Slightly stretched tires = stiffer sidewalls?
 

Can someone comment on running slightly stretched tires for stiffer sidewalls? What are the pros and cons of stretched vs normal?

Someone had posted it here in another thread but here's the quote from it again:

From here.

Anyway, most people would consider a 225 on an 8.5" rim, slightly stretched, and Andy Hollis recommends it here if you're autocrossing.

i also found this while googling stretched tires.

TLDR?

From the testing we've done, we find that a good rule of thumb is that the optimal rim is the same size (rounded up to the nearest half-inch or so) as the tread width (not section width!).

This assumes street tires though. If you're running Hoosier R-comps their sidewalls are much stiffer, and you're best off stuffing as much tire onto the rim as you can fit.

So on my 17x8" wheels, I should be faster with 205s than the 255s I'm running or 245s that others typically run? Like wise, a 225 tire on a 9" wheel?

a 215/45/17 tire would have a tread width of 8", so you'd want a 215 tire.

that makes sense now. the exmotion brz runs 17x8 wheels on 215/45/17 RE11 tires according to this thread.

http://www.ft86club.com/forums/showthread.php?p=462743

stick to driving to your local corner store for some milk

It depends on th construction of the tire generally. For instance the Dunlop Star Specs have very stiff sidewalls for a street tire. The Hankook RS3s do as well but not like the Star Specs. One datapoint is in the STR autocross class which is limited to 17x9 wheels S2000 guys at the top ran 255 Star Specs. The 245 has a little crisper turn in but doesn't have the ultimate grip of the 255.

In general, a stretched wheel will give you more response, less grip. A square or squished tire will give you more grip, less feel. The right combo is somewhere in between.

In the Miata community, 225 width tires work best on 9" width wheels. In the S2000 community, 255 width tires work best on 10" width wheels. The width of the tires is basically equivalent to the width of the wheel.

225/25.4 ~ 9", likewise, 255/25.4 ~ 10"

Anymore stretch than that is overkill and does not function as well as what I mentioned earlier. Most people will run whatever is recommended on the spec sheet, for example, 255 tires on 9" width wheels, 245 tires on 8.5" tires, 235 tires can be either on 8.5" or 8" wheels, etc.

For stock power, 215 width tires are good enough for the BRZ/FRS. We ran 215s on 8" width wheels to start off with on the track. This extra "stretch" helped improve some sidewall stiffness, but the car needed serious camber to fix most of our wear problems!

Autocross guys have to use the OEM wheel sizes for certain classes and will stuff as wide of a tire as possible. That's because those specific tires have super stiff sidewalls and at slower speeds, more tire equates to more grip for the given course. I'm not much of an autocross guy, so some autocross guys have to chime in on that idea. :thumbsup:

Hope this helps out! :)

thanks @CSG David, that's good to know that slightly stretching tires is common in other communities.

on an related note, the stock michelins have a tread width of 7.5'', so it appears that the OEM wheels are slightly undersized given the tire size.

Yeah, slightly stretching the tires is generally bad for ride comfort and protection of the wheels if you curb them.

so this begs the question of why are people 'overtiring' their wheels with 255 on 17x8 rims? for looks, i can understand, but there are some members who track their cars and still oversize their tires. hmm....

I autocross. People in my region have turned fast times with the setup. I knew it would be excessive, but I wanted to work my way down through the sizes so that I'd have experience with different size / width combinations. There's a saying to do your last experiment first, and 255 was the widest I'd ever try to cram on an 8", so I went for it. I'll go 245 next, and if I like the way it changes the car I'll keep going down in size until I start losing time. So I'm doing it for experience. I'm not going to be winning any events anyway, so nothing really lost.

keep us updated. i know that there are a lot of variables to account for, and that your experiment probably isn't very scientific, but it will still be interesting to know your results.

Very good information in this thread. Keep in mind that in most of this information be presenting it is in terms of "optimization" not "diminished results".

Example:
A: I have a 225/45/15 tire so running a 15x8 or even a 15x9 will improve the way that tire performs vs running a 15x7 wheel.
B: I have a 15x7 wheel. If I increase the size from 195 to 225 will I see a improvement in cornering forces.

Both A and B are correct and A does not cancel B. Just because it's optimal to run a 8 or 9" wheel with my 225's does not mean that the 225's on my 15x7 won't be an improvement over my 195's or 205's. I personally have racing data that backs up this point.

It makes sense to use a wheel width that is within spec of tire manufacturers recommended width. Example:
Hankook Ventus RS3 in 225/45/15 Recommends a 7"-8.5" wheel.
http://www.tirerack.com/tires/tires....num=245WR5Z222

If you look at that chart for that tire, Hankook doesn't recommend a 9" wheel until you reach a tire with a section width greater than 10". Interdasting! (255/40/17)

So running "over-tired" is not going to decrease performance until you reach the point of diminishing return: Tire breaks the bead, Tire can't actually be mounted, not all the contact patch is being utilized, engine isn't strong enough to turn those big heavy tires etc etc etc. Conversely running tires wider than your wheels is WAY better then running tires skinnier than your wheels. Just say no to Hella Flush.


Here's a picture of the 225/45/15 Toyo RA1 on my 15x7 wheels. As you can see they aren't exactly "struggling" on those wheels. (only pic I had of tire mounted on my wheels) BTW: that dent is thanks to a Spec Miata.

http://img.photobucket.com/albums/v8...c/DSCN2651.jpg

I love testing tires. In 2011 I raced on Toyos, Goodyears, Hoosiers and Hankooks. This year just Hoosier and Hankook but next year I'm looking forward to the new Toyo RR.

Tires as of mid season last year: (and those were just the Toyos!)

http://img.photobucket.com/albums/v8...c/DSCN2681.jpg

I agree and to put it more succinctly go wider unless you have a tire with floppy sidewalls. The increase in ultimate grip is going to outweigh the decreased feel and turn in response. Now if you are drifting or something that's another story since you actually want less grip.

Generally speaking, autocross is a very poor way to actually test tires. Too many factors that affect your consistency, the two biggest one is the fear of hitting a pylon and the long wait times between runs.

On track, having a 255 on a 10" width wheel is only a couple tenths faster on a consistent basis. How the S2k Challenge base their point system is through the testing on Buttonwillow Raceway. There is sufficient data to support the 255 on 10" vs the 255 on 9". The difference is approximately 0.25 seconds on a 2.68 mile track, hence the 0.25 point penalty. The key is consistency at this point. Also, I'd like to point out the S2k guys run either NT01s (R-comps) or Hankook RS3s/Dunlop Z1SS. Everything else is too expensive and unproven to go faster that those setups thus far.

On a side note, autocross guys stuff a lot of tire because in the end, grip is grip, so more tire helps give you as much possible grip you can get on your car for the autocross course. The speeds are not fast enough to cause you to slow down. On large tracks, too much tire on a particular car can actually scrub off speed in your corner, reduce the speed on the straights, etc. This is why some guys running 255s on the BRZ/FRS are not running any faster than guys with the 215 size tires. Now if the car was turbo'd, then 255s are the faster way to go...:p

I apologies in advance for bringing up a post from nearly half a decade ago but this is the first time I have encountered someone saying that 215's on 8 inch wheel causes tire wear problems. Is this true for stock suspension or if you are lowered? I just got this exact setup on my car and hoping this isn't going to screw with the tire wear.

When you stretch a 215 tire on an 8" wheel, you're putting nearly all of the tread width on the ground. So when leaning in a corner, you have very little excess tread before rolling onto the outer shoulder/sidewall. It's mitigated slightly with a relatively wider tire since there's more unused tread width to roll onto.

Regardless, without enough camber you'll have excess front shoulder wear. I've tracked on both 215 and 235 tires on 8" wheels (i.e., slightly stretched vs. slightly pinched), and either way I wore the front shoulders @ -1.5 degrees camber.

Don't worry too much about it. You can rotate the tires to even the wear (rear suspension gains significantly more negative camber on compression so wears more evenly) and your tires should last you a decent number of events. Just go have fun for now and add camber adjustment to your list of future mods.

:burnrubber:

You should not change too much the geometry of the tire. Saying that, a bit of stretch is not bad. Even factory does this. The new power package uses the same 215 tire on a +0.5'' wheel (215, 7.5'' width).

Back home the thumb of rule is the recommended rim size and upp to widest rim size, preferably the upper half of the recommended too get some stretche on the tier as you mentioned.



Toyo R888
http://www.toyor888.net/racing/index.htm
http://toyotires.com/tire/pattern/pr...petition-tires
http://rejsa.nu/im/user/19274/2012-0..._falgbredd.jpg

Kuhmo Ecsta V70A
http://www.kumhotyre.co.uk/Motorspor...rochure01.html
http://rejsa.nu/im/user/19274/2012-0..._falgbredd.jpg

Yokohama A048
http://www.yokohamatire.com/tires/detail/advan_a048
http://rejsa.nu/im/user/19274/2012-0..._falgbredd.jpg

I've had some good results running the stock high spec tyre sizes (215/45/17) on 17x8 rims.

My 2c.

While I don't think anyone can just go ahead and test a tire in autox, a good driver certainly can. I've ridden with drivers that can consistently turn in runs within a 10th of each other, and with a tire change their next set of runs is just as consistent at a different time.

Yep. Whatever is flawed or theoretically bad about my putting a 245 on a 7 doesn't seem to be causing me active harm. It's not a silver bullet, either, though.

The bolded is why I do it. Stock/Street doesn't give you a lot of knobs to adjust, so you do what's available. Doesn't matter if it feels bad or is a bass-ackwards methodology. It absolutely might not work for some folks' driving styles. In the end, use the scoreboard as your measure.

As a practical matter, it also lets me get away with some dumber/more aggressive tactics. Brake later without fear of abs intervention, get back on the gas sooner without wagging the tail so much, it's more material to heat up, so it's harder to overcook them (also harder to get heat into them...).