disc brakes

[Tynan]

I had discs briefy on a rather nice Cannondale Bad Boy before it went into very fast into the side of a Freelander.

I liked them, they were always ready to brake, rim brakes need adjustment and performance can vary a lot in the wet, dirty rims etc etc

I did used to get irritated by them picking up whatever and getting noisy until I was told how to clear it out (and indeed that that was the problem)

 

[Truth]

All excellent information as usual , thanks.
I nearly bought a Carrera Crossfire 3 hybrid yesterday in the Halfrauds sale, to replace my Kona Dew Hybrid, and one of the reasons was I fancied the change to disc brakes. I didn't because I spent quite a bit on getting the Kona sorted a couple of months back and want to get some "usage" out of it before changing. After reading this I will definitely go for discs when I am looking to replace my current bike ,whenever that may be.
Thanks again all !

 

[gaz]

Really? Surely the limiting factor is tyre/road friction rather than brake efficiency? I'll grant it's probably complicated, with the initial braking being pad/rim friction, but fractions of a second later when you've scraped the water off the tyre/road must predominate?

a disc brake is smaller in size and has holes in it and bigger brake surface, as such any water clearing is damn quick. A rim is much bigger and takes longer to clear all the water off and even then it isn't 100% gone. Do an emergency stop with rim brakes in the wet and you are going to struggle, do an emergency stop with disc brakes in the wet and you will have no issues coming to a stop.

 

[Tim Hall]

Discs, every time. Wouldn't buy anything else.

Sure, a well set up expensive pair of callipers will match them most (if not all of the time) but you won't touch the disc set up while you'll tinker with the callipers

Nothing to disconnect when you fix a puncture, I've had hydraulics for around two and a half years on the Sirrus and barely had to touch them, mechanicals on the Synapse have been similarly problem free. So far no need to replace pads either. Very good reactions in the wet (agreed I only have old tech callipers to compare them to, but one 'stop, stop, stop' moment approaching a roundabout in the heavy rain was enough for me there),

The clever science is that you take more pressure to lock a disc brake (roughly 1000nm vs 200nm for a calliper) which is where the extra modulation comes in, so you can part brake without causing problems a lot easier.

Definitely try some out before you dismiss them.

A pedant writes: Pressure is force per unit area, so would be N/m^2. Nm is a ,measure of torque (or indeed work - force times distance) (In fact nm stands for nanometres, it's a Capital N for Netwtons). Anyhoo, I think what you're saying is it takes five times as much grunt to lock a disc brake compared to a caliper.

 

[w00hoo_kent]

It is, and I'll put my hand up and say it's from an article read somewhere, I've not done the science :-)

Thanks for your pedantry though.

What I took from it (I read around a bit before plumping for disc brakes first time round, just to make sure I wasn't being charmed by the pretty) was that with a much bigger force needed (agreed, delivered by clever design rather than titanic lever gripping) it was easier to select 20% or 50% or 75% of the braking power. I'm sure a cycling god can hit that 60 N/m^2 spot to get into the ballpark of 30% braking out of their callipers, but I prefer having the five times larger target to aim for.

The only negative I've come across is the possibility of boiling the fluid in a hydraulic system but this is generally thanks to the twin issues of very long descents and weight weenie decisions on the brake set up. Even then if you don't brake like you are using rims, you can get around it. The suggestion was that rims tend to be dragged on long descents if the rider wants to keep their speed down while discs work better if pulsed every now and then so you can't get heat build up. Mechanical discs shouldn't suffer from this (I've not seen any suggestion that glazing is an issue) which I believe is one of the reasons there are so few hydraulic road bike solutions on the market.

But be warned, that's all worth exactly what you paid for it :-)

 

[runner]

I recently bought an equilibrium stainless with rim brakes. The idea of disc brakes on a road bike did not really alter my purchase, yes I know in wet weather they may not grip but they are easy to service and if I follow good advice...ie change my brake pads on a regular basis and use my brake levers properly...ie use the front not the back then I can look after my rims and they work fine...in fact using koolstops they are more than fine they are excellent.

 

[gaz]

Mechanical discs shouldn't suffer from this (I've not seen any suggestion that glazing is an issue) which I believe is one of the reasons there are so few hydraulic road bike solutions on the market.

It's because of the difficulties in getting the hydro system to work with drop bar leavers. with electronic shifting coming into play, i believe all the major players have a system out there or just about to come out.

 

[BrumJim]

Just had discs fitted to my front wheel. Love the improvement in braking performance, although very noisy in the wet at the moment.

 

[Profpointy]

Discs, every time. Wouldn't buy anything else.

snipped.....

The clever science is that you take more pressure to lock a disc brake (roughly 1000nm vs 200nm for a calliper) which is where the extra modulation comes in, so you can part brake without causing problems a lot easier.

Definitely try some out before you dismiss them.

I think you've misunderstood whatever point that is there. "Locking up" is when the braking exceeds the grip provided by the tyres. Same tyres, same amount of slowing down at "locking up". The pressure applied to the disc as opposed to the rim to actually do this, is managed by having the right amount of leverage / mechanical advantage in the mechanical arrangement. And as others have said, Nm is a measure of torque or energy so don't make sense either, nor do nm (nano metres).

 

[w00hoo_kent]

No, I've just explained it badly.

You lock the wheels using less pressure at the rim than at the disc. This means the range 0 to enough to lock is wider with discs than rims, in the region of 5 times wider, which means it's easier to partially brake with discs.

 

[gaz]

No, I've just explained it badly.

You lock the wheels using less pressure at the rim than at the disc. This means the range 0 to enough to lock is wider with discs than rims, in the region of 5 times wider, which means it's easier to partially brake with discs.

In other words. modulation.

 

[Pale Rider]

The only negative I've come across is the possibility of boiling the fluid in a hydraulic system)

If you think about it, that simply cannot happen on a bicycle.

Brake hard on a long descent and the rotor will be ruddy hot.

The calliper won't be anywhere near as hot, and the fluid behind the piston even less so.

Grip the hose as near as you can to the calliper and you will not detect any significant heat.

The fluid in the hose as it runs up to the reservoir won't rise in temperature hardly at all, ditto the reservoir.

Another point is the boiling point of most fluids is significantly higher than water.

The heat needed to boil the fluid is just not present.

 

[Profpointy]

No, I've just explained it badly.

You lock the wheels using less pressure at the rim than at the disc. This means the range 0 to enough to lock is wider with discs than rims, in the region of 5 times wider, which means it's easier to partially brake with discs.

OK, I now understand what you are saying. However I (still) don't think the explanation is actually right, or relevant. What matters is how hard you have to grip the handlebars - if you really had to grip the handlebars five times as hard, then I suggest you'd never manage to stop at all with discs - which is clearly not the case. The mechanical advantage in the system - whether this is leverage in the handlebars, amount of cable pull, or hydraulic equivalent, means you only have to apply a "sensible" amount of force to the handlebars - which is duly "multiplied" if you will, to whatever amount is needed at the rims - or to 5 times this if using discs. Now if I understand the word "modulation" correctly which has all the makings of a jargon word - this simply means how controllable they are. I'd argue this is very little to do with how much force is needed at the point of application (disc versus rim), but more down to other characteristics and general quality of built, amount of play, pad materials and what not.

And anyway, being able to brake just a little bit is hardly an issue with rim brakes - the issue is not being able to brake A LOT especially in the wet.

 

[Profpointy]

If you think about it, that simply cannot happen on a bicycle.

Brake hard on a long descent and the rotor will be ruddy hot.

The calliper won't be anywhere near as hot, and the fluid behind the piston even less so.

Grip the hose as near as you can to the calliper and you will not detect any significant heat.

The fluid in the hose as it runs up to the reservoir won't rise in temperature hardly at all, ditto the reservoir.

Another point is the boiling point of most fluids is significantly higher than water.

The heat needed to boil the fluid is just not present.

You only need localised heat enough to form bubbles inside the caliper to bugger up brakes to near zero. You don't have to boil the whole lot. Bear in mind motorcycle brake discs can get far too hot to touch - above boiling water easiliy from a a couple of quite modest stops and I dare say the weight of caliper is in proportion with the extra weight of bike, so not wholly an invalid comparison. I dare say it's not a huge issue on motorbike nor pedal bike if used reasonably, but I'd not dismiss it quite so glibly

 

[Pale Rider]

You only need localised heat enough to form bubbles inside the caliper to bugger up brakes to near zero. You don't have to boil the whole lot. Bear in mind motorcycle brake discs can get far too hot to touch - above boiling water easiliy from a a couple of quite modest stops and I dare say the weight of caliper is in proportion with the extra weight of bike, so not wholly an invalid comparison. I dare say it's not a huge issue on motorbike nor pedal bike if used reasonably, but I'd not dismiss it quite so glibly

The boiling points of some fluids are in excess of 200 C.

There will be a reason for that which I suspect is because the same stuff is used in more demanding applications.

I've seen the Moto GP racing on TV where the brake disc glows red hot going into a bend - you probably need high boiling point fluid for those brakes.

On t'other hand, a few years ago a lot of downhill mountain bikers used water - there are one or two members on here who can confirm that.

I think the reason was Magura was charging vast amounts of money for Bulls Blood, or it might have been in short supply.

Downhillers could get away with water because the last thing they want to do is brake, so their application is probably less demanding than a roadie going down his nearest 25 per cent descent.

Water is also not too clever in that it promotes corrosion.