Saturday, September 13, 2008

Pulse Braking

I use the technique of "pulse braking" for keeping a heavily loaded touring bike under control on steep roads without overheating the rim [v-brakes] or overheating the rotor/caliper [disc brakes]. The idea is simple you use one brake firmly at a time alternating between front and back. This allows your braking system to stay cool and keeps the bike's speed in check. Sheldon Brown and Jan Heine both subscribe to this technique and frankly that was good enough for me.

Recently there was an online discussion about the best way to brake on a long mountain descent and the theory behind why the pulse braking technique works was very well explained by Tony Raven. I've pasted in his comments below with permission - thanks Tony!:

"There is a very good physics reason for pulse braking. Heat is lost from the rim by conduction, convection and radiation. Conduction is your enemy because heat is dissipated from the braking surface to other parts of the wheel which heats the tyre up and can lead to the blow out. Convection is heat transfer to the surrounding air and radiation (which is relatively minor here) is the heat you feel when near a hot object.

Conduction happens slowly from the braking surface to the rim to the tyre so it takes time after the braking surface gets hot for the tyre to get hot and blow off. Convection and radiation happen quickly and you want to convect and radiate as much of the braking energy away from the braking surface as you can before it conducts into the body of the rim and on to the tyre.

The rate at which heat is convected away is, to first order, proportional to the temperature rise while the amount of heat radiated away is proportional to the fourth power of the absolute temperature of the braking surface. If you brake long and slow the heat will build up slowly and conduct from the braking surface to the rim and through to the tyre with the braking surface and rim body in quasi thermal equilibrium.

If you instead dissipate the same amount of energy by a short hard burst of braking the rim surface will get very hot - much hotter than the body of the rim - and much more of the energy will be convected and radiated away from the braking surface before it can be conducted to the body of the rim and on to the tyre. So pulse braking if done properly - i.e let the bike run until you are going as fast as you are comfortable with then a short sharp brake to bring the speed way down then let it run again - will convect and radiate a lot more of the braking energy away from the braking surface before it can conduct into the rim body. The net result is a lower rim body temperature and therefore less risk of a blow off.

For the same reason alternate pulse braking also works because it doubles the rim surface temperature of the wheel you are braking over braking on both wheels but it only really works on a tandem because the rear wheel will unweight and tend to skid on a single when you brake.

As a second order effect, letting the bike run and sitting up also dissipates more energy by wind resistance, which goes as the velocity cubed, in the faster bits of the cycle.


Here are few other things to consider about braking on long steeps roads:
  • sitting up and catching as much wind as possible is a good way to reduce speed without adding heat to your braking system
  • disc brakes can over heat so you need to manage their heat load - overheating rotors or calipers can cause permanent damage
  • lowering your tire pressure will raise the amount of heat your rims can absorb before you blow off a tire
  • when pulse braking keep in mind applying a brake firmly requires good traction with the road or you risk skidding a wheel - very bad on the front!
  • applying your brakes lightly, but consistently the whole way down is your worst case scenario for generating heat
  • you can always stop and let your rims or rotors/calipers cool down


Clancy said...

Some friends and I did a 16 mile descent (3600 vertical feet) on coaster brake bikes for fun. Going slow did not work as the hubs heated to a sizzle. We found going fast with few stops worked best. Even the occasional foot on the front tire was used.

Here is a google map/pic of the route.

Hocam said...

Interesting post but this seems to be a topic that generates more debate than practice.

Vik said...

Debating is great, but if you live and ride a heavily loaded bike in the mountains you have to have some strategy to deal with the heat build up from braking. The only brake system that can be used with impunity is a hub mounted drum brake that some tandems are equipped with. The rest of us have to manage the heat going into our brakes or we'll blow off a tire, melt a caliper, warp a rotor or worst of all have a sudden emergency and find our brakes won't stop the bike.

You don't need to use this particular methodology, but you do need to have some plan for braking a heavy bike in the mountains.

Anonymous said...

I went down the mountains in Pennsylvania with a bike loaded to 85 puonds my speedometer pegged 75 MPH at one point. I used constant pressure on the brakes, mostly rear.
Joel Moses