Yet Another Stoopid Question [rear v front brakes]

[Blodwyn Pig]

why are front brakes seemingly more powerful than rears, given that same brake fitted to both. Is it outer cable length/compression?

[Pancho]

When you yank the brakes, your mass piles forward over the front wheel? Giving more wheel/tarmac grip and stopping power?

[Blodwyn Pig]

yes, but apply the rear on its own and it should DO something? Sometimes tandems and trikes have 2 front brakes, but the rear never locks up, why is that , espescially if it becomes unweighted. Cars have brake balance gizmos, but bikes just dont seem to cut the mustard. I think its the outer cable length.

[Samuel D]

Compression itself should not reduce braking force (but it will require more work to be done – not force but work – by the hand at the lever, delaying the onset of peak braking and possibly tiring the hand more; plus, the lever will often end up closer to the bar where getting a good handful of lever can be more difficult).

To the limited extent that your observation is true, it’s caused by greater friction in the longer cable run to the rear brake. Friction has an enormous effect on braking force. The impression you get of energy wasted to friction when pumping the brake lever with the bicycle stopped is not accurate, because while braking heavily the friction in the cable, calliper, and even lever all increase tremendously.

[Polar Bear]

Roll backwards on the bike and test the braking.   A test is required.

[ACyclingRooster]

why are front brakes seemingly more powerful than rears, given that same brake fitted to both. Is it outer cable length/compression?

Hi Blodwyn Pig. I personal think that a lot of the effect/impression had to do with the fact that most cyclist are right handed and the brake lever is mounted on that side of the bars.
The leverage/fulcrum point of the lever/cable is also instrumental in the stopping power just as the fulcrum point of a car hand-brake either makes it easy to apply and also has a great-deal to do with the efficiency of the hand-brake in the first instance.
Also there is less cable distance and therefore less cable to be affected by stretching and therefore the power is increased also.

[Blodwyn Pig]

yes hydraulic discs are seemingly unaffected.

[Morat]

Locking up the rear on a tandem is difficult because there's so much more weight on the rear tyre with relatively little weight transfer due to the long wheelbase. Cable length won't help, but decently equipped tandems will have very high quality cables to help with efficiency.
I'd have to check with the captain but the rear brake is generally the principal stopper on a tandem.

[Highlander]

It is indeed cable friction and compression.

Even with really effective rear braking it's quite hard to lock up the back wheel because the weight distribution on a bike even under braking keeps enough weight on the back wheel (not like most (modern) cars).   By way of  comparison, it is easy to create a slide on a recumbent trike where there is relatively little weight on the back wheel that's why tadpoles don't tend to have rear wheel brakes.  Locking the back wheel on a tadpole is about the only way you can roll one.

Interestingly though, I learned to ride on a bike with rod operated brakes and seem to recall finding it easy to create a back wheel skid, maybe those brakes were really efficient compared with cables.  It would be interesting to hear whether users of hydraulic brakes can do the same.

[Samuel D]

It is indeed cable friction and compression.

Can you explain how compression reduces braking force? Not trying to start an argument. I’ve just never heard a satisfactory explanation.

Even with really effective rear braking it's quite hard to lock up the back wheel because the weight distribution on a bike even under braking keeps enough weight on the back wheel (not like most (modern) cars).

This depends on exactly what you mean by “quite hard”, I suppose, and additionally how hard you’re braking with the front brake and how quickly you apply the rear brake (this governs weight transfer to a degree), not to mention tyre friction on the road surface and the weight on your rear wheel. I can manage it on a dry road with no front braking and a single finger operating the rear brake, albeit indeed pulling quite hard with that finger in the hook of the lever. Shimano BL-R400 levers connected to Shimano BR-R650 callipers with Shimano cables and outers and Kool-Stop red brake pads operating on aluminium rims.

[sizbut]

Rubbish, you're just not fat/stoat/heavy enough. I can easily get the rear wheel on my bikes to start to skid. Its is as the first reply stated due to the momentum of the large mass (me) going forwards. Certainly can't risk applying as much force through the rear as through the front.

[hellymedic]

I think I agree with Pancho's answer.
If you brace your arms and shift your weight as far back as you can, the rear brakes should have more effect.
But the front provides most stopping IME.

[Jakob W]

The difference in effectiveness is mainly due to weight transfer under braking giving higher traction on the front. Assuming the front tyre doesn't lose traction you can brake as sharply using just the front as with both brakes. Both my town bike and my tourer have fairly indifferent brakes (rollerbrakes and Weinmann centrepulls respectively), but I can lock up the back on either with no difficulty.

[Pickled Onion]

hellymedic has it right.

It's simple physics. In conditions where there is sufficient friction between the road and the tyre, the maximum possible stopping power is when the rear wheel is just about to lift off the ground. At that point there is zero friction on the rear tyre so the rear brake is useless.

If the front wheel is going to slide before the back wheel lifts - ice/recumbents/tandems - then it gets more complicated, but the front is always going to give the majority of the stopping power because that's where the centre of gravity goes towards.

[Samuel D]

Blodwyn Pig’s original question was about power.

Obviously the front brake is vastly more effective at stopping the bicycle, since under maximum braking all the weight is on the front wheel, but that’s a different question (with an obvious answer).

If you brace your arms and shift your weight as far back as you can, the rear brakes should have more effect.

The same applies to the front brake, traction permitting (it usually does).

[sojournermike]

Agree with the later posts - locking the back wheel is a trivial party trick with hydraulic discs and easy enough with cables and calipers. Adding some front brake increases weight transfer, reducing rear loading and making it easier still


I read once that by braking gently with  rear and hard with the  front at the same time, you can use the onset of the rear slide to determine the point of maximum braking before the front locks. Not sure I have the skill set to deal with that in a situation where emergency hard braking becomes necessary.

[Samuel D]

On the topic of which brake is more effective (which I still consider a separate discussion, but maybe that’s what Blodwyn Pig really meant), Jan Heine did a useful brake experiment. Worth reading.

[ElyDave]

I've managed to lock up the rear on the recumbent on a number of occaisions, seems more likely as I come in for a stop and sit up, affective the weight distribution.

On cable operated brakes, I suspect that cable stretching has a lot to do with apparent brake effectiveness

[Ruthie]

The front cable is much, much shorter.  You need more force to tighten a longer cable, and more tension is lost along its length.

[Samuel D]

On cable operated brakes, I suspect that cable stretching has a lot to do with apparent brake effectiveness

Nonetheless, it doesn’t. Cables practically stretch not at all. Housing compression happens to a very minor degree, though there can be some movement where it seats at its ends (especially in a new installation) and loose, curved housing moves under tension to seek a shorter path (the extent of this path-shortening is far less than you might guess from the distance the housing moves laterally).

Most brake squishiness comes from metal bending at the calliper, plastic (where present instead of metal) bending in the lever mount, and pads getting squished, in about that order. Close observation will confirm all of this.

None of this squishiness reduces braking force, strictly speaking. If it did, someone would be able to explain why.

You need more force to tighten a longer cable, and more tension is lost along its length.

True, but only because of friction. Friction is the enemy, which is why hydraulic brakes feel stronger despite typically having more pad clearance, i.e. lower mechanical advantage.

[ElyDave]

Friction will not reduce the braking effort, only increase the apparent force you need to exert at the lever and we are only talking static friction here, dynamic must be half a gnats fart over the distance moved. Flexion as you cite, and stretching as I suggest must logically dominate.

[Samuel D]

Not so.

Imagine lifting a 1 kg object with, respectively, a piece of string and an elastic band. In both cases you will need exactly 1 kg of force to support the 1 kg load, but with the elastic band you will have to raise your hand farther before the object will lift.

Identically, things bending and stretching from lever to calliper do not reduce force at the pads but merely require your hand to pull more lever (cable).

Friction will not reduce the braking effort, only increase the apparent force you need to exert at the lever

Eh? A more powerful brake is precisely one that causes more retardation for a given force at the lever.

and we are only talking static friction here, dynamic must be half a gnats fart over the distance moved.

Not sure what you mean by that, but friction is the dominating factor in braking force whether you like it or not. Anyone who’s taken a rusty old bike out of a barn and tried the brakes can attest to that. Hydraulic brakes make it obvious.

There are some subtle friction effects that differ by brake type, notably the friction in the calliper pivots caused by the reaction load from the pads’ tendency to be dragged around with the rim during braking. This friction increases with pivot-to-pad distance (hence the power of V-brakes and the weakness of old, long-drop BMX callipers though the latter were made usable by small wheels) and with braking force, ultimately becoming the limiting factor in how hard the pads can be applied (i.e. eventually an increase in lever force results in no increase in pad force, stiction having locked the brake in position).

[Karla]

Imagine lifting a 1 kg object with, respectively, a piece of string and an elastic band. In both cases you will need exactly 1 kg of force to support the 1 kg load, but with the elastic band you will have to raise your hand farther before the object will lift.

The issue with bike brakes is that if the braking system has a lot of elasticity, the brake lever will hit the handlebar before you ever manage to apply as much force as your hand can manage, i.e. before you ever lift that 1kg mass off the ground.

Recall Hooke's law: F=kX (force applied = spring constant * extension)
Two equal springs (i.e. cables) in series have half the spring constant.
X_max is the X that will cause the lever to hit the bars.  F_max = kX_max
A spring twice as long will have k' = k/2.  F_max' = k'X_max
If our spring (cable) is twice as long, the maximum force that can be applied through that cable without maxing the lever travel is halved.
In reality, F_max won't actually be halved, because as you say, the remaining elasticity in the braking system, which is provided by e.g. the calipers, remains constant. 

Friction in the brake cables will also have an effect, even once you've applied the brakes and are holding them static in the 'on' position, because friction at the other end of the brake cable will require you to apply force to stretch the cable between you and the 'centre of friction' before you can move any wire beyond it. Even once you've applied a frictionate brake, you'll have to apply force to keep the cable taut to stop it slipping back - even if it wouldn't slip back immediately, but along a slow hysteresis curve.

The hysteresis effect will itself make it harder to control a brake with a longer and/or more frictionate cable: if the brake doesn't give a direct linear pull response to finger pressure, you won't be confident in how to use it under high load, and will stick to braking gingerly.

[contango]

hellymedic has it right.

It's simple physics. In conditions where there is sufficient friction between the road and the tyre, the maximum possible stopping power is when the rear wheel is just about to lift off the ground. At that point there is zero friction on the rear tyre so the rear brake is useless.

If the front wheel is going to slide before the back wheel lifts - ice/recumbents/tandems - then it gets more complicated, but the front is always going to give the majority of the stopping power because that's where the centre of gravity goes towards.

A simple test I did a while back (and I can't for the life of me remember what prompted it) was to stand beside the bike, apply the front brake and try to roll the bike forward. Then do the same but applying the rear brake. I found that applying the front brake stopped the bike moving, but with the rear brake on the bike would slide forward. Then I did the same thing but trying to roll the bike backwards, and found that with the back brake on the bike wouldn't roll but with the front brake on it would slide.

So the front brake appears to have more stopping power, where "front" is relative to the direction of travel rather than which end has the handlebars. It doesn't explain why though.

[Jakob W]

Weight transfer again - or rather, the height of the bike's CG above the tyre contact patch sets up a couple; if braking with the 'forward' wheel, it sets up a downforce on the wheel; if braking with the 'backward' wheel it reduces the downforce enough to cause a skid.

Trying to separate brake power from effectiveness seems to me to try and count the angels dancing on a cable end; given that most of us here seem to be able to lock up the back under normal circumstances, the force at the rear calliper is surely 'more than enough'?