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You want to run wide wheels and slightly stretch the tires onto the wheel. it gets rid of a lot of the movement of the tire. it also has more mechanical grip as the tire is held in place with great breakaway characteristics.
On my rx7 FD I run a 285/30/18 on an 18x11.5" wheel width. tread width of the tire is 11".
I am contemplating what to do with the RX8, probably 18x11 with a 275/35/18.
Not sure why you think those are stretched, or why you would want them stretched.......on a stretched tire the tread width is smaller than the wheel width. Stretched tires put stress on the sidewalls and beads that the tires were not designed for. Talk to some tire engineers about the way they are designed and you will see that is not ideal.
275-35-18's fit nice on a 9.5-10.5" wheel with the correct sidewall orientation..
Not sure why you think those are stretched, or why you would want them stretched.......on a stretched tire the tread width is smaller than the wheel width. Stretched tires put stress on the sidewalls and beads that the tires were not designed for. Talk to some tire engineers about the way they are designed and you will see that is not ideal.
275-35-18's fit nice on a 9.5-10.5" wheel with the correct sidewall orientation..
Actually Michelin engineers say to stretch the tires onto the wheels for various reasons. I found those reasons to be justified through my own findings.
a 275/35/18 tire should be on a 10.5 or 11" wheel to get full benefit from the tire. a 9.5" wheel would be better with a 245/40/18 tire.
There is a reason that Mazda engineers put a 225 wide tire on a 8" wide wheel.
If the tread width is wider than the wheel width the tire can move around. if the wheel width is wider than the tread width the tread is held in place and has many benefits.
There isn't a single race car that I know of that gets paid to race that has a tread width of the tire wider than the wheel width. I am sure you can find novices doing such things, but who own AutoX last year? how about most of the nurburgring guys? all stretched tires on the wheels.
notice how all the new top end cars keep going with wider and wider wheels and tires. More grip and stability they wider you go and fitting with a slight stretch.
It's because a 225/45-18 is specified for a 7" to 8.5" rim and a 275/35-18 is specified for a 9" to an 11" rim.
The Rolex GT RX-8's ran a 285-265-18 on an 18 x 11 rim. The Pirelli 285-265-18 slicks are specified for a 10" to 11.5" rim.
And what do the videos have to do with anything?
just showing that the fastest guys in the nation for autocross are running some massively wide wheels and tires and are stretched at that.
All of the time attack records are falling on wider and wider wheels.
the fastest miatas are now running 10" wide wheels, which are low WHP and torque engines.
So the trend and findings have been, if you want more grip (turning) run the widest wheels allowed/possible and slightly stretch the tire onto the wheel.
lookatme is right on this one. Using a wider rim than normally indicated for a given section width results in more usable contact patch and less sidewall flex. I see this on track and race cars all the time.
Maybe on a full on purpose built race car. I go to quite a few events myself and never see stretched tires. But racing is a monkey see, monkey do sport so it would not surprise me if someone legitimate did it, that many would follow just because.
Maybe on a full on purpose built race car. I go to quite a few events myself and never see stretched tires. But racing is a monkey see, monkey do sport so it would not surprise me if someone legitimate did it, that many would follow just because.
It is actually well-proven. You can find the concept in many racing books, including publications by Skip Barber Racing School, Penske, and many others.
I am in the process of setting up my 2003 Miata for track duty, and I went to a Miata time attack recently to see what the successful guys are doing. This is what I found. Lots of 15x9 or 15x9.5 with 225s and 235s mounted on them. Obviously, they could run much wider tires on those rims, but they achieve more contact patch and stiffer sidewall by stretching narrower tires.
The usual definition of "stretched" is trying to get a 225 width tire on a 10" wheel
Running a 275 or 285 width tire on a 10-11" wheel is not stretched it is the correct size tire..
I think comparing apples and apples is the first thing to do And running a wide wheel for the tire profile is much better than trying to stuff 285 width tires on a 8" wheel.
What people tend to forget is that racing has rules...and sometimes that results in strange wheel and tire combos as racers try to get even a slight advantage and still stay within those rules
And by my definition Steves pictures are not stretched
Yeah, we should probably agree on definitions. In my mind, "stretched" means a tire with the same or smaller section width compared to the rim width. For sake of argument, a 225-45-15 has a section width of 8.9". Mounting that on a 9" or 9.5" rim meets my definition of stretched--especially since most tire manufacturers specify a rim width of 7-8.5" for that tire size.
It may not be obvious due to the way I sneaked the photos, but all of the rims in those photos are wider than the tires.
Here is an example from Flyin' Miata and 949 Racing showing the difference in final tire width for the same size tire mounted on 7" and 9" rims. The "stretched" tire clearly has the advantage.
The examples given keep the tire section width the same while varying the wheel width. However, some rules sets (SCCA Street and Street Touring classses) wheel width is often limited.
For example, in STX, RWD cars are limited to 9" wide rims and 265 tires. In Street class, it's limited to the OEM wheel width (+/- 1" diameter) but any width tire. In these cases, there may still be a benefit to going with a wider tire (though you probably get into diminishing returns as width increases).
I think Andy Hollis or some other tire nerd did a comparison at one point where they varied tire width while keeping wheel width static and reported result. I wish I could find it again.
All of that said, if you're not driving the car competitively, then you're agonizing over meaningless minutiae. Oh, wait, this is the Internet... That's all we do here
Last edited by NotAPreppie; 12-08-2016 at 07:29 AM.
I am writing with my experience and what experience of top racers in the nation and those who set record lap times in the world do. Same with Formula 1 racing.
Having a wheel width wider than the tread width and sometimes section width is what I define as stretched. The recommended wheel widths aren't all that important to me because the most important part of all this is the shape of the tire and that the tire is put into a shape that holds it into position. you can even look at go kart racing and they stretch the tires onto the wheel. almost every competitive paid and sponsored racers stretch the tires onto the wheel to hold the tread in place.
As Michelin found in their Porsche Race cars who work along with the Porsche design team, they prefer to run a smaller tire on a relatively wide wheel than an overly fat tire on a skinny wheel due to steering reponse, tire temps across the tire, and less camber needed for better braking and acceleration.
Each 1" of wheel width is like jumping a tire class in grip.
I run a 18x11.5" on my FD with a 285/30/18 tire and I have tons of grip and progressive breakaway characteristics at the limits of the tire and car. I am setting up my rx8 track car and I am going wide wheels with slightly stretched tires. it just works.
the fastest autocross guys in the nation do it, they got to know something. Porsche on one of their production cars (forget the damn model) ran a wheel width a .5" wider than the recommended tire selection on say tirerack. they knew it and went ahead with it. those are only recommendations and running a slightly wider wheel than tread width is fine even if it falls outside of the recommendation of say tirerack or some other website.
Just trying to help fellow members on the forum be faster.
Without instrumented data to analyze, your claims are circumstantial, inferential or anecdotal. It's not that I don't trust you, it's that I don't trust anybody who makes claims without cited sources to back them up. I'm not saying you're right or wrong, I just can't judge the situation for myself without understanding it better (which requires data).
You and OP (at least starting at post #25) applying data from top-tier competitive driving scenarios to daily driven street cars that might see track time once in a while.
Also, there was a largely semantic and syntactic debate on just what the definition of "stretch" is.
Again, not saying that I think you're wrong but there's a lot of nuance to the situation so blanket statements and broad generalizations are contraindicated. Sure, optimizing tire to wheel width is important, but...
1) it's part of a larger system
2) there are often mechanical and rules constraints
3) I'd bet that the overwhelming majority of people on this board have standard deviations on their lap times that are orders of magnitude larger than a 0.5" wheel width difference would make.
12-03-2016, 12:31 PM
And running a wide wheel for the tire profile is much better than trying to stuff 285 width tires on a 8" wheel. 
