I Downhill Bicycle Braking Question

[RobRobinette]

TL;DR Summary

Do my bike's brakes absorb more energy if I allow my bike to accelerate downhill more between brake applications?

I ride gravel bikes on steep mountain roads and on the way down I have to ride the brakes pretty hard. I want to minimize brake heat so I have always thought that keeping the bike's velocity lower on average by applying the brakes more often will reduce the amount of energy the brakes will have to dissipate. I believe this because of the "velocity squared" in the energy formula. But it also seems that the bike and I have a certain amount of potential energy that will be transformed into kinetic energy going downhill and then transformed into brake heat and the velocity of the bike shouldn't matter. Which is it?

 

[Lnewqban]

The total amount of work that is converted to heat should be about the same.
The difference is in the time those happen, which means that the flow of energy through the brake elements, although of shorter duration, will be more intense, causing higher temperatures, mainly of the pads.

 

[A.T.]

But it also seems that the bike and I have a certain amount of potential energy that will be transformed into kinetic energy going downhill and then transformed into brake heat and the velocity of the bike shouldn't matter. Which is it?

If you arrive at the bottom with the same final speed, the total energy dissipated will be the same. Going faster and braking at the end dissipates more energy through drag, so the brakes have to dissipate less. But the dissipation power (energy/time) can be greater, so they will have less time to cool down by passing on the heat energy, resulting in higher peak temperature.

 

[jbriggs444]

At higher speeds, wind resistance increases. Some energy that would have been dissipated in the brake pads is dissipated into the air instead. However, the conditions at hand (steep, gravel) suggest that speeds high enough to make this significant are not safe.

 

[RobRobinette]

Thanks everyone, that makes sense but I still don't understand how the velocity squared in the energy equation does not come into play. When I used to aero brake the F-15 I always assumed the relationship between velocity, energy and the start of foot braking was nonlinear.

 

[jbriggs444]

Thanks everyone, that makes sense but I still don't understand how the velocity squared in the energy equation does not come into play.

It does. The rate at which you gain energy over time increases if your velocity is higher. But the rate at which you gain energy over distance stays the same.

The distance to the bottom of the hill is what is being held constant here.

 

[Dale]

Summary:: Do my bike's brakes absorb more energy if I allow my bike to accelerate downhill more between brake applications?

I want to minimize brake heat

I think that the others have covered the physics well. I would just recommend not trying to minimize brake heat. Just maximize safety and enjoyment! If you have to replace break pads a little more frequently to be safe then that is much less expensive than a trip to the ER

 

[RobRobinette]

MyMass * Velocity^2 / 2g = MyEnergy

"The rate at which you gain energy over time increases if your velocity is higher. But the rate at which you gain energy over distance stays the same."

How do we square this nonlinear equation with Jbriggs time and distance statement?

A related question, will 10 stops from 10 mph generate the same brake heat as one stop from 100 mph?

 

[Dale]

A related question, will 10 stops from 10 mph generate the same brake heat as one stop from 100 mph?

10 stops from 10 mph will generate much less heat than one stop from 100 mph. But they will also cover much less distance.

In other words, if you are going down a 100 mph hill then you will need many more than 10 10 mph stops to cover the distance.

 

[RobRobinette]

Thank you Dale, I think you finally got through, that makes sense to me. Again, thank you everyone for the help.

 

[berkeman]

I ride gravel bikes on steep mountain roads and on the way down I have to ride the brakes pretty hard.

I would just recommend not trying to minimize brake heat. Just maximize safety and enjoyment!

When I ride steep downhills (on pavement or dirt/gravel), I will apply my brakes about half of the time. For me, that gets better airflow to the pads and disks, which helps to reduce brake fade for me. When the pads are against the disks all of the time, that significantly reduces airflow to the pads (and a little bit to the disks).

And yeah, you have to be smart about where that half-time braking takes place (where there is actually traction!)

 

[cjl]

Applying the brakes more often and keeping your speed lower will, as everyone above said, actually increase the energy dissipated by the brakes, but it will also give them substantially more time to dissipate that energy, so it's very likely that the maximum brake temperature will be lower despite the extra energy. Often, with braking systems, the peak temperature is the limiting factor, so purely from a brake health standpoint, maintaining the slower average speed and braking more often is likely the way to go.