Calling all the handling geniuses, another Newbie Question (not found in faqs and forums)

What is the relationship between rim width and tire width as it affects handling. Example, on street tires (namely kumho mx or falken rt615) will a 245 width tire on a 8.5 inch rim provide more lateral grip or less than a 265 on the same rim?

Note: I am not talking anything else on the car (fitment, etc, or even front and rear stagger). I just want to know the science of when a tire is too big for a rim, meaning it results in worse grip rather than better, for whatever reason.

It seems that on r-compounds, people are sticking on bout the widest they can fit, but is that because r's have superstiff sidewalls that don't need rim support???

Does a wider rim in the same size tire stiffen the sidewall of a street tire? If one could fit 245's and 265's street tires with no issues on an otherwise stock AP2 would it have potentially better lateral grip than the same car on 215's and 245's? Or would their be more sidewall flex due to the tire overextending the rim so much?

Note: Assuming all tire pressures are kept the same

Thanks

 

All else being the same, use the widest rim you can for a given tire in order to maximize grip. However, wider rims cost more and weigh more.

In practice, it usually ends up being that people have rims that fit and then try to squeeze the widest tire they can onto them.

 

Also bear in mind that the stiffness of the sidewall for a given brand/model of tire here can make a huge difference. With a stiffer sidewall tire you have more room to play with rim size while one that is more flexible requires a closer match of rim width vs tire width or you might experience a squirmy ride by going with too narrow a rim.

This question does not have a simple answer that is applicable to all tire brand/model choices. Sadly most of the information available at the consumer level offers far less enlightenment for these types of decisions than what we really need

 

Thanks for the info, guys. That leads me to a sub-question.

So in an attempt to reduce understeer, when someone increases the width of the front tires of an AP2 from 215 to 225, and leaves the rear 245, they can actually be making the understeer worse (if the sidewalls of chosen tire aren't strong enough) by allowing the sidewall to flex more easily because the rim is no longer supporting it as well?

Could this actually be used as an effective method to reduce oversteer? i.e. 235's up front. Nevermind, I don't think that would be worth the trade-off-I'm guessing sloppy turn in, etc?

As my sig says, I am running 225/255 kumho mx currently. Does that mean that I would get more benefit of increasing my tire pressure than someone who is running stock size 215/245? (to compensate for my "softened" sidewalls?

Thanks again

 

You have it backwards Thomas. All other things left unchanged, decreasing front to rear stagger will bias the car towards oversteer (more front grip). If you're experiencing oversteer, I would tend to think it's driver-induced...especially on an AP2. You might want to try 215/255, but I would suggest trying to settle out the driving issues first.

Best bet is seat time with an instructor riding along...an autox school would be perfect. Local PCA Club has autox classes in the Spring and Evolution does an annual school in DFW.

Regarding rim widths...it's usually advisable to stay within the rim width range specified by the manufacturer. As mentioned above, however, you can run safely outside those specs as long as the sidewall (construction and amount available) will allow it. There is almost always a measuring rim width listed, which I take to be the measurement they designed the tire around.

 

Any time you make a change almost anything can happen. But that's not a very likely thing to happen.