Can changing contact patch size change handling balance?

Say you have a car, any car running a square wheel setup. Say you change the width of all four wheels by the same amount, in either direction. Could any such change alter the handling balance of the car? I don't think so, I think it would be pretty much the same, you'd just have more or less grip overall. That could mean behaviors that used to change at certain speeds now change at higher or lower speeds, but I don't think it could make a car that normally oversteers start understeering or something like that. Am I wrong?

Changing tire width can, but does not necessarily, change handling characteristics. Narrower tires are typically less sensitive to a larger range camber variation than wider tires, but wider tires are capable of added grip when in the 'correct' camber range.

Keep in mind that a wider tire effectively shortens the length of the contact patch and that moves the center of the contact patch (trail - effectively the dynamic caster due to tire deflections) slightly. In a straight ahead no lateral load condition, that would not be felt. Once the wheels are steered off center, the change could absolutely manifest itself as different behavior, both in speed and steer/camber response. Tires are way more complicated than most people suspect. I work with several tire engineers who are never caught up on the analysis for the relatively limited set of conditions we analyze.

I think the most notable difference when keeping it square would be a lower offset yielding a wider track.

That could be a benefit but I didn't say anything about changing offset...by "square" I meant the same size wheels and tires all-around.

OK, if you go from one comparable square setup to a different square setup, you won't notice much......different widths but same offset do add small amounts of track width though. Otherwise the only thing that would really mess with your vehicle setup would be if you expected to run a staggered combo on the same setup as the square combo......If not that, you're probably thinking too much.

also consider that if overall grip is increased:

• lateral weight transfer is increased
• body roll is increased

so you're going to run into some kinematic effects which could change the balance of the car.

Sure it can. Let's say your car tends to understeer, you upgrade to 2"-wider wheels on all-4 corners, presuming your alignment was set pretty well to begin with & the new tires offer at least as much grip as the narrower ones, what happens when you go into a corner?

Now you can brake/turn harder due to the increased front grip. Sure, you've increased rear grip too, but consider what's happening dynamically with the chassis - once you factor in roll-couple, greater dive & roll from increased front grip, etc. - the rear may behave completely different than it did before the wheel/tire upgrade.

If the larger contact patch results in greater mechanical grip, and offset doesn't change, you'll be inputting greater forces. Anti-roll bar rates may no longer be correct. Spring rates, depending on who's talking, should be driven by weight, so damping shouldn't change much, but if you're using spring to control roll, that may become an issue.

GameboyRMH wrote: Am I wrong?

Yes, if the roll stiffness is nonlinear with increased roll. Usually is due to running against a bumpstop but some sway bar mounting scenarios can cause this too.

Or if bushing compliance is nonlinear with respect to side loads - say you run out of give at a certain point and suspension toe changes go backwards. VWs can be like this - the rear beam flexes for outside wheel toe-out, so the mounting bushings are "cammed" to pull the outside wheel forward thus making toe-in... until you run out of bushing compliance, anyway.

So it seems like the short answer is that the increased grip can get your suspension into new situations which it might not handle well, therefore you can get different behavior when driving near the new limits. Is that right?

In reply to GameboyRMH:

Yes, but that's true of all tire changes.

It depends. Are you going from 195 to 205 tire without a change in the aspect ratio? Wider is better so there may be a slight difference imo but I doubt that the ass dyno would feel it. You would also be dealing with a taller tire so there might be different sidewall effect. You are also changing the front end aspect of the car so there may be a difference in the aero drag of the car.
Most cars benefit from the widest tire you can get under a car in theory but backing that up with good numbers is going to be difficult.

Im in the camp that says yes, the handling balance can change. I've noticed it to some extent on my own cars.

I drive mostly FWD. On a FWD, the rear end does nothing except what is started/caused by the front end. So, if you change how the front end acts, you change how the back end acts, although the changes might not be the same.

For example, i take a stock super-floppy old FWD family car and bolt a crapload of grip to all corners. The thing now rolls over so hard that the outside rear smacks the bumpstop hard and the inside wheel picks up, so now it rotates even with more rear traction than it had before.

Or, go the other way and install bicycle tires on all 4 corners. Take a sharp turn at 50mph, or TRY to. Understeer city. But, would it understeer WORSE if you bolted some 305 hoosiers to the back? Not really. The front not generating enough action to begin with, so the back end traction over some tiny minimum, is almost irrelevant.

In reply to GameboyRMH:

With good geometry and/or tuning, yes it can simply be the result of increased grip at both ends leading to other component interactions affecting the balance. But it can come from the tires themselves too, as the geometry and/or tuning may not take advantage of the tire width equally on both ends of the car. It's even possible that with poor geometry and/or tuning, the wider tires could theoretically produce less grip at one or both ends than the narrower tires, per my previous post.

Also wider tires tend to be able to take more heat, and might not come up to temperature the same as narrower tires. Maybe the wider tires interacting with everything else noted in this thread will still heat up the same way front and rear...Maybe not.