Tire bead seating angle

[cranetruck]

I'm not a tire expert and it seems difficult to find answers to some questions no matter how hard I go ogling. Here is one question;

When a tire is mounted on a wheel that is too narrow, what is the seating angle of the bead? Would it not be tilted outwardly? Could it "slip off" the edge of the rim and tear as shown in image #2 straight up along the cords (posted B4, I know, but I can't get any good replies on it).

Another question I have relates to driving on side slopes. The old bias ply tires can run all day long on the side aired down to 10 psi, while the new radials need to be at much higher pressure "highway pressure" when doing so (don't have the reference handy, but I'll look, FMTV?). Why is that?

 

[gringeltaube]

... no expert either but trying to use info and data collected over the years...


From your first Q., may I assume that that blown tire came off of a Deuce wheel...?
The M35 wheel is to narrow for a 12.5R20; the specified rim width is 11". But that is only part of the problem; it is clear that MPT tires should only be mounted on MPT wheels! The diagram below shows what I think happens when a stock Deuce wheel is used in place of a MPT type wheel: rim shoulder - and tire bead angles are the same at 5º but its flange height is 1.5" instead of 1". TP will force the bead to seat all the way against the flange but since parts don't match serious deformation (stretching) of the relatively stiff bead section and adjacent sidewall area will occur, eventually leading to premature fatigue and failure...


BTW: comparing both wheels in your first pic it looks like the flange height of that 12" wheel is much less than the Deuce's. If it was 1" it would very well be suitable to mount a MPT tire like the A3's 14.5R20 XL..?

As to your second Q.: compared to a bias-ply, we know that a radial of similar size has a much larger contact pattern and adapts better to any kind of ground, level or inclined, for better traction and flotation without having to air down that much. Now driving on sideslopes for longer periodes at low TP should not hurt a modern radial tire itself. I don't know if the FMTV's 395/85R20 XMLs come with beadlock inserts but without them and being that they are tubeless it could happen that sand and dirt gets deep enough in between tire bead and rim (on the downhill side) to the point that they would start leaking.



G.

 

[cranetruck]

Thanks for your reply, was actually going to ask you about produce one of your beautiful drawings to illustrate the problem.

Yes, from the info provided, the damaged tire was mounted on a 7" deuce wheel.
The 12" wheel was made for tubeless tires (m656/xm757).

..."compared to a bias-ply, we know that a radial of similar size has a much larger contact pattern and adapts better to any kind of ground, level or inclined, for better traction and flotation without having to air down that much."

You still need to air down to get the desired large contact pattern (lengthwise).

 

[1stDeuce]

Just a thought, but I'd bet that the requirement on FMTV is potentially due more to the relatively tall vehicle profile than any aspect of the tires. when I worked for Oshkosh, we didn't have any wording about not using lower pressures on sidehills, and we were running mostly 16.00 XML radials. The sand and dirt in the beads issue becomes critical if the FMTV tires are not beadlocked as well. (The Oshkosh tires were beadlocked)

g-t, your MS Paint art work is awesome as usual!
You would be dangerous with a copy of Solid Works!!
C

PS, evidently MPT is for Multi Purpose Tire, which is evidently the standard for normal bead flanges now??

 

[hklvette]

one other issue with radials on side-hills is their willingness to flex sideways. keeping the pressure up alleviates this problem, but also reduces flotation. Bias tire belting helps to prevent the sideways flexing (part of the reason they can be difficult to mount) so low pressures on side-hills isn't nearly as big of a problem.