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Old April 13th, 2006, 08:22   #72
Ceilidh
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Join Date: Mar 2006
Default tires and handling balance

Quote:
Originally Posted by cartog
This is quite interesting. I wonder if it relates at all to the earlier reminiscence for old English sports cars with slippery tires. Could slightly less grip help the rear come about a bit better?
Hello cartog,

There are actually (at least) three somewhat independent phenomena going on here:

1) One concerns the Honda minivan:

Quote:
.....Then there was another interesting piece, on a Honda Minivan, where they tested both the basic model and the high tier model together. They both had all season tires, the exact same brand and model, but the basic model had them H rated and the high tier model had them V rated. The basic H model did very well in the lift-throttle and change lane test (the one we were talking earlier in this thread) and went through at speed of 102 km/h. The V rate tire model, thought, barely passed at only 100 km/h and the tester was commenting that due to the soft suspension and specific geometry on such vehicle, the car rolls a lot more (it is a minivan!) and at that point the softer tire wall H rated tires actually help a lot more (work a lot better) than the stiffer side wall V rated tires, therefore the higher tier car could not go as fast as the lower tier counterpart.


I'm not familiar with this minivan, nor do I know the specifics of how its particular test went, but from Peter's quote it sounds like the V-shod minivan simply had less grip and agility than did the version with H-rated tires. Surprisingly, this sort of reduction can sometimes happen: different tires respond to camber in different ways, and some (A) have more camber thrust and others, and/or (B) have more loss-of-ultimate-grip due to camber. In general (though with tires, there are exceptions to almost any rule of thumb!) a stiff-walled tire tends to generate more camber thrust, so if you lean it away from a corner (as seems to be happening with the Honda), it'll steer away from it as well; and with stiffer walls, you also have more likelihood of lifting the inside portion of the tread off the road, thereby reducing your ultimate grip.

As an aside, these effects can also apply to low-profile tires: in general (again with exceptions!), the wider and lower a tire's aspect ratio, the less the tire enjoys being cambered adversely in a corner. For some -- perhaps even many -- innocent little FWD cars that get tarted up with big-wheeled "performance" packages, the cars would actually grip better if shod with taller, narrower versions of the superwide rubber that comes with these packages. Of course if you did that, people wouldn't buy the packages! (The marketers have done a good job convincing everyone that wider is better..) And if you make sure that you spec your option list such that the choice is between a narrow all-season versus a wide high-performance tire (i.e., nobody actually ever compares a narrow versus wide performance tire), the big performance tire will in fact grip better than does the narrow all-season (simply because of the rubber tread compound), so no one is ever the wiser.

In any case, suspensions that are designed for stiff and/or low-aspect-ratio tires usually keep the tires a bit more upright in a corner, either via suspension geometry, or by reducing overall roll. If you don't do this, and you continue to let the car roll, you often don't gain much from the stiffer/lower tire, and sometimes you actually lose...

2) The second one is tricky, and we won't get much into it here....

Quote:
Then they tested the upper tier model, which was basically the same car (suspension wise) but had performance tires, which is a trick many car companies do – just put bigger wheel and sticky tires on their “sport package” and that is it. Anyway, due to the stickier tires, not the car has actually issues in the Elk Test as it started to lift BOTH inner wheels (!)
....but what MIGHT be happening to the Fiat Panda referred to above is the sort of dynamic instability alluded to by Nate several weeks ago. That is, the Panda might be rocking up onto two wheels simply because it now has better grip and is cornering faster (and thus generating more g-forces), but it wouldn't be surprising if a more subtle phenomenon is also at work here.

Quote:
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... __________________
Nate
I'm not sure we'll ever get time to discuss this issue very much (this thread seems to have taken on a life of its own, with multiple subthreads going every which way!), but the resonance effect Nate refers to above is quite real, to the extent that -- if you believe rumors -- almost any car can be rolled if steered in just the right way at just the right speed...

(sigh)

Ok, I'd better explain a little bit right now! :

For the young whippersnappers in the audience who have no idea how the Mercedes A-class enters into this discussion: the famous-in-Europe Elk Test was just an obscure handling test performed by a more or less obscure Scandinavian motoring journal (I'm sure the Swedes & Norwegians et al read this journal, but hardly anyone else did back then!), until the day that a revolutionary, long-awaited, much ballyhooed Mercedes shocked everyone by rolling over in the middle of it. This was the A-class, and before that fateful day, Mercedes had poured millions and millions of dollars (deutsche marks back then...) into advertising how it was a safer, better-handling, more-comfortable, roomier, higher-quality, and simply better-engineered small car than had ever existed before. The advertising runup took literally months and years, and in Europe (where smaller cars are king) there was a lot of anticipation for this radical vehicle (the A-class' architecture is very unusual and clever) from the world's exemplar of engineering prowess. So the last thing anyone expected was for the Mercedes to roll in a test that more mundane cars routinely pass, and photos of the Scandinavian test driver being carted into an ambulance, with the partially destroyed Merc lying sideways in the background, were about the worst thing marketing-wise that could have happened to the folks in Stuttgart.

There were a lot of reasons why the A-class rolled, but much of it comes back to what Nate brought up: there's a certain resonance in a vehicle's handling, and the Elk Test happened to hit it on the A-class. Until that day, Mercedes never ran its cars through that particular test (it's been argued that it's quite contrived, and real people don't actually respond to an obstruction in quite that way), and thus didn't know of the problem; and hence they got very, very unlucky.

Where is this leading? Well, back to the rumors: one of the many interesting things about the whole A-class debacle was that absolutely nobody -- not a single European, Asian, or American car manufacturer -- ever came out and said anything evenly remotely critical of Mercedes or of the A-class. Not a single cheeky advert boasted that "Our cars stay planted!"; not a single press release referred to "Our engineers perform a battery of tests, to ensure that no handling surprises will catch anyone out..."; not a single commercial talked about "our cars' wide stance and stability, which keeps our cars upright where competitors fail....". If anything, an occasional competing exec would say something like (when interviewed and essentially forced to say something): "Well, Mercedes is a fine company, and I'm sure that they'll sort everything out...". So what was going on here?

Rumor has it that the reason that nobody took advantage of Mercedes' predicament was that everyone knew it could easily have happened to them. A few engineers murmured something to the effect that "...mumble...mumble...well all cars can roll if you do the right th.....mumble...mumble" before being silenced by their respective companies and having their quotes expunged from the records. And then there was a very strange rumor that shortly after the Elk Test disaster occurred, Mercedes bought an assortment of cars, trucks, vans, and hatchbacks from all its competitors, and instructed its presumably-exceptionally-highly-paid test drivers to find a way to roll each and every one of them in front of the video cameras, with the resulting footage going into a Mercedes vault somewhere, with word sent out to the various competitors that they might not wish to capitalize on Mercedes' predicament.

Interesting, non?

In any case, going back to the Fiat Panda on 2 wheels: as Nate says, there's a particular frequency where cars begin to go unstable. And when you change the tires, you change the response rate to steering motions, and sometimes -- if you're lucky/unlucky -- that's enough to nudge a car towards instability in a particular test. So it's possible with the Panda that the 2-wheel result is not just from the tires being stickier, but also from their being more responsive.


3) And finally (though I'm going to have to pick this up in a future week, as I'm out of time)...

Quote:
Originally Posted by cartog
This is quite interesting. I wonder if it relates at all to the earlier reminiscence for old English sports cars with slippery tires. Could slightly less grip help the rear come about a bit better?
On many cars, less overall tire grip can lead to reduced understeer (especially with the old English sports cars!!) -- but we'll explain that some other day.

Cheerio lads!

- Ceilidh
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