fooferdoggie 

the stoker has a standard v brake on our bike. I want my wife using it more on long downhills to keep the back disc from so much heat but it takes a lot of work to use. I think it is just cheap brake housing/cable and was thinking of replacing it. but what about using a shifter for that? and if so what to get? her bars are mountain bike not road so thats what I have to deal with.

Leisesturm

We don't have a drag brake on any of our tandems but if we did, I would control it! Not because I don't trust my wife, but because I feel that all braking functions fall under my responsibility. Using a v-brake to take heat load off of your disc is like using a space heater to take load off your furnace! It should be the other way round. The pads of a v-brake used like a drag brake would last about 15 minutes and they would likely blow your rear tire off the rim before they died. I am not sure why you are having problems keeping your rear disc from overheating but using the v-brake to help it out is a bad idea.

Tony Marley

Leisesturm is correct that using a V-brake as a drag brake will likely blow your rear tire pretty quickly on a serious mountain downhill.

IPassGas

You show a heavy load. V-brakes and a standard disc are simply not sufficient braking power. Either stop and let the brakes cool or get hydraulic brakes with large/thicker rotors. In previous days, you would have had a Arai drag brake...problem solved.

fooferdoggie 

I would not have her use it constantly. I usually go from back to front to lettuce rotors cool adding the rim brake gives more time for the rear rotor to cool off. I have 4 piston deore calipers with fins and ice tech rotors. but I know I have been using the back more then I should too. but we have the brake so I use it. the pads are still original after 6000 miles. but the lever takes a lot of effort to use so I want to make it easier for her. I am not worried about overheating it wont get used the whole time but it gives us 3 braking surfaces to use to keep everything cool.

cyccommute 

Blowing a tire off the rim due to overheating is far more difficult than people think. The pressure change with temperature is far smaller than most people realize. The pressure that a tire blows off a rim will vary of course but if you have a tire at 80psi at 70°F and you heat the system to 130°F, the pressure rises to 89 psi. If you heat it to the boiling point of water, the pressure rises to 100 psi. That might be enough to blow off the rim but it’s going to very difficult to get the entire wheel to that temperature. The wheel is very large and radiates a lot of heat.

The whole system would have to be at that temperature to get those kinds of pressure changes as well. The actual temperature and associated pressure change in the tire is going to be less than the calculated value.

How large is the rotor? It looks like a 180mm, although it is hard to judge from a picture. If it is a 180mm rotor, a 203mm rotor will radiate heat a little better.

Effective braking isn’t a “use more front/less rear” thing. It’s also not a “smash the brake on at the top of a hill and leave them on to the bottom” kind of thing either. Effective braking on any bike should be used in pulses…i.e. brake hard, scrub a lot of speed, then get off the brake. Do this will both brakes…or, in your case, all three brakes. If you apply the brakes and keep them on for long periods of time, you will build up heat. Disc rotors are particularly bad with regard to heat build up because the rotor has a low mass and has a small surface area. It can build a lot of heat up because it doesn’t lose heat as fast as a wheel can (larger surface area and greater mass).

Have your stoker use the rear brake…I dislike calling it a “drag brake”…in the same manner. If she is currently using it as a drag brake and keeping it on for extended periods of time, of course her hands get tired. Tell her to brake hard when you brake hard but tell her when to get off the brake as well. If you need more control speed of speed, use the stoker brake alternating with the disc brake. Just don’t have the stoker pull the lever and leave it on.

fooferdoggie 

I tend to use the back more on the long descents and I am trying to depend more on the front. I think this was because if we go offroad you cant use much front and turn or the front wheel will slide out. the lever is really hard to use its so stiff. so I was debating on if I should get good brake housing or replacing it with a different method. but maybe just get the braking working well. we have 203 rotors. it took about 1000 miles before I needed to bleed the back but I get black gunk on the back rotor from heat. so I need to use it a bit less. the front has been fine the whole time.

diabloridr

Often overlooked is the effect of heating on the kevlar bead of the tire.

I've also seen the adhesive on Velox rimstrips soften on hot rim, allowing the Velox to migrate and expose portions of the spoke hole in the rim leading to tube failure.

IPassGas

Perhaps overlooked, but Kevlar shrinks when heating (negative coefficient of thermal expansion). The rimstrip perhaps, don't know, but lots of pressure to hold it down.

cyccommute 

This reference says that Kevlar 29 and Kevlar 49 yarns shrink less than 0.1% at 212°F in water and 375°F in dry air. It may shrink but not by much at at far higher temperatures than when the rubber begins to soften (about 200°F).

I agree that the adhesive on the rim strip may soften but, again, that would be highly localized and would rapidly cool once the rim has passed the brake surface, in addition to being held in place by a lot of pressure.

diabloridr

More importantly, tensile strength drops by about 10% as Kevlar is heated to 200°F, so stretching should be expected.

That air pressure pushes the rim strip down into the spoke pocket within the rim. If the rim strip is insufficiently wide, this will cause the corner between the pocket and the rim bed to become exposed where it can cut the tube. I've witnessed this phenomenon myself, the solution is choose a wider rim strip.

cyccommute 

The problem is getting a rim to 200°F. Getting a disc to 200°F isn’t hard but a rim with a tire is a much larger heat sink and, more importantly, sheds heat more efficiently.

Even if you could get the rim to that temperature, the link I posted above says on page 13.

An associated chart does show a decrease in tensile strength vs time at various temperatures but it should be noted that the time scale is on the order of hundreds of hours at a much higher temperature (320°F) over the course of 500 hours (3 weeks). Even if you could get a rim to 200°F, it would only be for fractions of a second to perhaps a few seconds. Hardly enough to cause damage to the bead even over years of use.

As you noted that is a user error problem. In the case of a too narrow rim strip, the adhesive can squirm without heat. But with a sufficient size, the pressure will keep the rimstrip from moving.

Leisesturm

All of this point/counterpoint misses the point that using a v-brake in this application is pointless. Personally I would be wanting to find out why two 203mm rotors are having so much trouble with a less than 400lb. team and light loading. Also, coordinating the use of a third brake in a non-drag situation with another rider ... well, I wouldn't do it. Which is worth what it's worth.

79pmooney

I rode down Mt. Washington on a single. Tubulars. Blew the front. Mediocre tire, high pressure. (I didn't think to drop the pressure before I started down.) That rim was hot! Glue fully melted. I don't think that Velox tape would have been much different. The idea that significant localized cooling happens in the rim after it passes the brake shoe? Nah. And yes, based on what I saw I would fully expect the Velox to slide into the rim box on any modern rim. What would stop it?

Oh, the usual "good practice: of high speed then brake hard didn't work in those days on Mt. Washington. Very little of it was paved. 11% average grade, no level for almost 8 miles and dirt curves you didn't do fast. Most of us went up on light road wheels and tires and came down on the same. (Except for the annual time trial when they wouldn't allow you to ride down).

diabloridr

Figure 2.7 isn't valid (as you point out). Figure 2.8 is.

fooferdoggie 

because I was overusing the back over the front I think. plus I keep the speed to 25 or so that takes more braking. my wife has no problem using her brake its just so hard to use. I think she likes to use it. I will just replace the housing and see how well it feels. I did not overheat it in one try it is over time. we also have a lot of steep drops 20% grades and that takes a fair amount of braking to keep from freaking my wife out.

fooferdoggie 

because I was overusing the back over the front I think. plus I keep the speed to 25 or so that takes more braking. my wife has no problem using her brake its just so hard to use. I think she likes to use it. I will just replace the housing and see how well it feels. I did not overheat it in one try it is over time. we also have a lot of steep drops 20% grades and that takes a fair amount of braking to keep from freaking my wife out. Plus wife and I are about 350 and the bike with nothing but the battery is 65# with the bag and stuff I bet its at least 80

Carbonfiberboy 

The only issue here is your 25 mph limit, an unsustainable practice without an Arai drum. The only way to descend on a tandem, reliably, is to let the bike run and brake hard before corners. If there are many corners, I alternate between front and rear brakes. If the straights are steep enough that we'll hit well over 50, I alternate front and rear short, hard braking efforts. You need to keep the pads off the discs and rims. You need to control all the brakes. Rim-braked tandems have had better success in descending Ventoux because their riders knew to keep the pads off the rims. Disc users had a tendency to try to drag brake, which is not good. I had a friend, an expert tandem captain, have his front brake cable go out on a short steep descent, which quickly turned his rear disk red-hot and they went off into the blackberries on a hard right-hander. They were really lucky. Everything has to be perfect on a tandem.

We ride a rim-brake tandem. Starting out on it, I blew 3 tires off the rims and was lucky every time. The secret to not blowing off tires is to run deep section, like 28-30mm, alu rims. The extra material is both a heat sink and a heat radiator. We never blew another tire off after swapping out the rims and spokes.

When we went on loaded tours, we swapped out our rear wheel for one with an Arai drum on it. Around here doing sport rides, we have done many, many mountain pass descents without issue with our rim brakes. Going over 50 is not uncommon. Max speed on one descent was 65. It gets easier with practice.

fooferdoggie 

because I was overusing the back over the front I think. plus I keep the speed to 25 or so that takes more braking. my wife has no problem using her brake its just so hard to use. I think she likes to use it. I will just replace the housing and see how well it feels. I did not overheat it in one try it is over time. we also have a lot of steep drops 20% grades and that takes a fair amount of braking to keep from freaking my wife out. Plus wife and I are about 350 and the bike with nothing but the battery is 65# with the bag and stuff I bet its at least 80

cyccommute 

You were talking about tensile strength. Figure 2.8 is about the modulus…or stiffness…of the fiber. That doesn’t have anything to do with the dimensional stability. Look at the section just below Figure 2.8. It says that there is very little change in dimension of Kevlar when exposed to heat and even that change is negative. In other words, it would get tighter, not looser.

cyccommute 

I didn’t say the rims don’t get hot. I said that the temperature increase isn’t enough to change the pressure that much. Velox would be different from tubulars as well. I don’t know what the softening point of tubular glue is but a rim tape is trapped inside the tire with a bead keeping the tire on the rim. Tubulars are only held onto the rim with pressure and glue but can roll off the rim.

I’ve got years and years and years of mountain biking experience including many years of riding rigid on steep downhills. I learned how to brake on mountain bikes and that’s still the way that I do it. Speed, brake hard, get off the brake, get back on, ad infinitum. It works better than trying to keep the brakes on from top to bottom.

diabloridr

Modulus is stress divided by elastic strain.

Figure 2.8 show that the kevlar bead will inherently stretch and become looser as temperature increases.

fooferdoggie 

Dont think my wife would be happy with those speeds or me. Dont even know how the bike would handle it since it was designed as a cruiser bike more then anything else. but ya I need to work on how I brake.

cyccommute 

Again, in post 11, you mentioned tensile strength, not modulus.

Elsewhere in the link, they discuss the coefficient of thermal expansion in the longitudinal direction and compare it to other materials. The coefficient has a negative value…i.e. it shrinks with temperature. Steel has a positive coefficient of expansion of slightly larger magnitude in the positive direction. In other words, heat steel and it expands slightly. Heat Kevlar and it shrinks slightly.

Out where the rubber meets the road, Kevlar bead tires have been around for decades without issues. They have been used in high speed, high friction rim brake applications thousands, even millions, of times. They aren’t any more prone to blow off than a steel bead is. It’s a nonissue even if the Kevlar were to do what you say it does, probably because the tire never gets all that hot to begin with. Tires blowing off the rim because of braking is one of those things that is far less common than people think it is. The heat that builds up just doesn’t have the effect that people think nor does the pressure increase as much as people think.

Carbonfiberboy 

It's all about entropy and the heat death of the universe. All energy decays into heat, one way or the other. At the top of a descent you have hard-earned potential energy. As you descend that energy has to turn into heat. When you go fast and sit up, you heat the air rather than your brakes. I ride with a team whose stoker has a special speed brake. She sits up and opens her jacket wide, holding it out with her hands as a speed brake. Works for them, but don't you try it. Really easy to get blown right off that saddle. But that's the idea. Sit up, put your knees out wide and let the bike run.