Cycling UK Forum

It's obvious, but I'll say it anyway: every cyclist should practise emergency stops. It's important to get a feel of how far it takes, and how to do it, under varying conditions. It also tests various components. If my brakes are going to fall apart under stress, I want to know about it before I need them.

Yes, perhaps trivial and simplistic. That's me!

There's no black art to bicycle design. They are simple - but not trivial! - and brakes are simple as well.

Any moving object that has to be stopped has to use a mechanical device. A wheeled device derives friction for traction via the wheels and the surface onto which it is moving. In order to stop, the wheels must be stopped from turning.

Momentum wants to keep the object moving. Friction on the surface that the wheels are rolling on has a finite limit as do the brakes. Infinite braking and infinite friction on the surface do not exist. We have to look at this practically.

Lets say we have a four wheeled vehicle. This vehicle is moving forward and the brakes are applied at maximum force. The vehicle will skid to a halt with its weight pushing forward with the rearmost wheels lifting. How much they lift is proportional to where the centre of gravity is.

If the COG is low or to the rear, the rearmost wheels won't lift at all. If the COG is high or at the front, the vehicle may even tip over forwards.

Now we come to a bicycle. Same scenario.

Does the bike tip over forwards?
Does the rear wheel lift?

It depends on the bike, the rider, the skill of the rider, and how the COG moves.

If we're talking physics, we have 2 scenarios:

1) The front brake only applies force F1 so that the rear wheel is just lifting i.e. its best braking ability
2) Both brakes are applied, with front applying force F2 and the rear applying force R2, to maximise their combined braking ability

Assume the same bike and rider position.

F2 can never be greater than F1.

Mick's experiments suggests that F2 + R2 can be greater than F1 i.e. the bike can brake faster with both brakes applied. Sheldon implies that F2 + R2 cannot be greater than F1*.

Someone with more inclination could do some calculations (coefficients of friction, turning moments lifting the rear wheel etc) to see which scenario is best.

Cars need brakes on both front and back wheels because even on dry roads you can lock the front wheels. On a bike the front does not lock, since the bike tips forward before that happens (assuming dry road, good tyres etc).

I will reiterate, however, that experiments are great and provide the ultimate answer, assuming they have been performed correctly.


(*Actually, Sheldon implies that at the limit, F2 + R2 = F1 but you have the disadvantage of a locked rear wheel.)

mark a. wrote:Assume the same bike and rider position.
assuming they have been performed correctly.

mark, i reckon that what you're saying is correct, with the proviso of the two assumptions.

i don't doubt mick's tried his best, but an experienced cyclist would have to override years of learnt behaviour/instinct to not alter their position if they're trying to stop suddenly with the application of front brake only. i'd suggest that it's a lesson you learn at c. age 4, and it's so ingrained that it simply can't be overridden

mark a. wrote:Mick's experiments suggests that F2 + R2 can be greater than F1

Actually, this is probably wrong because the experiment probably had the actual front-only braking force being less than the optimal F1 value, unless Mick is very good at endos.

yep, but sheldon says in theory and in practice.

he may be right, but you'd need an inexperienced/suicidal/masochistic cyclist to do the practice, not someone who'd ridden a bike and doesn't enjoy pain

You're right. I'm sure most of us has had an over-the-bars moment at some point in our lives, and don't want to repeat it!

I use both, because my front isn't powerful enough and because the rear can act as an indicator when reaching the limit (as what andrew_s does).

mark a. wrote:Cars need brakes on both front and back wheels because even on dry roads you can lock the front wheels. On a bike the front does not lock, since the bike tips forward before that happens (assuming dry road, good tyres etc).

That is precisely the difference between bicycles and cars. On a bicycle (dry road etc) you will not get anywhere near tyre grip limits on the front before the effective CofG is ahead of the front hub, and I suspect this is even the case when you only have the normal 40% of weight there. A car has a far longer wheelbase, greater braking capacity so tyre grip can be the weak link. The rear brake pressure adjuster is IMO to limit rear braking under forward load transfer, not to increase the brake rear bias if a vehicle is heavily rear-laden.

(*Actually, Sheldon implies that at the limit, F2 + R2 = F1 but you have the disadvantage of a locked rear wheel.)

That's pretty much it, but if your back wheel is in the air there's no real downside of a locked wheel.

mark a. wrote:If we're talking physics, we have 2 scenarios:

You are ignoring the time between starting to apply the front brake and the rear wheel starting to lift. I apply the rear brake during this time, and this seems to help the deceleration.

I suggest we are all correct, though SB less so. He is (was) a typical expert. What he and other experts say is absolutely correct, no argument, no compromise.

I say it's more complicated than what he says. The F1 R1 etc formula is a great way of looking at this. Thank you!

F1+R1 must be greater than F1 because of dynamics. At the end of the braking sequence, R1 is zero perhaps, but before we get there, R1 is greater than zero.

I learned from a very early age that headers hurt - didn't we all! - but I always shift my weight about on my bike. Braking as well as cornering, so when I brake hard, I shift my weight rearwards and push forward with my arms.

Cycling is a dynamic activity. We don't sit on a saddle like sitting on a car seat. We ride a bicycle. We ride horses too so try just sitting there like a stuffed lump and see how long you can stay on. Brake hard - really hard on a bike - and do nothing else - and you'll be off!

On my flat bar bike I use the back much more but then I tend to only use it with my kids on the back with a tandem tag-along so there is a good amount to weight on the back wheel so it does something whilst not locking up.

On my drop bar bike the back brake isn't great (it's not got a great routing) but does tend to do little to slow down and then it locks. However when I have more weight on the back (touring or towing a trailer) it becomes more useful. So yes I do use both brakes but I'm not sure that it would matter much if I didn't have the back one.

To me the important part of braking and stopping quickly is the weight shift.
If you do not move your weight lower and backwards the center of your mass will be higher and closer to the pivot point, where the front wheel is touching the road.
When you move the weight lower and back the center of mass is obviously lower in relation to the pivot point. If you add pushing the weight back with the feet this lowers it even more.
This I feel reduces the tendency to pivot over the front wheel, as you have effectively lengthened the lever lifting the weight. To pivot on the wheel contact point the weight must be lifted.
If by doing the above weight shift the rear wheel will have more weight on it and not be so inclined to lift.

A simple experiment would show the effect I am on about. Put a weigh in some panniers on the rear then ride down a hill at a set speed. At a known point apply the front and rear brakes and note the distance it takes to stop. Then repeat the above with the panniers on the front the higher the better and note the difference.

hubgearfreak wrote:but you'd need an inexperienced/suicidal/masochistic cyclist to do the practice, not someone who'd ridden a bike and doesn't enjoy pain

Would a suicidal experienced cyclist who is not worried about the pain because he has already said OW do. If so that's me.

I have deliberately ridden down steep twisting hills then shifted the weight forward just before and during fast cornering for the fun of it. It is an interesting thing to do as you soon find out the rear brake does lock the wheel up and lift.
Great fun was had.

Mick F wrote:I suggest we are all correct, though SB less so. He is (was) a typical expert. What he and other experts say is absolutely correct, no argument, no compromise.

I agree with your comment Mick F, the sadly deceased SB is often quoted here as if his opinion is definitive. Although his knowledge was encyclopaedic his opinions on some matters was IMO erroneous. (As one finds errors in most encyclopaedias.)
There is no such thing as an expert just various levels of expertise.

Mick F wrote:I always shift my weight about on my bike.

we all do, unless there's the occasional 4 year old reading this?

which makes any experiments that you might do to gather evidence to either support or disprove sheldon's theory entirely pointless

(Pedantic head on)
No, it's not pointless.

This discussion has a great deal of point, it's helping us all to understand the dynamics of riding - and stopping - a bicycle. SB states that in an ES situation, using the rear brake is pointless.

SB wrote:Conventional Wisdom
Conventional wisdom says to use both brakes at the same time. This is probably good advice for beginners, who have not yet learned to use their brakes skillfully, but if you don't graduate past this stage, you will never be able to stop as short safely as a cyclist who has learned to use the front brake by itself.
Maximum Deceleration--Emergency Stops
The fastest that you can stop any bike of normal wheelbase is to apply the front brake so hard that the rear wheel is just about to lift off the ground. In this situation, the rear wheel cannot contribute to stopping power, since it has no traction.

I take issue with the above statements.

Beginners? Who does he think he's talking to?
Using the front brake by itself? Piffle IMO.

The rear wheel does not contribute to stopping power at the last second I might agree. Before that it contributes quite a bit! So yet again, his Absolute Statement is just plain wrong because there is nothing in that statement about speed or body weight. If he were still alive, I would ask him to come into this discussion to defend himself.