Originally Posted by
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.
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.