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Blowing tube descending



 
 
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  #31  
Old April 21st 17, 04:12 PM posted to rec.bicycles.tech
Sir Ridesalot
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Default Blowing tube descending

On Friday, April 21, 2017 at 10:23:43 AM UTC-4, wrote:
Snipped
And I do not think that going too slow is a problem. Going too fast and then trying to slow up often is. If you ride more slowly there is a lot less energy going into the brakes and hence the rim.

Snipped

The problem is that in order to go slow on a lot of long descents a fair number of people ride the brakes constantly and that's what heats up the rims enough to cause a tire to blow or in the case of a tubular to roll off the rim.

Going fast, braking hard, going fast and so on allows the rim to cool between applications of the brake.

Cheers
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  #32  
Old April 21st 17, 04:19 PM posted to rec.bicycles.tech
Duane[_3_]
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Posts: 1,900
Default Blowing tube descending

On 21/04/2017 11:12 AM, Sir Ridesalot wrote:
On Friday, April 21, 2017 at 10:23:43 AM UTC-4, wrote:
Snipped
And I do not think that going too slow is a problem. Going too fast and then trying to slow up often is. If you ride more slowly there is a lot less energy going into the brakes and hence the rim.

Snipped

The problem is that in order to go slow on a lot of long descents a fair number of people ride the brakes constantly and that's what heats up the rims enough to cause a tire to blow or in the case of a tubular to roll off the rim.

Going fast, braking hard, going fast and so on allows the rim to cool between applications of the brake.

Cheers



Well that was the part that I didn't get. The advice to ride more
slowly on a long descent in order to not burn up your brakes. What
would be the other technique, aside from braking, to ride more slowly on
a long descent? Parachute? Get off and walk?

You're right about not riding the brakes but when you have curves
involved it's more difficult. I've never blown a tube like that though
so it's not impossible. Just more difficult.
  #33  
Old April 21st 17, 04:44 PM posted to rec.bicycles.tech
Frank Krygowski[_4_]
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Default Blowing tube descending

On 4/21/2017 11:19 AM, Duane wrote:


Well that was the part that I didn't get. The advice to ride more
slowly on a long descent in order to not burn up your brakes. What
would be the other technique, aside from braking, to ride more slowly on
a long descent? Parachute? Get off and walk?


Using the brakes to ride descents more slowly does reduce brake
temperatures. Think of a limiting case: applying brakes hard enough to
descend a hill at 3 mph. The temperature rise won't be great, because
the rate of energy input is low.

More detail: On a given slope, if aero drag is small enough to neglect,
the brake force needed to maintain constant speed is the same whether
the descending speed is 3 mph or 30 mph. That's simple physics.

But the power that must be dissipated by the brake system is that
braking force times the speed; so 30 mph instead of 3 mph puts ten times
as much power (that is, energy per unit time) into the brake system.

In that case, temperatures of rims and brake blocks, or discs and pads,
will rise until heat loss rate equals input power. Higher temperature
differentials are needed to reject heat faster. So equilibrium
temperature is higher for higher constant descending speeds - that is,
until you start getting significant help from aerodynamic drag.


--
- Frank Krygowski
  #34  
Old April 21st 17, 06:55 PM posted to rec.bicycles.tech
Ned Mantei[_2_]
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Default Blowing tube descending

On 21-04-17 17:12, Sir Ridesalot wrote:
On Friday, April 21, 2017 at 10:23:43 AM UTC-4, wrote:
Snipped
And I do not think that going too slow is a problem. Going too fast and then trying to slow up often is. If you ride more slowly there is a lot less energy going into the brakes and hence the rim.

Snipped

The problem is that in order to go slow on a lot of long descents a fair number of people ride the brakes constantly and that's what heats up the rims enough to cause a tire to blow or in the case of a tubular to roll off the rim.

Going fast, braking hard, going fast and so on allows the rim to cool between applications of the brake.

Cheers


I think that the timing of rolling down vs. braking may need to be
better defined. For disc brakes a local bike parts company suggests 2
second bursts of braking and 2 seconds of rolling, which might also be
appropriate for rim brakes. Some 10 years ago I was rolling down about
500 meters on a road with 10 switchbacks or at least sharp turns. I was
letting the bike roll on the straighter stretches and then braking hard
for the turns (with rim brakes). To again link to a photo that I already
referred to earlier in this thread, the tube on the front wheel didn't
like this at all: https://flic.kr/p/9XmmWn . Fortunately the tire stayed
on the rim, and I was able to stop safely. Since then I have been much
more cautious.

Ned
  #35  
Old April 22nd 17, 09:01 PM posted to rec.bicycles.tech
[email protected]
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Posts: 3,345
Default Blowing tube descending

On Friday, April 21, 2017 at 8:19:15 AM UTC-7, Duane wrote:
On 21/04/2017 11:12 AM, Sir Ridesalot wrote:
On Friday, April 21, 2017 at 10:23:43 AM UTC-4, wrote:
Snipped
And I do not think that going too slow is a problem. Going too fast and then trying to slow up often is. If you ride more slowly there is a lot less energy going into the brakes and hence the rim.

Snipped

The problem is that in order to go slow on a lot of long descents a fair number of people ride the brakes constantly and that's what heats up the rims enough to cause a tire to blow or in the case of a tubular to roll off the rim.

Going fast, braking hard, going fast and so on allows the rim to cool between applications of the brake.

Cheers



Well that was the part that I didn't get. The advice to ride more
slowly on a long descent in order to not burn up your brakes. What
would be the other technique, aside from braking, to ride more slowly on
a long descent? Parachute? Get off and walk?

You're right about not riding the brakes but when you have curves
involved it's more difficult. I've never blown a tube like that though
so it's not impossible. Just more difficult.


It's all a math problem. If you go slow enough although you're dragging your brakes, too little energy is going into the brakes to burn them or the rims enough to blow a tube.

Faster (20+) on a long descent and you can overheat everything.

Going very fast and braking for the turns also reveals less energy into the rims (high energy short duration). It also increases the cooling air over the rim.

So blowing tubes is from not going slow enough.
  #36  
Old April 23rd 17, 09:48 PM posted to rec.bicycles.tech
[email protected]
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Posts: 275
Default Blowing tube descending

It doesn't seem to me that the discussion is taking into account the fact that the bike's kinetic energy (to be dissipated by the brakes as heat) is proportional to its velocity squared. So it is better to keep your velocity as low as possible, right?
  #40  
Old April 24th 17, 09:45 AM posted to rec.bicycles.tech
John B Slocomb
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Posts: 356
Default Blowing tube descending

On Mon, 24 Apr 2017 04:21:11 -0000 (UTC), OSHA
wrote:

On Sun, 23 Apr 2017 21:06:44 -0400, Frank Krygowski wrote:
On 4/23/2017 4:48 PM, wrote:
It doesn't seem to me that the discussion is taking into account the fact that the bike's kinetic energy (to be dissipated by the brakes as heat) is proportional to its velocity squared. So it is better to keep your velocity as low as possible, right?


Keeping your velocity low is one way to prevent blowouts from
overheating brakes & rims. Letting your velocity get very high is
another, assuming the road allows that. At high speeds, the aero drag
does a lot of the braking for you. If you're gutsy enough, you may not
need to use the brakes at all.


A long time ago one of the technical mags (Cycling Science? Bike
Tech?) had an article where the author did a bunch of calculations to
characterize heat generated while braking and then found the maximum
to be about 1/2 the free decent velocity. (So, if you descend a hill
at 30 kph without any braking, maximum heat will be generated when
holding the bike to 15 kph).

Sadly, I don't recall the actual math.


Didn't Brandt participate in some brake/temperature tests? I seem to
remember mention of it but I never read any results.
 




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