Wider rear tires/wheels then front, Why?? Options

[Jetsetsurfshop]

Ive been running a "square" set up for years. 15x7 with 205/50/15 Hoosiers.
I'm now contemplating running wider rears then fronts and started to ask myself why? At the HSR races all the 914 guys are staggered. It looks like the PCA guys stagger too.
Whats the performance advantage here? Is it handling? Something to do with getting hard on the brakes? Putting the power down? Fitment on the front suspension?

Teach me something.... (IMG:style_emoticons/default/beerchug.gif)

[wndsrfr]

Such a great question....love it. I have 15 inch X 8's.
I've run square 225/50/15 radials, 225/45/15 radials to gear it down a bit now on staggered Hoosiers (SVRA req'd) which are 7.5/23 fronts (close to 225's) and 8.5/24 rears (close to 245's) so the rear is not only wider but also taller.
At my talent level it's hard to say anything definitive, but here's a few thoughts and then feelings/observations.

Your comment about more weight per square inch needs more thinking because these are pneumatic--35psi is the force per square inch, so the tire will have a contact patch of the number of square inches to equal the corner weight on that wheel. Sooo, a real narrow tire will have a long front to back and narrow side to side contact patch, a wide tire will have a shorter but wider contact patch, see that picture? Staggered setup will give a more uniform patch in the direction of travel. I think steering effort is more influenced by camber/caster than tire width within these limits.

Do a corner balance-- on my car it works out to about 56%rear 44% front so the rear tires carry more of the weight. My fronts are like 460 pounds each and rears are like 600 pounds each. A little math comes up to have contact patch be 13.1 square inches on fronts and 17.1 square inches on rears at 35psi.

Braking and cornering have more effect than acceleration in our cars--we can corner at 1g putting much more weight on the outside wheels than steady state would lead you to think. Hard braking lifts the rear--we want as much contact patch as we can get there to avoid getting squirrely.

My lap times are pretty similar between radials and bias Hoosiers. The biggest difference that I notice is (other than the ridiculous cost) that the bias tires will hold the same cornering force when they slide than they did just before breakaway--the radials get more slippery after breakaway so harder to catch. So the Hoosiers are more "forgiving" or provide more time at the limit than I can get with radials. That said, radials have less rolling resistance so will give higher top speeds on long straights and better acceleration times throughout the lap. Pressures are radically different--radials start at 29 to finish at 36psi in a 30 minute sprint session, bias start at 16-17 to finish at 22psi. That means larger contact patch for the bias ply tires all around and when sticky give more cornering force.

You can do your own tire temps. I do it and I'm 75 years old for gods sake. Get an inexpensive probe type unit, hop out of the car and what you're looking for is the temperature profile across the fronts and rears to decide on camber & toe setting. Push the probe in 1/8 inch or so. Doesn't matter that it's instantaneous, just fairly quickly after the session, best if pitting without a cool down lap. Profile should be close to uniform, say within 10-15 degrees from outside to center to inside of the tread.

Wow--sure got going on that one, eh? Need to get a life....