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Rim brakes and heating concerns

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Rim brakes and heating concerns

Old 06-18-21, 03:32 PM
  #1  
reburns
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Rim brakes and heating concerns

Our tandems use linear pull rim brakes coupled with an Arai rear drum drag brake that the stoker invokes on command to control downhill speed. Using SwissStop pads on the rim brakes, I am completely happy with braking performance: powerful, quiet, low maintenance, etc.



When it comes to safety related issues on the tandem, my philosophy tends to be that measures that reduce risk canít be overdone. So even with the drag brake and judicious use of the rim brakes on long descents, I still look for any advantage that might reduce the likelihood of the dreaded tire blowout at speed.



Our wheels are Spinergy tandem Z-lites with tubeless-ready rims, but I use tubes. The rims are certainly light and consequently not a lot of thermal mass. I run 700/34 tires at 90 psi, Continental 5000/4 Seasons F/R. A few months ago I decided to install Schwalbe Aerothan tubes due to their claims of high temperature tolerance, flat resistance and comfortable ride. So far they have been trouble free.



So I started thinking about the rim tape as a possible weak point. The wheels came with so called rim strips, one piece loops that are stretched onto the rim. But the Spinergy rims have extra large spoke holes and I had problems with these strips moving slightly and exposing the edge of a hole, causing a flat. So I installed the tried and true Velox cloth tape. That works fine but the thickness makes removing and installing tires a little tougher. So I decided to try DT Swiss rim tape, mostly because I could get it in the 21 mm width I wanted.



Iíve had no problems since I installed the DT Swiss tape, but I recently got to thinking about whether it stands up to high temperatures, so I decided to try a quick, not very well controlled experiment. I took a spare, naked rim and covered a couple of spoke holes with DT Swiss, Velox and Stanís yellow tubeless tape. I then took a heat gun and directed it over each section of tape. Stanís melted pretty quick. The Velox started to toast, but seemed to retain integrity up until I backed off, as I didnít want to ignite it. Surprisingly, the DT Swiss tape never showed any sign of changing under the same assault. Poking it with a tool, it remained solid. Itís expensive tape, but I guess Iíll stick with it.



Just FYI for anyone with a similar braking scheme. As always, YMMV.
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Old 06-19-21, 10:05 PM
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Interesting to read in your post how differently the rim tapes reacted to heat. I will have to see what rim tape is installed on my tandem rims.

I have an 80's Jack Taylor Tandem that I have converted to Campy 10 speed. It is currently running Mafac tandem cantilevers and an Arai drag brake.

The drag brake was never very effective when used with the original 7 speed freewheel. This required that the rim brakes to be relied on more than I would like for long descents.
The drag brake effectiveness changed dramatically for the better when I switched to a Phil tandem rear freehub when converting to 10 speed. From this, it became clear that the slight bend in the rear axle of the old hub had a significant impact on the effectiveness of the drum brake.

Now that I'm using 10 speed Ergopower, I actuate the drum with the front derailleur control of the left Ergopwer brifter. The front derailleur being controlled with a downtube shifter.
With the drag brake using the Ergopower shift control, I can set it to a specific drag with a few clicks and then revise to more or less drag quickly without changing my hand position.

I feel the stoker maintaining the drag brake via a brake lever can lead to hand fatigue and more reliance on the rim brakes to control speed on a descent.
A bar end shifter is another way to do this without hand fatigue, but requires requires moving your hand to adjust the drag. Also if the bar end shifter is controlled by the captain, another cable is required under the bar tape. Two cables under the tape is enough for me.

This has improved the safety factor as all brakes are at hand and rim brakes do not have to be overused on long descents.

Another safety factor to consider is using wider tires to take advantage of using lower tire pressures. I run 700 x 44 at 55 PSI. I expect the lower pressures will reduce the strain on the rubber in the tube and tire as the rim temps rise.

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Old 06-19-21, 10:38 PM
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I have two sets of wheels for my 1990 Burley Bongo. I ordered in 48 spoked dish less Phil Wood hubs for it the first month I had it. They use the 7 speed freewheels. The Arai drum brake is hooked up to a thumb shifter and does its job quite well. The Tandem came with the self energizing cantilevers. When I wore them out I replaced them with the Magura hs66 hydraulic rim brakes. They are setup square with the rims no towing in of the pads is needed. They are simple to maintain and you change the pads with your fingers. I run 26"x 1.5-2" tires. I have come down canyons pulling a two wheeled cart with three children and not had problems with heat.
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Old 06-28-21, 01:41 PM
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Originally Posted by ReidH View Post

Another safety factor to consider is using wider tires to take advantage of using lower tire pressures. I run 700 x 44 at 55 PSI. I expect the lower pressures will reduce the strain on the rubber in the tube and tire as the rim temps rise.

Reid
Not necessarily.

The formula for pipe stress (which is what your tube is) is Stress = (Pressure X Tube Diameter)/2 X Tube Thickness

Since your decreasing pressure while simultaneously increasing tube diameter, without plugging in the exact variables it cannot be said if stress will decrease, increase, or stay the same.
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Old 06-29-21, 09:11 AM
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I've had good experiences on tight rims with installing Velocity's VeloPlugs instead of rim tape. I haven't tested them for heat. I have had problems with plastic rim tape and found Velox the only reliable tape.

I have had tires blow off on lightweight, shallow rims. Deep alu rims, say 28mm, are the solution. It's not the mass, it's the exposed heat-dissipating part of the deep rim which makes all the difference, plus they're stronger. We have a touring wheel with an Arai drum, but only use it for loaded touring. We've never had a problem with our rim brakes on our usual hilly rides. We sit up as necessary, let 'er run as possible, brake hard before sharp corners, and alternate front and rear brakes. Speed is your friend. BTW, on our bike, captain controls the drum with a bar-end lever. We think that makes more sense, since the captain can feel the balance between the rim and drum brakes.
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Old 06-29-21, 01:35 PM
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Originally Posted by diabloridr View Post
Not necessarily.

The formula for pipe stress (which is what your tube is) is Stress = (Pressure X Tube Diameter)/2 X Tube Thickness

Since your decreasing pressure while simultaneously increasing tube diameter, without plugging in the exact variables it cannot be said if stress will decrease, increase, or stay the same.
I don't think this equation has much to do with heat-related TIRE failure on tandem wheels. Inner tubes only provide an air-tight air seal; they do nothing to resist or prevent blowout. The rim strip similarly plays almost no role in tire blowouts on a tandem (assuming it's a quality rim strip to begin with, and not butyl rubber). It's the tire itself and its inability to withstand high temperatures and pressures that are created through extended braking sessions. Once the tire pressure exceeds 2X stated "max pressure" on the sidewall, all bets are off, especially considering the tire is undergoing heat stress at the same time. Adhesives and butyl rubber get very weak at high temperatures.

Plus, this equation is explaining tube WALL THICKNESS as it relates to diameter and pressure maximums. This simply does not apply to bicycle tires. There are no "extra-thick, extra high-pressure" tires sold with thicker sidewalls designed to prevent temperature-related blowouts. So tire sidewall thickness plays almost no role because you can't pick a thicker tire sidewall. (With the exception of, perhaps, choosing a black sidewall that has a rubber coating from bead to bead. But this is merely cosmetic and plays no role in air pressure ratings for tires.) And one could argue that when one does find a thicker tire sidewall, it's usually referred to as "gumwall," and these tires are of the lowest quality, and thus a terrible solution to the problem at hand.

To reduce heat buildup at the rims, improve the efficacy of your drum brake, first and foremost. I had the opportunity to overhaul an Arai drum brake and the results were impressive. Although I've never been unhappy with the 30 year old Arai on my first road tandem, I was impressed with the performance of the rebuilt Arai on the recent Co-Motion purchase and overhaul.

If you're THAT concerned about tire blowouts, the best thing you can do is ensure your Arai is performing at its best. New, compressionless housing, properly installed (housing edges cut and ground flat & square) and lubricated with an appropriate shifter lever that provides enough leverage and stays in place once brake pressure is set is crucial. No Arai will perform well with old, fouled cable housing & cables.

The second measure to take is simple: stop frequently on long, steep descents to reduce the time your wheels are heating up. That alone is probably the easiest way to prevent catastrophe. Either that, or simply increase your speed tolerance. After all, the best way to cool your wheels is through conductive air transfer - the faster you ride, the faster your wheels return to ambient temps. Stay off the brakes may end up being a safer strategy than riding the brakes in fear of going too fast. Plus, air resistance grows exponentially with speed, so terminal velocity can be reached pretty quickly without having to rely too much on the brakes.
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Old 06-29-21, 04:00 PM
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Originally Posted by LV2TNDM View Post
I don't think this equation has much to do with heat-related TIRE failure on tandem wheels. Inner tubes only provide an air-tight air seal; they do nothing to resist or prevent blowout. The rim strip similarly plays almost no role in tire blowouts on a tandem (assuming it's a quality rim strip to begin with, and not butyl rubber). It's the tire itself and its inability to withstand high temperatures and pressures that are created through extended braking sessions. Once the tire pressure exceeds 2X stated "max pressure" on the sidewall, all bets are off, especially considering the tire is undergoing heat stress at the same time. Adhesives and butyl rubber get very weak at high temperatures.

Plus, this equation is explaining tube WALL THICKNESS as it relates to diameter and pressure maximums. This simply does not apply to bicycle tires. There are no "extra-thick, extra high-pressure" tires sold with thicker sidewalls designed to prevent temperature-related blowouts. So tire sidewall thickness plays almost no role because you can't pick a thicker tire sidewall. (With the exception of, perhaps, choosing a black sidewall that has a rubber coating from bead to bead. But this is merely cosmetic and plays no role in air pressure ratings for tires.) And one could argue that when one does find a thicker tire sidewall, it's usually referred to as "gumwall," and these tires are of the lowest quality, and thus a terrible solution to the problem at hand.

To reduce heat buildup at the rims, improve the efficacy of your drum brake, first and foremost. I had the opportunity to overhaul an Arai drum brake and the results were impressive. Although I've never been unhappy with the 30 year old Arai on my first road tandem, I was impressed with the performance of the rebuilt Arai on the recent Co-Motion purchase and overhaul.

If you're THAT concerned about tire blowouts, the best thing you can do is ensure your Arai is performing at its best. New, compressionless housing, properly installed (housing edges cut and ground flat & square) and lubricated with an appropriate shifter lever that provides enough leverage and stays in place once brake pressure is set is crucial. No Arai will perform well with old, fouled cable housing & cables.

The second measure to take is simple: stop frequently on long, steep descents to reduce the time your wheels are heating up. That alone is probably the easiest way to prevent catastrophe. Either that, or simply increase your speed tolerance. After all, the best way to cool your wheels is through conductive air transfer - the faster you ride, the faster your wheels return to ambient temps. Stay off the brakes may end up being a safer strategy than riding the brakes in fear of going too fast. Plus, air resistance grows exponentially with speed, so terminal velocity can be reached pretty quickly without having to rely too much on the brakes.
diabloridr intended that we take the word "tube" to mean the tire, which technically could be described as a tube in the context of his equation. Note that his equation is the same as what I've been talking about - multiply tire diameter by pressure and keep the result below 3000. That works, the "3000" constant having more to do with rim strength than sidewall thickness. However if one does the calc on a number of tires, one sees that it's a good approximation. Note that 44 * 55 = 2420, which is below the critical 3000 constant and is thus about right for a tandem.

Personally, as I've mentioned above, I've found rim brakes to be adequate if combined with a deep alu rim (not carbon!) and a good bit of speed. As you say, "Stay off the brakes may end up being a safer strategy," except that "may" should be replaced by "is." Never, never ride your brakes, rim or disc. You can't blow a tire by overheating a disc, but the disc will fail if it gets hot enough and away you go. That's been proven by accidental experiment. I've never heard of an Arai overheating when used as a drag brake.

On the famous and hopefully well-known group tandem descent from Mt. Ventoux, rim braked tandems were the only ones to descend without issues. Why? Because they knew to stop and let them cool! No drums were used.

A tandem who posts here made a loaded descent during a New Zealand tour, 17% with hairpins IIRC, on rim brakes, no drum. That said, going into unknown territory fully loaded, I always fit our drum wheel.
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