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PostPosted: Thu Sep 13, 2012 5:19 pm 
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Joined: Fri Mar 26, 2010 4:24 pm
Posts: 91
fdegrove wrote:
Regarding point #2. IMHO quite a few of the experienced blow outs are not so much temperature related but due to weak spots in the bead caused by overstretching it locally: use of levers, bad technique, etc. or a combination of all the above.
Most higher end clinchers and so called open tubulars use kevlar beads. Not sure how much those can take temperaturewise before they'd start to stretch. Once they do then yes, that would be a sure recipe for a blow out/ blow of especially when a latex inner tube would be used.
Not to mention carbon clinchers.... I've seen a few selfdestruct at room temperature which led me to the theory of the overstretched beads.



I think you may need to re-examine your theory of over-stretched beads causing blow-offs. High pressure clincher tires do not rely on the circumferential strength/stiffness of the beads to contain tire pressure - instead, they rely on the air pressure to force an interlocking between the bead and the "hook" on the inside of the sidewall. In other words, the outward force of the air pressure causes the bead to be "clinched" by the sidewall hook, and hence the name "clinchers". The circumferential strength of the bead only plays a role when there is no internal pressure.

The effect of the "clinch" in holding the tire can be easily demonstrated. Here's an example of a simple test:
http://www.sheldonbrown.com/rinard/tirebead.htm

I have also repeated this test, but I went a bit further. Starting with a 700c x 23 mm tire, I cut each bead in 8 places, 45 degrees apart. Clearly, the bead had no circumferential strength at this point. I then mounted the tire on a wheel and inflated it to 150 psi. The clinch held the pressure and tire stayed on the rim just fine. I left the tire at this pressure and put the wheel aside for a day. The next day, the tire was still firmly mounted on the rim and holding the air pressure. I then deflated it to a normal riding pressure (about 100 psi), installed the wheel on my bike and went for a ride on it. In my 30 mile ride, a did some hard braking and some hard cornering to test the tire's hold on the rim. The tire stayed on just fine all through the ride.

Here's another test, which doesn't require damaging a tire: Take a wheel with a normally installed tire. Release the pressure holding the clinch between the bead and sidewall by deflating the tire. Now try to lift the bead out of the rim using just a few fingers. On many tire/rim combinations, the tire bead of a deflated tire will slip on and off the rim easily, despite the strength/stiffness of the bead - afterall, if the bead strength/stiffness was the thing that kept the tire on the rim, we wouldn't be able to install tires or fix flats so easily. No, it is not the bead strength/stiffness that keeps an inflated tire on the rim, it is the clinch between the bead sidewall hook, held in place by the internal air pressure.


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PostPosted: Thu Sep 13, 2012 10:56 pm 
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Tubbie Guru

Joined: Tue May 03, 2005 2:20 am
Posts: 5786
Location: Belgium
Hi,

Quote:
I think you may need to re-examine your theory of over-stretched beads causing blow-offs. High pressure clincher tires do not rely on the circumferential strength/stiffness of the beads to contain tire pressure - instead, they rely on the air pressure to force an interlocking between the bead and the "hook" on the inside of the sidewall. In other words, the outward force of the air pressure causes the bead to be "clinched" by the sidewall hook, and hence the name "clinchers". The circumferential strength of the bead only plays a role when there is no internal pressure.


We're all well aware of how clincher tyres should work in theory but does that not conflict with what was said earlier on here:

Quote:
The primary cause of tire blow-off under braking is changes in the tire bead properties under heat - specifically, an increase in bead elasticity (allowing it to be squeezed and flattened) and a decrease in the friction coefficient between tire and rim (allowing the bead to slide more easily over the edge of the rim). Combined with the circumferntial shearing action between tire and rim under braking forces, a hot tire bead can more easily creep out of the rim "hook". The extra pressure with temperature provides additional force to push the tire off the rim, but it is not the primary cause of tire blow-off under heat.


If it is the beads increased elasticity due to heating then surely an overstretched bead would not even require overheating to form a source of trouble?

Which in turn brings us back to the statement that an overstretched bead caused by inappropriate mounting of the tyre say, may cause enough slack in the bead for the inner tube (latex one in particular) to go pinched inbetween it and the rim during the inlation process and subsequently be the cause of an unexpected blowout later on in time....

Does that make sense?

Ciao, ;)

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PostPosted: Fri Sep 14, 2012 9:56 pm 
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Joined: Fri Mar 26, 2010 4:24 pm
Posts: 91
fdegrove wrote:
If it is the beads increased elasticity due to heating then surely an overstretched bead would not even require overheating to form a source of trouble?


Only if the bead was not merely stretched, but deformed so that it no longer engaged properly into the bead hook.

But even so, overstretching is not likely to occur because bead materials are selected specifically to prevent stretching. For example, Kevlar can only stretch 3% before breaking. So even if overstressed while levering the bead over rim edge, it shouldn't stretch enough to make a difference in the ability to engage the bead (or at least not without breaking the bead) and causing other casing damage).

fdegrove wrote:
Which in turn brings us back to the statement that an overstretched bead caused by inappropriate mounting of the tyre say, may cause enough slack in the bead for the inner tube (latex one in particular) to go pinched inbetween it and the rim during the inlation process and subsequently be the cause of an unexpected blowout later on in time....


It doesn't take a stretched bead for a tube to get pinched between the tire and the rim. In fact, a tight tire/rim combination often results in pinched tubes, as it is can be hard to lift the bead high enough over the sidewall to be sure it clears the tube before it settles into the rim cavity.


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