On the subject of tires.

[IOPort51]

The physics of tires and traction and weight distribution are facinating, to say the least. I have spend more than a little time lately thinking about the tires available and what is important to me.

I would like to ask a question or two, for my continuing education.

On the matter of tire width, with factors like weight, rolling resistance and side wall flexibility to consider.

which tire will resist breaking the bead seal better at low pressure, a 10.50 or a 12.50? Both tires identical otherwise.

Is the "flotation" of a wider tire a greater plus than the lower rolling resistance of the narrow tire?

 

[richardoutwest]

IO, I can only answer to what I am first hand aware of. Flotation for me is the only way to go for off road use. Two of my trucks that I have changed over from flotation to metric and larger wheel dia. have all had tire troubles! Road chunking and side wall splits have been a nightmare!! All my tires have been from Discount Tire. They use to replace tires for road chunking and now they say it is normal wear and abuse. After several phone calls to BFG and talks with managers ended with no results in my favor. I will never buy another BFG and I have ran them for 20 plus years! The F-250 Supercrew diesel was the worst by far! Never had any trouble with BFG's until I went metric and larger wheel dia. So I guess in conclusion, either flotation is the way to go no matter the weight or BFG changed their rubber compound or larger wheel dia is a bad idea.

 

[Incommando]

which tire will resist breaking the bead seal better at low pressure, a 10.50 or a 12.50? Both tires identical otherwise.

I think that this decision cannot be made without considering rim width.

 

[IOPort51]

fair point.

8"

I think that this decision cannot be made without considering rim width.

 

[Purple77CJ7]

IO I read some recommendations, that said a 4" difference in wheel and tire width is best. A wide rim gives a straighter side wall at street pressures which leaves a stiffer sidewall. For airing down I read a narrower wheel helps maintain the bead better. I run 33's with 10" wheels, but I only air down about 15 psi. We're I wheel I don't need to air down to single digits. So I have had no issues.

 

[Old Dog]

I always ran 8# in the rear and 10# in the front of my 33X12.5 mtz's on a 10" wheel without problems
being radials they had a good bit of sidewall buldge.
As for off road traction I'm not sure there is much differance between a 10.5 and a 12.5. At least in the terrain I run. I believe tire compound make more of a differance.

 

[007]

Rim width is definitely necessary to answer your questions... There is a law of diminishing returns on rim width, both at the wide and narrow ends. Too wide a tire will leverage the sidewall to pull the bead loose as will too narrow a tire for different physical reasons.

Increased width "usually" accompanies increased diameter / circumference and both will aid in pulling the bead loose in a rim that's too narrow. With Jeeps I think we rarely experience scenarios where too wide a rim is an issue, but it is possible.

 

[bondo258]

I have found...

The larger the sidewall is, in proportion to the tread width, the more likely you are to throw a bead.

IF you air down.

Ideally tread width and rim should match as close as possible, as well.

Wheel width being around 80% of tread width.

Aired down, the tread has more lateral movement and can unseat easier if on a wheel where the tread width is much greater than the wheel width.

Wheel Diameter is really only an issue where sidewall proportions are concerned.

 

[IOPort51]

this is interesting, saying that on a 15" wheel 8 inches wide any tire smaller than a 35x10.50 will be good.

Is this an intelligent opinion or do you have sources of information.

I have found...

The larger the sidewall is, in proportion to the tread width, the more likely you are to throw a bead.

IF you air down.

Ideally tread width and rim should match as close as possible, as well.

Wheel width being around 80% of tread width.

Aired down, the tread has more lateral movement and can unseat easier if on a wheel where the tread width is much greater than the wheel width.

Wheel Diameter is really only an issue where sidewall proportions are concerned.

 

[bondo258]

My sources of information are primarily observations of my rig, and of the others I wheel with.

Glad my opinions sounds somewhat "intelligent" to others.

To sum it up, a tire without full pressure flexes side to side. (duh)

More sidewall equals more flex.

More tread is more grip, meaning the tire tread is more likely to stay in place, as the sidewall flexes, and driver turns.

Both of the following issues occur on the bottom of the wheel/tire.
My terms just refer to the angle of operation.

Sidewall flex on the "upper' side creates leverage against the upper bead retainer (behind where the wheel weights go), with that part of the wheel as a fulcrum, it can pop your bead and cause run off.

Conversely, the lower side can be pulled away from the bead retainer, and unless you get some 'tire glue' or buy internal retaining seat wheels like circle track racers run.

Most folks know and understand the lower side unseatings.The rim more or less "drives off of" the tire. No mystery there, we've all done it, seen it.

The Upper side ones are usually less frequent, but quite often f* the sidewall up. Looks like you hit a curb, without a tell-tale dent or ding in the wheel.

With this stuff in mind it may be easier to picture what different proportions do to low-inflation tires.

The reason I believe 80% percent to be a good number for wheel-to-tire width, is to avoid the upper side unseatings. The leverage is already built in with a tire that is larger than the wheel. It holds the tire on, under normal pressures, but once you go off-camber with low air, it only gets worse.

 

[Turbogus]

Tires can turn into a conundrum for certain. A while back I purchased a couple of new tires for "Easy bake"

and I was accustomed for decades that for the benefits of steering control, new tires should go up front. I was taught this not only in the service (I was in Motor T) but later as a civie. Also that tire pressures (for the road) should be maintained within 15% of max pressure (checking pressure periodically along with tread depth & wear patterns). I was puzzled when after mounting a new pair of BFG TA's on the rear of my van that the tech said it was the law.
I posted this concern on another forum and the arguments became so vehement that the moderator had to close the thread. I reckon we have to post with caution these days.

 

[bondo258]

the tech said it was the law.

Yeah, I had a tech tell me they couldn't let me leave the shop unless I had brand new rotors and pads installed right there and then.

"Luckily" they had a set on the shelf. I took my keys, paid for the oil change (this was years ago on a rainy day) and left.

Had to have an Airbag unit reset at the dealer. My free front end diagnosis required a total rebuild (We can't let you drive out of here like that, it's a liability.) FYI the front end was OK, I'm compulsive about that stuff.

Now tire shops aren't supposed to move plugged tires to, or mount retreads in the front. That's pritnear it for tire shops and "the law".

The chains that warranty tires for X amount of miles, always put the new meats in the rear. They'll rotate them as well, if needed. Less road hazard and wear claims that way.

Citing "The Law" is a BS move. Or an attempt to have you buy two more tires.

 

[PaRenegade]

To throw in my 2 cents to the question at hand. The disclaimer: I'm no tire tech, nor have I ever played one on tv or in my garage. I would think the 12.5 would break a bead with an 8" rim quicker then one on a 10" rim. And I base that simply on the angle of the sidewall and the variation of flex from tire to tire. The 12.5's sidewall would be closer to horizontal on the 8" because it has to reach an additional 1" to get to the rim. To picture what I'm saying, imagine a keystone shape, where the top flat is 24" and the bottom flat is 12". Compare that to one with a 24" top and a 20" bottom. Now squeeze the top and bottom sides closer and you can see which one will be closer to horizontal and therefore leverage the bead the most.
Bottom line is, imho, it depends on how low you will be airing down whether breaking a bead is of concern. If your going down to single digits I think the closer the tire and rim width are to each other, the better. If you stay in the teens I think 12.5's on an 8" rim would be fine. Merely my opinion. Interesting topic though.