Wheel Sizes Again.....

[bolarbag]

Ok I'm currently speccing my wheels, sizes and offsets and being an anal c*nt I like to go over the info I have learned in the past aswell as any new or existing info that will affect my choice

 

When I initially joined this forum I was always of the opinion that wider wheels and wider tyres gave you more grip and better handling - CW and the Car Bible soon put a stop to that

 

But I frequent the U.S forums and their road race(track) guys all seem to run 10j and 11.5j front and rear respectively

 

There are a lot more road race guys over there than here, they all run my new power aim and then some, and they all seem to run good times in their chosen events

 

This is the response I got from a pretty respected road race member and supra owner for like 10years:

With stock-width rear tires (255s) on an APU Mkiv Supra Turbo (eg. 750rwhp+), the puppy will just spin/slide...you'll have almost zero control in the rear when boost hits. Please note that I actually agree with you, IF (and only if) you keep your Mkiv TT bone-stock, then the oem rims & offsets are ideal. But if you reengineer the driveline, you also have to reengineer other parts of car, like the rim widths (along with the alignment and suspension to match, of course). This is speaking from hard-learned evidence and the twisted scraps of metal that once-were beautiful Mkivs. Of course, the choice and the freedom to experiment is yours. Alternatively, if you want to go with what has been proven to work in a 750rwhp+ Mkiv, get some 11.5" width rims in the rear, with a tall profile (eg. 35s or 40s) tire, preferrably race rubber (assuming dry conditions), proportionally wide front rims&tires to match, and watch it grip and handle (on a roadrace course - not a dragstrip) after the upgrades. Don't and ... well let's just hope your car insurance AND your life insurance policies are both up to date.

 

Your post flies in the face of almost all roadrace modifications that are done, and wider tires do have more lateral grip for a given vertical load. For example, most modified Vettes and Vipers all run 335s all the way around. Porsches with flared fenders to accommodate extremely wide rubber is commonplace on roadrace tracks. Agreed oem handling is good, but my personal experience says wider is a LOT better. Despite all of this, I really do welcome you to prove otherwise. Put a 750rwhp+ Mkiv on the skidpad with oem rims&tires and g-meter, and post the results here. Then slap some 10s&275s up front and some 11.5s&315s in the rear, with proportionally similar sidewalls (i.e. don't put some super-low profile rubber on the rear), same brand/model/rating of the rubber too, and re-test. My money's on the retest with wider rims giving you a much higher G rating.

 

Here's a quote that might help:

"...when you look at lateral grip (side force) other factors start to matter. The tyre develops side force because of the slip angle between the tyre and the road. This slip angle means the tread is being pulled sideways by the road surface. At the front of the contact patch the deflection is relatively small. As you move back along the contact patch the deflection increases steadily. At some point, the sideways forces in the tyre exceed the friction between the tread and the road and the tread starts to slip relative to the road. When the tread is slipping like this it produces less grip on the road. Now imagine increasing the slip angle and imagine what effect this has on the side force. As the slip angle increases the sideways deflection builds up quicker so the front of the contact patch works harder. But more and more of the back of the contact patch is sliding and losing grip. At some point you reach a maximum point where more slip angle means less side force because you are losing more grip at the rear of the contact patch that you are gaining at the front. This is often referred to as 'breaking away' where you ask the tyre for more grip and end up getting less.

 

The longer the contact patch is, the more gradually this break away occurs. If you shorten the contact patch, the break away occurs more abruptly but you get more absolute grip at the peak. This is because there is less variation in sideways distortion between the front and back of the contact patch, more of the contact patch reaches maximum grip and starts to slide at the same point. Having a shorter contact patch also means you get less self-aligning torque so there is less feedback through the steering about how close the tyres are to breaking away.

 

When you fit wider tyres, what you're doing is making the contact patch wider and shorter for the same tyre pressure. This means you get a more abrupt breakaway but more grip right on the limit. The disadvantage is more expensive tyres, more tramlining and steering kickback, more wind and rolling resistance and noise, less grip in slippery conditions, a more abrupt breakaway to catch out the unwary driver and less warning through the steering about how close the tyres are to breaking away. "

 

Lastly, consider a couple of things:

1) The contact patch actually isn't the same with narrow vs. wide, since the steel belts in the radial change the shape of the contact patch.

2) Wider rims & tires means a wider wheelbase...which in turn adds noticeably to the lateral stability (you can actually feel a difference, through the first hard corner you turn after running wider rims).

 

Again, imho, wider is always better.

 

Please discuss again...

 

Stock or Wiiiiide?