Stretched Tire Theory.

[Condor1970]

As I talked with a guy who tracks his Cayman and BMW quite a bit, I started understanding some of the theoretical mechanics behind why a "slightly" stretched tire may actually contribute to better handling, and a more comfortable ride for a lot of European cars. It makes sense, but only to a certain point, and the amount of stretch can quickly become very problematic vs a benefit as it stretches further. This idea may also be why the GT350 had a slightly stretched tire from the factory

The idea being if the sidewalls are slightly stretched, then the angled sidewall provides more compression strength when cornering hard. Whereas a vertical sidewall will give by flexing a little more. The vertical sidewall however, will theoretically maintain a slightly better contact patch when cornering because of this, as both inside and outside sidewalls flex at the same angle. As the stretched tire sidewall flexes, the outside sidewall angles more, and the inside sidewall becomes more vertical raising/applying more pressure the inside of the tire contact patch if the sidewall isn't strong enough, or not enough air pressure is present. This means sidewall strength needs to be engineered better, and proper air pressure applied for optimum contact and grip. Though the potential benefits are there.

Another potential benefit, is because the sidewall is angled inward, it contributes to more easy compression for a smoother ride over rough roads. Vertical sidewalls may flex slightly less when hitting a bump, whereas and angled sidewall will have a bit of angular preflex to allow for further compression and more absorption. The idea being a tire with a hard vertical sidewall would provide a slightly better ride when slightly stretched.

I'm certainly no expert on racing tires, but these were just some of the thoughts we discussed, but I'd like to know what you guys think as well.

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[AlbertD]

Great discussion. I am certainly no expert on the topic, but I do generally pick a tire size that gives a slight stretch for improved tire response. Ideally .5" narrower than my wheel width. This is a great read on the topic... https://motoiq.com/how-to-properly-select-and-size-tires-for-performance/2/

 

[Condor1970]

Great discussion. I am certainly no expert on the topic, but I do generally pick a tire size that gives a slight stretch for improved tire response. Ideally .5" narrower than my wheel width. This is a great read on the topic... https://motoiq.com/how-to-properly-select-and-size-tires-for-performance/2/

It's ironic that you mention 0.5" more narrow. We were tossing that same number around as well, just as a thought process, but admittedly with no real data to support the idea. In my case, I recently bought a full set of SVE R350 wheels for my base GT. The stock 235's on the 18x8's are highway comfy, but just light up way too easy, and ice traction is beyond sub par imo.

Anyway, the new R350's are all 19x10 rims. So my thoughts for quite a while, is which Michelin AS3's do I get for best comfort, and best performance? Everyone keeps screaming 285/35's!!! Which they may be right, but with such a tire, I know for a fact my overall ride comfort may not be achieving what I would prefer for daily driving.

So, here's a thought. For the 19x10 rim, why not go with a 265/40 AS3? Yes the tread is 20mm less than the 285, but it will have a very slight stretch that may really help compensate for cornering performance, yet provide a better daily ride. The 265 Michelin AS3 has 10.7" measured width and a 9.4" tread width. This puts it very close to the 1/2" more narrow desirability vs running a wide 285.

Does that make sense?

 

[AlbertD]

I completely agree. I am actually running 275/40 (9.5 thread width) on my 19x10s squared for street. For track I recently went with a 285/35 NT01 (10.7 thread width) on 19x11s squared.

 

[NightmareMoon]

I’ve run the same 285s RE71Rs pinched 9/9.5” and now Im running the same set on 11” wheels, and the differences feel slight, almost imaginary, but I wont go that far. Felt like the same tire, but maybe a little more response feel on the stretched tire, but its hard to tell with all that stuff between the steering wheel and the actual tire rubber. I was honestly hoping for a little bigger difference. Now Im just hoping it helps with tire wear patterns when being driven hard.

I will say from video of my own car, that badly pinched tires dont deform much from the rim until you are really into it. Not just “driving hard for the street” 7/10ths, but closer to the 9/10ths of the grip limits and loaded up /heavily/ with cornering weight. Any benefits for straight line driving have to be slim. Even a 30 degree change in angle from vertical is only a 14% change in cosine. You need a lot of stretch or pinch to make a noticeable difference in structural properties that might make a tire more bump compliant

Real data on contact patches (and how they shape, deform, and affect traction) is hard to come by. Someone explained that its a bit of a tire manufacturer or race team trade secret, so those who know, generally dont share much.

Ford definitely chose to stretch the GT350 tires. Maybe they thought it would give them an edge in respose feel, maybe not. The PP2 GT on the other hand is mostly square to the wheel width 305s on 11s and 10.5” fronts, so thats a little interesting.

 

[Condor1970]

Ford definitely chose to stretch the GT350 tires. Maybe they thought it would give them an edge in respose feel, maybe not. The PP2 GT on the other hand is mostly square to the wheel width 305s on 11s and 10.5” fronts, so thats a little interesting.

It's also possible they just went nuts and wanted to see how wide of a tire they could fit on there before it looked too ridiculous to sell. LOL

 

[Mountain376]

A 285 on a 10” wheel is going to be faster than a 265 on the same wheel.

Contact patch > sidewall stiffness

If you want better steering response, go 275 on a 10” wheel. With a 265, yeah, you’ll get sharp response, but you’ll be down on contact patch (grip).

Also, none of this is that simple: you stiffen the tire sidewall and more load goes into the suspension. If you’re on soft/factory springs...

 

[sdiver68]

I run 275/40 on my 19x10 square set-up.

 

[Grintch]

A 285 on a 10” wheel is going to be faster than a 265 on the same wheel.

Contact patch > sidewall stiffness

If you want better steering response, go 275 on a 10” wheel. With a 265, yeah, you’ll get sharp response, but you’ll be down on contact patch (grip).

Also, none of this is that simple: you stiffen the tire sidewall and more load goes into the suspension. If you’re on soft/factory springs...

It all depends on the tire.

But in ten years of racing I have had zero discussions of stretched tires or bulged tire with tire experts. For powerful cars, you fit as much wheel & tire as you can. If possible choose the stickiest tire first, then the widest tire available that will fit, then the widest & lightest wheel that will fit (that you can afford). The whole stretched or squished discussion is purely how it looks, and the "standards" change almost as often as women's fashions.

A tire with soft sides walls (which excludes most serious performance tires) will be more sensitive to wheel width than tires with stiff side walls. But stick to the manufactures fitment range and it shouldn't be an issue.

 

[BmacIL]

A big reason to have a slight stretch is break-away characteristics. It will be more progressive/smooth with a slight stretch, and more abrupt if straight up or bulge.

 

[dmann]

this is a very interesting topic. So with a 10 inch wheel whats the max stretch one could make in tire size?

 

[SteveW]

The rule of thumb is basically a given tire will be fastest when mounted on a wheel that's .5" wider than tread width. However, the same model tire but in a wider size will be faster on the same wheel.

I've run a 275/35/18 RT615K+ on an 18x11" wheel and that tire was stretched a lot. It held up fine though. I currently have some Comp-2 A/S in 255/40/19 on a 19x10" wheel and that is almost ideal for that tire. Maybe a touch too wide but when I've run the tires on track or autox because the weather was really chilly they did great and I did not shred the tread.

So, if the class you run in limits tire size then stretch that puppy out a little to maximize its performance potential. If you are limited on wheel width and it's kinda narrow then you'll probably be better off stuffing a wider tire on the wheel.

 

[CTH621]

How bout 285 re71r on 11 inch wheel vs 10 inch wheel? How much faster will the 11 inch wheel setup be, very roughly speaking?

 

[Grintch]

The rule of thumb is basically a given tire will be fastest when mounted on a wheel that's .5" wider than tread width. However, the same model tire but in a wider size will be faster on the same wheel.

Those two statements contradict each other. How many people are running 255 tires (~10" tread width) on 10.5" wheels? That is one persons rule of thumb that is now being parroted by others. With no sign of testing to back it up.

You final statement is more correct. A wider wheel is better for a given tire, and a wider tire is better for a given wheel. Within the manufacturers recommended range.

 

[BmacIL]

Those two statements contradict each other. How many people are running 255 tires (~10" tread width) on 10.5" wheels? That is one persons rule of thumb that is now being parroted by others. With no sign of testing to back it up.

You final statement is more correct. A wider wheel is better for a given tire, and a wider tire is better for a given wheel. Within the manufacturers recommended range.

He's saying that a 255 on a 9.5" will be faster than a 255 on a 8.5" AND that a 285 on a 9.5" will be faster that a 255 on a 9.5".

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