29er braking performance

[droo]

I think you mixing up a few concepts.

 

Force, power and energy are all different.

 

Mechanical leverage is used often, and its a trade off.

 

For example, with a lever you can lift a heavier object. That doesn't mean you suddenly applying more energy. Energy is the same, because you are moving the heavier object a smaller distance (Ep = mxgxh, mass increases but h decrease, energy is conserved)

 

Other examples of leverage are pulley systems, hydraulics, gearing on a bicycle.

 

So in this case, 29er brake force must be higher because the disk is travelling less of a distance (wheel rotation is slower at the same bicycle speed). Energy is still conserved.

 

I studied engineering, so I understand all these concepts. I was just trying to link them, which I've just managed to do. Also, 1998 was a long time ago.

 

W = Fs, which remains constant because on a larger wheel the force is increased due to the larger moment around the axle, but the distance travelled by the rotor is less.

 

Ping.

 

I will sleep better tonight.

Edited September 5, 2014 by droo

[nolipoli]

Whether he was an engineer or not had nothing to do with whether or not he was right, more to do with the ability to do the maths. Engineering dynamics is beyond the understanding of many, including some engineers...

 

I'll accept your reasoning, I'm just trying to reconcile it with conservation of energy, as per the post you quoted. There must be a factor I'm missing...

 

Everything is very interlinked so it is easy to miss the crux of the conservation of energy. Interestingly. Eldron is right on the money with it, but hasn't taken the last step.

 

The disk brake setup simply converts kinetic energy into heat and noise energy. With the same amount of force applied between the pad and the disk (same pull force on the brake lever), but with the pad not covering as much distance on the disc surface (slower rotation because of big wheel), it has less opportunity to generate heat, therefore converting energy slower, and having a reduced effect on the bicycle's speed.

 

Don't get confused between application of force and conservation of energy.

[Captain Fastbastard Mayhem]

Everything is very interlinked so it is easy to miss the crux of the conservation of energy. Interestingly. Eldron is right on the money with it, but hasn't taken the last step.

 

The disk brake setup simply converts kinetic energy into heat and noise energy. With the same amount of force applied between the pad and the disk (same pull force on the brake lever), but with the pad not covering as much distance on the disc surface (slower rotation because of big wheel), it has less opportunity to generate heat, therefore converting energy slower, and having a reduced effect on the bicycle's speed.

 

Don't get confused between application of force and conservation of energy.

 

Exactly. For a given force on the lever, if your disc only rotates 90% of the amount that it would under regular (26") circumstances, there's no way you can get the same amount of total braking force (total work done - joules over a certain timeframe) on the niner as you would on the sixer.

 

Therefore, the only way to get to a stop on a niner as you would on a sixer, given the very same weight, tyre and brake combination, is to increase the size of the rotor so that the total work done is the same across both bikes. Which means your distance travelled on the rotor has to be the same.

 

Short answer - the ratio between disc size & rotor size needs to remain constant between the 2 bikes.

[Captain Fastbastard Mayhem]

Also - that's why a well set up pair of V brakes, or those Magura Hydraulic Rim brakes, work as well as they do, and manage to generate so much stopping power - the amount of material rotating through the pads is far higher than the amount of material moving through a disc caliper.

 

So if your pads and brakes are set up properly, they have 3 times the amount of real estate to facilitate the braking force as you would with a 180mm rotor.