Braking

[AndyB]

Okay, so essentially a force is being applied towards the bike (from front or rear braking). Since no force is being applied to the rider, if he/she is to avoid continuing at the same speed (and parting company from the bike!) an equal force must be applied to him/her. This can only come from the rider bracing against the handlebars, which applies force onto the front wheel of the bike. No?

Having thought about this some more, I'm utterly convinced. If I may rephrase it. The retarding force is at the tyre/road surface. This is below the centre of gravity of the bike/rider, so will cause the bike/rider to start to rotate. This is opposed by force up on the front wheel: this wheel is more heavily weighted and may therefore have a greater braking effect (since friction between two surfaces is proportional to the force pushing them together). Simple mechanics.

All the usual caveats about surfaces, as per Sheldon...

[Mick F]

Yes, I believe you're right, but SB wants no rear braking.

My point is that BOTH is better than front only.

[meic]

Yes you are right that the majority of the vertical force while braking (with either brake) will be on the front wheel and for that reason the front brake will always have a greater effect.
The point I was making is that by using a rear brake you can have some of the vertical force being applied through the rear wheel, to the extreme case that if you only use the rear brake you could not easily generate enough turning force to do a cartwheel.
In physics you have taken into account the direct linear forces in your description but failed to consider the rotational forces about the front and rear axles. Also that we can and do apply forces through our feet and bums.
I think that we are arguing about different degrees of braking (which is my fault) as you and Sheldon are talking about the limiting case where enough braking is applied on the front wheel to make the rear wheel ineffective. I personally believe that that case is rarely achievable with average brakes, tyres, roads and riders.
Now Mick F and I probably dont have V brakes that can flip you over the bars in an instant. With our much lesser brakes there is plenty of room for the back brakes to do some work.
Undeniably the best braking is from a powerfull front brake and a rider hanging of the back of the bike. Most of us would come a cropper if we tried that however.

[Auchmill]

Have you considered momentum? The problem with front wheel braking is that if the front wheel is locked up then if there is sufficient momentum the bike becomes like a lever rotating around a fulcrum and you go over the handlebars. No amount of rear wheel braking force can cause this to happen.

[AndyB]

In physics you have taken into account the direct linear forces in your description but failed to consider the rotational forces about the front and rear axles. Also that we can and do apply forces through our feet and bums.

Erm... I did - I said that the bike/rider will start to rotate, which can be stopped by bracing against the handlebars.

I think that we are arguing about different degrees of braking (which is my fault) as you and Sheldon are talking about the limiting case where enough braking is applied on the front wheel to make the rear wheel ineffective. I personally believe that that case is rarely achievable with average brakes, tyres, roads and riders.

In that case, why all the worry about cartwheels? All Sheldon is saying (and I agree) is that there is no need to use the rear brake unless you are unable to get sufficient braking from the front brakle due to it slipping.

Now Mick F and I probably dont have V brakes that can flip you over the bars in an instant. With our much lesser brakes there is plenty of room for the back brakes to do some work.
Undeniably the best braking is from a powerfull front brake and a rider hanging of the back of the bike. Most of us would come a cropper if we tried that however.

I hope you're not suggesting my Mafac Racers are "lesser"!

[Auchmill]

In physics you have taken into account the direct linear forces in your description but failed to consider the rotational forces about the front and rear axles. Also that we can and do apply forces through our feet and bums.

Erm... I did - I said that the bike/rider will start to rotate, which can be stopped by bracing against the handlebars.

Sorry, can't see that happening if the front wheel is locked or the braking force is such that the momentum of bike and rider causes unavoidable rotation. The bike and rider are "one machine" and only by shifting the centre of gravity backwards, as previously suggested, can rotation be counteracted if braking is not released.

[AndyB]

Sorry, can't see that happening if the front wheel is locked or the braking force is such that the momentum of bike and rider causes unavoidable rotation. The bike and rider are "one machine" and only by shifting the centre of gravity backwards, as previously suggested, can rotation be counteracted if braking is not released.

In those circumstances, I'd agree. The rotation is precisely why it is better to brake on the front wheel (as I explained above), and going over the top can only be avoided by bracing and/or modulating the braking. But locking the front wheel and going over the handlebars is very unlikely for an experienced rider. The other circumstances for front wheel lock are going to be lack of friction, which would cause a front wheel skid (obviously a Bad Thing). I still say Sheldon's article is correct - the physics supports it. Also, contrary to the impression you might get from some posts here, he doesn't say never use the back brake, just that for skilled users in good conditions the front is sufficient (and better). It will take some good physics to convince me he's wrong.

[Mick F]

It will take some good physics to convince me he's wrong.

Prove it!

I've just back from a ride with a couple of fast descents. Rear Only doesn't provide much retardation, Front Only is quite good. Both is marvelous.

Speed MUST come into the equation.

[AndyB]

If the front brake is not sufficiently powerful to lift the rear wheel, the rear brake can help, but the best thing to do is to repair the front brake.

Typical rim brakes lose a great deal of their effectiveness in rainy conditions, so using them both together can reduce stopping distances.

It sounds like a case of the former to me Mick, unless you're not enjoying the beautiful sunny day we're having up here. If only I could get out for a ride...

I just cannot see how he is wrong. The only point I would concede, is that his 95% figure (amount of time experienced cyclists need use the front brake only) is too high - maybe he is not taking account of the amount of bad weather we have here, or heavily laden bikes, or poor surfaces. The principles of what he says are sound though.

[Mick F]

I agree that in PRINCIPLE he's right, but not in practice.

But in his statement about 95% of experienced cyclists, he's wrong! 95% use BOTH brakes IMHO.

It's patently obvious that the front brake is far more effective than the rear. But that doesn't mean that the rear can't contribute. It MUST!

I know that when a wheel skids, it has less grip than if it's rolling. Good braking is using both brakes and "feeling" what is going on. When the technology gets round to it, bikes will have ABS.

The bike and rider are "one", not just coz the rider is sitting on the thing, but because the rider is part of the whole and is an intelligent being(?) controlling his steed, he "knows" the road and the conditions, predicting the results of anything he does.

You can break down the physics of riding and stopping a bike, but there's more going on than can be studied with simple equations.

As I keep saying: Carry out your own practical experiments, and report back!

[AndyB]

But that doesn't mean that the rear can't contribute. It MUST!

I agree with everything you say, except this! In good conditions, and with good brakes, the rear brake should not contribute at all. Are you disagreeing with Sheldon when he says what happens at the limit of braking, or are you saying that that limit is never reached?

I'd love to get out and experiment, but with work and all... and I don't think these are the sort of tests to be done with my son on the back!

[Mick F]

Sheldon when he says what happens at the limit of braking, or are you saying that that limit is never reached?

In practice, the limit is never reached, I suppose. Maybe that's what I'm going on about. If you're braking that hard, you may as well jump off! Actually, I don't think my front brake plus my effort could exert enough force to throw me over the bars. SB suggests that I should fix my brake! Ha Ha, have a word with the designers! Dual Pivot brakes are as good as they get on a standard road bike. When, or if, I have to brake hard, I always throw my weight rearwards, getting out the saddle and stopping the rear from lifting, thereby allowing grip for the rear wheel. Man and machine in perfect harmony.

I'd love to get out and experiment, but with work and all... and I don't think these are the sort of tests to be done with my son on the back!

Perhaps not!
But then again, he could hold the chalk, the notepad and pen, and the tape measure!

[Velo]

Sheldon when he says what happens at the limit of braking, or are you saying that that limit is never reached?

In practice, the limit is never reached, I suppose. Maybe that's what I'm going on about. If you're braking that hard, you may as well jump off! Actually, I don't think my front brake plus my effort could exert enough force to throw me over the bars. SB suggests that I should fix my brake! Ha Ha, have a word with the designers! Dual Pivot brakes are as good as they get on a standard road bike. When, or if, I have to brake hard, I always throw my weight rearwards, getting out the saddle and stopping the rear from lifting, thereby allowing grip for the rear wheel. Man and machine in perfect harmony.

If your DP brakes cannot throw you over the bars, they are not properly set-up!

Putting more weight rearwards helps a little in emergency braking, but better to brace your arms body to absorb the braking forces.

[AndyB]

I'd love to get out and experiment, but with work and all... and I don't think these are the sort of tests to be done with my son on the back!

Perhaps not! But then again, he could hold the chalk, the notepad and pen, and the tape measure!

And the first aid kit for when I go over the top!

[stoobs]

Seems to me that the real test (cos I'm too chicken to try it myself) is to have someone who is an endo specialist do it. If you're not doing an endo, then you have to be one of the two-brake brigade, because there will be a little friction to be had, however small.

On the other hand, an endo is the limiting case. It's just not far from doing an endo to doing a faceplant, no?

Mortals do tend to use 2 brakes for modulation and a bit of control, but SB's assertion is based on good physics, and great riding skills.