Cause of blowout when heating rims?
03-31-21, 08:49 AM
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I'd wager hot rim tire blow offs are less common with new rubber.
03-31-21, 08:53 AM
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Most people live to tell the story when their tires come off the wheel for any reason. Go ride. Use good practices when going downhill.
Seems the safer thing to do would be to discuss safer operating procedures for going down a hill and not nitpick the science behind it.
Seems the safer thing to do would be to discuss safer operating procedures for going down a hill and not nitpick the science behind it.
03-31-21, 09:21 AM
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Anecdotal comments of tire blowouts with hot rims are often associated with a raise in tire pressure or tire rubber weakening in some way. Statements from Wilson's Bicycling Science book suggest tire patches failing or glue weakening for tubular tires as the cause of blowouts for a hot rim. This seems reasonable especially if the patch was poor.
Focusing on clinchers with new inner tubes, heating the rim by 100C (180F, braking on step downgrade) will cause a 20 psi pressure increase, which is insufficient to blowout the tire. As the tire get hots, the kevlar bead (folding tires) has a negative expansion coefficient with temperature. Whereas for steel bead, the expansion is negligible, which suggests that a bead change does not cause a blowout. I have double-walled rims, the spoke ends do not protrude into the rim tape and cannot cause blowout due to weakened rim tape.
Are the above statements are correct? If so, it would appear that heat induced blowouts are due to improper bead seating or poor patches or spoke protrusions, rather than the limitations of tires and tubes. I have not experienced this problem, but I would like to better understand the cause so as not to have an experience.
Focusing on clinchers with new inner tubes, heating the rim by 100C (180F, braking on step downgrade) will cause a 20 psi pressure increase, which is insufficient to blowout the tire. As the tire get hots, the kevlar bead (folding tires) has a negative expansion coefficient with temperature. Whereas for steel bead, the expansion is negligible, which suggests that a bead change does not cause a blowout. I have double-walled rims, the spoke ends do not protrude into the rim tape and cannot cause blowout due to weakened rim tape.
Are the above statements are correct? If so, it would appear that heat induced blowouts are due to improper bead seating or poor patches or spoke protrusions, rather than the limitations of tires and tubes. I have not experienced this problem, but I would like to better understand the cause so as not to have an experience.
03-31-21, 09:45 AM
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But this is not a clear cut, pro disc point. Like choice of tire and rim for rim brakes, it will depend of choice of disc. Standard discs have much less heat capacity than a robust rim. The same energy dissipation causing the 100C increase I mentioned would fry a standard disc. The newer downhill discs have twice the mass, which among other changes, mitigates the heating problem to some extent.
Last edited by IPassGas; 03-31-21 at 09:53 AM.
03-31-21, 10:02 AM
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I suppose you could get one of those bluetooth temp sensors and put in your wheel rim and monitor that with a phone app. Then you won't have to speculate if you are even near a temp to be concerning.
03-31-21, 10:27 AM
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Food for thought:
A 180mm disc has 1/7th the surface area of a 700c rim brake.
My work should be easy enough to follow.
(I treated the disc as a single, dual sided thing, rims as 2, 1 sided things because they are so far apart.)
Rim_vs_disc by Richard Mozzarella, on Flickr
Area of 622mm diameter circle minus area of 602mm circle gives us the area of the 10mm wide brake track "donut", then times 2 (because seperate 2 brake tracks.)
Then convert mm˛ to inches˛ =59.6 square inches of brake track surface area to act as a heat sink..
Same basic procedure (except not x2, because both brake surfaces are actually 1 & the same) for disc =8.27 square inches of area to sink heat.
Then rim brake sink area divided by rotor sink area = 7.21, the inverse of 7.21= ~1/7
180mm discs have 1/7 the heat sink area of 700c rim brakes
Conversely 700c rim brakes have 7.2 times the area, but are in intimate contact with heat susceptible compenents.
Of course, material mass to heat & thermal coefficients are excluded. Anyone care to carry on the engineering torch?
A 180mm disc has 1/7th the surface area of a 700c rim brake.
My work should be easy enough to follow.
(I treated the disc as a single, dual sided thing, rims as 2, 1 sided things because they are so far apart.)
Rim_vs_disc by Richard Mozzarella, on FlickrArea of 622mm diameter circle minus area of 602mm circle gives us the area of the 10mm wide brake track "donut", then times 2 (because seperate 2 brake tracks.)
Then convert mm˛ to inches˛ =59.6 square inches of brake track surface area to act as a heat sink..
Same basic procedure (except not x2, because both brake surfaces are actually 1 & the same) for disc =8.27 square inches of area to sink heat.
Then rim brake sink area divided by rotor sink area = 7.21, the inverse of 7.21= ~1/7
180mm discs have 1/7 the heat sink area of 700c rim brakes
Conversely 700c rim brakes have 7.2 times the area, but are in intimate contact with heat susceptible compenents.
Of course, material mass to heat & thermal coefficients are excluded. Anyone care to carry on the engineering torch?
Last edited by base2; 03-31-21 at 11:23 AM.
03-31-21, 01:39 PM
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If so, it would appear that heat induced blowouts are due to improper bead seating or poor patches or spoke protrusions, rather than the limitations of tires and tubes. I have not experienced this problem, but I would like to better understand the cause so as not to have an experience.
You don't account for cooling. The reason "feathering your brakes" works, is because the rims cool off pretty quickly once you let off the pads by transferring heat into the air - relative flow rate of air over the rims is higher than air over disks, so disks cool off slower. I always tell new folks to think of "not braking" as "adding cold" to the rims.
03-31-21, 10:08 PM
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