Upgrading Brakes - advice please

[GrahamNR17]

....... as I said .....

I spent ages talking to a chap the other evening about braking. Rim versus Hub ............

A wheel rotates on an axle.
The bottom of the wheel is stuck (hopefully) to the road and therefore is at Speed Zero.
The axle is moving forward at a certain speed - 10mph?
The top of the wheel is moving at double that speed - 20mph.

Think of any wheel of any diameter and the above is true:
The top of the wheel is moving at double the speed of the axle
So, for any given speed, the top of the wheel will be double. Therefore a rim brake is as efficient on any diameter wheel.

As for spokes, if you have a small wheel, your spokes are shorter, so are less affected by ovalisation or stretch.

[MikewsMITH2]

I hate to disagree Mick but the following truths are self evident:
1. If your chopper is travelling at 10MPH the bottom of the wheel is travelling at zero MPH relative to the road, provided it is not skidding, because it is in contact with the road.
2. The top of the wheel is therefore travelling at 10MPH relative to the road.
3. Relative to the bike, the tyre is travelling at the same speed as the bike is travelling aloong the road, so the rim is travelling a little slower

None of this alters the fact that rim brakes function equally well whatever the size of the wheel because of (3)
For hub brakes the torque is greater at the hub for larger wheels. The friction provided by the brake is dependent uopn the coefficient of friction multiplied by the force applied by the lever, so the barkes will initially feel sharper, but the efficiency of the brakes is ultimately dependent on it's ability to dissipate energy in the form of heat, so going down a long hill the brake should perform equally well whatever the wheel size. It is like having power assistance on weedy brakes on a car.They wil feel great at slow speeds but will not perform when braking from high speeds.

[stoobs]

I hate to disagree Mick but the following truths are self evident:

2. The top of the wheel is therefore travelling at 10MPH relative to the road.

Oh dear, this needs more work. There's no therefore about it, please stay after class:

http://www.animations.physics.unsw.edu. ... olling.htm

[Mick F]

Hi Mike.
If you are travelling at 10mph, your frame is doing that speed, consequently your axles are too.

The top of the wheel MUST be going faster than that so it can go round! In fact it is moving at double the travelling along speed.

In fact, the wheel rotates of course, but as it stationary at the bottom, any single point on the rim has to accelerate from zero to double the speed of travelling along to be able to stop again at the bottom.

Than you, Stoobs, for the link.

[Mick F]

Done it!

Out for a test ride this afternoon!

A quick test showed that the brake lever wasn't man enough for the rollerbrake, so I used an old MTB one. Whilst I was at it, I fitted one for the rear too.

Initial checks and tests are VERY optimistic. This front brake is going to be phenomenal!


(Been working in the kitchen, as it's a bit chilly outside!)

[MikewsMITH2]

The top of the wheel MUST be going faster than that so it can go round! In fact it is moving at double the travelling along speed.

You are confusing speed with velocity. The speed of the surface of the tyre is exactly the same as the speed of the bike over the road. The difference in velocity of diametrically opposed portions of the spinning wheel is indeed 20MPH this is because the top of the tyre is doing 10MPH forwards and the bottom is doing 10MPH backwards. This doesn't alter the speed of the tyre going round. So your brakes are coping with a rim speed of 10MPH. (not 20MPH) which was the root of the question.

It's all to do with relativity; velocity is relative, speed isn't. It's just academic semantics reallt, just don't run away with the idea that the brake rim is travelling at a peripheral speed of 20MPH.

[Mick F]

I think we're saying the same thing, but differently!
Anyway, although I agree with you and understand fully what you're saying, you may not be understanding me.

I maintain that Speed is "speed over the ground".

The bottom of the wheel is stationary, and the top of the wheel is moving at double the Speed of axle.

This is shown very well by the ancient Egyptians moving heavy blocks of stone on rollers. As the block moves forward, the rollers come out the back, as they are moving at half the speed of the block.

[MikewsMITH2]

Yes I agree. Relative to an observer on the road, but not to the rider. To the rider (and the brake caliper) the wheel is whizzing past the fork crown at the same speed the as road is whizzing past the BB. As I said its relativity - blame Einstein.

However, the relativity of this to the differences in braking performance of hub and rim brakes on small and large wheels is somewhat tenuous. I trust that you are going to perform some braking tests and report back. Otherwise I'm going to write to Mythbusters!

[GrahamNR17]

Isn't cheese getting expensive?!

[random37]

Disc tabs and a disc hub might be OK, but I think you might struggle to fit the disc tab. I think there's adaptors, but don't they all need straight bladed forks?

I really think a hub brake has to be the way to go.

[Mick F]

Sorry, I should have given a ride report!

I went out briefly, and gingerly! for a couple of miles. I deliberately chose a hilly ride.

The final leg was down a 1in4 hill.
The roller brake was hot and smelly by the time I'd done a few "Emergency Stops" down there, but it worked!!

Hopefully, I'll be out for a longer test-ride later today.