Why have disc brakes become so popular?

[[XAP]Bob]

Nah, it’s all in here...

And discussions of heat are tangentially relevant

[Brucey]

re heat; disc brakes don't always cope very well. Two instances spring to mind

1) a tourist in the south downs found that they managed to heat the (mechanically actuated) discs so much that they went blue.... and
2) a journalist riding a twisty-turny 1000' descent managed to boil up his hydraulic brakes. He lost his brakes, lost control, and ended up in hospital.


When disc brakes were still quite a new idea for road-going bicycles a standardised test was devised. This test was based on a real road, a real descent. When no extant disc brake could pass the test, instead of making better brakes, they made the test half as hard.

Now all brakes have their failure modes and characteristics when they get too hot. For example most rim brakes start to get noisy in a very obvious fashion, so you know to back off and give them a rest. Hub brakes (SA , Arai, Sachs, SRAM etc) tend to 'go away' and then if you ignore that you get smoke coming out of them. Again this is quite obvious and once they have cooled down they work normally again.

But disc brakes don't behave in a similar fashion; there may be little warning if the discs are getting too hot and there is certainly no warning before a boil-up; one pull of the brakes you have a brake, and the next pull you don't.

Now 99% of riders are not going to get their brakes hot on a regular basis, but that is perhaps scant consolation for the other 1% who do. Shimano have made efforts beyond the usual 'fit a bigger disc then' approach with their sandwich rotors and so forth but these come with other compromises too.

cheers

[busb]

A browse around a LBS in Reading this afternoon would suggest the answer to the original question is: they haven’t! Well, not on road bikes anyway. As for MTBs, we didn’t see any with cable actuated discs. I’d estimate that over 75% of the drop-bar bikes had rim brakes with a couple having V brakes mounted behind the forks. I am surprised that more so-called racing bikes don’t use V brakes instead of dual-pivot side pulls, especially as the benefits of aerodynamics are being touted over pure lightness.

[Bmblbzzz]

V-brakes are not compatible with the pull ratio of most drop-bar brake levers. You make a good point that in fact disc brakes, despite the noise about them, have not yet really taken off for road bikes. I guess (some) people feel they have because they're a newish thing; but it's the new appearance they're noticing, not the numbers.

Also interesting that you saw no MTBs with cable discs; presumably they were all hydro (and maybe Vs on the very cheapest)? It seems possible that road bikes might largely (obviously not entirely) skip the cable-disc stage and hydros will end up becoming the standard on all bikes. Maybe; we'll have to wait and see how it pans out.

[amediasatex]

Different bike shop different stock, I’ve just come back from mine where all but the cheapest (and poshest) road bikes on display are disc, probably 75% or more were disc, and no MTB with rim brakes >80% were hydro disc.

One bike shop is a very limited sample
Set and tells you more about the market that shop caters to than the market in general.

[busb]

... It seems possible that road bikes might largely (obviously not entirely) skip the cable-disc stage and hydros will end up becoming the standard on all bikes. Maybe; we'll have to wait and see how it pans out.

My observations of the last couple of years in many shops was that most budget to mid-range drop-bar bikes with disc brakes were using cable operation - probably because cheaper hydro levers/shifters just haven't been available. Shimano's 105 groupset includes hydro with Ultegra & Dura-Ace no longer sharing hydro parts.
Although most low/mid MTBs & upwards use hydro, I don't think it's inevitable that rim brakes will disappear so quickly. Sure, different shops carry varying stock but roadies are traditionally more cautious regarding new tech than MTBers or CXers.

[Peter W]

What brake fluid do Formula 1 cars use?

Why do their hydraulic brakes not suddenly fail from overheating when their carbon fibre discs glow absolutely red hot during successive multi G force slowings from very silly high speed?

[Bmblbzzz]

... It seems possible that road bikes might largely (obviously not entirely) skip the cable-disc stage and hydros will end up becoming the standard on all bikes. Maybe; we'll have to wait and see how it pans out.

My observations of the last couple of years in many shops was that most budget to mid-range drop-bar bikes with disc brakes were using cable operation - probably because cheaper hydro levers/shifters just haven't been available. Shimano's 105 groupset includes hydro with Ultegra & Dura-Ace no longer sharing hydro parts.
Although most low/mid MTBs & upwards use hydro, I don't think it's inevitable that rim brakes will disappear so quickly. Sure, different shops carry varying stock but roadies are traditionally more cautious regarding new tech than MTBers or CXers.

Agree with previous points on sample size. As for roadies, yes; perhaps I should have made it clear I meant hydros becoming standard on disc-braked bikes, ie cable discs being a minority. But it's not just technological conservatism that will keep roadies on rim brakes; the advantages of disc brakes are not so beneficial to racing as to touring, leisure riding and off-road.

[Brucey]

What brake fluid do Formula 1 cars use?

Why do their hydraulic brakes not suddenly fail from overheating when their carbon fibre discs glow absolutely red hot during successive multi G force slowings from very silly high speed?

A synthetic version of something like DOT fluid is used. They have a strong (up to 200mph) breeze to keep the brakes cool enough, and the ducts are arranged so that if the discs are happy, so are the calipers.

You could have ducts on a bicycle brake, but the brakes are already sources of parasitic aero drag....

cheers

[PaulCS]

Hi
In the dry rim brakes are as good as hydraulic disc brakes but not so in the wet. Put simply I just feel safer with the latter! Its a good enough reason for me.

[busb]

... It seems possible that road bikes might largely (obviously not entirely) skip the cable-disc stage and hydros will end up becoming the standard on all bikes. Maybe; we'll have to wait and see how it pans out.

My observations of the last couple of years in many shops was that most budget to mid-range drop-bar bikes with disc brakes were using cable operation - probably because cheaper hydro levers/shifters just haven't been available. Shimano's 105 groupset includes hydro with Ultegra & Dura-Ace no longer sharing hydro parts.
Although most low/mid MTBs & upwards use hydro, I don't think it's inevitable that rim brakes will disappear so quickly. Sure, different shops carry varying stock but roadies are traditionally more cautious regarding new tech than MTBers or CXers.

Agree with previous points on sample size. As for roadies, yes; perhaps I should have made it clear I meant hydros becoming standard on disc-braked bikes, ie cable discs being a minority. But it's not just technological conservatism that will keep roadies on rim brakes; the advantages of disc brakes are not so beneficial to racing as to touring, leisure riding and off-road.

As Has been pointed out earlier in this thread, there were more bike than wheel changes in this year's TdF due probably to some riders of certain teams using disc brakes rather than whole teams using them exclusively. I'd imagine that all teams would prefer to carry more wheels than bikes on logistical grounds! So unless there's a move to have all riders using the same size rotors (let’s say 160mm f & 140mm r) on the same thru-axles (with neutral service bikes having the same), coupled with the increased time for wheel changes, I’d not expect GTs to adopt discs en mass for a long while - perhaps never if also enough non-professional cyclists refuse to adopt discs in sufficient numbers to keep production viable.

However, I think tubeless may well be adopted over tubs before discs to cut down on the need for so many bike/wheel changes far sooner than many would expect, especially as tubeless tech matures. My personal view is that hydro discs coupled with tubeless tyres definitely have a place on racing bikes, despite Continental stating otherwise!

[Brucey]

…. I think tubeless may well be adopted over tubs before discs to cut down on the need for so many bike/wheel changes far sooner than many would expect, especially as tubeless tech matures. My personal view is that hydro discs coupled with tubeless tyres definitely have a place on racing bikes, despite Continental stating otherwise!

I think the chances of the pro peloton, en masse, adopting technologies that are heavier, slower, less aero and result in slower wheel changes and more punctures are vanishingly small.

More punctures? I hear you say. Well yes; tubs are far less likely to pinch-flat than any other tyre.

If there were advantage in using sealant to fix punctures then it would be added to tubs (they may do this already for certain events for all I know). The result would still be faster and better than tubeless or any other form of HP.

Don't get me wrong; HPs have been 'almost as fast as tubs' for decades, and you can ride a good TT on the right HPs and only be at a significant disadvantage to those spending a small fortune on their tubs. But the idea that they will displace tubs in the pro ranks is a bit of a stretch.
Ditto disc brakes; IME even with 'identical wheelsets' swapping wheels usually results in some brake rub, and is a frightful palaver by comparison with a traditional QR wheel.

cheers

[Peter W]

Brucey wrote - 'A synthetic version of DOT fluid is used.' (Formula 1.)

Presumably that fluid is more 'boil proof', and fade proof, which would be of great advantage on a hydraulic disc braked system road bike. I take it that said fluid is prohibitively expensive, requiring special components (seals etc) and may even be corrosive and dangerous to handle?

Any chance of a more reasonable 'half way house' fluid finding its way into road bike hydraulic systems? Surely, they will evolve, and steadily improve with continual development? Things rarely progress backwards! (Can of worms! )

[RickH]

I think the chances of the pro peloton, en masse, adopting technologies that are heavier, slower, less aero...

That's probably why 4 out of the first 5 stages of this year's Tour de France were one by riders with disc brakes, Gaviria & Sagan 2 wins each (the stage 3 TTT being the exception)!

I think 8 stages were won on disc bikes in all. It didn't seem to have slowed them down.

I was reading a review of latest iteration of Giant's aero bike, the Propel Advanced. That has gone disc AND has a claimed 10 Watt advantage over the previous version (I presume that means it needs 10 watts less to pedal it at a given, but unstated in the review, speed).

I suspect we will see more uptake next season. The official UCI ruling to allow discs, rather than it being a trial, came well after the start of the season & I doubt world tour teams would be switching bikes until the riders have had plenty of time to get used to any differences in handling in different situations.

...and result in slower wheel changes and more punctures are vanishingly small.

I think the wheel change issue will die down. I suspect a standard through axle design will be adopted - one company, Focus I think, has produced one which is a 1/4 turn bayonet & they said it can be used freely by others. That could even speed up front wheel changes as there won't be any need to wind out the QR to pass the "lawyers lips" since the UCI changed the rules & they aren't allowed to take them off (the frame has to be "as sold").

I can't see how disc brakes cause more punctures!

[[XAP]Bob]

Brucey wrote - 'A synthetic version of DOT fluid is used.' (Formula 1.)

Presumably that fluid is more 'boil proof', and fade proof, which would be of great advantage on a hydraulic disc braked system road bike. I take it that said fluid is prohibitively expensive, requiring special components (seals etc) and may even be corrosive and dangerous to handle?

Any chance of a more reasonable 'half way house' fluid finding its way into road bike hydraulic systems? Surely, they will evolve, and steadily improve with continual development? Things rarely progress backwards! (Can of worms! )

It’s more to do with the cooling ducts... and the fact that cyclists won’t accept the mass of calliper, or the aero penalty of cooling ducts.