Tire stretch past rim - where do you draw the line

[dre99gsx]

I'm experimenting with some new wheels and tires. I'm trying to run a 10.5" rim with a 295 size tire (R888R), which to my surprise, runs very wide. In fact, the sidewalls bulge outward about 0.5" on each side.

My regular 9.5" rim on a RE71R 275 tire had the sidewall perfectly perpendicular with the rim width.

I've tried this tire on a 10" wide rim and it is very exaggerated, so much so that I don't feel comfortable HPDE'ing it. I actually think Tirerack shouldn't recommend 10" as the minimum, they are crazy.

Any of you guys run tires which protrude from the rim without any issues on the track? Looking for feedback, and perhaps some photos for reference.

[dc_wright]

I've run a 275 Hankook RS3, 275 RE71, 265 RS4, and 265 RE71 on an 8.5" rim without any issues. In the end the 275 width didn't buy anything because the pinch fit caused the shoulder of the tire to be more rounded coming off the tread surface toward the side wall, and the contact patch width was actually slightly narrower then with the 265s.
Note that there is a trade off in pinch fit vs. response. As you increase the pinch, turn in gets less responsive because there's less sidewall support, and there's a point where increased grip from the wider tires doesn't offset it. It's probably more noticeable in autocross than on the track because there's more transient response demand.

[dre99gsx]

Attached an image of what I'm talking about...

10.5" rim, and 295 R888R.

[tiltmode43]

Originally Posted by dre99gsx

I'm experimenting with some new wheels and tires. I'm trying to run a 10.5" rim with a 295 size tire (R888R), which to my surprise, runs very wide. In fact, the sidewalls bulge outward about 0.5" on each side.

My regular 9.5" rim on a RE71R 275 tire had the sidewall perfectly perpendicular with the rim width.

I've tried this tire on a 10" wide rim and it is very exaggerated, so much so that I don't feel comfortable HPDE'ing it. I actually think Tirerack shouldn't recommend 10" as the minimum, they are crazy.

Any of you guys run tires which protrude from the rim without any issues on the track? Looking for feedback, and perhaps some photos for reference.

Presumably you're running 295/30/18?

The TireRack recommendations should based on the tire manufacturer recommendations, though I've seen a couple cases where TR recommended spec differs from the tire manufacturer. To be sure, I always pull up the tire manf's site to confirm. In this case Toyo matches TR, 10-11" rim width range.

Tire manufacturers test their tires when ensuring/determining the safety of a recommended rim width range, I have heard of tire companies purchasing wheels specifically for this sort of physical testing. I'm not sure what the exact tests are, but it's likely a combination of requirements such as retaining bead seat with min/max xx.xx PSI combined with x.xx lateral G's and xxxx radial load Lbs.

That said, I would be 100% confident running tires on wheels within the recommended rim width range. Visually, tires run different because of varying sidewall shapes, tread block shapes/sizes, and tread widths (different from section width, which is the entirety of the tire, effected by the factors just mentioned and what the overall visual impact of a tire is usually based on). Toyo is the manufacturer of the tire, who has gone through the process of engineering that tire. If they are providing a recommendation on what width wheels to run an awesome competition tire like the R888R on, I wouldn't hesitate to take their word for it and push that tire to it's limits in the harshest of environments.

Originally Posted by dc_wright

I've run a 275 Hankook RS3, 275 RE71, 265 RS4, and 265 RE71 on an 8.5" rim without any issues. In the end the 275 width didn't buy anything because the pinch fit caused the shoulder of the tire to be more rounded coming off the tread surface toward the side wall, and the contact patch width was actually slightly narrower then with the 265s.
Note that there is a trade off in pinch fit vs. response. As you increase the pinch, turn in gets less responsive because there's less sidewall support, and there's a point where increased grip from the wider tires doesn't offset it. It's probably more noticeable in autocross than on the track because there's more transient response demand.

These factors are so often overlooked when picking tire sizes. A narrow tire on a wide wheel will have a wider section width and typically a wider tread width/footprint when mounted on a wider wheel, sidewall flex/responsiveness being the other factor.

What blows my mind with all of this is that some car manufacturers have actually strayed from the typically approved sizes with their OEM tire options. Seems crazy to me that Chevy, Porsche, and BMW have actually gone outside the typical recommended ranges. Perhaps they worked with the OE tire supplier at the time to make sure it was safe for those applications based on GVWR & GAWR? Who knows. But for what it's worth, here are some examples to show the lines are sometimes even further blurred (list yanked from TireRack):

1995 BMW M3 combined 235/40R17 sized tires on 17x7.5" rims (which are 0.5" less than the narrowest 8.0" wide rim listed for the size).

Corvette C3 models combined P225/70R15 sized tires on 15x8" rims (which are 0.5" wider than the 7.5" wide rim now listed for the size).

Corvette C4 models combined P255/50R16 sized tires on optional 16x9.5" rims (which are 0.5" wider than the 9.0" wide rim now listed for the size).

The rear tire application for Porsche 911 GT3 cars during 2010 - 2012 model years combined 305/30R19 sized rear tires on 19x12" rims (which are 0.5" wider than the 11.5" wide rim listed for the size).

[inTgr8r]

IMO you need to dig deeper than just looking at whether it's a 275/285/295.
265/35 etc... are just rounded ratios.

What is far more important is tread width and section widths.
I've seen where one brand's 265 is the same width as another's 275
They can vary a lot.

[3002 tii]

Honestly that doesn't seem that bad...

And I also echo Ian's comments about looking at section widths in the specs. In this case your looking at 11.9" section width on a 10.5" rim which tells me going up to 11-11.5" run would've given you that more square look you're going for. Typical starting point for r-comp fitment among my track buds is 255/9.5, 265/10, 275/10.5... you get the idea.

Me personally, I've run tires (not oversized but on the conservative end of width) and it just meant more rollover effect. Playing with pressures can help but I wouldn't worry about what you have.

[dre99gsx]

Thanks for the feedback. The fronts will be more square, these are just rears.

I'll work with it and see how it feels this season.

[dc_wright]

Originally Posted by dre99gsx

Thanks for the feedback. The fronts will be more square, these are just rears.

I'll work with it and see how it feels this season.

After you do your homework, this is what it comes down to. You try to dial in based on the published specs, but those don't tell you about dynamic characteristics which are what count when you're rolling. You pick something that looks like it's in the ball park and then you have to test. Same thing the professional teams have to do. Sometimes it works better than you expected, sometimes not.

Currently I'm running RE71s on the front and RS4s on the rear as my autocross set. The idea is to help the car rotate, and it does.......for two runs. 1st run the RS4s have no grip because they need more heat than the RE71s. 2nd run the RE71s have developed their grip and the RS4s have a bit but not enough to push hard. 3rd run the RE71s are still good and the RS4s now have some heat to where they've got some lateral grip and you can move the rear of the car and have confidence it's going to move and then hold. 4th run without a spray down the RE71s are getting too hot and the RS4s are now getting to where they've got pretty good grip. The car shifts from being slightly tail happy to understeering and then it's figuring out how much to cool the RE71s so that they have optimum grip, and not too much otherwise you're back to pushing again. The set up achieves what the intent was, but essentially the 3rd run has to be the one or it's sub optimum. I'll be going back to RE71s on all 4 corners when this set wears out.