Breaking force

[Jdsk]

I never noticed that. But it recently came up in another thread about recumbents and power loss upstream of the driven road wheel.

Jonathan

PS: I miss my KingCycle.

[Mike Sales]

It seemed to me to twist in a spiral sort of way.

[Red Kite]

The force starts with the 'grip' the force at the brake lever. Then it's mainly a question of mechanical advantage. That can be limited by travel required at the brake end and available at the lever end.

Thinking aloud, the pads in a disc brake don't have to travel very far at all so it is possible to design for a relatively high mechanical advantage for a short lever travel - easier than with typical rim brakes where more clearance is probably needed. I think it's probably easier to design for higher brake force with discs, whether it's needed might depend on the friction material.

Actual braking effectiveness (deceleration force) other things being equal is proportionate to the friction coefficient and the force. Pressure, in simple theory for a single condition, doesn't come into it. In practice, an adequate brake contact area is necessary to keep temperatures manageable and give consistency.

This is musing, not knowledge.

[Jdsk]

Thinking aloud, the pads in a disc brake don't have to travel very far at all so it is possible to design for a relatively high mechanical advantage for a short lever travel - easier than with typical rim brakes where more clearance is probably needed. I think it's probably easier to design for higher brake force with discs, whether it's needed might depend on the friction material.

Imperfections in wheel truing can necessitate play in rim brakes.

Jonathan

[Jdsk]

Actual braking effectiveness (deceleration force) other things being equal is proportionate to the friction coefficient and the force. Pressure, in simple theory for a single condition, doesn't come into it. In practice, an adequate brake contact area is necessary to keep temperatures manageable and give consistency.

There are many problems with the "law" of friction. It's better thought of as a rule of thumb.

The coefficient isn't necessarily constant:
https://en.wikipedia.org/wiki/Friction# ... f_friction

Jonathan

[NATURAL ANKLING]

Hi,
So the pad material on both tetra and Shimano were of the same compound Mick?

OP- Simply too many variables to make any type of comparison.

MA equals mechanical advantage.
Mechanical advantage equals load over effort.
In calculations it's normal to exclude friction and of course any type of deflection, collapsing cables and bending brake arms et cetera.
So it is normal to use the VR which is of course velocity ratio.
Velocity ratio equals distance moved by load over distance moved by effort.
It will be common to see MA quoted as equal to VR!

Any quoted MA Even if calculated is theoretical.
Also it must be remembered that velocity ratio varies as the lever comes further back to the handlebar.
There are many variables which affect the velocity ratio of a cable operated caliper brake, Includes brake lever, brake caliper (rim & disc) And deflection of any other moving parts including the cable.
Hydraulic brakes velocity ratio, Will be controlled by lever, ratio of cross-sectional area of master and slave cylinder, deflection of caliper and expansion of brake line.

Therefore :
Any calculated is theoretical MA.
Actual MA is and will have to be measured as a force at his place of origin.

Simply too many variables.

Good lubricated cables and correct set up of brake parts with proper clearance et cetera may have a very dramatic affect on performance of your brakes, along with good choice of pad material of course.

Good Brake modulation or feel, should mean that normal brake operation should be able to be carried out with just two fingers.
If you need your whole hand or the lever comes all the way back to the handlebars without locking the wheel, Then work needs to be done on the brake system ideally.

Agreed it's very surprising on what level of performance of brakes people tolerate.

[Mick F]

So the pad material on both tetra and Shimano were of the same compound Mick?

I tried Campag Record (I have spares) and then tried Koolstop. Both made no difference.

The 105 brakes on now still have the original Shimano blocks - hardly worn since July 2017 - and work beautifully.

[NATURAL ANKLING]

Hi,
I only ever use Shimano pads on Shimano brakes.
Comes from the days of working on motorbikes when all the aftermarket brake shoes were complete and dangerous rubbish.

[MartinC]

Today it's wet and horrible and my mind has wandered to the question of whether anyone has seen a comparison of the actual braking force achieved by rim, v and disk brakes? Just to be clear this is not a x is better than y question, just a question about numbers.

You need to be more precise about what you're asking. Do you mean the braking force at the pad interface, at the tyre/road interface or the deceleration force applied to the bike/rider combination (and before or after these part company) or somewhere else?

All of these quantities will vary with many external factors and the variations may not be linear - they may have abrupt transition points. All these braking systems work by turning kinetic energy into heat so how the system dissipates this heat is a big factor in the process.

Irrespective of whether they're rim, disc or drum brakes the operation method (cable or hydraulic), the pad/shoe material and the design/manufacture/installation/braking technique will make an enormous difference too.

[Spen]

Today it's wet and horrible and my mind has wandered to the question of whether anyone has seen a comparison of the actual braking force achieved by rim, v and disk brakes? Just to be clear this is not a x is better than y question, just a question about numbers.

You need to be more precise about what you're asking. Do you mean the braking force at the pad interface, at the tyre/road interface or the deceleration force applied to the bike/rider combination (and before or after these part company) or somewhere else?

All of these quantities will vary with many external factors and the variations may not be linear - they may have abrupt transition points. All these braking systems work by turning kinetic energy into heat so how the system dissipates this heat is a big factor in the process.

Irrespective of whether they're rim, disc or drum brakes the operation method (cable or hydraulic), the pad/shoe material and the design/manufacture/installation/braking technique will make an enormous difference too.

What I was wondering if anyone had read anything which compared rim, side pull and vee, with disc in terms of how much braking force was applied to the bike, not just the force applied gripping the rim or disk, under controlled conditions. I would imagine that tyre set up etc would have to be consistent to make comparison valid. There are plenty of vids comparing them on the road but there is no way of knowing how reproducible the results would be

[MartinC]

What I was wondering if anyone had read anything which compared rim, side pull and vee, with disc in terms of how much braking force was applied to the bike, not just the force applied gripping the rim or disk, under controlled conditions...........

I haven't. It seems unlikely to me that this is either possible or useful because the braking force applied to the bike will depend on so many other things. In my experience a decent brake of any description can lock a wheel so the type of brake isn't the limiting factor. If you have a strength limitation with your hands then the mechanical advantage delivered at the brake may make a big enough difference to determine the answer.

[mjr]

Hi,
I only ever use Shimano pads on Shimano brakes.
Comes from the days of working on motorbikes when all the aftermarket brake shoes were complete and dangerous rubbish.

Pretty much every Shimano pad I have seen has been awful. Hard and slippy instead of soft and grippy, probably so it lasts a long time and impresses fools and magazine reviewers. Koolstop, BBB Techstop, Clarks, Fibrax: all far better. I have Shimano shoes on some bikes (Fibrax on more) but don't use the pads if I can get any others.

[roubaixtuesday]

Some time back, when I bought my Moulton ........ it had Tektro calliper brakes.
Twice - or was it three times? - descending a steep and long Cornish hill, the front rim overheated and burnt out the inner tube. Inner tubes were scrap after that.

The issue was that the Tektro callipers were too bendy and not giving enough of a powerful grip and therefore dragging.
Changed to Shimano 105 callipers, and even though I've REALY tried through experimentation, the rims don't overheat at all and braking is much much improved.

I know we've been over this at length before Mick, but "bendy" brakes do not change the laws of thermodynamics. The gravitational energy from descent either goes into the rim as heat, or into air resistance. The latter is dependent on speed.

If you descend the hill at the same speed, the characteristics of the brakes cannot make any difference to where the heat goes. All the energy not going to air resistance goes ton the rim. If you descend slower or faster, there will be a change in the balance of where the heat goes, but how "bendy" the brakes are does not matter one iota.

I know you don't agree with this, and I won't be replying further, but the laws of physics are prescriptive.

[MartinC]

I know we've been over this at length before Mick, but "bendy" brakes do not change the laws of thermodynamics.......
If you descend the hill at the same speed..........

I think I agree with you both and the conflict is inherent in the second point. With efficient (stiff in this context) calipers you're able to cadence brake so the heat build up is more rapid but interspersed with periods of cooling. With "bendy" brakes you'll be drag braking for extended periods. The laws of physics don't necessarily say the peak rim temperature will be the same in both scenarios.

[Mick F]

Yes, we had a long thread and discussion about all this.

My original thought was that the rims were black that cause the overheating, and not the brakes.
I changed the callipers as well as changing to shiny silver (non black) rims at the same time.
I was told by the "physicists" on here, that black rims don't get hotter than shiny rims .......... I don't actually know, but my hunch is, that they do.

The consensus with the "bendy" Tektro callipers was that the flexing in them didn't allow for enough force being applied to the blocks and the rim, meaning that they were continually dragging rather than gripping tightly.

I still believe that the black rims were to blame, and that the bendy Tektro only exacerbated the problem.
Six of one, half a dozen of the other.

Whatever ............... all is well, and the rims don't even get warm even on the self-same hills that I had blow-outs.
Same bike, same bloke, same hills, same speeds.