture until we can build a spreadsheet model (if ever). Next installment will finally get into GT suspensions, now that we have all the conceptual tools. See you all next week!

- Ceilidh
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Old March 30th, 2006, 16:58   #33
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Oh my -- I'm not keeping up with Peter's cutting and pasting!!

Here's what we'll do for now: let's conceptually divide this thread into two separate subthreads. One subthread is the cut & paste, consisting of rather long, quite detailed explanations of suspension theory -- call that the "Deep Knowledge". The other thread is what we're composing in 2006 for you, dear TDI reader , these posts are intended to be a little more accessible to the normal non-physicist enthusiast. Hopefully if you can keep these themes separate in your mind, you'll be able to make sense of things!

Anyway, moving right along (with the backlogged comments):

3) bhtooefr and 4Vdubs:
We'll (hopefully) get to your topics, all in good time! bhtooefr, I had an A2 before my current A4 GTI, and I know exactly what you mean: the A2 was a much better darter chassis! It was better not because it had less inherent understeer, but because of its much more immediate agility (which we'll get to in a few weeks -- sorry for the delay), which among other things allowed one to throw it briefly into oversteer almost at will (while retaining lots of background understeer so that these brief fun oversteer moments didn't lead to catastrophic trips into the shrubbery). Along those lines, 4Vdubs, if you want to do some experimenting and reporting back to the masses, try this sometime: find a nice, big, empty parking lot with nothing to hit, and drive a smooth big circle (essentially, do a skidpad run) until the car is maxed out; then do sort of the same on a smooth dirt road curve (again, with nothing to hit!!). If your experience is similar to those of Peter's (in a slightly different context), you should find that you have more oversteer on dirt than you do on smooth asphalt (though if the asphalt is bumpy, you might have oversteer there too). If it's not perfectly safe, don't try any of this! But if you have a chance, let us know what happens (and if you get a different result, please let us know exactly how you performed the test, as these variances are extremely useful!).

In any case, I have a feeling both of you might be interested in the agility section, which is forthcoming...


4) Golf_GTDI

Yes, let's ask Peter for pictures & animations! (And I really look forward to hearing more reports-from-the-field from you -- thanks for posting!)


5) BoosTDIt

Agreed -- A2s make great darters!! (I so miss my A2 Golf sometimes....)


6) oldpoopie and IndigoBlueWagon

Sigh. I never thought of myself as an older veteran, but your posts made me realize that, yes, I'm no longer one of them young whippersnapper types with their new-fangled iPod thingamajigs, no sirree....

Anyway, very glad to have met some folks who can understand!! I learned to drive in a Slant Six Dodge Dart, and my first car was a Spitfire (with 145/80-13 tires in the rear, and 155/80-13 in the front -- talk about oversteer!! I used to love roundabouts (there were tons of them in Newcastle upon Tyne): downshift to second, turn in, nail the throttle, and gosh the rear end would skate! A friend had an 850 Mini, which was a riot on the singletrack along the Scottish borders, and another friend had a (post-Frogeye) Sprite, in which he took me on the fastest slow-motion city drive I've ever been on: it was in London, and at every intersection he had the rear end far, far out (he was a better driver than I) -- and at no time did we deviate from the normal traffic flow: as far as the Bobbies could see, we were just driving long with everybody else.... Oh(!) -- and the high point of my university year was a visit to the Westfield factory (dragging along another non-motoring roomate), where they gave us test rides (they drove, we rode) in their Lotus XI replica (no windscreen, no helmet): during my ride, I asked the driver "Can you hang the tail out?", thinking he'd do it at the next corner, and the guy said "Sure!", jerked the wheel on the straight, and we were suddenly at a 20-degree drift at 80mph...

(Oh, for all you young whippersnappers: with these old sportscars, oversteer didn't have to be noisy. You could bring the rear end out, and hold it out, without the tires making any unusual sound at all (at least nothing loud enough to be heard over the general sound of an English car continuously falling apart -- these things didn't have Japanese or German build quality). I wish there were some way to magically teleport you all back in time, and put you in these cars (someplace in Europe): they were safe, slow, fun, and incredibly, incredibly responsive to what you did as a driver. Drive them badly, and they handled really badly; drive them well, and they made you feel like a hero.)

Digressionary Note #5:
Even among automotive journalists, there's a generational divide on what makes for a good-handling car. The older folks (e.g., George Karcher with "Car" and "Autoweek", Dennis Simminaitis and Peter Egan with "Road and Track") don't care very much about how much grip a car has; in many ways, they even prefer a car that slips fairly early, so long as it does so progressively & predictably, while remaining responsive to driver inputs. In contrast, the younger writers are all about grip and agility -- how well the tires stay glued to the road, and how much a tug on the steering wheel leads IMMEDIATELY to a change in direction and attitude. Goodness knows why this generational divide exists, but I've a hunch some of it has to do with the cars and tires the writers grew up with: when the older guys came of age, tires didn't grip, and trial lawyers hadn't yet met Ralph Nader, so cars were constantly slipping and sliding, and a fun car was one that did what the driver told it to do (regardless of whether the instructions were good or not); conversely, the young guys all grew up when tires were sticky and literal responsiveness (Driver to Car: "I'm going to use my pedals and steering wheel in a way that will spin you off that high cliff above the petrol station next to the orphanage." Car to Driver: "Yes sir!! Right away, sir!!") was no longer a legal option -- with cars like that, g-forces and (safe) responsiveness to the steering wheel are what make for thrills and fun.

Anyway, we'll never get a Golf/Jetta to handle like a 1950s Austin Healey (heck, even the Miata apparently no longer tries to handle like that), but we'll see if we can get a little more (safe) responsiveness into our cars.
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Old March 30th, 2006, 17:14   #34
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Default mea culpa! mea culpa!

Quote:
Originally Posted by peter pyce
(continued from above on Roll Centers and Weight Transfer)

......the Golf/Jetta IV has a twist beam rear suspension, about which Milliken & Milliken are only able to say (p. 661): "This family of rear suspensions is....used on front wheel drive cars. The only time one would be on a racing car is in a showroom stock-type class."

For this suspension, the roll center (up in the twist beam) is unusually high. On my GTI I've measured it at 8.5"; on a brand-new Golf it'd be at around 9.5". This height is not quite so extreme as what you'd find on a vintage British race car (on my MGB it was about 12"), but it's still high enough to cause a lot of weight transfer that has nothing to do with the springs.....

- Ceilidh
Alas, there was an error in that Vortex quote of 2 years ago: when I wrote the above paragraphs, I had been lazy, and had simply read some numbers off a published suspension diagram without working through the geometry on my own. And, well, laziness is a bad thing -- when I subsequently plotted things out on a sheet of paper and thought it through, I discovered that (1) there was a misprint on the published diagram, and (2) I was completely wrong.

So here's the correction: the roll center at the rear of the A4 suspension is very close to the ground (just a fraction of an inch above), and it doesn't move very much when you lower the car. Hence with the stock car (which has the front roll center also near the ground), there's relatively little roll-center effect in determining the lateral-weight-transfer distribution, and the latter IS controlled primarily by the springs and bars (i.e., Quiz Question #5 should be answered "True"). But the end conclusion is still valid: when you lower an A4 chassis, the front roll center drops and the rear center stays constant, so you wind up shifting the distribution rearwards.

Anyway, my apologies for the error!

- Ceilidh

Last edited by Ceilidh; March 30th, 2006 at 18:00.
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Old March 30th, 2006, 17:20   #35
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Sorry Winston, I tried to read everything once again before transferring it here, but that part slipped. I do even remember the following discussion with Dick about this and yet simply did not see through when was reading it again today. Good catch and thanks for reading it again in detail
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Old March 31st, 2006, 01:44   #36
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Thank you folks again so much for bringing this info here. I wish I had better input but for now I'm gonna try to shut up as much possible and just listen and try to understand. As the weather is better and the road surfaces improve I feel I am getting a better understanding of how my current system is working and look forward to working with Peter to help build as true a GT type system as we can get to with our cars. I tend to think that I may be a bit of a GT type in that I find myself enjoying covering distance on fun roads at speeds that tend to be a bit quick but feel that the car is well collected and safe. With that said I think I would like to push past this seeing as I want to get past these limitations as I just don't feel I can find the limits of what I have at this point in a safe way on public roads and aim to spend some time on real tracks this summer.

In short I aim to lend some seat of the pants thoughts with a system that hopefully ever closer to what these folks are trying to build. If you don't mind Peter I may try to give you a call in the next few days to these ends.
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Old March 31st, 2006, 08:38   #37
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Quote:
Originally Posted by Ceilidh
...

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...
Yes, Ceilidh - a very interesting course. Unfortunately it's been over for a year, and I'm pretty busy finishing up my last three to graduate in May.

It was by far the most fun class I've ever taken. It was actually a course in Modeling & Simulation, but using an auto as the subject. As an added bonus, the prof is a bona-fide car nut. For that semester wei used a Saleen S7... the end goal was to critique/compare our results to the advertised performance specs. Turns out its top speed is RPM-limited... and it could drive upside down if you could find such a road while going fast enough. We played with steady-state grip and transition/emergency lane change maneuvers to investigate effects of springs, dampers, roll bars, and geometries on handling and ride quality. We also did the same for school buses, tractors, etc... the trick is getting all the data to plug in.

A pretty comprehensive overview, but in one semester you have to make some generalizations to keep things simpler (we neglected the effects of changing camber and modeled the clutch as an on/off linkage, for example). Once I finish my degree, I'd like to continue my model-building on my own and add that fidelity... then the hard part is quantifying the actual products out there (including OEMs - they don't like to share, and would tend to wonder about "consumers" asking what the actual damping/spring rates, etc are)

We used Matlab/Simulink... it doesn't get much better than that - endless possibilities. If you can get the student version, you can buy a copy for around $99 (that's what it cost then)... it just doesn't include much support or any updates (really the only way to get that is to be the independently wealthy, mad scientist type, or work for a company that owns a license - I might be mad, but not wealthy, so I'm stuck with my older version, but it works!)

I lived in Cloudcroft and drove US 82 between there and Alamogordo to work every day: 16 miles + 4,600 ft elevation change + lots of curves = FUN! I've been up around Durango/Ouray/Silverton, CO a couple times - also breathtaking.

Well, I've gotta get back to work now... I really enjoy the discussion - Thanks!
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Old March 31st, 2006, 09:09   #38
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Ceilidh - I forgot:

One interesting thing I haven't heard here yet is talk about natural frequency in steering response... I forget the actual physical ways it's controlled/derived right now, but remember the lesson well:

As vehicle speed increases, natural frequency decreases. In other words, rocking the steering wheel back and forth at, say 1 Hz, doesn't do much but weave the car around at low speeds, like on a city street.... but... if you do the same thing at the right speed, say 70 or 80 mph, you can get a real tank-slapper (to borrow a term from motorcycling) going with one or two cycles. Our professor urged us to try it sometime... it's eerie.

Any fully-engineered setup mitigates pathological behavior, but that natural frequency is still there beneath, no matter what you do. Worn-out components can negate proper controls, though... my old IROC (sold it to buy my TDI ) with worn tie rods and wheel bearings could get very excited and start a nasty steering shimmy when going just the right speed and hitting a bump just so... usually at low speed, 25-30mph, solved by lightly accelerating. But then I once hit the next harmonic at about 60 mph once in a sweeping off-ramp... the rear end started sliding around - scary religious experience... dumb me for driving it worn.

When I get some time I'll look back through my notes and post a little more about it - not sure how much a factor it is in these discussions, but it's interesting...
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Old March 31st, 2006, 13:58   #39
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This is an awesome thread. Sticky it is ...
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Old April 3rd, 2006, 08:31   #40
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Quote:
Originally Posted by Nate_Grauvogel
my old IROC (sold it to buy my TDI ) with worn tie rods and wheel bearings could get very excited and start a nasty steering shimmy when going just the right speed and hitting a bump just so... usually at low speed, 25-30mph, solved by lightly accelerating. But then I once hit the next harmonic at about 60 mph once in a sweeping off-ramp... the rear end started sliding around - scary religious experience... dumb me for driving it worn.
I must say that I still honestly wonder about some of the things I am finding in my own system. I hope to see this touched on at some point in more detail.
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Old April 3rd, 2006, 11:59   #41
peter pyce
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Let’s put some of the key scenarios discussed here into pictures, so it will be easier to comprehend and build from there….

Quote:
Originally Posted by Ceilidh
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.


Here is the front of our cars. Little bit schematic, but the dimensions are correct, so the movements are pretty close to reality. The driver is sitting on the Left side of the animation, so the front is towards the screen you are looking at and the rear of the car is behind your back. The car has both wheels steering to the right, making a right curve, going forward and rolling to the left. So, the left wheel is the left front wheel of the car and the right is, of course, the right front wheel of the car:

(Note: It is a GIF animation, so wait till it loads for smooth view. Sorry for the folks with Modems, there is really no way to animate those things in large format and make them small at the same time)



So, above we have the car from flat (straight line) rolling to about 6 degree towards the left (in a right curve), which is more or less the max roll angles ones would get when going wild. To relate to the text above – we can see how the left tire goes into positive camber pretty dramatically. At the same time, the right tire (inner to the curve) goes into negative big time. Now, with the help of those packages used to model those things, we can record the degrees of total inclination the tire/wheel has in relation to the road. Below is a diagram that puts those numbers (generated from the animation above) in a simple chart, so ones can see what degree of camber we have on the front wheel when the chassis rolls degree by degree:



Let’s concentrate on the dark blue line in the above chart (that line represents the A4 front geometry). This chart is exactly for the above scenario (in the animation) as we have the steering at 10 degree. So, at 0 degree body roll (horizontal axis on the chart) we have about -1.5 degree front camber (This is all on the outer, left wheel only!). The 10 degree steering is the reason the camber is already negative, all due to the caster on these cars, but perhaps will go in depth on that later….. We roll the car one degree and according to the chart the total camber the wheel has now (related to the road!) is about -0,6 degree. Then we roll another degree (to 2) and the camber is already positive (!) at about +0,3 degree. Then 3 degree roll gives us +1.2 degree positive. 4 degree of roll puts us at +2,3 degree and so on, to a total of about +4.4 degree of positive camber at full roll when the car will stop rolling as it will start sliding and no more roll gain will be possible.

(As a side note, a stock car on stock tires may not even get that far as the all season M+S tires should give up a lot earlier and the car will slide instead of rolling. But let’s leave this discussion for later).

Let’s move to the rear now…

Quote:
Originally Posted by Ceilidh
….. 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, let’s look at this animation, representing the rear twist beam. This time the camera looks from the rear right corner, down on the assembly. The lower right “disc” represents the rear right wheel and the upper left “disc” represents the left rear wheel. The red “beam” is nothing else but representative of the chassis (the car) that we see here rolling left and right. This animation is not as clear as the first (of the front) so if anyone has trouble visualizing the movements we are going to talk about below, let me know and perhaps we can do a different angle animation:

(Note: it is another heavy GIF file, so wait till it loads for smooth view)



Let’s observe the lower right “disc” (as it is closer to the camera and easier to see its movements). It is pretty visible how the “disc” (representing the wheel) gains and loses camber as the read beam (the car) rolls. If we do the same data collecting as per the Front Geometry, then the following chart comes out:



Here we have both wheels (inner and outer) and their related camber gain/loss. We can see the outer wheel starts from -1.65 negative (that is what our cars come with in general as static rear camber), and by rolling at about 5 degree (body roll) it goes to about +1.62 positive! (we are looking at the blue line as that is the one of our interest in this scenario).

So, to put it now together. As per above quote, when the car rolls, the front has to gin more positive camber than the rear per every degree of roll. Let’s put now all the graphs together and see if we have it that way:



The dark blue line is the outer (left) front wheel and the light green line is the outer (left) rear wheel. (both lines derive from the other two diagrams shown so far). So, we can see how the blue line gains a lot more positive camber than the green line as the car rolls, to a point that at pretty full roll at about 5 degree (car roll) we have more than one degree of difference in front to rear camber!

Last edited by peter pyce; April 3rd, 2006 at 12:03.
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Old April 3rd, 2006, 13:23   #42
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Thanks, guys! I have to confess to reading the VWVortex Suspension FAQ on geometry more than once, and spend entirely too much time thinking about these things. But Peter, you wrote something in your first post that just helped me figure out something about my wagon. I had the SRS setup and thought it rode too harsh, but now understand my problem was the tires. I took out the springs and dampers and now run Koni Reds with the stock springs--but the Shine rear bar is still in place. I'm taking a driver's school this weekend at NHIS and today decided to bring my A3, which has Neuspeed Sofsports, Koni Reds, and a Neuspeed rear bar on the middle setting. Why? For reasons I couldn't explain (until now) I feel more comfortable in the A3. I just felt like I didn't know (or wasn't going to like) what the car would do at the limit. It feels great at 5/10s, but occasionally I feel like I'm headed into no-man's land. And I have lost the rear end (once) in the snow, even with the Shine springs.

Now I know what I was feeling was the approaching terminal understeer when a rear wheel lifts. It tempts me to take the Shine bar out and see what I have.

EDIT: A walk and a cocktail and now I have more ideas. What happens if I keep all else the same but install TT spindles that will keep the front camber (and therefore the grip) more favorable? Will the car still transition to terminal understeer, just at a higher cornering load? Or will it become more neutral at the limit?
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Old April 3rd, 2006, 20:46   #43
Ceilidh
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Quote:
Originally Posted by peter pyce
.......So, we can see how the blue line gains a lot more positive camber than the green line as the car rolls, to a point that at pretty full roll at about 5 degree (car roll) we have more than one degree of difference in front to rear camber!
Despite best intentions, this thread is looking like it's going to jump around a bit(!), but rather than let something pass for too long, here's a little preview:

At some point (a point that steadily recedes in time, what with all the interesting diversions along the way), Peter and I hope to write up a little guide explaining what different sorts of modifications do to different aspects of handling. That "guide" is still a long ways off, but now that Peter's posted these excellent graphs of the A4 camber change with roll, it seems a good time to (briefly) mention the TT front spindle option.

For those not familiar with this option: there exists a reasonably straightforward OEM (VW) modification that will "cure" the "excessively understeering" camber curve in the A4 Golf/Jetta chassis. This mod basically consists of taking the front spindles off an Audi TT, and installing them in place of the stock Golf/Jetta items. Do that, and you have a slower front camber "degradation" with roll, with much less difference between front & rear camber angles even at extreme roll angles. Since the front camber plays such a major role in producing roll-understeer in our cars (because of the camber-thrust effect discussed in the cut&paste), reducing the front-vs-rear camber difference will significantly reduce the understeer in a hard corner, while still keeping the front tires more or less upright when driving in a straight line (which is good for tire wear, straightline braking, street steering feel, etc., etc.). Moreover, because the spindles are actual VW parts, the conversion can be done fairly neatly, with none of the cursing & filing & fudging that normally comes with installing aftermarket "performance" parts.

Sounds good eh?

Now, the reason we're bringing up this option now (immediately after Peter's posts and cut&pastes) is that it's a great opportunity to try applying some of the concepts posted up over the last two weeks. The TT spindle option (as regular Vortex readers will likely know) is heavily favored by some tuners & autocrossers/track enthusiasts, and indeed Peter and I would never argue that it isn't a plausible mod for racers and the like.......

But --

How will this mod behave on the street?

Let's apply the concepts. If we look at Peter's graphs, we can see that the TT spindle buys you a little less than 1 degree of camber when the car is rolling 5 degrees -- and (not) coincidentally, 1 degree is roughly the front-vs-rear difference in camber gain for the stock chassis at 5 degrees of roll. Thus the TT spindle comes close to wiping out the differential camber gain: when you install it on the Golf/Jetta chassis, you basically make the front and rear wheels behave similarly in a corner.

Now, earlier we talked about how a racecar suspension (one that keeps both front & rear tires essentially upright in a corner) will feel very strange for the normal non-racer driver: if set up to understeer, it will seem to track solidly up to a point, and then suddenly and irretrievably break away at the front; if set up with oversteer, it will feel solid until the moment that it seems to snap into a dramatic and breathtaking spin. (This behaviour arises because (1) in an upright-wheel car, over- or understeer will set in only when g-forces overwhelm the tires; (2) the transition to the tires' non-linear state is very hard for anyone to detect until he/she has spent lots of time lapping a track at high speeds; and (3) once the overloaded tire "lets go", it doesn't respond very well to further driver inputs.)

In the preceding posts, we discussed this "bad" behaviour solely in the context of an upright-tire racecar suspension -- but the behaviour doesn't hinge upon the tires remaining upright. Instead, the "sudden" breakaway is something that occurs when we rely only upon tire characteristics for producing under- or over-steer. Such is certainly the case for an ideal race car suspension in which the tires are kept bolt upright -- but it is also the case for a suspension in which both the front and rear tires are allowed to lean the same amount: basically, if both sets of tires "lose" the same amount due to camber, then the camber is not affecting the handling balance.

Or in other words, the TT-spindle, when mounted on a Golf/Jetta, makes the car behave in a way reminiscent of an upright-tire racing suspension.

This is great on the track. It is great on an autocross circuit. It is even great for a very attentive, trained driver on the street, or perhaps for a slightly less attentive, less trained driver who never approaches the limits of his suspension. But what will it do for a "normal" driver (or a fatigued highly-skilled driver) at the limit? Well, if the grip is lost via understeer, the car will plow straight off the road, with the steering wheel rather useless. If it's lost via oversteer, it will be the sort of oversteer that most non-race drivers have never experienced: if the driver tries to slow down, via braking or even via reducing engine power, the car will not stabilize (the way that the stock Golf/Jetta is designed to stabilize), but will instead snap spin (this because any attempt to slow down will transfer more weight to the front tires, giving them more traction, while further unweighting and reducing the traction in the rears). Indeed the only way to stop a spin once oversteer sets in (with a FWD car on such a suspension) is to apply more power -- not something that comes naturally to anyone(!).

It's for the above reason that yours truly (Ceilidh) has no intention of ever trying a Golf/Jetta with a TT suspension: I'm sure the car will feel absolutely marvelous up to the moment that the tires let go, and I'm equally sure that (my racecar days being many years ago) my current skills and reflexes will not suffice to catch the slide or spin. On a racetrack my incompetence will be embarrassing; and on the road, well, I don't like hitting things.

Final Point:
Regular Vortex readers might recall a sort of TT-spindle forum war about two years ago: basically, there were respected individuals who had worked out how to perform the conversion, who had tried the mod and loved it, and who were urging others to try it; on the other side of the fence was/is yours truly, who feels that this is essentially a dangerous mod that should not be sold for street use. The war came out as a sort of draw (nobody changed their positions), and you, dear TDIclub reader should form your own opinions (should you ever contemplate this mod for yourself), but I'll leave you with an historical tidbit, coupled with a bit more theory:

1) VW owns Audi, and is entirely aware of what the TT spindle will do for the A4 chassis (they have to be aware -- they're the ones who designed and built the spindle!). And yet they have chosen not to mount it on the ordinary Golf/Jetta -- presumably there's a reason.

2) Besides shifting the Golf/Jetta towards an upright-tire suspension feel, the TT spindle conversion will reduce the Golf/Jetta's inherent understeer -- particularly at high cornering loads. Thus in contrast to the stock suspension, which badly wants to understeer when cornered hard on dry pavement, a TT-modded car will be much more easily prodded into oversteer, should the driver do something wrong....

3) The classic oversteer-promoter on dry pavement is a driver who, upon realizing he's come into a fast corner with too much speed, attempts to slow down by easing off the power. When he does so, the forward weight transfer (from decelerating) reduces the rear grip, causing oversteer. The amount of weight transfer (and hence the violence of the oversteer) depends partly upon the severity of the deceleration, and partly upon the car's CG (center of gravity) height: the higher the CG, the more violent the weight transfer, and the more sudden / stronger the transition towards oversteer. Because the CG in a Golf/Jetta is higher than the CG in an Audi TT, the "lift-throttle-oversteer" in a TT-modded Golf/Jetta will hence be stronger and more sudden than it would be in a comparable Audi TT....

4) When the Audi TT was first introduced, journalists and reviewers praised the sharpness of its handling. But then TTs began crashing, with a lengthy spate of accidents (with injuries and deaths) all stemming from sudden oversteer and spins, particularly in Germany, among drivers who were not trained racecar drivers, but who were instead individuals who had learned to drive fast on street-tuned cars (i.e., cars with progressive and stabilizing understeer). Within the year, Audi recalled the cars, enacted a comprehensive suite of suspension modifications, and installed electronic stability control. The public explanation for the oversteer was aerodynamic instability (which probably played a factor), which was "cured" by a retrofitted rear spoilier --- and yet, all subsequent tests of the TT showed the car to have lost its handling sharpness at all speeds: the car simply understeered more, had slower responses, and was much harder to steer on the throttle.

So in putting pieces together: if someone looks at Peter's graph and tells you "Obviously you'll want to fit TT spindles to your A4 Golf/Jetta, as those spindles will cure your understeer and make your car handle like a dream!", consider the above: VW itself is unwilling to fit this modification, one that will dramatically reduce the car's stabilizing understeer at all speeds -- but particularly at the highest speeds and cornering loads. The modification will likely give the Golf/Jetta a more violent and more sudden transition to oversteer than is felt by the Audi TT -- and the Audi TT itself, when first given these spindles, had an oversteer transition too violent for German drivers, and presumably too sudden even now (after a host of chassis mods) for the car to be sold in the U.S. without electronic stability control.

Kind of interesting, what one can deduce via Peter's graphs, yes?

Cheers folks!
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Old April 3rd, 2006, 20:58   #44
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Quote:
Originally Posted by IndigoBlueWagon



EDIT: A walk and a cocktail and now I have more ideas. What happens if I keep all else the same but install TT spindles that will keep the front camber (and therefore the grip) more favorable? Will the car still transition to terminal understeer, just at a higher cornering load? Or will it become more neutral at the limit?
Hello Indigo!

Your edit was well-timed! -- you must have put it in while I was writing the above post.

In answer to your question: whilst I'm uncertain whether the final state will be terminal understeer or terminal oversteer (it'll probably still be steady-state understeer, given the car's weight distribution), we'd expect the on-the-limit handling balance to be less understeering than stock (i.e., probably more neutral at the limit).

Once again, this would be an interesting experiment to try for a track or autocross car -- but on conceptual and historical grounds, it's one that would make me very, very nervous on the street.
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Old April 4th, 2006, 00:12   #45
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As a person who not only has a TT front end but one who has it without a FSB on OEM springs and dampers I will say that it can be a bit of a handfull. The "snap" feeling is in fact there and it seems to happen to me most often in the 65mph range in soft turns (I don't have the nuts nor have I been in a situation to turn hard at this speed on public roads. Please don't ask me too). I can now replicate it almost at will and can drive it out without too much fuss but I can recall the effect it had on my colon the first time I felt the rear go away... gulp.

I can also say that for me I would not go back to a stock front end. I adore the way the car tries to claw to the inside of most corners and the flat crisp grip the car gives no matter how far it rolls over. The only thing that the car seems to be limited by is tire grip and my skill... I'll not give that up easily.

One observation that my wife made the first time she drove the car that I was that it seemed to wander more on the freeway. I did not notice it as much at first because I was so smitten with everything else but I must admit that the car does need more attention in some situations and her observation has merit. It may also have to do with the winter/ice tires on the car at the time.

I don't tend to like to see my wife drive the car as much as I liked too in the past and agree that this is now a car that demands respect and care. Some of us with RallyVW aim to work with Peter on the side to get these issues in check and after trying to spin my car like a top many many times I think that better dampers will do a lot for the attitude of the car in this dynamic situation.
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