Tire sizing question for track car
 

I've decided to get some new track-only tires. I'm not going to go with r-comps or slicks, so I'm leaning towards the Falken Azenis Rt-615 because it's good and grippy and inexpensive. Lots of people use them for lapping days and HPDE events with good success.

I have a set of Forgeline wheels I plan to stuff them on, 17x8 front and 17x9.5 rear. Currently I have:

235/45-17 front (643.3 rolling diameter)
275/40-17 rear (651.8 rolling)

stock tire size is 225/50-16 (631.4 rolling) so these current tires that I have are significantly larger. I've had some minor scrubbing issues with my stock suspension and these bigger tires at the track when I lost a bumpstop.

The problem is, while the Azenis are available in 275/40-17 for the rear, there's nothing similar to that for the front. I can choose from:

235/40R17 (619.8 rolling) <-- (.2" ride height less than stock)

or

245/45R17 99W (652.3 rolling) <-- too big

I'm inclined to go with the 235s because they are a lower profile, but then my car will be jacked up by over half an inch in the back if I use the 275s which I assume is bad for weight distribution. I could use the 245s in the front but I risk scrubbing the fender liner because they're so large. Alternatively I could run a 255/40-17 in the back to even it out a bit but I'd be losing a lot of contact patch.

Suggestions/recommendations?

Matt,
I ran 235/45/17 and 275/40/17 for years. It used to be the standard "upgrade" size for FD's. It'll be fine... EDIT: 235/40/17 is very small diameter... but I remember Paul ber ran these w/ 275's on his Forgelines, because AO32's didn't come in 235/40 either.

With so many choices available, I'd go with a different tire.


Most dedicated track setups go with a larger or same-size front tire, but your wheel sizes won't allow it. You can tune the resat of the car (shocks, springs, anti-roll) to work with your setup.

You could go slightly smaller on the rear, if ballance is an issue, and they are availble in the tire you want. You could go down to 265/40 or 255/40.

I understood the 615 were more of a autox tire that got a bit greasy after extended sessions on track. I think I saw threads about that here and on bimmerforums under the track sub-forum.

Yes, the Falkens get greasy if you push them hard for more than a couple of laps. But if you're just out to have some fun doing lapping days they're fine. Personally I prefer the Toyo RA1 tires for lapping days since they are very consistant and wear pretty well.

Just watch out for the rubbing on the fronts. Realize that the engine wiring harness runs right across the top of one of the fender liners. I seen people rub right into and through their wiring.

Good luck,

-bill

I have done, when I lost a bumpstop. The harness in question was for the airbag sensor. Chewed right through it.

I have a number of friends that use them on different types of cars from a MR2 Sypder to an E46 M3. As an autox tire they reach operating temps very quickly, and then start getting a bit greasy as they heat up more and more. I've never heard anyone complain about it being a real problem for them though, even on a heavy car like the M3. I'll look into the RA1's, but I'm not sure I want to make the jump to r-comp yet. Anyone else have a tire they prefer that's good for the track but still streetable?

Don't have much input for you there as I'm running Kumhos and I'm still getting up to speed. SleepR1 is a big fan of the new-ish Bridgestone RE 050(?) on his M fwiw for street/track use. Another friend of mine has the Bridgestones on his 993 and loves them but he's only used them on the street thus far.

If you've got a second set of wheels, i don't know why you wouldn't use R's... particularly something like the Toyo that will last a while.

Yea I'm running a set of Bridgestone Potenzas on stock wheels which are fine for beating around town but aren't satisfying my need for speed at the track. I'm looking into the Hankook Z211 DOT r-comps which Fritz pointed out are 40% off on close-out (and have been since the winter) but I don't know if I can find the sizes I need any longer.

I know several people who have used the 615's on the track and auto-x. They also mentioned on the track that they aren't too friendly if/when they overheat, but what street tire is? :) I plan to run them next year on my M3. IMO, for the price its one of the better street/performance tires from what I've observed.

On my street car on the track, I actually used Goodyear Eagle F1 GS-D3's and they were fantastic!

As ptrhahn suggested, I could step down to a 255/40/17 in the rear and then run the 235/40/17 in the front, but it seems like I'd be giving up a lot of contact patch just to get the rotating diameters closer together... Alternatively, I could go with a 245/45/17 up front, but I'm thinking that'll probably cause rubbing issues as it's close to 22mm diameter wider than stock. Any experience with a 245/45/17 on a 17x8 front wheel? I don't know the offset of these Forgelines, it's not listed on their website :(

Matt,
Are you making big power? the 255/40 rear ought to be fine.

Nope, stock twins with an M2 ecu and basic bolt ons. I only run 9psi on the track for reliability reasons anyway. I guess 255's would be perfectly adequate, but I am a little concerned about stretching a narrower tire on a wider wheel.

255 will do you. A 255 on a 9.5" wheel won't be bad at all... it's well within the manufacturer's acceptable range.

you'll definately want to go with as squared of a setup as possible, so you're on the right track with the 255 rears and 235 fronts.

I've used the 615s for track days without problems. As a matter of fact I was at one on Saturday with ambient temps around 90 degrees, sunny, and running 20 minute sessions. They are very tempermental toward pressures as far as getting greasy, and you need to run them lower than comparable street tires.

I run mine at 37lbs HOT pressure, and that's where I've had the least problems with them. I would think that those pressures would be a good place to start with on an Rx7 as well since it's lighter than my e36 M3. The tires take the abuse quite well, and are a hell of a deal price wise. I'll move to an R-comp next, but these offer a great bank for the buck, and are suprisingly close to an RA-1 traction wise (which I've also driven on).