Others (Duhh! comes to mind) have mentioned this, but I chose to ignore it in the hope that I just wouldn't have to deal with it and that my lack of driving skill would render it meaningless. While the latter may still be true, I was surprised by the differences that I measured today. (Hey, what else should one be doing on Christmas but measuring the sizes of their SRF tire inventory, huh? Well, I was playing Kinect Dance Central with my daughter. I'm doing my best to keep the video replay off of YouTube!)

I have 3 sets of mounted dry tires. Here's how they compared in diameter (I set all of them to 30 psi prior to measurement):

Weld wheel, front: 22 3/4 and 22 1/8 (5 cycles)
Weld wheel, rear: 22 1/2 and 22 3/8 (5 cycles)

SCCA set 1, front: 21 7/8 and 21 5/8 (Stickers)
SCCA set 1, rear: 21 3/8 and 21 3/8 (Stickers)

SCCA set 2, front: 21 1/2 and 21 5/8 (15 cycles)
SCCA set 2, front: 21 3/8 and 21 1/2 (15 cycles)

Considering that we adjust ride height to 1/8" and go "all anal" about corner weights, those differences in mounted tire sizes seem pretty darn significant.

So, does anyone try to manage these discrepancies, and, if so, how?

I don't think my Goodyear rep would take kindly to me asking him to make sure that the mounted tire diameters are within some tolerance, like 1/8". Maybe I should expect that of him?

Merry Christmas to all of you and your loved ones!

Ed

Ed--The difference in sizes will definitely cause problems. Don't get too worried abouit the 1/8" difference. Do get worried about the 1/2 or 1" difference. Usually Sauce doesn't have any excess tires, but if he did he should trade tires out. At the run-offs I have had Goodyear swap out one tire each year. They always do it immediately, with no questiohns.

Thanks Tom. I've never measured loose tires and then measured them after they've been mounted, but I wonder if these variations could be caught before mounting? If so, then I could ask Mike to show me which tires he has "reserved" for me and I could measure them beforehand. Maybe he'd be willing to swap them around before he mounts them? At the very least, I could mark which ones I want on which rim to try to minimize the variation. Of course he could take the hard line and just tell me, these are yours, take them if you want them, if not, go get your SRF tires from someone else. Oh wait, there is no one else.

Unfortunately you need to measure the circumference with the tire mounted and inflated. The trick would be to be the first to get your tires, and measure them while still at Sauces. Then he would be able to swap for you. Sometimes he could swap around to best match the front and rears. An added advantage if you get a different tire is that maybe your competitor might get the unmatched set...

Try this. Take one under-sized tire and inflate to 50psi and leave overnight. Then drop to 30lbs and re-measure (I expect you'll be surprise, especially on a newer tire.) Also, measure circumference, not diameter, as it's a more precise measurement of stagger. Because these are bias-ply tires, they are subject to more variation in circumference (stagger) and sensitive to TP variation. Variation in diameter between difference wheels is normal, I believe, as the wheels are not precisely the same width (which is why front tires have higher diameter because of the narrower width).

If you're running 15 cycle tires, tire diameter is the least of your concerns!

Thanks Bob, I'll give that a try. I would run the 15 cycle tires at a DE, but not in a race. They were in the garage, so I measured them.

This is an interesting thread. I have a few follow up questions. First, do the tires grow larger after use? Is the growth fairly equal (e.g. 1/8, 1/4)? In short, can stickers be accurately measured or do the tires need to be heat cycled first? Second, how much do tires grow if left over night at 50 psi? Is that growth permanent? It would seem they must grow at least much during use due to centrifugal force. I would need to look at a physics text book, but my guess is that the outward force on the trend at 100 mph is greater than the static force at 50 psi (of course, we don't drive at 100 mph for an entire night). Thanks for the input.

I try to get my measurements with the starting pressures, pre-race. That way, you have the added advantage of adjusting your setup during the race by "creating" stagger by "over"heating a particular corner, thereby increasing the diameter of that tire.

H.(unfortunately, rarely races on ovals)B.

Hal B. wrote:I try to get my measurements with the starting pressures, pre-race. That way, you have the added advantage of adjusting your setup during the race by "creating" stagger by "over"heating a particular corner, thereby increasing the diameter of that tire.

Pure, unadulterated genius! These are the kind of nuggets that I read this forum for!

Careful, Hal, some people will believe anything...