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[brakes] Overheating vbrakes on descents

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[brakes] Overheating vbrakes on descents

Old 07-22-19, 09:32 PM
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BromptonINrio
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[brakes] Overheating vbrakes on descents

Hy there,

im facing a problem with overheating rims/pads from shimano alivio vbrake groupset and doublewhalled alloy rims.
its in a 20% incline hill for 700mts.
Bikes are Dahon Curve D3
It gets so hot the brakes become impossible to touch.
is there any thermal pad?
disks whould solve those issues in 16" bikes?
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Old 07-22-19, 11:19 PM
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A descent of 20 percent for 700 meters, especially if it is not a mostly straight or semi-straight is going to very tough on any rim brake system. (I don't have any serious riding experience with disc brakes.) The heavier the rider the greater the problem.
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Old 07-23-19, 10:59 AM
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Originally Posted by mtb_addict View Post
Disks would be worse because they have even less metal surface area to thermaldynamically transfer heat away. Rim should be superior in way of heat transfer.
With disks there is not risk overheating the tire and they a designed to transfer the heat.
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Old 07-23-19, 11:04 AM
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The risk with rim brakes on steep descents is blowing out the tire and tube from the heat buildup, which is exactly what you don't want in that situation. Don't want to go careening off a cliff or mountainside if you can help it.

I prefer discs on steep descents. The discs may heat up from continued friction, but you don't stand much chance of blowing out a tire or tube because the discs and tires are separated by quite a distance, unlike rim brakes where you're cooking the tires and tubes directly.
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Old 07-23-19, 11:33 AM
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Originally Posted by mtb_addict View Post
Disks would be worse because they have even less metal surface area to thermaldynamically transfer heat away. Rim should be superior in way of heat transfer.

The correct course of action is to pause at halfway, and let the brake cool down before continuing the descent.
Disc brakes would be highly recommended INSTEAD of rim brakes when a steep decent is encountered. The disc brakes will get hot, but there won’t be a risk of transferring all the heat from the pads to the rims, or to the tire and inner tubes. The inner tubes can weaken, and can rupture, or the tire can split due to weakening from the excessive heat with rim brakes. Rim brakes also have way less stopping power when excessively hot, and can melt.
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Last edited by tds101; 07-24-19 at 10:45 AM. Reason: typo
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Old 07-23-19, 01:29 PM
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Rim brakes are lighter. It is easy to replace a wheel if a tire goes flat due to less adjustment problems.
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Old 07-23-19, 02:15 PM
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Originally Posted by BromptonINrio View Post
im facing a problem with overheating rims/pads from shimano alivio vbrake groupset and doublewhalled alloy rims.
its in a 20% incline hill for 700mts.
At first: How do you define "overheating"? 700m is not a long distance.


Originally Posted by BromptonINrio View Post
Bikes are Dahon Curve D3
It gets so hot the brakes become impossible to touch.
What is "it"? The bike, the break or the rim? And why would you want to touch the rim? Plus: Just you feel it would be hot does not mean it would be overheated.

Originally Posted by BromptonINrio View Post
is there any thermal pad?
Never heard of something like that for rim brakes on bicycles.

Originally Posted by BromptonINrio View Post
disks whould solve those issues in 16" bikes?
Possibly, if there is a problem at all. Mainly because you can break more carelessly with disk brakes. But probably it is just a matter of breaking technique and riding adapted to the material and terrain. It makes a huge difference in temperature if you stand on the brakes all the time vs. giving massive break impact from time to time and letting it roll (and the rims cool down) in between. With bikes the same than with cars and close to a necessity with small wheeled bikes on longer, steep descents.
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Old 07-23-19, 02:26 PM
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Originally Posted by mtb_addict View Post
If rim brake is so terrible, then why is most of TdF running it.
They don't brake as much. They don't mind going 50+ mph downhill.
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Old 07-23-19, 02:36 PM
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Originally Posted by BromptonINrio View Post
Hy there,

disks whould solve those issues in 16" bikes?
Yes. A 16" wheel has less than 2/3 the braking surface as a 700mm wheel, so it's going to overheat faster. A 140mm rotor would be plenty (possibly overkill) for a 16" wheel and would manage heat better than a rim brake.
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Old 07-23-19, 02:44 PM
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There are things you can do as a rider to minimize the heating and consequences. Ride as fast as possible. If you can ride at terminal velocity, ie the max speed wind resistance allows, you are doing no braking and cooling previously heated rims. Stay at speed and brake fairly hard and decisively for turns and required slow-downs. Also, before you start down, drop your tire pressure.

Here, wind resistance is your friend. Sit up. Elbows out. The more wind resistance, the less your brakes have to do. And remember what brakes do is convert your kinetic energy (your speed times you and your bike's weight) into heat. A "thermal pad" would just be a pad that doesn't degrade when hot. It still has the exact same tast of converting your speed to heat.

When you brake, do it with quick, hard pulls. I like to alternate front and rear so the overall braking doesn't change but each rim is only "on" half the time. It is continuous lesser braking that gets rims really hot. Another "trick" - ride a bigger tire. Two benefits. You can drop the pressure more to start your descent and still have plenty of cushion for any road irregularities you hit before your rims are hot and you have more air in the tire to warm up so a given amount of heat does less to your tire pressure.

berlinonaut - if the OP is talking about descending 700 vertical meters at 20%, that is very real. 2300' vertical. 2+ miles.

Ben
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Old 07-23-19, 02:52 PM
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Originally Posted by 79pmooney View Post
berlinonaut - if the OP is talking about descending 700 vertical meters at 20%, that is very real. 2300' vertical. 2+ miles.
Indeed - in this case I misunderstood the post.
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Old 07-24-19, 06:01 AM
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Originally Posted by 79pmooney View Post
There are things you can do as a rider to minimize the heating and consequences. Ride as fast as possible. If you can ride at terminal velocity, ie the max speed wind resistance allows, you are doing no braking and cooling previously heated rims. Stay at speed and brake fairly hard and decisively for turns and required slow-downs. Also, before you start down, drop your tire pressure.

Here, wind resistance is your friend. Sit up. Elbows out. The more wind resistance, the less your brakes have to do. And remember what brakes do is convert your kinetic energy (your speed times you and your bike's weight) into heat. A "thermal pad" would just be a pad that doesn't degrade when hot. It still has the exact same tast of converting your speed to heat.

When you brake, do it with quick, hard pulls. I like to alternate front and rear so the overall braking doesn't change but each rim is only "on" half the time. It is continuous lesser braking that gets rims really hot. Another "trick" - ride a bigger tire. Two benefits. You can drop the pressure more to start your descent and still have plenty of cushion for any road irregularities you hit before your rims are hot and you have more air in the tire to warm up so a given amount of heat does less to your tire pressure.

berlinonaut - if the OP is talking about descending 700 vertical meters at 20%, that is very real. 2300' vertical. 2+ miles.

Ben
great post.
thanks for the tips and tricks.
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Old 07-27-19, 05:21 AM
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How often are you doing a descent like this? When I have a steep descent with a lot of elevation change with my loaded touring bike, I stop every few hundred meters of elevation and check my rims for temperature. If they feel hotter than I think they would from sitting in the sun on a hot day, I give them a five minute rest to cool and start rolling again. I might do this a couple times a year, not often enough that it becomes much of an inconvenience.

If you have a rim braking surface that is too hot to touch, you are fortunate that you did not blow out the tire(s).
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