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Preventing rim overheating during long descents

Old 08-14-12, 09:50 PM
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Preventing rim overheating during long descents

My girlfriend is new to cycling and is afraid to descend without using the brakes.

I assume that too much braking can cause tubes to pop during the descent.

Any tips to minimize rim heating?

Is it possible to install disk brakes to the front wheel of a road bike that was not designed for disk brakes?
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Old 08-14-12, 10:00 PM
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FBinNY posted some good tips in a thread I started about a similar problem.

Originally Posted by FBinNY
Absolutely, Just as heat causes brake fade on cars and trucks, it increases the transfer of metal from the rim to the shoe through abrasion. If you're riding long downgrades, lighten the load on brakes by raising your body profile which will lower your terminal velocity, possibly to where you don't even need bakes, but at lest to lessen your reliance on them. This is the bicycle equivalent of downshifting and letting engine drag spare the brakes in a car or truck.

Then when you do brake, use them to scrub off speed in bursts, letting the bike accelerate between them. This both gives the rims a chance to cool, but also takes advantage of the added wind drag at the times of higher speed. Long term, the best thing you can do is improve your technique so you can coast at terminal velocity more of the time, scrubbing off some speed only now and then as you need to.

Since wind drag increases with the square of the speed, you may find that the terminal velocity is only slighter faster than what you're riding at while burning up your brakes. Obviously you need to balance speed against road conditions and your bike handling skill to stay safe, but I expect that even small improvements will work wonders in reducing shoe glazing.
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Old 08-15-12, 12:23 AM
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with out replacing the fork with one with disc caliper mounts, no. but they do make road compatible disc brakes now. YEs you can overheat a rim and blow a tube.
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Old 08-15-12, 01:50 AM
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Or, get a front wheel built with a ceramic(coated) rim.
Better braking, particularly in the wet, and less heat transfer.
They're kinda pricey, but probably less hassle and overall less cost than getting a new fork and all that.

But I've gotta ask you where you're riding. I've only ever encountered significant heat build up in the alps, losing something like 1/4 - 1/2 mile in altitude in one go.
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Old 08-15-12, 05:24 AM
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pulse brakes and alternate front and rear. you can overheat a disc brake too
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Old 08-15-12, 05:34 AM
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starving, You can teach her to only use one brake at a time, alternating between front and rear which allows the rim to cool. This was the preferred method when I lived in Connecticut as a boy. As my skills improved it evolved into more of FBinNY's comments.

Brad
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Old 08-15-12, 05:38 AM
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This is more an issue with tandems - with around twice the weight for the one set of brakes. A drum drag-brake was an option on some bikes in the past. The manufacturer of my newish Tandem does rear disk brakes but won't let a disk braked bike out of their shop without a backup v brake on the rear wheel. I believe that their concern is that you can kill a poorly maintained disk brake by boiling the traces of water in the hydraulic fluid.

Back to solo bikes - I'm a timid comfort braker - hottest I've ever got my brakes was descending Mont Ventoux - 20 or 30 minutes of dragging the brakes on and off. I run my tyres at 100psi - no explosion to report. If your gf is new to cycling then she won't be climbing mile high mountains I would guess. Probably on more comfortable tyres than 100 psi also.

If you bully her into descending, brakes off, faster than she is comfortable with then there is the risk of running wide on a corner and a nasty crash. In the (unlikely) event of a blowout theres a chance she'll hold it up.

Here's the thing. If you make riding unpleasant for her by haranging her about things then she'll stop the riding. I'd suggest leave her be about braking.

Just my 2c - I could be missing something. I hope you continue enjoying bikes together.
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Old 08-15-12, 06:11 AM
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Originally Posted by jolly_ross
This is more an issue with tandems - with around twice the weight for the one set of brakes. A drum drag-brake was an option on some bikes in the past. The manufacturer of my newish Tandem does rear disk brakes but won't let a disk braked bike out of their shop without a backup v brake on the rear wheel. I believe that their concern is that you can kill a poorly maintained disk brake by boiling the traces of water in the hydraulic fluid.

Back to solo bikes - I'm a timid comfort braker - hottest I've ever got my brakes was descending Mont Ventoux - 20 or 30 minutes of dragging the brakes on and off. I run my tyres at 100psi - no explosion to report. If your gf is new to cycling then she won't be climbing mile high mountains I would guess. Probably on more comfortable tyres than 100 psi also.

If you bully her into descending, brakes off, faster than she is comfortable with then there is the risk of running wide on a corner and a nasty crash. In the (unlikely) event of a blowout theres a chance she'll hold it up.

Here's the thing. If you make riding unpleasant for her by haranging her about things then she'll stop the riding. I'd suggest leave her be about braking.

Just my 2c - I could be missing something. I hope you continue enjoying bikes together.
yes hydraulic fluid can boil. it should be flushed once a year if you ride regularly. most tandems and road bikes are using mechanical disc brakes. these can over heat and have brake fade or some stuff can melt.
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Old 08-15-12, 06:54 AM
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Thanks for the advice, guys.

Originally Posted by dabac
But I've gotta ask you where you're riding. I've only ever encountered significant heat build up in the alps, losing something like 1/4 - 1/2 mile in altitude in one go.
Here is a ride we plan to do.....
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Old 08-15-12, 07:20 AM
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Or switch to Road Tubeless. No chance of a blowout with that if properly installed.

I actually descended with a friend a while back on a ride new to me; he had done it many times, so he got way ahead of me and then waited at the bottom. I caught up and stopped, we chatted for a moment, and POW his rear tube blew out from heat. Mine were fine of course.
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Old 08-15-12, 07:26 AM
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I've ridden many years in the Rockies and Sierra, and I wouldn't worry about. There are many much more likely reasons she'll crash than blowing a tube.
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Old 08-15-12, 08:10 AM
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Those descents are not horribly long, you would almost have to be trying to overheat your rims to the point of it being a problem. Let your girlfriend just ride and do not freak her out with little details that are likely not going to matter at her current riding level. As she gets more comfortable with her bike she naturally drag the brakes less anyways.
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Old 08-15-12, 09:07 AM
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On a longer descent, would it help to take a break every few minutes and spray the rims with a water bottle?
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Old 08-15-12, 09:07 AM
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Originally Posted by starving
On a longer descent, would it help to take a break every few minutes and spray the rims with a water bottle?
No. Wet braking sucks.
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Old 08-15-12, 09:32 AM
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Originally Posted by jolly_ross
This is more an issue with tandems - with around twice the weight for the one set of brakes. A drum drag-brake was an option on some bikes in the past. The manufacturer of my newish Tandem does rear disk brakes but won't let a disk braked bike out of their shop without a backup v brake on the rear wheel. I believe that their concern is that you can kill a poorly maintained disk brake by boiling the traces of water in the hydraulic fluid.

Back to solo bikes - I'm a timid comfort braker - hottest I've ever got my brakes was descending Mont Ventoux - 20 or 30 minutes of dragging the brakes on and off. I run my tyres at 100psi - no explosion to report. If your gf is new to cycling then she won't be climbing mile high mountains I would guess. Probably on more comfortable tyres than 100 psi also.

If you bully her into descending, brakes off, faster than she is comfortable with then there is the risk of running wide on a corner and a nasty crash. In the (unlikely) event of a blowout theres a chance she'll hold it up.

Here's the thing. If you make riding unpleasant for her by haranging her about things then she'll stop the riding. I'd suggest leave her be about braking.

Just my 2c - I could be missing something. I hope you continue enjoying bikes together.
Emphasized for truth.

And the technical solution is easy enough: drum brakes.
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Old 08-15-12, 09:38 AM
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I would do flatter rides until she gets more comfortable with the descents.
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Old 08-15-12, 09:56 AM
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THis is a good question for me. I have wondered just how long of a descent do I have to go down before issues like overheating rims become an issue to worry about?
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Old 08-15-12, 11:09 AM
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OK, 6 miles of 4% grade. No problem. With my 250 pounds on a road bike (25 mm tires and 19 mm rims) I've only had significant heating on my rims during a 4 mile long 11% average grade (Oregon cascades forest service road with lots of switchbacks, so I couldn't go very fast and take advantage of the "terminal velocity"). Just use high-quality tires and change them when they get worn or damaged- it takes a pretty hot rim to melt the bead/sidewall enough to blow a tire.
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Old 08-15-12, 11:22 AM
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I think maybe the steel bead expands and you get blow-offs. Does a kevlar bead help?
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Old 08-15-12, 11:31 AM
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Originally Posted by starving
Here is a ride we plan to do.....
With that, I wouldn't worry at all.

Well, maybe if visibility, traction, or something else became real bad and prompted an extra-slow descent...
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Old 08-15-12, 11:34 AM
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Originally Posted by cycle_maven
OK, 6 miles of 4% grade. No problem. With my 250 pounds on a road bike (25 mm tires and 19 mm rims) I've only had significant heating on my rims during a 4 mile long 11% average grade (Oregon cascades forest service road with lots of switchbacks, so I couldn't go very fast and take advantage of the "terminal velocity"). Just use high-quality tires and change them when they get worn or damaged- it takes a pretty hot rim to melt the bead/sidewall enough to blow a tire.
Originally Posted by Kimmo
I think maybe the steel bead expands and you get blow-offs. Does a kevlar bead help?
The heat expands the air within the tubes, causing the pressure to rise inside them. The tires aren't rated for such high pressures, so they blow off the rim.
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Old 08-15-12, 11:41 AM
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Originally Posted by AlphaDogg
The heat expands the air within the tubes, causing the pressure to rise inside them. The tires aren't rated for such high pressures, so they blow off the rim.
Exactly. Which is why hub brakes are safer.
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Old 08-15-12, 12:19 PM
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Originally Posted by starving
On a longer descent, would it help to take a break every few minutes and spray the rims with a water bottle?
That is totally unnecessary.

For one thing, the descents in the ride you linked above are just not all that steep. It's entirely possible that you could descend these without braking at all (assuming there are no traffic controls or corners that require braking). Also, with this kind of grade, simply sitting more upright and using your body to 'catch the wind' would give you a substantial decrease in speed without even touching the actual brakes.

There are several websites out there that directly investigate what it takes to make this happen.

This guy ( https://mrbill.homeip.net/rimTempStudy.php ) used various rim and tire combinations to do several descents and recorded peak temperatures and the results. In only one case did he have a blowout and that was using a 700x28 tire near its max pressure on a loose fitting rim and strangely, it happened not while riding, but while he was stopped.

This guy ( https://www.bicyclesource.com/brake_performance ) directly compared rim braking to a hub-based coaster brake (like on your kid's bmx bike or a beach cruiser). The rim brakes interestingly seemed to demonstrate thermal equilibrium - once they got to around 175F they didn't increase in temperature any more. The coaster brake on the other hand, when asked to provide the same braking force, completely destroyed itself. It should be noted that these tests were performed on a 2040ft descent with an average 9.6% grade - much more extreme of a case than the ride you linked.

In addition to the above, I will make the assumption that your girlfriend's total mass is below that of an average male cyclist which means her brakes (and rim) will be asked to provide far less heat absorption to slow her since her lower weight will cause her to not go as fast and also her lower mass will mean slowing her requires less power than a heavier person would require. Less speed, less braking to reduce speed. Win-win.

As for getting her comfortable going downhill, find some lower angle hills where if she brakes, she'll be going slower than if she was pedaling on flat ground. That should help build her confidence. Otherwise, let her brakes all she wants. She'll get there. Just be in her corner - don't bark at her to let go of the brakes, just let her get down the hill on her own terms - like letting a new skier go down a steep ski run. Better to have her go slowly than get beyond her comfort level and become dangerous.
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Old 08-15-12, 01:02 PM
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nhluhr, thanks for those studies. I was looking for previous references to do my own tests after building up some brackets to hold an infrared pyrometer.


starving,

My wife was in a similar situation many years ago when we started riding. I would get her used to speed on downhill by doing smaller hills and having her go down them without brakes and getting up to higher and higher speeds. So a 1/4-mile hill would start at 5mph and end at 20mph. Then the next hill, will start at 6mph, then 7mph, etc. Gradually she got used to the speed and wouldn't bother with the brakes once we crested the hill.

For the profile shown, I wouldn't bother with brakes. Sit up on the bike and your top-speed will be limited by aero-drag to 30-35mph max. Then hit the brakes at 100% at the very last moments before entering the turns, but let off before actually diving in. For example, if a turn can be negotiated at 20mph, and you're heading towards it at 35mph, wait until the last 100ft before the corner. Then hit the brakes all-out (front 90%, rear 10%) for 1-second to slow from 35 to 20mph. Let up and go around the corner with no brakes. Then sit up and catch as much wind as possible so you don't accelerate too much.

The basic idea behind keeping the brakes cool is to go as close to a free-fall speed as possible to use as little brakes as possible. And when brakes are necessary, use them for as little time as possible. A 100% effort for 1-second going into the corners will heat up the brakes and rims less than dragging it 5% the whole time. That's because it takes time for the heat to travel from the braking-surface to the interior of the rim & tyre. The least amount of time spent on the brakes with maximum cooling-time in between yields the coolest brakes.

Personally on a hill like that, I'd do a full tuck hiding my entire body behind the bars to get maximum speed. Even then, I doubt I'd go over 40-45mph on that hill. Nowhere near as fun as hitting +70mph on steeper hills and drafting off mini-vans & trucks.

Last edited by DannoXYZ; 08-15-12 at 01:06 PM.
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Old 08-15-12, 01:20 PM
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Any tips to minimize rim heating?
Stop completely more often, and make it into a series of short descents..
hit the brakes hard , then let go, don't hold them on constantly.

the view at the top and stopping, looking around more often on the way down, is a benefit.

Last edited by fietsbob; 08-15-12 at 01:23 PM.
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