the how wide can I go debate

I'm not reading hundreds of words to tell me how which tires sizes I should run.
Aspect ratio most definitely has a lot to do with how well a tire will fit onto a wheel.
It's part of the reason a 305/40 or a 305/45 "fits just fine" on a 18x9.5 wheel, like so many SVT guys do (and myself with my 305/40/18 drag radials on my 18x9.5 wheels).
Now if you try to fit a 305/30/18 onto that same 18x9.5 wheel, you'll quickly fin that sidewall bulging, the tread patch oval not flat against the ground, and if you do that shit on a front tire and actually corner, you'll probably even end up damaging the tire or the wheel or both.

This is a 305/35/18 on a 18x9.5, can easily see how much tread is not hitting the ground, at the outer parts of the contact patch, because the wheel is too narrow:



That's gonna be even worse when the shorter 305/30/18 sidewall "pulls up" on the edges of the tread/contact patch.
But a taller sidewall allows the tire to relax and it doesn't affect the contact patch reduction as much.

Gabe said:

I'm not reading hundreds of words to tell me how which tires sizes I should run.

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thats the great part, basically, the safety factor is so big. You can pretty much run whatever you want.

Racer47 said:

“Understood completely, with one caveat. A M&H drag slick is really not intended for general road use, where the conditions under which the tire is asked to operate are far less controlled than they are at a drag race.”

T&RA does not care about intention. It’s either DOT or not. I believe stkjock’s tires are DOT legal drag radials so they fall under T&RA rules and pass all the requirements.

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If his tires are in fact DOT-legal drag radials, of course. I wasn't sure that they were - and I'm not at all certain that any bias-ply true slick is. Maybe he can confirm this one way or the other.

“I know the T&RA's "narrow-side recommended limit" isn't a fixed pass/fail boundary. And I'm pretty certain that carcass overstrain and the consequential fatigue effects are involved in how that was determined. What I don't know is how much margin necessarily exists in any given tire make/model regarding fatigue life, or how fast you sacrifice that per half inch of lesser wheel width.”

You are giving the government regulators way too much credit. There is absolutely no stress / strain type data as any part of T&RA wheel sizes. There is simply a formula that for a given width and aspect, you get the “measured” rim size and the accepted range. Those are then rounded off to the nearest half inch.

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So what was the basis for the formula? Ultimately, wouldn't it have to have been based on stress/strain and fatigue data?

“What I'm seeing is when a tire is mounted on a narrower than recommended width wheel, it starts out with a slightly higher amount of carcass strain, …., somewhat faster accumulation of fatigue damage”

Fatigue strength is tested for tires. ½” too wide or too narrow adds zero fatigue damage. 1” may add trivial amounts of fatigue damage. Cornering forces put much more load into a tire than being mounted on a wheel that’s slightly too narrow.

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OK. But wouldn't the fatigue damage effects be additive? Even if the rim width contribution is small? While the fatigue life might not be as sensitive as inverse 5th power like one estimate for steel happens to be (where a small increase in strain has a significant effect on fatigue life), I doubt that it's any better than inverse square.

We need to take a wider view of a tire’s limits. Passenger car tires in general have more load capacity than is required for the weight of the vehicle they are mounted on. This is because styling likes bigger diameters and wider tires.

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And I hope at least to cover for lateral and longitudinal load transfer. I've seen a formula related to wheel design that accounts for lateral acceleration.

But the load capacity requirements go up for those larger tires per T&RA. So we have load capacity (and fatigue damage) to spare.

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Is this because the tires themselves are "taller"? Or because the vehicles that they might be fitted to are (and have higher CG heights)?

But then, say we go from a 255 to a 275. Now we have even more excess capacity. So whatever trivial amount of damage that 1” too narrow wheel imparts on the tire, it does not matter, we have plenty to spare.

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Makes sense.

“A tire with a 4.5" tall sidewall (from bead to tread) and a vertical spring rate of 1500 lb/in supporting a 1050 lb corner weight is going to deflect (roughly) 0.7". That does alter the sidewall shape into sharper curvature. Has to. Just like the greater deflection resulting from underinflation does (I did rather recently stumble across an accepted formula for estimating tire vertical spring rate from tire size and inflation pressure).”

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I’m not sure where you are going with this but those numbers are believable.

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I'm looking at the amounts of carcass strain as an indication of fatigue life. A tire with a lower vertical spring rate will distort to a greater extent. And that could either be because of lower inflation pressure or - probably of secondary importance - due to a lower inherent amount of vertical stiffness inherent in the carcass itself by reason of being squeezed in at the beads and ending up with a more sharply curved shape (the visible 'bulge', plus possibly a reverse curve closer to the bead region).


Norm

The M&H Dragmaster radial is a DOT tire. I've had them on the car for 10,000 miles.

DOTMlefal bias

http://www.mickeythompsontires.com/strip/et-street-r-bias/

oldVOR said:

General rule is to stay within 1/2 inch +/- of the tread width (not section width) of the tire. Considering most 275's float around 10 inches of tread width, wheels ranging from 9.5 to 10.5 inches would work best.

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The only "rule" I've ever seen is that for tires of 50 and higher profile, "measuring width" is 70% of "advertised" section width. For 45 and lower profiles, it's 85%. Both being rounded to the half inch.

From memory, but I think I've got that much right.

I do not know how the acceptable range either side of that is determined.


Norm

Well, this is all a great read! I am also an engineer but I had never really looked into the best combination before on my cars. The stuff I work on flies high and REALLY fast so they have different tire needs. I did discuss cornering with a colleague who ran a 250 cc race kart that did a bunch of testing for Hoosier at the Motorsport Ranch a while back. Right now I just need to get the car to hook best I can. Another colleague with a 2012 ZL-1 wants to race so I have to do whatever I can to get off the line on street tires.

[FONT=&quot]Norm,
[/FONT]
[FONT=&quot][FONT=&quot]H[/FONT]ere’s one of the pages of load tables. The loads are also calculated by a formula. Basically the load rating is proportional to inside tire volume and air pressure. More on formulas later. [/FONT]



[FONT=&quot]Size P255/40R18 is rated for 1521 lbs at 35 psi. 1500 lbs is a much higher than the actual tire load on a supercharged, iron block, Shelby (guessing 3800 lbs, 55% front wt, divide by 2 = 1045 lbs per front tire). And yes the load ratings factor in cornering and accel loads. So you see there is plenty of extra capacity.[/FONT]

[FONT=&quot]Now lets put a P315/40R18 tire on the rear. That tire is rated for 2205 lbs at 35 psi. Rear tire static load is 855 lbs (same car as above). So that tire has 2.6 times more load capacity than it needs. Even at only 20 psi it can carry 1552 lbs. Even the low load versions (LL) at 20 psi can carry 1433 lbs. My point is to show just how much excess tire capacity there really is. It’s a lot more than most people realize.[/FONT]

[FONT=&quot]A curve like this is what I’m talking about for fatigue damage.
[FONT=&quot]

[/FONT][/FONT] [FONT=&quot]The damage due to a too narrow wheel is down near the dashed fatigue limit line and may even be below the dashed line. Everything above the dashed line is defined as fatigue damage and yes it is cumulative. Everything below the dashed line does not add up to anything. A flat tire is somewhere in the middle of the Y axis. It can accumulate enough damage to cause a tire failure in a relatively low amount of miles.[/FONT]

[FONT=&quot]The strains and you are talking about, narrow rim vs low inflation, are at least 1 order of magnitude apart, probably more. There will be very little difference in actual axle height for a tire on an 8” wide wheel vs that same tire on a 10” wide wheel. There will be a significant difference in axle height at 10 psi vs 35 psi. It takes this level of deflection to cause tire damage.[/FONT]

[FONT=&quot]But you also need to understand tires are non-linear in every aspect. Its not as simple as metal fatigue or this S-N curve implies. Additionally high deflection builds heat which causes extra damage and fails the tire sooner.[/FONT]


[FONT=&quot]So how are the formulas calculated?? [/FONT]
[FONT=&quot]Radial tire formulas were done in the 1970’s and maybe into the 80’s. So I don’t know ‘exactly’ how they were calculated but I know the process. T&RA is made up of mostly ex-tire engrs and lawyers. They send out proposals to the tire companies. The tire companies either approve the proposals or edit them and send them back. Eventually everyone agrees and a standard is put into place. Things like rim width and loads have a lot of test data generated. So yes, if you dig deep enough there is data to support the standards and formulas. But these standards are ultra-conservative for liability reasons. [/FONT]

[FONT=&quot]T&RA does not list a “recommended” rim size. They don’t actually recommend anything. They call it the “measured” rim size. All tire size measurements, loads and inflations are based on this wheel width. [/FONT]

[FONT=&quot]Plus T&RA starts with a tire size not a wheel size. So the widths listed are what they and the tire companies deem as accepted usage that is based on the very conservative formula they all agreed to that was at least loosely backed up with actual tire test data (from the 70’s or 80’s). [/FONT]

[FONT=&quot]So if a car company knows it wants to use a 255/40R18 tire it does not make sense to put it on a 7” or 11” wide wheel. But that does not automatically mean that it is unsafe to do so.[/FONT]

Thanks, Racer.

Unfortunately your pictures aren't getting through - only "minus signs in circles".

I'm particularly interested in the fatigue curve and I'm really curious as to how well (?) what I expect it to look like matches up with what it actually does look like. FWIW, it's not a constant inverse 5th power thing with steel over the entire range of alternating stress intensity either. That was just the simplest approach in general use over the 30+ years I was involved in that industry (working to ASME III, if it matters).


As a general comment, it's clearly a good thing that margins are high, given the lack of attention that most people have paid to maintaining tire inflation pressure.


Norm

Did this work??

stkjock said:

The M&H Dragmaster radial is a DOT tire. I've had them on the car for 10,000 miles.

DOT legal bias

http://www.mickeythompsontires.com/strip/et-street-r-bias/

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Those sizes are all old school light truck bias tire sizes. Thats what the LT at the end means. Small tire companies push these T&RA rules to the limit.

Still nothing.

Can you post a screenshot of the S-N curve as a .jpg? I do understand the notion of protecting proprietary information.

I think this snippet of the SL Load-Inflation table is correct, and I've intentionally left the warning (and the "Single Load" note) intact.




Norm

Odd I saw the pics yesterday and now they are gone.

Racer - maybe host them at Photobucket or imgur

I have always been able to see his images. Except for post#47. Those only show up as a click to view link.

The load table and SN curve should be visible now.

Gabe said:

This is a 305/35/18 on a 18x9.5, can easily see how much tread is not hitting the ground, at the outer parts of the contact patch, because the wheel is too narrow

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I suspect that contact patch shape is more due to high inflation than rim width. Look at the light reflection off the sidewall. There isn't any deflection from load. The approved width is 10" to 11". A 9.5" wheel at 30 psi would be fine.

Gabe said:

Now if you try to fit a 305/30/18 onto that same 18x9.5 wheel, you'll quickly fin that sidewall bulging, the tread patch oval not flat against the ground, and if you do that shit on a front tire and actually corner, you'll probably even end up damaging the tire or the wheel or both.

Click to expand...

......or most likely neither will be hurt. But I agree that 9.5" is not wide enough for a 305/30. Plus it would be extremely difficult to mount up that combination in the first place.

Racer47 said:

Did this work??

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It does now (both pictures).

The S-N curve - about what I expected.


Norm

Racer47, to take this off-topic a bit, can you explain the difference between the euro metric and the P metric size designations please?


thanks,
Ed

They are more the same than different.
The load ratings are a little different. The sizes are the same.
ETRTO is the Euro version of T&RA and is the one who comes up with non P tire specs. Since the tire's load capacity is usually more than required, the load differences are not that important. Most everyone uses P metrics and non P's interchangeably.

There really only needs to be one standard. But its almost impossible to get the entire world to agree on a common standard. Remember the Michelin TRX all metric tire? This is what we should all be using by now. But politics, marketing and company profits dictated that we stick with the current inferior design and goofy half metric half inch system.

I do remember the TRX system - and that there were at least two features about it designed to preclude unintentional installation of non-TRX tires on TRX wheels (and vice-versa) that may have contributed to acceptability being killed here.

I'm actually a little surprised that at least for the US tire market they didn't go from the all-inch 8.00-14 kind of designation straight to something like 8.00/83-14 once profile designations started showing up in the latter half of the 1960's.


Norm

Yesterday morning I ended up putting on Bridgestone S04 Pole Position, treadwear 280, $200 a piece from Discount Tire in Fort Worth. So, the size I went with is 265/45/18 and they look great, not too much bulge and fill out the fender well nicely. I went with 265/45 so its just a little bit taller than the 255/45 that were on it. In my opinion 275/45 would have been fine but I didn't want to go that much taller. In my short drive to and from work I am pretty happy with the increased grip. 2nd gear is now useable rolling into it instead of instant tire spin.

Also, I couldn't make it to Ennis (Texas Motorplex) this past Friday but after calling them Friday afternoon they have added a Fast Friday test and tune for this coming Friday 7 April and the weather, so far, looks to be perfect. My coworker is bringing their 2012 Automatic ZL1 so should be a good race. Can't wait to see how well the new tires hook. And just hope they don't hook so well that I find the next week link in the driveline. The car just rolled over 73,000 miles and the Roush TVS 2300 has been on it since January 2012 when the car had about 54,000 miles.