AutoCross Tire Width

[67 Camaro]

Hey Guys,

As my build progresses I find myself wanting to compete more and more in autocross when I'm done. Thus I'm at the cross roads, I am not entirely sure weather WIDER is nesscesarily better. I originally wanted 19 x 10's with a nice low profile tire. I now find that I really don't know much about how width really affects cornering. I am steeping back from the idea of mini tubbing (a step back in the build b/c I already have a 4 link system installed) and considering something like 18 or 17 x 8 or 9's... any insight as to the applicability of tires dimensions v.s. their application would be a great help.

Thanks

[JayR]

A very good basic rule of thumb is to not have too big of a disparity from front to rear when prepping a car for serious cornering.

For instance, a mini-tubbed car with 335s out back that can only accommodate a 245 in front and maybe an iron headed BBC sitting over those 245s is pretty much guaranteed to "push" or understeer at the limit because the 335s will stay hooked and if the car wants to slide the fronts will be doing all the sliding which makes steering difficult at best. There's exceptions to every rule which is why I prefaced that sentence with "for instance" but it illustrates what you should be wary of when planning and building. You can tune out some of those characteristics with sway bars and spring rates, etc but physics are physics.

[68Formula]

Keep this in mind, low profile tires help with cornering, but decrease straight line bite (stiffer sidewall).

So one way to lessen imbalance due to a big difference in front/rear width is to run a shorter sidewall in front than rear. While using a taller rim in the front would stupid, you could for example run a 245/40R18 in front and a 285/40R18 in the rear. On the first gen F bodys, this works aesthetically as well, since the rear wells are a little bigger.

Also, anti-rollbars are intented to balance the vehicles. A thicker rear bar will increases the tendency to oversteer and a thicker front bar increases understeer. Many first gen owners don't find it necessary to run a rear bar. So adding a large rear bar, would balance the effect of the staggered with tires. It will also help plant the car for staight line traction.

Finally, anytime you can remove weight from the front (or shift it from to the rear) will increase oversteer. If you can further take advantage of the reduced weight by lowering the front spring rate, this will help as well. Conversely a higher spring rate in the rear biases the car to ovesteer. And again, this should increase your straight line traction.

So while width is a big factor in the balance of the car, you can counteract it by adjusting weight, spring rates, anti-rollbars, and sidewall stiffness. It's also important to make sure you have the correct camber gain and static alignment settings, so that you're getting as much bite out of those front tires, as possible.

Food for thought.

[DriverzInc]

Keep this in mind, low profile tires help with cornering, but decrease straight line bite (stiffer sidewall).

So one way to lessen imbalance due to a big difference in front/rear width is to run a shorter sidewall in front than rear. While using a taller rim in the front would stupid, you could for example run a 245/40R18 in front and a 285/40R18 in the rear. On the first gen F bodys, this works aesthetically as well, since the rear wells are a little bigger.

Also, anti-rollbars are intented to balance the vehicles. A thicker rear bar will increases the tendency to oversteer and a thicker front bar increases understeer. Many first gen owners don't find it necessary to run a rear bar. So adding a large rear bar, would balance the effect of the staggered with tires. It will also help plant the car for staight line traction.

Finally, anytime you can remove weight from the front (or shift it from to the rear) will increase oversteer. If you can further take advantage of the reduced weight by lowering the front spring rate, this will help as well. Conversely a higher spring rate in the rear biases the car to ovesteer. And again, this should increase your straight line traction.

So while width is a big factor in the balance of the car, you can counteract it by adjusting weight, spring rates, anti-rollbars, and sidewall stiffness. It's also important to make sure you have the correct camber gain and static alignment settings, so that you're getting as much bite out of those front tires, as possible.

Food for thought.

That was some excellent advice right there.

[68Formula]

That was some excellent advice right there.

Thanks!

Also should mention, if the OP is still looking into a new front suspension, some companies offer setups with more clearance for wider wheels. This is because the limitation on the inside is the subframe itself.

For example, I believe the DSE subframe allows up to a 10 inch rim, and has improved suspension characteristics that I mentioned in my previous post.