# Downhill Bicycle Braking Question

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• RobRobinette
In summary, according to the "velocity squared" relationship in the energy formula, it may seem that applying the brakes more often at a lower speed would reduce the amount of energy the brakes have to dissipate. However, the total amount of energy dissipated will be the same if you arrive at the bottom with the same final speed. The difference lies in the time frame in which the energy is dissipated, with higher speeds resulting in shorter durations but more intense dissipation, leading to higher peak temperatures. Additionally, wind resistance at higher speeds can also dissipate some energy, but in steep and gravel conditions, this is not a significant factor. Ultimately, it is recommended to prioritize safety and enjoyment while riding, rather than trying to minimize brake heat.
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?

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.

russ_watters and RobRobinette
RobRobinette said:
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.

RobRobinette and Lnewqban
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.

russ_watters, Dale and 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.

RobRobinette said:
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.

Lnewqban
RobRobinette said:
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

diogenesNY
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?

RobRobinette said:
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.

etotheipi and jbriggs444
Thank you Dale, I think you finally got through, that makes sense to me. Again, thank you everyone for the help.

Dale
RobRobinette said:
I ride gravel bikes on steep mountain roads and on the way down I have to ride the brakes pretty hard.
Dale said:
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!)

RobRobinette, Lnewqban and Dale
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.

Lnewqban

## What is the purpose of downhill bicycle braking?

The purpose of downhill bicycle braking is to slow down or stop the bike while riding down a steep slope. This is important for safety and control while navigating through challenging terrain.

## What are the different types of downhill bicycle brakes?

The two main types of downhill bicycle brakes are rim brakes and disc brakes. Rim brakes use pads that press against the rims of the wheels to slow down the bike, while disc brakes use pads that press against a disc attached to the wheel hub.

## How do I know when to use my front or rear brake while descending?

The front brake provides the most stopping power, so it should be used to slow down the bike. However, using only the front brake can cause the bike to flip over. It is important to use both brakes together, with more pressure on the rear brake, to maintain balance and control.

## What are some common mistakes to avoid while braking downhill?

Some common mistakes to avoid while braking downhill include using only the front brake, braking too hard and locking up the wheels, and not anticipating the terrain ahead. These mistakes can lead to loss of control and accidents.

## How can I improve my downhill braking technique?

To improve your downhill braking technique, practice using both brakes together with more pressure on the rear brake. Also, anticipate the terrain ahead and adjust your speed accordingly. It is also helpful to have a proper body position and to keep your weight centered over the bike. Regular maintenance of your brakes is also important for optimal performance.

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