Why no Disc Brakes on road bikes?

Chris5150

On my mountain bike I find the disc brakes have far superior stopping power. So why has no one ever put them on a road bike?...ok I know the weight thing needs to be considered, but I dont really see that would make much of a differance to most people, they dont exactly add pounds to a bike.
Maybe its a silly question, maybe not, maybe its an asthetic thing, but I would love the stopping power of my mountain bike when charging down steep hills at 40 mph etc

LangerDan

Limiting factor is the tyre contact patch which is much smaller for a road bike. It is quite possible even in the dry to lock-up a road wheel using conventional calipers - adding even more braking power wouldn't do anything. They may make sense for heavily loaded road bikes e.g. tourers, tandems where they is usually a larger contact patch and rim brakes often perform poorly.

Chris5150

Ahhhh you see I knew there would be a simple answer,,cheers!

aracer

It's not even tyre contact patch. There's actually plenty enough rubber on the road even with a skinny road tyre that when the wheel locks up you get thrown over the bars rather than the tyre sliding. There is also plenty enough power in road caliper brakes to lock the front wheel and send you over the bars. The reason for discs on MTBs is largely to do with coping with poor conditions where they do work a lot better - but not an issue on the road.

Noit quite sure given that how you find your road brakes don't have as much power as the discs on your MTB - either they're rubbish brakes or poorly set up, or you don't know how to use them properly. I can certainly stop at least as quickly - if not quicker due to lower CofG - on my roadie as on my MTB.

eh

Actually having descended a big road descent into Boulder, Colorado with old Giant MPH disk brakes, I'd say road calipers are way better. My disc brakes due to heating pushed the pads nearer to the disc resulting in very scary, unpredictable braking, my mate on his crossbike with cantilevers had much more control.

I admit that the newer disc brake systems sold by the likes of Shimano don't have this problem being an open design, however, it does show that without carefull design disc brakes are not an immediate replacement for calipers.

Another interesting thing is that because calipers are out at the rim, you get some advantages over discs in terms of the leverage and hence stopping power.

nicklouse

eh
thats MPH's for you. (but i bet you would have caused some serious wear in the blocks if you had bracked the same way.

there are disc brakes available suited to road use and CX use (forgetting any rules)

discs give a more predictable braking in Poor conditions.

simbil1

I reckon in 5 - 10 years you will see more and more minimalist road discs.

The advantage of a disc for road use is that the disc will heat up rather than the rim - this means the exploding clincher / melted tub glue problems will disappear.
The other advantage is that the rotor gets worn out rather than the expensive rim - that alone sells it for me.
Carbon rotors with a mechanical (rather than hydrolic) light weight brake system - I can't wait

Eat My Dust

I always though that the brakes on road bike's were rubbish (I had low end Tekro brakes) then I upgraded to 105's, first time I pulled the levers, both wheels locked up and I nearly came off!! Now I'm using Ultegra and they are amazing compared to the original Tekro's!!

Kléber

If the contact patch is smaller, then you can make the discs and pads smaller - in other words adjust the disc brake system to suit the road set up. There could be advantages, the rim surface would no longer be a braking surface so you could use lighter rims, saving rotating weight, adding a small carbon disc for braking.

I think discs are revolutionary for MTB riding but the advantages don't translate so effectively to road bikes. That said, road bikes are practically the only vehicles left - apart from milk floats and cheap scooters - which don't use discs.

nicklouse

Kléber wrote:

That said, road bikes are practically the only vehicles left - apart from milk floats and cheap scooters - which don't use discs.

and most HGV's

Monty Dog

I have a cyclo cross bike with Avid cable discs and believe me they do make a considerable difference - I've ridden it on fast group rides with road tyres and it amazes people the fact that you can brake harder, later and mid-corner - regardless of how much rubber you have on the road. To prove the point whilst on a ride we had a number of 'brake-offs' into fast corners - I could hold a faster speed into the corner, whereas the rim brake guys were backing-off and having to hold a straight line - these were Elite cat road riders too. I expect in a few years, we'll see lightweight, compact disc calipers, carbon-ceramic discs on some road bikes - the UCI weight limit actually works in your favour. Shimano have already got a disc specific road hub in their line-up and expect SRAM are cooking up something with Avid. Look at the way MTB XC bikes have gone with tubular tyres and carbon rims. When screaming down an alpine decent in the pouring rain, I know which brakes I'd rather have.

alfablue

LangerDan wrote:

Limiting factor is the tyre contact patch which is much smaller for a road bike. It is quite possible even in the dry to lock-up a road wheel using conventional calipers - adding even more braking power wouldn't do anything. They may make sense for heavily loaded road bikes e.g. tourers, tandems where they is usually a larger contact patch and rim brakes often perform poorly.

It is easier to lock-up a fat tyre than a road tyre - a larger contact area means less pressure between rubber and road (less psi, the same weight is distributed over a wider area) and they will skid much more easily. With powerful disc brakes on an mtb I have found I need to be really careful when descending at speed on the road, the back in particular locks up with the lightest touch (obviously the vast majority of the load is on the front tyre under braking). Of course the amount of tread/depth of knobbles also has an effect in reducing grip.

I think well set up calipers are fine for the road, the only downsides are rim wear, and the reduced braking caused by wet rims (though the latter is not really a problem in my experience). Discs could be attractive, but they are probably bound to be heavier, and hydraulics would not be possible with current STI shifters. There are a growing number of Urban / Flat bar road bikes with mechanical discs, and on cyclocross bikes as above, so there clearly is a place for them, but I do't expect to see them on all-out road bikes very soon, mainly on weight considerations.

stevec205gti

Kléber wrote:

I think discs are revolutionary for MTB riding but the advantages don't translate so effectively to road bikes. That said, road bikes are practically the only vehicles left - apart from milk floats and cheap scooters - which don't use discs.

Road bikes do use a disc of sorts - a caliper with pads to grip a rotating disc slowing it down. The problem is you don't consider it such because it doens't look like the disc/caliper arrangement you see on MTB's, cars, motorbikes etc - but from a design point of view it's doing the same thing.

portuguese mike

alfablue wrote:

LangerDan wrote:

Limiting factor is the tyre contact patch which is much smaller for a road bike. It is quite possible even in the dry to lock-up a road wheel using conventional calipers - adding even more braking power wouldn't do anything. They may make sense for heavily loaded road bikes e.g. tourers, tandems where they is usually a larger contact patch and rim brakes often perform poorly.

It is easier to lock-up a fat tyre than a road tyre - a larger contact area means less pressure between rubber and road (less psi, the same weight is distributed over a wider area) and they will skid much more easily. With powerful disc brakes on an mtb I have found I need to be really careful when descending at speed on the road, the back in particular locks up with the lightest touch (obviously the vast majority of the load is on the front tyre under braking). Of course the amount of tread/depth of knobbles also has an effect in reducing grip.

I think well set up calipers are fine for the road, the only downsides are rim wear, and the reduced braking caused by wet rims (though the latter is not really a problem in my experience). Discs could be attractive, but they are probably bound to be heavier, and hydraulics would not be possible with current STI shifters. There are a growing number of Urban / Flat bar road bikes with mechanical discs, and on cyclocross bikes as above, so there clearly is a place for them, but I do't expect to see them on all-out road bikes very soon, mainly on weight considerations.

I think you're logic here is flawed. Yes, a larger contact patch means less force per unit area through the tyre but this is compensated for by the fact that there's more of it (my maths isn't good enough to work out if it adds up to exactly the same but i suspect it is , more or less). but more crucially as you brake and therfore load up the front tyre you are apllying more and more pressure/force on the contact patch and since there is a larger amount of rubber on the road the total amount of grip available will be greater (assuming the coeffiicent of grip is the same for the thinner and wider tyres), so the force required to cause the tyre to break adhesion will be greater i.e. you can brake harder before skidding. if this wasn't the case then performance cars and bikes wouldn't need such wide tyres - imagine the non existant braking performance of a motorbike if it had the same contact patch as a push bike.

simbil1

I think you're right Mike. The issue with MTB tyres is the nobbles which actually decrease your contact surface and deform causing corner creep. Nobblies don't corner well like slicks do on a road surface.

maddog 2

its because roadies are a conservative bunch

discs are the future. People scoffed at discs when they first arrived on mtbs. Now they're standar issue, even on cheap bikes.

Anyone who's used a decent disc setup knows they are superior in evey way to rim brakes (except weight). More power, better modulation, better in the wet, more consistent... (and of course you can lock up rim brake, that's not the point. The point is control)

But until there are some decent disc forks for road bikes, and compatible levers, no one will/can touch them.

Crossers can manage it though, and many now use discs.

With discs getting lighter and better all the time, it won't be long 'til they are a practical option, even if it's only for the front.

portuguese mike

I agree, Simbil 1. Although, to be fair, Alfablue does mention the reduced grip of nobblies compared to road tyres

Eat My Dust

I believe there's the weight issue as well. If we go to such lengths as carbon bottle cages, imagine the horror of weighty disc brakes!!!!!!!!!!

Kléber

Eat My Dust wrote:

I believe there's the weight issue as well. If we go to such lengths as carbon bottle cages, imagine the horror of weighty disc brakes!!!!!!!!!!

A carbon disc and a lightweight caliper: no problem.

Some MTB systems are getting very light. And you can make your rims lighter, even shallower, because they no longer need to be so thick as a braking surface that wears out.

Monty Dog

DT and TUFO have developed an XC MTB wheelset that weighs something like 1200g complete - as Kleber says, there's no need to make the rim take the load of the brakes. There'll probably be a nominal weight penalty of a couple of hundred grammes, but given it's quite possible to build a sub-6kg bike these days I think the performance benefits probably outweigh the disdvantages - you could end up with riders in the tour swapping their uber-light climbing bikes for something with disc brakes for a fast descent - the last thing you want when blasting down the Tourmalet or the Galibier at over 80kph is some whippy bike that refuses to slow down or go round corners without shimmying all over the place.

OllyBianchi

The most important thing which needs to happed before discs are used on high end road bikes is the UCI rules. I'm not sure of the road rules but the definition of a 'cross bike to be used in ranking cross races specifically excludes discs.

So you won't see any Record or Dura-Ace discs until they can race them - which also means you are unlikely to see Mr Colnago, for example, putting disc mounts on his frames.

The development in road racing equipment tends to be focussed on performance gains and in road racing, unlike MTBs, people don't really win races on having the best brakes - the development budgets of the big boys will be aimed at stiffer frames, lighter wheels, slicker gears etc for the future.

alfablue

Thinking about it, a wider contact area will not change matters, 50kg (per tyre) divided by a contact area of 2cm square, versus 4cm square, should mean the same amount of friction in total (the friction being a function of contact area and downward force).

Other factors come in to play, such as the smoothness of the contact surface and the tyre pressure (rougher surfaces need softer tyres to provide the same actual contact with the surface - and to achieve this whilst protecting the shape of the tyre and preventing punctures they therefore have toi have a higher volume), the stickiness of the rubber, tread pattern (if any) and dryness of the surface (narrow slick road tyres need speeds in excessof 100mph to aquaplane apparently).

On the other hand, I may be wrong....but (just from personal experience) I don't think I have ever managed to skid on 700x23c slick tyres in the dry, whereas it is relatively easy to do on 26x1.75 tyres.

simbil1

OllyBianchi wrote:

<snip>
The development in road racing equipment tends to be focussed on performance gains and in road racing, unlike MTBs, people don't really win races on having the best brakes - the development budgets of the big boys will be aimed at stiffer frames, lighter wheels, slicker gears etc for the future.

The big boys want to make money, so as soon as they think people will buy discs on road bikes, they will put them on. It's as simple as that. Point taken about the UCI - they won't put them on fully fledged racers if they are banned equipment.

You may not win a race on discs but you can sure loose a race when your tub glue melts and you fall off!

I'm not convinced discs will win you an MTB race either - some XC racers still run v-brakes for the weight saving.

LangerDan

Kléber wrote:

Eat My Dust wrote:

I believe there's the weight issue as well. If we go to such lengths as carbon bottle cages, imagine the horror of weighty disc brakes!!!!!!!!!!

A carbon disc and a lightweight caliper: no problem.

Some MTB systems are getting very light. And you can make your rims lighter, even shallower, because they no longer need to be so thick as a braking surface that wears out.

Still heavier than a rim / calliper combination. Most of the rims on the market (exlcuding CF specials etc) that have a rim-brake / disk option save, at most 25g per rim. Many have no weight savings. You could make a rim lighter but most manufacturers won't because it will mean having two different alloy extrusions - easier to make one and machine a braking surface if required. Even if you do make a saving, most if not all of it will go in providing a disc-compatible hub.

There would be distinct advantages if you go to carbon rims because a) you can tailor the rim to a disc application more easily b) the cost implications are less significant and c) the difficulties of rim braking on a carbon surface are gone.

aracer

Kléber wrote:

A carbon disc and a lightweight caliper: no problem.

Apart from that carbon discs don't work in bicycle applications (and the carbon discs used on cars aren't carbon in the same sense as carbon bike frames, and cost a fortune).

The tyre thing is simply irrelevant (hence whilst I could post corrections I won't) - ultimate stopping force on a road bike on normal tarmac even in the wet is limited by geometry - ie not going over the bars - not traction.

The point is they really are uneccesary - normal caliper brakes do slow you down just as fast as the geometry allows, and modulate just fine. I really can't say I miss having disc brakes on my road bike, despite being totally committed to them on an MTB.

alfablue

aracer wrote:

Kléber wrote:

A carbon disc and a lightweight caliper: no problem.

The tyre thing is simply irrelevant (hence whilst I could post corrections I won't) - ultimate stopping force on a road bike on normal tarmac even in the wet is limited by geometry - ie not going over the bars - not traction.

The point is they really are uneccesary - normal caliper brakes do slow you down just as fast as the geometry allows, and modulate just fine. I really can't say I miss having disc brakes on my road bike, despite being totally committed to them on an MTB.

Agreed on both counts.

aracer

LangerDan wrote:

Still heavier than a rim / calliper combination. Most of the rims on the market (exlcuding CF specials etc) that have a rim-brake / disk option save, at most 25g per rim. Many have no weight savings. You could make a rim lighter but most manufacturers won't because it will mean having two different alloy extrusions - easier to make one and machine a braking surface if required. Even if you do make a saving, most if not all of it will go in providing a disc-compatible hub.

Lightweight disc hubs actually typically weigh getting on for 80g a pair more than the equivalent non-disc hubs, so you actually lose out on wheel weight, even with a lighter rim. I'm not actually convinced you'd save anything at the rim anyway, given top road wheels have an aero rim section which inherently includes a braking surface.

The weight really is a big issue - over on MTBR the comparison was done between the lightest rim brake setup and the lightest disc setup (including differences in wheels etc), and the conlusion was that discs added nearly 400g. That's quite a chunk, and more than most top-end bikes have to spare under 6.8kg.

eh

Three further thoughts:

1) I doubt a disk and caliper is as aero as standard brakes.

2) You'd need to beef up your carbon forks a fair amount I'd expect, to take the forces involved in braking.

3) How often do tyres roll of the rim due to overheating? I can't think of ever having come across this problem.

maddog 2

aracer wrote:

I really can't say I miss having disc brakes on my road bike

for most riding that is true. I don't think anyone would dispute that.

The main gains are for commuting (less rim wear, better wet performance, more consistent) and riding downhill quickly, where rim brakes really start to struggle.

I run a flat bar roadie based on an Airborne Carpe Diem CX frame. I looked into fitting discs a while back but decided against it at the time, for a number of reasons. But I'm still tempted - if the right kit came along and I had the cash then I may well do it. Instead I run Avid Ultimate Vs and ceramic OP rims so braking performance is pretty good. Stiill not in disc-territory but quite a bit better than standard rims and DPs. On the Fred Whitton I was out-braking pretty much everyone on the descents. You should have seen the blokes with carbon rims....they were comically slow :oops:

I think the point about carbon rims is interesting. If you ran discs, you could use carbon rims without having to worry about crappy braking performance.

JWSurrey

Disk brakes on road bikes could be the next unique selling point after STI shifters that has everyone racing back to the shops!
Imagine Mr. shopkeeper when he sees the small change from a new fork, wheelset and actuator mechanics.

One factor not mentioned is that, if I have got my schoolboy physics right, you'd have to run a 3x spoke pattern, or poss. something like a funky 3 spoke carbon setup due to the different torsional forces at work..... It's the equivalent of your drive-side acceleration in reverse.

At least it would do away with all that fine-fettling of block-rim distance and cursing when your wheel goes off-true.

alfablue

JWSurrey wrote:

At least it would do away with all that fine-fettling of block-rim distance and cursing when your wheel goes off-true.

and substitute fine -fettling of disc pad alignment.