AZDoug

Lets say you have a neutral steering car with same size tires on front and rear, now lets say you change tires so that fronts have 6" wide tread and rears have 10" wide tread, logic tells me that the car will now understeer,

Or, lets say you have a car that in theory will oversteer, will wider tires in the rear be a way to overcome that tendency?


Is there any mathematical calculation of how much the tire size differential changes the steering condition? Is this a simple ratio calc or a combo of roll centers, roll stiffness and roll couple, etc that gets into differential equations?

Too bad a skid pad can't be had for use by peons like me. That is one thing I miss about not living in Montana anymore: large snowpacked empty parking lots. I learned a lot about driving and past the limit recovery on those.

Thanks,
Doug

66since71

Doug,

oversteer and understeer become most evident when tires become non-linear in their slip angle vs load vs cornering force behavior as they near their limits. All else being equal (compound, construction tread pattern, i.e. the same brand and model of tire, all the same except size) the higher load rated tire (bigger/wider), properly/similarly inflated will stay linear longer. That means the smaller tire will seem to go away first.

Roll couple, roll stiffness and roll centers are just used to manage the distribution of loads to the tires, to manage this behavior. The tires have to support the entire weight of the vehicle, so if you force all of the weight to one corner, it gives up first!

What are you trying to fix?

Harry

vetrod62

I do not know where you are doing with this. But all this year I have been driving with a 4.30 spool and 10" wide drag tires and 7 " wide front radials. Talk about under steer. But you know what, it is easy to get used to. I do not even notice it any more and my upper body is much stronger.

Pop Chevy

Get yourself an older Chevy Performance manual, it's explained pretty well in there. It's been so long I have forgotten most of that stuff ! CRS !! Harry hit it pretty good, I'm impressed. If ya get serious about it you will need chassis scales so you can get the corner weights right, it makes a big difference(even for drag racing).
Pops

AZDoug

Not trying to fix anything at teh moment.

Just educating myself on suspension for possible future upgrades.

That said, I have an essentailly stock front suspension, but with the '59 heavy duty front coils.

At rear, I have 5 1/2 leaf springs, with lift bars, and sway bar, but radius rods removed.

Theory tells me, I have a very stiff rear in relation to the front, and should get over steer.

Now, I also have M/T SR tires, 26X8.0 front, 26X10.0 rear, which should counter act some or maybe all of that oversteer condition.

But what it really comes down to, until I actually get the car at its limit in a corner, which end is going to break away first? I don't like surprises, and would like to get an idea if the car is going to plow straight ahead, or if it is going to want to swap ends.

And of course, decel in a corner vs accel ina corner, will yield different results.

Doug

Matt Gruber

my 61 is the easiest car i've ever owned to steer with the throttle. so it doesn't matter if it understeers, which of course it does(primitive front end)
just give it gas and it gets pointed where u want. i've never lost it. your results may vary
does take practice and a bit of nerve on a wide empty turn to learn. would be fun to race up pikes peak passing cars sliding the whole way

most guys won't dare try it, but if u do, practice in 1st or 2nd gear at slower speeds until u get the hang of it.

AZDoug

You can throttle steer with a car that is neutral, or under steers, but if your car oversteers, the only thing you can do is hope that the curve has more room on the outside to widen your turning radius, because once the back end breaks loose, more throttle ain't gonna help.

Doug

Matt Gruber

so your 61 oversteers?

66since71

Doug, some of the other guys will need to speak to specific set ups for C-1s. And the bottom line is that without sophisticated models, you'll just have to balance the car by testing.

If this helps your thinking, consider that a tire (outside) that sees increasing loads (weight) will gain traction, but not as fast as the opposite (inside) unloaded tire loses it. Tuning chassis roll stiffness (or couple) exploits this. So the axle with the least weight transfer (the "softer end") will have the most grip.

The acel/decel thing in corners works because, regardless of the how fast or slow a tire gains traction with load (up to a point), it still gains traction. So putting weight on the front (decel) will make it bite relative to the rear, so the nose will turn in, and make you feel like the back end is coming out, and vice-versa.

I hope this helps a little..

Harry

Edit: You can go too soft in the front, and loose traction because of unwanted camber change

AZDoug

Don't know.

I need a place to find out what it does, before I find out in real life.

Doug

AZDoug

So application of throttle in a car with oversteer tendencies will cause the rear to hook up again, even as the rear is already breaking loose in a corner?

Thanks,
Doug

66since71

Actually, yes but gently. That's exactly what they teach at Bondurant's road race school! Hardest thing I ever learned to do in a car.
Let up on a car that is set up for oversteer, and you'll spin it for sure. Of course, if you are already spinning, all bets are off.

Harry

Plasticman

Harry,

You must have a much gentler foot than me! I learned anytime I was already feeling oversteer, further throttle would put me in a spin for sure. I understand roll into the throttle, but adding that to an oversteering situation would spin me every time.

Something about tires don't like to do 2 things well, at the same time, such as turn and accelerate (or brake). If the rear tires were on the edge of breaking loose sideways, adding throttle sure never helped me.

Now throttle induced oversteer I can appreciate!

Plasticman

Matt Gruber

in an emergency, don't be afraid to crank on the wheel and try to go around. if u r going to crash anyway, it just might work. Many just slam on the brakes; before u hit , let up on brake, crank wheel, go around. No amount of testing will guarantee success. just do it.
.
one time a moron ran a red light; i was going to T-bone him for sure i made an extreme full lock left; then right; missed him by inches. well, a miss is a mile took 5 min for my pulse to slow down
never touched brake; that would of made me crash

66since71

I guess I should have qualified it. If the oversteer is throttle induced (ie. the wheels are broken loose and spinning because of judicious use of horsepower), then you will have to back off to hook up again. I think we have all experienced that.

About two things at once (turn and acel/decel): the engineers talk about the friction ellipse or circle. One axis is acel/decel, the other is turn right/left). But the ellipse gets bigger as the tire gets loaded.

Harry

63 340HP

You might place a call to the local Sheriff of Highway Patrol and ask where they train drivers, and then call to see if you can use the same facility. The LEO's typically have a location with a skid pad or private lot (closed to the public) where they can set up an autocross course. A little water on the course adds drama without the speed (and the techniques for driving and handling tuning are the same).

Much of what you will read regarding handling relates to modern vehicles with limited camber suspension & a stiff chassis - and wide & low aspect ratio tires. The C1 (and C2) have neither beneficial chassis design attributes that are comparable to a modern sports sedan, and the C1 cannot fully exploit the benefit of wide tires or the limited sidewall flex of a low aspect ratio tire (a 50 series on 17's, etc.). Be careful applying everything that is currently in print to the C1 (& C2), because the old chassis is not as stable of a platform (what is needed to control the tire contact patch).

The entire goal of chassis tuning is to gain the best tire contact patch adhesion through maximizing the contact patch and pressure consistent with the tire compound & ideal temperature (you can get too big a tire so that it never reaches it's ideal temperature). With older chassis setups a more compliant (taller) sidewall may help the ultimate grip with less than ideal camber, but add delay in transition to steady state handling. Better steady-state grip may not result in better transitional handling, when working with the old chassis (modern sports cars handle better in all aspects because they are designed and built better).

The Chevy Power manual is great for the C2/C3 because it deals with the older chassis, and what works for these unique chassis's. I do not know if a similar manual is available for the C1. There are however good books on the 1st & 2nd generation F-body camaro, similar solid rear axle chassis cars, that might apply and general suspension design books.

A good technical book is Race Car Vehicle Dynamics. This book has become somewhat like the textbook bible of suspension design and analysis over the past five years.

I understand that Chassis Design: Principles and Analysis is a good read with some historical prospective (I have not read it yet myself).

These are available on Amazon or through SAE.

Good Luck!