|
|
Thread Tools | Display Modes |
#31
|
|||
|
|||
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 |
Ads |
#32
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
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?
|
#37
|
|||
|
|||
Blowing tube descending
|
#38
|
|||
|
|||
Blowing tube descending
|
#40
|
|||
|
|||
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. |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Pics/video of descending while sitting on top tube? | jeff potter | Racing | 11 | July 22nd 09 03:30 AM |
Tube squeezing out of tyre and blowing out | jc | UK | 1 | July 2nd 08 01:54 PM |
Descending the Poggio... | ronaldo_jeremiah | Racing | 6 | March 27th 08 08:39 AM |
Descending Snowdon 28-12 | Steve Ray | Unicycling | 9 | January 16th 06 06:03 PM |
Mtb'er descending stairs :) | gescom | Australia | 15 | February 9th 04 05:21 AM |