Originally Posted by
IPassGas
Not to inflame an already heated discussion. It would be good for people to reference more authoritative work. For example Wilson's Bicycling Science is a great source. Here are figures for (1) rim temperature increase during constant braking versus speed and (2) terminal velocity versus %slope. From these graph 1 you see for constant braking to maintain 40 MPH, a single bike does not heat rims more than 50C even on a 20% slope. My concern would be temperature increases closer to 100C (melting temperature of butyl rubber, 120C). I have been concerned of late with this on a tandem since we often bike in mountains. For a tandem, >10% slope can cause temperature increases that are much higher. Wilson does not mention pulsing brakes. I have heard anecdotal talk that pulsing does allow air flow between pad and rim/disc to prevent brake pad melting.
While I admire Dr. Wilson’s work, I think he has drawn a wrong conclusion in this case. He seems to assume that the temperature decreases with speed is due to the increase airflow. However, he neglects the effect of the brakes on the rim itself. On a 20% slope (or any slope for that matter) maintaining a slower speed requires more of the forward momentum to be converted to heat than does a faster speed. You can’t maintain close to terminal velocity with the brakes on as hard as they would be at 20 mph. The temperature increase at slower speed would be friction related rather than related to the air flow. At higher speed, there is a lower temperature increase above ambient because there isn’t any friction being put into the system.
He makes the statement “…going fast avoids heating the rim unless emergency braking has to be applied, in which case the danger of overheating the rim is sudden and serious.” The problem with that statement is that if you are going 60 mph, you aren’t using the brakes so the heat being put into the system is zero. Hard application of the brakes to slow for some reason would be transitory and would not cause the temperature of the system to rise to the same level as if the brakes were being used to maintain a given speed.