Yet Another Stoopid Question [rear v front brakes]

[Blodwyn Pig]

Olive has TRP Spyre cable discs. Front works fine, but rear is not as poweful. There is a thread on CTC about cable end routing, and it transpires about half leave the bike factories with the cables ends in the wrong place. The correct position would give less mechanical leverage. Basicallythw cable end should sit on the lower/innerside of the bolt,and the cable should sit in the groove. But a lot are fitted to the outer side.
Anyway this got me thinking about external cable length, and cable discs seemingly have a full length outer cable. Now I seem to remember mtbs used cable stops and bare cables to improve efficiency, over 20 years ago, so why the regressive move.

[Morat]

I would suspect that the advantages of exposed cables would soon be reversed after a good dunking in all that beastly mud and oomska.

[ElyDave]

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.

Exactly.

[Veloman]

yes, but apply the rear on its own and it should DO something?

It will do something and if you apply enough it will lock the wheel.

That's why front brake is often dual pivot for maximum braking effect and rear is single pivot to avoid wheel lock.

[Highlander]

Olive has TRP Spyre cable discs. Front works fine, but rear is not as poweful. There is a thread on CTC about cable end routing, and it transpires about half leave the bike factories with the cables ends in the wrong place. The correct position would give less mechanical leverage. Basicallythw cable end should sit on the lower/innerside of the bolt,and the cable should sit in the groove. But a lot are fitted to the outer side.
Anyway this got me thinking about external cable length, and cable discs seemingly have a full length outer cable. Now I seem to remember mtbs used cable stops and bare cables to improve efficiency, over 20 years ago, so why the regressive move.

I would guess bikes intended for the British market are more likely to have full length cables simply because of our climate.

I wouldn't claim to know what is more important, friction or compression, but the cable length is the answer to your question.  You could easily demonstrate this by taking a length of rear brake cable and routing it to the front brake.  But anyone who rides a bike regularly knows instinctively this is going to reduce the braking efficiency. 



 

[Samuel D]

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.

That may be a problem in some cases but rarely is on a high-quality bicycle. Certainly I have never run out of lever travel, though I have on occasion adjusted the cable tension mid-ride to make sure I don’t. The barrel adjusters on rim-brake callipers are conveniently located for this purpose.

That’s not to say squishy brakes aren’t problematic; they are because the hand has its strongest grip at a set distance. Ideally the bite point would be at that amount of lever travel and the lever would not move much more with increasing force, but that’s not the case with most bicycle brakes.

Elasticity in the cable and conduit is not a big contributor to this squishiness anyway, at least with good cables and installation. This can be seen on my bicycle, which has very little difference in lever feel between the front and rear brakes despite a large difference in cable length. New bicycles usually have cheap cable housings and replacing them with high-quality Shimano housings can often improve the brakes to a surprising degree.

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.

All of this can be understood by observation. In contrast, much of what is written about brakes online is flatly at odds with observed reality. This doesn’t bother most people, apparently.

[sizbut]

Whilst the additional cable run to the rear has some effect, if it is significant it will be more likely due to poor setup or maintenance. On a well setup bike it will be minor.

The biggest positive improvement in braking that could be applied on the vast majority of bikes I see is learning to brake on the drops (apology to those who ride straight bars). The difference over braking from the hoods is significant.

Yes, you sometimes just have to brake and if that means braking whilst on the hood then brake and pray. But if you have a choice moving to the drops when you anticipate needing to brake makes a phenomenal difference in both fine control and all out braking power if needed. When hurtling downhill it can seem counter-intuitive to go onto the drops but it gives you so much more power.

And I'll admit that I find it hard at times so my regular compromise is to go on the drop for the front brake and stay on the hood for the rear (since I only use the rear for feathering the speed).

[Highlander]

Whilst the additional cable run to the rear has some effect, if it is significant it will be more likely due to poor setup or maintenance. On a well setup bike it will be minor.

The biggest positive improvement in braking that could be applied on the vast majority of bikes I see is learning to brake on the drops (apology to those who ride straight bars). The difference over braking from the hoods is significant.

Yes, you sometimes just have to brake and if that means braking whilst on the hood then brake and pray. But if you have a choice moving to the drops when you anticipate needing to brake makes a phenomenal difference in both fine control and all out braking power if needed. When hurtling downhill it can seem counter-intuitive to go onto the drops but it gives you so much more power.

And I'll admit that I find it hard at times so my regular compromise is to go on the drop for the front brake and stay on the hood for the rear (since I only use the rear for feathering the speed).

Descending at speed absolutely should be done on the drops for all sorts of reasons.

I once had the pleasure of riding with "super" Sid Barras and in the cafe afterwards we were discussing descending and in particular why he always without fail went "on the hooks" downhill.  Sid explained that early in his racing career he was in a bunch heading downhill at speed just riding with his hands on the brake hoods.  Suddenly his front wheel hit a pothole and the resulting shock knocked him forwards so his hands lost their grip and he ended up chest down on the handlebars at 40mph...   When this conversation took place Sid had retired from racing and though to my eyes he was till a demon descender he reckoned ho went "considerably slower" these days, - but always on the drops.

This was getting on for 20 years ago and in those days a lot of "leisure" cyclists rode routinely on the top of the bars simply because they couldn't actually reach the drops comfortably.  Nowadays with the "sportive" style frame and short reach drop bars there's no reason why anyone should have this problem. 

[Kim]

Anyway this got me thinking about external cable length, and cable discs seemingly have a full length outer cable. Now I seem to remember mtbs used cable stops and bare cables to improve efficiency, over 20 years ago, so why the regressive move.

Because MTB frames are designed for hydraulic disc brakes - which are superior to cable discs for actual mountain biking - and therefore have studs for securing hoses to rather than cable stops.


More generally, and geometry issues aside, I'm not sure why so many people seem to have such rubbish rear brakes.  I don't think mine are particularly worse than the front when properly fettled, though it's hard to tell on the recumbents, as the most braking they get to do is holding the weight of bike + rider statically on steep hills (as all but the lightest application of a recumbent's rear brake when moving locks the wheel).

Admittedly, I don't have any of those curly handlebars and associated levers, and it's unfair to infer anything much from the braking performance of a Brompton (though for reference, both its brakes seem equally 'adequate').

[mzjo]

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.

The continental norm is to have the front brake on the left hand lever but the front still seems to be more powerful.
I would have opted for cable flexing; this certainly was the case on bikes with exposed cables. The fixed that I used as a schoolboy had the most powerful front brake that I have ever known. The lever was in the middle of the bars; a centrepull brake made for a ridiculously short cable run. The difficulty in keeping the rear wheel on the ground probably added to the sensation (and the purists will spot that there was no rear brake for making the comparison but compared with all other front brakes this was the top).
In the modern era of cables routed under tape and through frames I would guess (but cannot prove) that cable drag is the main source of differences in power

[Biggsy]

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.

You can apply the brake just as hard with the bike stopped.  Movement of the bike doesn't increases the friction.  The extra friction from a longer cable is noticeable when pumping the lever with the bike stopped anyway.

[Samuel D]

In fact, you can apply the brake a good deal harder with the bicycle stopped.

When you brake with the wheels rolling, the pads are not only compressed against the rim but dragged around with it. (You can plainly see this under heavy braking: the arms of the front calliper bend forward.) Only a reaction force in the arm, transmitted to the pivot, prevents the pads from spinning around with the rim. This reaction load introduces its own friction (stiction) at the pivot, and it turns out to be significant.

Understanding this demystifies otherwise inexplicable differences in performance between long- and short-drop callipers, single- and dual-pivot callipers, callipers and V-brakes, etc. The greater the distance between pad and pivot, the greater the reaction load in the pivot and thus friction.

This stiction increases with pad force until it eventually prevents you from increasing the pad force on the rim no matter how hard you pull the lever.

The extra friction from a longer cable is noticeable when pumping the lever with the bike stopped anyway.

It may be noticeable but its magnitude is not intuitively understood by pumping the lever through the pad-clearance range. Friction in the cable is proportional to cable tension (and since levers have their own return springs, cable tension in the clearance zone is even lower than suggested by the already low lever pressure there). When the pad hits the rim, cable tension and thus friction increase, absorbing more of your lever force. If when braking you pull the lever 20× harder than you do in the pad-clearance zone, then cable friction is 20× greater than that which you feel when playing with the brake lever.

[Biggsy]

I'm talking about pumping the lever HARD.  Then cable friction is the same whether the bike is moving or not.  The rim dragging the pads happens with the front brake as well.  It may happen more with longer calliper arms, but the discussion is about when essentially the same brakes are used at the front and rear.