JKinOB

I am trying to figure out what front tires to run on my 718 Cayman for A Street this year.

The following are the stock tire sizes and outside diameters
20": Front - 235/35R20 with 26.5" outside diameter; Rear 265/35R20 with 27.3" outside diameter
19": Front - 235/40R19 with 26.4" outside diameter; Rear 265/40R19 with 27.3" outside diameter
18" Front - 235/45R18 with 26.3" outside diameter; Rear 265/45R18 with 27.4" outside diameter

The stock setup puts a rake in the car that lowers the front wheel hub by 0.4" to 0.55" (half the difference between the front and rear outside diameters).

I have 19" wheels that are 8.5" wide in front and 10.5" wide in back

I plan to run 305/30R19 Bridgestone RE71Rs in back. They have a 26.2" outside diameter.

I've narrowed it down to three candidates:

255/35R19 Bridgestone RE71Rs with an outside diameter of 26.0"
Pros - Proven tire, max tread width recommended for 8.5" wide wheels
Cons - changes the rake of the car to raise the front so it's almost flat (0.1" lower than the rear)

265/35R19 Bridgestone RE71Rs with an outside diameter of 26.3"
Pros - Proven tire, max tread width that can be squeezed onto 8.5" in wheel
Cons - not recommended by Bridgestone for wheels narrower than 9", changes rake of car to raise the front so its almost flat (0.05" higher than the rear)
Other - tallest sidewall

255/30R19 Yokohama ADVAN Neova AD08 Rs with an outside diameter of 25.2"
Pros - max tread width recommended for 8.5" wide wheels, maintains rake with front 0.5" lower than rear
Cons - not used by anyone at nationals
Other - shortest sidewall

Thoughts?

Thanks!

John

chriswd62

I would strongly recommend against a different tire compound front to rear.

I too am considering new tire sizes for this season and would recommend 265/35r19 up front in an attempt to keep the overall balance in check. I've been running 255/285 for years now on the 981 and 718 base Caymans, and feel these work well. If I were to go with the wider rear tire and 255 fronts, I would recommend a stiffer rear bar to improve rotation (something else I'm considering).

JKinOB

Thanks!

Why keep the compound the same front to back? I am genuinely curious because I don't know enough about tire technology.

PedalFaster

The concern with mixing tire compounds is that it will change your car's handling because different tires behave differently -- they warm up at different rates, have different grip levels, different breakaway behavior, etc.

I find this concern hugely overblown. We generally make sway bar adjustments one end at a time. We generally make shock adjustments one end at a time. We generally make tire pressure adjustments one end at a time. Changing the tire compound on just one end of the car will affect its handling, yes, but not in any uniquely challenging way. I've frequently mixed tire compounds front to back in the past with no problems.

Having said that, you don't want to mix AD08 Rs with RE-71Rs. The AD08 R is a few generations old now, and has significantly less grip than the current state-of-the-art tires (Rival S and RE-71R). If you run AD08 Rs on one end and RE-71Rs on the other, the end with the Yokos on it is going to be severely lacking in grip.

JKinOB

Thanks! That is very helpful.

What are the disadvantages (dangers?), if any, of running the 265 RE-71R on an 8.5" wide wheel?

Thanks!

John

PedalFaster

The biggest disadvantage is that your tire guy will swear at you the next time he sees you.

As you increase tire width while keeping wheel width the same, there's a point beyond which grip no longer increases (or could theoretically even decrease). Stuffing a wide tire onto a relatively narrow rim can also worsen steering response. A 265 RE-71R on an 8.5" rim is probably in the vicinity of the no-gains-in-grip point.

I used to run a C5 Corvette in BS, which had 8.5"-wide front wheels (although most of us ran 18" wheels, not the 19s that you'll be running). Almost everyone tested both 255 and 275 RE-71Rs in front, and opinion seemed to be split about 25% / 25% / 50% between "the 255s are faster", "I can't tell the difference", and "the 275s are faster". That indicates to me that the differences were subtle at best. For what it's worth, I was in the "I can't tell the difference" camp.

JKinOB

Thanks!

Fortunately, my tire guy likes me (and my relatively new hobby of buying tires).

How so? Does it take longer to respond, or need a greater input to get the desired response, or something else?

Thanks again!

John

PedalFaster

All of the above -- longer to respond, and more steering input required. The sidewall of a wide tire stuffed onto a relatively narrow wheel will flex more than that of a tire that's "properly" matched to the wheel.

The RE-71R's a pretty stiff tire, though, so the differences are relatively subtle -- I don't recall noticing any significant difference in response between 255 and 275 RE-71Rs on an 18" x 8.5" front wheel.

JKinOB

Thanks!

JKinOB

One follow up question, has anyone running the 305/30R19s with either the 255/35R19s or 265/35R19s in front had any issues with PSM (either on course, if used, or during daily driving) as a result of the larger front tire diameter causing the front wheel to rotate slower?

motoring

GT4 but close enough... i ran the kumho v720 245/305 combo for a significant amount of street driving, the 245 is nearly .5 inches taller than the 305, never had an issue with the psm. the ABS was not all that happy, it was pretty easy to shock it into ice mode. The 255/305 re71r combo did not have the same ice mode issues.

JKinOB

Thanks!

Mport68

I'm kinda with PedalFaster on this, and I'm in the same boat as the OP... I want to step up my rear tire size to the 305/30-19 (from a 285/35-19) but I'm not sure moving from a 255/35-19 front tire to a 265/35-19 is going to be helpful. TR says it has 0.3" more tread width but that's when mounted on a 0.5" wider wheel so I'm not sure we (on an 8.5" wide wheel) will get any measurable increase in tread width and because its a bigger, heaver tire the net effect could be slower. So that leaves the decision to the diameter... its taller and given that we cant adj ride height, that has to have a negative effect on balance by taking weight away from the front end. The only thing holding me back is as far as I can tell, every nationally competitive GT4 is running the 265/305 set-up... so maybe there is something more to this than I considering. I realize they have fully adjustable suspensions so maybe that's the only discriminator that matters?

chriswd62

The more I've been researching, the more I think I'm going to stick with the 255/285 setup for my 718GTS. The GT4s come from the factory with 245/35r20 and 295/30r20, so moving to a 265/35r19 and 305/30r19 is only a a small variance in rake change front to rear. Plus the GT4 has fully adjustable coilovers, camber and sway bars so it's easier to tune out any potential negative effects. The 718 base, S and GTS don't have these adjustments and the last thing I want to do is to introduce more understeer. Although I haven't tried out the larger setup myself, based on my experience I don't think there will be much benefit to the 265/305 vs 255/285 on a 19x8.5/19x10.5 wheel setup.

edfishjr

I've run 255-17 on 8.5, 265-18 on 8.5 and 275-18 on 8.5, all on a Corvette similar to PedalFaster's. (I never tried the 255-18 tire, which may not have been available then.) I liked the 265 the best. Felt the 275 was too wide and no faster (maybe even slower) than the narrower tires, but others might disagree. Plenty of good drivers beat me on 255-17 when I was on the 265-18, however. The 255 had better feel, but I felt less total lateral grip and definitely poorer braking ability. (Back then I was a really late-braker.) I never tested them back to back new, however.

One thing autocrossers (unlike drag racers) don't often talk about is that a larger overall outside diameter increases the total contact patch, independent of the width. (An infinitely tall tire would have an infinite contact patch, no matter the width.) It's not as important as tire patch width, but it still has an effect. In the case above, the 265-18 was slightly taller than the 255-17, by about the same amount as the 255 vs 265 19's being discussed here.

Also, the difference in tire height front to rear has a really very minor effect on front to rear static weight distribution. (Unlike corner balancing in which small changes can create big differences in static weight distribution.)