Slakker

I read about a lot of people trying to max out the contact patch on their cars with the widest tires possible. But how big of a difference does it make? On a 996 I've read of people running from 245F/275R up to 275F/315R. Is it more important to have the right front to back balance or just get all of the tire you can fit? For lap times, are we talking a difference of tenths or of seconds? All opinions and experience are appreciated, no matter how far out there.

Veloce Raptor

I'd say it matters to some extent, however there is also the factor of aero drag of the front tires to consider for top speeds in relatively lower powered cars. For example, in an E36 M3, going beyond 245 or 255 cross section fronts on longer tracks is actually slower, whereas on tighter, more technical tracks is can be an advantage presuming the driver knows how to take advantage of the extra width.

IMO finding a tire package that remains compatible with suspension & driving style is key, rather than obsession with maximum widths at all costs.

AudiOn19s

Wheel widths are going to make a difference here possibly as much as tire width. Shove a shove 275/315 tires on the stock 8/10" 996 wheels might work in an auto-x setting but gives very little benefit on the track.

9/11 inch wheels are the standard go-to setup for narrow 996 cars but if you're ordering custom wheels you can go 9.5 / 11.5 with little sacrifice and if you're creative about things you can to 10/12 on a narrow body car

I run 245 / 305 on 9/11" wheels and it's a great combo.

NYC993

Wider tires don't necessarily give you bigger contact patch. They change the shape of it. Patch is rectangular. As tires get wider the amount of pressure decreases and the patch becomes shorter. That shape helps in cornering and keeps tire cooler iirc. But then contact patch is also impacted by alignment settings. So simply putting wider tires may not add more grip and may reduce it. Then you can get into slip angles and scrub radius blah blah blah. It's a very complex topic so I would be careful reading the answers you get. And grip doesn't only depend on tires, if depends on how well car is balanced in the corner.

Matt Romanowski

The contact patch formula is area (meters^2) = vertical load (Newtons) / tire pressure (bar)

The length of contact patch = area (m^2) / width of the tire (meters)

Camber changes it some, but this is a general size.

certz

Interesting topic and, as said, there are a lot of variables that effect contact patch. In PCA Clug Racing we are limited by the width of the rims we are allowed to run. As AudiOn19's said, you do not want to run an overly wide tire on a given rim width as it adversely effects sidewall stiffness. So, when I discuss contact patch with the coaches I have used, we discuss optimizing the contact patch for a given section of the track. In general and high level, under braking as "weight" shifts to the front you have a higher contact patch on your front tires than rear, and under acceleration the opposite is true. It is mid-corner that things get really interesting as you trail brake, unwind steering and apply throttle - this is where the fast guys show you why they are the fast guys...

mark kibort

ive alway gone as big as possible with good luck. never have i seen a benefit for going smaller. but, sometimes the gains are small. sometimes aero dynamic losses should be considered. even when i ran WCGT and had only a choise of a 275/335 (F/R), i took the 335 on the rear vs the 305 that was available even though it wasn't a great fit on the rear rim (1" too narrow). But, in the end, those tires would be consistently faster on the tracks i visit. (lots of back to back , same day testing) basically, it was one size wider than the rim was spec'ed for. in the front, i later was able to go larger as well. i found that a big tire up front tended to cure some push issues and wear was less (maybe because there was just more rubber). i ended up back with a slightly larger front tire and the tire that fit the rim in the rear. that seemed to be the best mix, both handling and economically. you could see the wear patterrn on the oversized rear tire, didnt use all of the tire, so it probably wasnt as effective of a change. however, up front, that wasnt the case.
lots of discussion here on the topic. i have no science to offer, other than experience here.

rlm328

What ever happened to the English system that we all know and love in this country.

Matt Romanowski

I'm sure you can work it in!

I blame Danny Knowlin.

Deadeye

Not sue how it effects the contact patch, but I am seeing rims quite a bit wider that the tire tread in the paddock. Are these ill fitted tires or is this a trend?

NYC993

stretched sidewall is stiffer providing more feedback and preventing rolling the tire on its side.

I run wheels that are and inch wider than stock (9 and 11) with stock tire width (225 and 275). Actually rear is 10mm narrower than stock due to availability issue.

Of course the civic guys take this and neg camber to a whole new level-but those don't make it to track.

Slakker

Awesome information. This helps me out a lot. Thx all!

johneecatt

And now for something completely different. Tried a bunch of things, but now I run a 235 up front and 315 rear. No exotic suspension tuning or camber - turns really quickly, tracks arrow straight and I rarely get a push condition...and on that rare occasion, can usually tune it out with tire pressures. Having meat in the rear is great though (significantly!!).

mark kibort

and that's the Holy Grail, right? i find that to be the case with my car, but only when the fronts are new. always slides to a problem with grip in the front. ive done it all to losen the rear up, without losing the grip i need for those high speed turns an exits out of slow turns. such a chase , right??
good for you. and for the weight disty of the 911 and cup cars, this makes sense. 50% more rear braking capability in the rear vs front engine cars, 15% less up front possible...... if i could get just 20% of my weight out of the front, i would be in heaven!! i think i really need 340s up front and 315s in the rear. hahahah actuallly my issue is a geometry one with such a long wheel base and stock rear swing arms that fight oversteer on a street car. i have to get rid of that and maybe the car will turn in those medium speed turns!

johneecatt

This month!! Haha!! But your 340s up front is intriguing!!