Slightly stretched tires = stiffer sidewalls?

[ayau]

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

[korinfox]

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

Optimal wheel size
Many folks entering the ST category are refugees from Stock looking to make some performance upgrades to their car and/or reduce their tire budget. Unfortunately, they carry with them some baggage on concepts that may work well in Stock, but not in ST. One of these is optimal wheel/tire fitment.

Since Stock limits you to the OE wheel sizing, astute competitors have learned how to stuff increasingly wider tires onto those skinny rims. And tire makers have been happy to make wider and wider tires, likewise charging more and more money. But, is wider better? In Stock, the answer is often "yes". The sidewalls on these DOT-approved "Not for Highway Use" R-comps are super stiff and allow for "overtiring" much in the way that a cantilever style slick works to get around wheel width limitations for formula cars.

In ST, drivers in most classes have limits on both wheel width and tire section width, both of which are much wider than OE. The tendency is to cram the widest legal tire on the widest legal rim that will fit and go for it. But that is not always the optimal solution, as street tires do not have the same super-stiff sidewalls as do the best R-comps. Sometimes it makes sense to run a narrower tire for a given rim fitment. 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!). So a 205/50-15 tire with a typical tread width of 7.5" is best on 7.5" rim. An 8" rim will work, too, but it will be majorly stretched on a 9. Likewise, it will lose some performance on a 7, and will lose a LOT going down to a 6 or 6.5. Similarly, a 225/45-15 with an 8.2" tread width works best on at least an 8.5" rim and loses a fair bit on a 7.5".

A practical example of this is in STC, STF and STS where the rim limitation of 7.5" keeps the 195/50-15 Toyo R1R at the top despite it not being the widest allowed tire size. The 225/45-15 R-S3 mounted on that same 7.5" rim is not quite as fast. But put that same R-S3 on an 8 and it comes alive. And it's even better on something a bit bigger. So in STR, where wider rims are allowed, the common 225 R-S3 on 9's is a faster combo than the 195 R1R on a 7.5 (assuming nominal weather). And putting the 195 on an 8 or wider does nothing to enhance performance and close the gap.

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.

[ayau]

i also found this while googling stretched tires.

If you are further interested in this topic I suggest you pick up some basic literature on vehicle dynamics or google cornering stiffness.

The sidewall flex that everyone else is talking about is parameterized by cornering stiffness in the tire world. What it describes is the rate at which the tire will produce lateral force. Physically this can be seen as the initial linear slope of slip angle vs. lateral force for a given tire. Many factors affect cornering stiffness and the "stretch" or angle of the sidewall is an important factor. In testing the cornering stiffness will continue to increase until the angle of the sidewall approaches that of the tread section.

Construction of the tire itself will contribute to the cornering stiffness and things like tread compound actually play a much more important roll than the sidewall stiffness. It's easy to understand as sidewall flex but this is just one part of the interaction. For instance the relaxation time of each material included in the tire will have an effect. The stiffness of the summit in bending and in shear will have a large effect. Even the size of the bead mounted to the rim will have an effect.

As everyone has stated before some "stretch" may be good. In fact ALMS teams use it as a balance and tuning parameter. Take a look at the Pratt & Miller C6R's they usually run a wheel larger than the measuring wheel up front.

At the control end for the driver it's down to the required response of the vehicle. A vehicle with higher cornering stiffness will respond faster to input but will also drop off quicker at the limit (unless this is countered by material choices). Actually changes in mechanical grip are negligible unless extreme differences in cornering stiffness are had.

In the case of every street tire it will most likely be perceived that handling has been improved with higher cornering stiffness (especially by gear-heads). Until you run the risk of unseating the tire or scrubing your wheels handling will improve with stretch (as others have stated before comfort will not).

As with anything you can have too much of a good thing...

[Nitro]

TLDR?

[korinfox]

Originally Posted by Nitro

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.

[Mitch]

Originally Posted by korinfox

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?

[ayau]

Originally Posted by Mitch

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.

[ayau]

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

[7thgear]

Originally Posted by Nitro

TLDR?

stick to driving to your local corner store for some milk

[xwd]

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.

[mla163]

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.

[CSG David]

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.

Hope this helps out!

[ayau]

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.

[korinfox]

Originally Posted by ayau

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.