Z06

I come from a long history of Corvettes..Z06....where everything was about larger..we would upsize to 295x18 on the front so going to a Porsche GT3 with a 235 seems so small. I know there is more to this....tire size...

What is it when it comes to front tire size....is larger more?

The GT3 comes with 235x18...some have fit a 275 on the front...

I am looking at a choice of a Kuhmo 245X18..24.6 inch 21lbs vs. 265x18...25.1 inch 25lbs.....or should i look for larger...275. or maybe 285..

I like to run in Solo 1 and the Kuhmos come up to temp quicker.

Do you want a larger tire for more grip or a smaller lighter tire?

Greg Fishman

I ran a 275 on the front of my 993 and there are two main reasons I quit running that size. One was that it was tough to get them up to temperature and the second was due to the larger/taller sidewall compared to the 245 (Hoosier). With the 245 I was able to lower the car another 15mm or so and the better weight distribution offset the larger tire size.

I would also look at the overall diameter and how it will affect your gearing.

Z06

Greg Fishman,

I was thinking that as well.....that lowering the CG with the smaller tire would help without affecting the geometry of the suspesion...I would guess the extra 4 lbs saved will make a difference as well.

Why does Porsche & Ferrari...use a small tire as opposed to larger with Corvette..Viper?

Greg Fishman

Think about how little weight there is up front on the Porsche and Ferrari. The Corvette and Viper have a lot more weight to deal with and more momentum for the tire to control.

Just my thoughts on this, don't take it as gospel.

Glen

I loved the fat fronts on the other hand!

Greg Fishman

Glen is probably one of a few drivers that is crazy enough to get them up to temp!
Plus my car was a bit tail happy anyway so the smaller fronts help balance it out.

SundayDriver

Pretty much everyone uses as much tire as they need. With a front engine, there is more weight of there. Add the weight transfer under braking the front tires take a real beating. Rear / mid engined cars don't need as much front tire.

Bigger tires take more energy to spin up and if they are sticky, take far more power down the straights to keep them spinning. There is a point of diminishing returns - even ignoring balance, bigger tires are not always better. With the low grip of street tires, I have never found that much difference in handling by changing tire size (but others will strongly disagree with this).

Z06

If i was to tune the balance of the car (sway bars) tire size would not make a difference in lap times?

Glen

In the Cup platform, we basically struggle with push and have seen that by making wing changes all we are really changing is the front bite . So if I were able to run a wider front on the Cup I would but am limited by the 9 inch front wheel width limit. Even then we still throw the 250 nslicks up there and those are the widest available that fit well on the 9's. But slicks are different than DOT's of course. Donohue always ran the biggest available with the logic being that You really can only win so many checkbook races without a bigger checkbook(call those drag races) the races You can win without a bigger checkbook we will call cajone races and those require going through the corners faster. the balance however is tricky as Mark alluded to, You can get through the corners faster with wider rubber and arguably get a better exit speed as a result. the downside of course is bigger fronts require more energy and have a higher drag and friction coefficient. My hands on experience started with a 2.2L 914/6 and the end result from bunches of races was the wider we made the fenders and the fatter we made the tires the lower the laptimes went but the slower our naked (not able to steal air froma faster car down the straight) terminal velocity was at the end of straights. So there you have it, wider tires are faster up to the point they are slower.... I am sure that makes Your decision much clearer now doesn't it....lol I myself always go for more rubber when I can but I am used to stealing air as I am running a lower HP car vs my competition. the GT3 is very slippery anyway so I think I would still be inclined to run as much rubber as I could fit under the fenders on Your car.

RJay

So this discussion reminds me that I was wondering the about the effect of tire width on braking during a recent discussion up here on wet tires and contact patches. Given equal compounds, if the contact patch is more longitudinal ie narrower width, does that generally mean better traction for braking, higher threshold until lockup? Or is there no difference? Did you happen to recall noticing any positive or negative difference in brake reponsiveness, feel, stopping distances etc when you went to wider fronts?

APKhaos

Nice post Glen. That demystified a few things that always had me confused. The trade off is workable if you understand the parameters. "wider tires are faster up to the point they are slower....due to drag and friction effects" works for me. Many thanks!

Dbltime

When choosing front tires think of breasts and let your personal preference take over. I run D cup 305's all the way around so I can rotate the tires.

just kidding....could not resist... lighten up

I have switched to 275's up front on a 2600# 993 and love the change.

Glen

Hence my concern about PUSH in the CUP platform along with wing ANGLE and BITE....

SundayDriver

That is easy enough to take care of. You need a much stiffer suspension. You might also want to equip yourself with enduro rubber.

Glen

Sorry I overlooked this as I was fixated so to speak...lol

Ok back on track, in the rain actually the best tire will be the tallest tire with a narrower footprint so that You still have a longer conatct patch but narrower to prevent the hydroplaning. This is a very often overlooked factor even in dry tire choice and one of the reasons 18's work better than 17's etc...It is not just the sidewall height it is actually that the contact patch can be made bigger in 2 ways, width and length. So in the dry You want the tallest, widest and longest tire You can run with gearing usually being what limits Your choices. in the wet You want the tallest tire with the longest resulting contact patch and give up width to reduce the hydroplaning. Hope that made sense.