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Hard braking down hill blowouts



 
 
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  #11  
Old March 28th 08, 04:26 PM posted to rec.bicycles.tech
Marz
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Posts: 610
Default 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  
Old March 28th 08, 04:34 PM posted to rec.bicycles.tech
Marz
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Posts: 610
Default 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  
Old March 28th 08, 05:29 PM posted to rec.bicycles.tech
Hobbes@spnb&s.com
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Posts: 200
Default 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  
Old March 28th 08, 05:30 PM posted to rec.bicycles.tech
Jay Beattie
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Posts: 4,322
Default 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.
  #15  
Old March 28th 08, 05:49 PM posted to rec.bicycles.tech
sergio
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Posts: 504
Default Hard braking down hill blowouts

On Mar 28, 5:29 pm, wrote:

Actual blow outs or is the tire moving on the rim and tearing the stem?


It certainly depends on whether one has clinchers or tubular tyres.

Sergio
Pisa
  #16  
Old March 28th 08, 06:21 PM posted to rec.bicycles.tech
[email protected]
external usenet poster
 
Posts: 2,322
Default 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  
Old March 28th 08, 06:33 PM posted to rec.bicycles.tech
Ben C
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Posts: 3,084
Default 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  
Old March 28th 08, 07:48 PM posted to rec.bicycles.tech
Larry Dickman[_3_]
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Posts: 18
Default 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
  #20  
Old March 28th 08, 07:59 PM posted to rec.bicycles.tech
Hank
external usenet poster
 
Posts: 887
Default 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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