Pretty simple question with a not-so-simple answer:

Why is it that a 205/40/16 tire is safe (according to the manufacturer) on an 8" wide wheel -- but a 205/45/16 is not? Why is an 82mm sidewall ok to stretch, but a 93mm sidewall is beyond the scope of safety?

Is there any science to this? Or am I just going to receive the answer of "You need to follow the manufacturer's safety recommendation"?

 

i wonder.

 

I do not know the exact reasoning myself, to be honest. It has cropped up in my research, as well. I would assume that it has something to do with the construction of the tire, and how the sidewall height has a certain amount of elasticity that can either make or break the tire's durability.

 

So the current theory is that with a larger sidewall and the same treadpatch, it may give it too much play?

 

It doesn't make sense, at first look at least.

From what I know 7.5" is the max. wheel width for 205 tire.
Maybe the manufacturer's concern is more with incorrect speedometer reading than stretching.

When sidewall thickness is the same the bigger sidewall will be more flexible,
which means that will stretch easily, I don't know how much this will affect the cornering stability of the tire, though.

 

To be honest with you bud, I'm just theorizing. I think that, because the sidewall is taller, the angle of deflection at the lip is one that doesn't endure the tests they undergo to state that the tire is unsafe.

 

The wheel and tire setup I'm proposing (size for size) is not unheard of either. I know this forum does not advise or condone anything beyond the manufacturer's setting -- but I think this is a grey area.

Not stretched -- but not recommended.

 

I bolded the incorrect portion.

The first number in the size of a tire is the section width, not the tread width. The section width is the widest part of the sidewall.

Given the same section width, the tire with the larger aspect ratio will generally have more curve to the sidewall, reducing the width of the wheel mounting area. The smaller aspect ratio tire will generally be "more square", allowing wider wheels to be used.

Then there's also the intended use portion of the design. Lower profile tires are generally reserved for more performance oriented use, and thus tend to be designed for wider wheels.

 

My mistake then.

Seems odd though to have section width as a static number ... wouldn't the section width change depending on the width of the wheel it's mounted on? I understand that the measurement is originally taken without load, but it changes dramatically once mounted. A tread width, on the other hand, should stay relatively constant unless you're doing some ungodly stretch.

Not saying this is wrong -- just that I am now perplexed by something that used to be a pretty cut and dry bit of info before.

 

The section width is measured with the tyre mounted on a standard measuring rim, which the manufacturer should state. I am fairly sure that the measuring rim width for a given tyre size is an industry standard so that a manufacturer can't mount their tyre on a wider wheel for example to give the impression of a wider tyre.

 

Tire sizing is, in general, a rather screwed up thing.

Section width in millimeters / aspect ratio / wheel diameter in inches
mm / % of mm / in

Yeah, that completely makes sense to me...

Once upon a time, there was no aspect ratio. All tires used to come with an aspect ratio of 82 (such an absurd choice imo).

And that's not even getting into the reality of just how inaccurate the posted section widths of tires are. I currently have a set of 205/45-16s on my Integra that measure wider (tread and section widths) than the set of 225/45-17s on my wife's car...

 

I ran this past our resident Tire Guru, and he felt the shorter sidewall provides greater lateral stability (in terms of the tire’s ability to stay properly seated when it encounters a side force) that could push the bead of the tire off the bead seat of the wheel (very bad when it happens). The taller sidewall can mechanically move (hinge) further under the side force, raising the risk of coming unseated if mounted on a wide(er) base width.

in other words, sometimes the shorter sidewall fits a wide wheel better than the taller sidewall .....


who knew ?

 

Great info, thank you.

 

if thats true zilvia would probably explode, and i wouldve died in a tire explosion/debeading a few years ago.....