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Hard braking down hill blowouts
On Mar 28, 3:49 am, Ben C wrote:
On 2008-03-28, Ben Kaufman wrote: Is it normal to have blow-outs on a road bike from prolonged hard braking going about one mile down a steep hill or should superior wheels and tires be able to deal with the generated heat? I have an old panasonic but keep the tires and tubes up to date (PerfomanceBike GT2 Kevlar, rated at 105 lbs, 26TPI, which are not the best in the world but a heck of a lot better than what my LBS sold me. It is a 27 inch rim and I could not find any better quality tires). But is it the tire/wheel quality at issue? I have been thinking about getting a new bike rather than trying to upgrade this one for a number of reasons (I don't think it's even possible to switch to the current wheel size) but the blow-outs are my biggest justification of expenditure to my wife. Some math I did earlier: http://groups.google.co.uk/group/uk....1d32532f671264 It's probably all wrong off course, but my tentative conclusion from it is that the rims act as heat sinks-- they can soak up quite a lot of energy, enough for most purposes, but they can't really cope with sustained braking. Disks on the other hand have much less heat capacity (because they're smaller) and probably work hotter, so I reckon they might get up to temperature quite quickly and then dissipate more because of the bigger temperature difference with the surrounding air. This would mean they should be better for continuous braking. In any case they won't make the tyres pop off. Disk brakes have their own problems with heat and can 'fade' (fail) on long downhill sections. From hayes... "Brake Fluid Fade - This type of fade occurs when the brake fluid inside a hydraulic caliper boils. An important characteristic of brake fluid is that it is incompressible. When a brake fluid boils, gas is formed within the system that is compressible and any lever stroke available goes toward compressing the gas instead of generating brake power. Interestingly enough, when a fluid is under pressure, it is very difficult for the fluid to boil. If a brake system is under pressure, the fluid temperature can rise above the boiling temperature without the fluid actually boiling. Once the pressure is released, the fluid will instantly boil and fade will occur." A common practice for long downhill sections when using disk brakes is to brake hard and release as opposed to constant braking to control speed. The idea being, that when you release you allow the disk to cool slighty. I've experienced disk brake fade more often than rim brake blow outs and that the disk brake fade occurs quicker during a descent than a rim brake blow out. But, the offroad trails I ride are on average alot steeper than the roads I ride and also require alot more braking. Laters, Marz |
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#12
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Hard braking down hill blowouts
On Mar 28, 9:03*am, jim beam wrote:
wrote: Ben C? wrote: Disks on the other hand have much less heat capacity Nor do they exchange heat with the tyre. Indeed. *I wonder if they ever melt the grease in the hub though. Most bicycle disks are mounted on support spiders so flimsy that heat conduction to the bearings is insignificant. "flimsy"??? *is that stanford parlance for "stainless [disk] steel is a poor conductor"? *or is it presumptive nonsense from someone that doesn't know what they're talking about? Disks are very flimsy and are not constructed to resist side to side forces. They're only 'stiff' in the direction of the rotating wheel. And not all disks are contructed with stainless steel spiders (see Hope). For example you could poor water on a hot rim to cool it, but the same action on a disk may (and has for me) warp the bloody thing. laters, Marz |
#13
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Hard braking down hill blowouts
On Thu, 27 Mar 2008 22:06:48 -0400, Ben Kaufman
wrote: Is it normal to have blow-outs on a road bike from prolonged hard braking going about one mile down a steep hill or should superior wheels and tires be able to deal with the generated heat? I have an old panasonic but keep the tires and tubes up to date (PerfomanceBike GT2 Kevlar, rated at 105 lbs, 26TPI, which are not the best in the world but a heck of a lot better than what my LBS sold me. It is a 27 inch rim and I could not find any better quality tires). But is it the tire/wheel quality at issue? I have been thinking about getting a new bike rather than trying to upgrade this one for a number of reasons (I don't think it's even possible to switch to the current wheel size) but the blow-outs are my biggest justification of expenditure to my wife. Actual blow outs or is the tire moving on the rim and tearing the stem? Just asking because the latter is more common but the former more often blamed. |
#14
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Hard braking down hill blowouts
On Mar 28, 8:26*am, Marz wrote:
On Mar 28, 3:49 am, Ben C wrote: On 2008-03-28, Ben Kaufman wrote: Is it normal to have blow-outs on a road bike from prolonged hard braking going about one mile down *a *steep hill *or should superior wheels and tires be able to deal with the generated heat? *I have an old panasonic but keep the tires and tubes up to date (PerfomanceBike GT2 Kevlar, rated at 105 lbs, 26TPI, *which are not the best in the world but *a heck of a lot better than what my LBS sold me. It is a 27 inch rim and I could not find any better quality tires). *But is it the tire/wheel quality at issue? *I have been thinking about getting a new bike rather than trying to *upgrade this one for a number of reasons (I don't think it's even possible to switch to the current wheel size) but *the blow-outs are my biggest justification of expenditure to my wife. Some math I did earlier: http://groups.google.co.uk/group/uk....1d32532f671264 It's probably all wrong off course, but my tentative conclusion from it is that the rims act as heat sinks-- they can soak up quite a lot of energy, enough for most purposes, but they can't really cope with sustained braking. Disks on the other hand have much less heat capacity (because they're smaller) and probably work hotter, so I reckon they might get up to temperature quite quickly and then dissipate more because of the bigger temperature difference with the surrounding air. This would mean they should be better for continuous braking. In any case they won't make the tyres pop off. Disk brakes have their own problems with heat and can 'fade' (fail) on long downhill sections. From hayes... "Brake Fluid Fade - This type of fade occurs when the brake fluid inside a hydraulic caliper boils. *An important characteristic of brake fluid is that it is incompressible. *When a brake fluid boils, gas is formed within the system that is compressible and any lever stroke available goes toward compressing the gas instead of generating brake power. *Interestingly enough, when a fluid is under pressure, it is very difficult for the fluid to boil. *If a brake system is under pressure, the fluid temperature can rise above the boiling temperature without the fluid actually boiling. *Once the pressure is released, the fluid will instantly boil and fade will occur." A common practice for long downhill sections when using disk brakes is to brake hard and release as opposed to constant braking to control speed. The idea being, that when you release you allow the disk to cool slighty. I've experienced disk brake fade more often than rim brake blow outs and that the disk brake fade occurs quicker during a descent than a rim brake blow out. But, the offroad trails I ride are on average alot steeper than the roads I ride and also require alot more braking. I think that is an important point -- road versus trail. I have done long road descents on my mechanical disk equiped cross bike without any brake fade. In fact, in the rain, the disks are far better than my rim brakes. There is a local 14 mile descent that I did in the rain/snow on my cross bike with good stopping all the way to the bottom -- which was a good thing because my hands were frozen, and I couldn't generate much force on my levers. -- Jay Beattie. |
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Hard braking down hill blowouts
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#16
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Hard braking down hill blowouts
On Mar 28, 12:35*am, wrote:
I instrumented a wheel with pressure sensor and temperature thermocouple to monitor a blow-off but was unsuccessful getting the tire to separate even though temperature reached 150 degC and pressure 125 psi. *The mechanism for blow-off is still unclear but it seams the bead material softens and creeps of the hooked bead of the rim. Some people, more successful than you (by report) (ha ha) (sorry, that one kinda just popped out) in getting tires to blow off, use the advantage of starting with much higher pressures. "I run 140". Yo! Obeying the advertising... At some point, I saw HED warning against use of a certain tire brand. Seemed to imply that prolonged braking might not be entirely necessary to provoke a blowoff. Since I had some of each in the fleet, I looked at a Specialized/HED Trispoke rim and a (IMS) Brand C tire. I noticed the bead was much too large to fit all the way, so to speak, into the hooked bead area in the rim. "Already on the way out?" when inflated-- or was that over-inflated? A nice, rounded slope on the tire bead, not much to hook into there. Other tires, with much smaller beads, seemed to fit nicely, with the rim perhaps (no x-ray machine on the premises) finding purchase where the bead stops and the sidewall begins, as would seem to be the intent of the design. --D-y |
#17
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Hard braking down hill blowouts
On 2008-03-28, Marz wrote:
On Mar 28, 3:49 am, Ben C wrote: On 2008-03-28, Ben Kaufman wrote: [...] Disk brakes have their own problems with heat and can 'fade' (fail) on long downhill sections. From hayes... "Brake Fluid Fade - This type of fade occurs when the brake fluid inside a hydraulic caliper boils. An important characteristic of brake fluid is that it is incompressible. When a brake fluid boils, gas is formed within the system that is compressible and any lever stroke available goes toward compressing the gas instead of generating brake power. Interestingly enough, when a fluid is under pressure, it is very difficult for the fluid to boil. If a brake system is under pressure, the fluid temperature can rise above the boiling temperature without the fluid actually boiling. Once the pressure is released, the fluid will instantly boil and fade will occur." A common practice for long downhill sections when using disk brakes is to brake hard and release as opposed to constant braking to control speed. The idea being, that when you release you allow the disk to cool slighty. I've experienced disk brake fade more often than rim brake blow outs and that the disk brake fade occurs quicker during a descent than a rim brake blow out. But, the offroad trails I ride are on average alot steeper than the roads I ride and also require alot more braking. I have heard of fluid fade affecting bicycle brakes before. It is shocking in my opinion because that's the easiest kind of fade to fix-- you just need brake fluid that doesn't boil, which exists. There's no tradeoff or anything, it's a win-win no-brainer. Cars haven't suffered from fluid fade for years with modern decent brake fluid. I mentioned this before and someone said it's because bicycle brakes use crappy brake fluid because it doesn't strip the paint. My advice: use proper car brake fluid in your bike brakes but pour it in carefully and don't spill it. |
#18
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Hard braking down hill blowouts
Ben C wrote in
: I have heard of fluid fade affecting bicycle brakes before. It is shocking in my opinion because that's the easiest kind of fade to fix-- you just need brake fluid that doesn't boil, which exists. There's no tradeoff or anything, it's a win-win no-brainer. Cars haven't suffered from fluid fade for years with modern decent brake fluid. I mentioned this before and someone said it's because bicycle brakes use crappy brake fluid because it doesn't strip the paint. My advice: use proper car brake fluid in your bike brakes but pour it in carefully and don't spill it. Many (probably most) hydraulic disc brakes for bikes use DOT 3 or 4 brake fluids. The problem still exists, especially if there is water in the system. Also, one should never put DOT fluid in a system designed for mineral oil and vice versa. The seals may not be compatible. All this said, I've never had any problems on my mountain bike equipped with hydraulic brakes. -- Larry Dickman |
#19
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Hard braking down hill blowouts
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#20
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Hard braking down hill blowouts
On Mar 28, 9:30 am, Jay Beattie wrote:
On Mar 28, 8:26 am, Marz wrote: On Mar 28, 3:49 am, Ben C wrote: On 2008-03-28, Ben Kaufman wrote: Is it normal to have blow-outs on a road bike from prolonged hard braking going about one mile down a steep hill or should superior wheels and tires be able to deal with the generated heat? I have an old panasonic but keep the tires and tubes up to date (PerfomanceBike GT2 Kevlar, rated at 105 lbs, 26TPI, which are not the best in the world but a heck of a lot better than what my LBS sold me. It is a 27 inch rim and I could not find any better quality tires). But is it the tire/wheel quality at issue? I have been thinking about getting a new bike rather than trying to upgrade this one for a number of reasons (I don't think it's even possible to switch to the current wheel size) but the blow-outs are my biggest justification of expenditure to my wife. Some math I did earlier: http://groups.google.co.uk/group/uk....1d32532f671264 It's probably all wrong off course, but my tentative conclusion from it is that the rims act as heat sinks-- they can soak up quite a lot of energy, enough for most purposes, but they can't really cope with sustained braking. Disks on the other hand have much less heat capacity (because they're smaller) and probably work hotter, so I reckon they might get up to temperature quite quickly and then dissipate more because of the bigger temperature difference with the surrounding air. This would mean they should be better for continuous braking. In any case they won't make the tyres pop off. Disk brakes have their own problems with heat and can 'fade' (fail) on long downhill sections. From hayes... "Brake Fluid Fade - This type of fade occurs when the brake fluid inside a hydraulic caliper boils. An important characteristic of brake fluid is that it is incompressible. When a brake fluid boils, gas is formed within the system that is compressible and any lever stroke available goes toward compressing the gas instead of generating brake power. Interestingly enough, when a fluid is under pressure, it is very difficult for the fluid to boil. If a brake system is under pressure, the fluid temperature can rise above the boiling temperature without the fluid actually boiling. Once the pressure is released, the fluid will instantly boil and fade will occur." A common practice for long downhill sections when using disk brakes is to brake hard and release as opposed to constant braking to control speed. The idea being, that when you release you allow the disk to cool slighty. I've experienced disk brake fade more often than rim brake blow outs and that the disk brake fade occurs quicker during a descent than a rim brake blow out. But, the offroad trails I ride are on average alot steeper than the roads I ride and also require alot more braking. I think that is an important point -- road versus trail. I have done long road descents on my mechanical disk equiped cross bike without any brake fade. Well, yeah - if it's mechanical, the fluid definitely won't boil! |
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