Disc brakes and road bikes

[lokstah]

What characteristics of current disc brake technology prevent them from being a natural choice for road bikes? Just curious. I know there are a few odd road bikes out there which offer them, and some cross bikes as well. But not being a moutain bike rider, I don't follow the technology well enough to understand why they haven't become more universal.

Is it weight? Do they require a unique enough lever mechanism that they can't be adapted to work with STI/Ergopower levers? Or is there something about the character of disc brake stopping action that makes the current technology just wrong for road riding/racing? Too powerful? To abrupt?

As disc brakes become more and more fancy, and more and more standard on non-road bikes, you've got to wonder these things, and wonder if they won't become standard road equipment some day.

 

[Aztec]

1) Weight.

2) Efficiency -- I don't think discs would make much difference. You can already lock up at least your rear wheel easily, I'm sure.

3) Discs were likely put on MTBs because the rims get dirty/wet so much more easily that a disc that is close to the center of the wheel. And, it's cheaper to replace a chewed up disc than a rim.

 

[Duckwah]

1. Good discs weigh a ton

2. Good discs are hydraulic and no one has designed a brake/gear lever for hyraulic brakes

3. As stated above there is no real need for them on most road bikes

4. They need special hubs plus frame mounts and no one seems to want to take the step to put all that extra stuff on bikes yet

 

[filtersweep]

the others already said it well-

mtn biking requires a bit more technical braking than road biking, and discs also don't fade like rim brakes- the contact patch on a road bike is so tiny that you already have more than enough braking power.

There's already all sorts of controversy about using QRs on mtn bikes with discs...

 

[rljones]

This is taken from Velonews about Interbike. Andrew's report indicates disc brakes are what all respectable bikes will be soon wearing...

**********

Tech Talk: Reports
Interbike Daily Update - Sunday
Five Cool Bikes
By Andrew Juskaitis

Morati SC 1.3 Ti Pro disc brake road bike

Built in the Czech Republic by aerospace manufacturer Mora Aerospace (now owned by Honeywell) Morati produces some of the most innovative titanium bicycles on the market. Most striking on this, the opening day of the 2003 Interbike trade show, was a full equipped Hayes mechanical disc brake road bike. Built using the company's SC 1.3 Ti Pro platform, the frame boasts the addition of a rear disc brake, a 130mm spaced VeloMax wheelset and reinforced Morati disc brake specific titanium road fork. While we're seeing disc brake technology slowly creeping onto cyclocross bikes at this year's show, the Morati is the first fully raceable disc brake adapted road bike we've spotted. With the proliferation of carbon clincher wheelset now available (which are susceptible to overheating under prolonged heavy braking), disc brakes make more sense than ever for road bikes. My guess? In less than five years more than half of the road bikes on the market will sport these superior stoppers

 

[lokstah]

Good post, rljones.

I had actually caught that blurb a few days back; it's what inspired my question. I had assumed the primary reason was weight, but wondered if, beyond that, the only think holding road disc brakes back was the time it takes the industry to adjust with compatible technology (forks, wheelsets, etc).

 

[JoeRider]

Another major factor in putting disk on MTB's was that the rims tend to get bent or lose true much more often. In XC events, the last thing you want is brake rub problem. Not that you can't dent or bend a disk, I have, it's just not as common.

JR

 

[hedgehog]

I read on one of the magazines (I believe it was bicycling) that campy has a disc brake under development, don't know if it's just a rumour or not.

I have disc brakes on my mountain bike and I would love to have them on the road bike, I have the hayes hydraulics on my mt bike and though a little heavier they are worth the weight penalty.

Probably not as necessary on a road bike but the power would be nice (it's the first thing I miss when I jump on the road bike) and obviously no melted tube flats on those long downhills.

 

[Julian Radowsky]

Lets think about this:
The braking surface of a road wheel rim IS A DISC. that gets pressure applied simultaneously from both sides by pads.

I am also pretty sure that because of the larger circumference and radius from the hub of the rim braking surface over a small hub mounted disc, that the stopping power of a standard road wheel rim surface, with good brake pads, is SUPERIOR to the stopping power of most, if not all hub mounted discs (assuming the same force applied at the levers, and same friction rating on the pads) - it is VERY easy to lock up BOTH wheels on a road bike at even 60kph or above.

The benefit of hub mounted disc brakes is really that the braking sufrace is more likely to remain clean and true, especially in MTB conditions.

Additionally, hub mounted discs, being replacable, can have higher friction pads (more abrasive), since the cost of replacing only a disc is a lot lower than replacing rim or wheel.
As a matter of fact, bacause of the high torque (rotational force) on a disc close to the hub, the pads would have to be more abrasive and have a higher friction rating to provide the same braking rate as standard pads at the outer rim surface (this is also why high end disc braking systems use hydraulics - to greatly magnify the force applied [so that even light force on the levers produces high braking force])

In other words: Disc brakes on a road bike are really unnecessary, would add to the weight and complexity of a road bike, and would probably only be used by those who are victims of marketing spin.

Of, course, not being a mechanical engineer I could be totally wrong!

 

[hedgehog]

Originally posted by Julian Radowsky
Lets think about this:
The braking surface of a road wheel rim IS A DISC. that gets pressure applied simultaneously from both sides by pads.

I am also pretty sure that because of the larger circumference and radius from the hub of the rim braking surface over a small hub mounted disc, that the stopping power of a standard road wheel rim surface, with good brake pads, is SUPERIOR to the stopping power of most, if not all hub mounted discs (assuming the same force applied at the levers, and same friction rating on the pads) - it is VERY easy to lock up BOTH wheels on a road bike at even 60kph or above.

The benefit of hub mounted disc brakes is really that the braking sufrace is more likely to remain clean and true, especially in MTB conditions.

Additionally, hub mounted discs, being replacable, can have higher friction pads (more abrasive), since the cost of replacing only a disc is a lot lower than replacing rim or wheel.
As a matter of fact, bacause of the high torque (rotational force) on a disc close to the hub, the pads would have to be more abrasive and have a higher friction rating to provide the same braking rate as standard pads at the outer rim surface (this is also why high end disc braking systems use hydraulics - to greatly magnify the force applied [so that even light force on the levers produces high braking force])

In other words: Disc brakes on a road bike are really unnecessary, would add to the weight and complexity of a road bike, and would probably only be used by those who are victims of marketing spin.

Of, course, not being a mechanical engineer I could be totally wrong!

yes, you are completely wrong. Ride a bike with disc brakes sometime, that seems to be the only way to convert people. They are WAY more powerful.

I have no idea how you can lock your brakes on your road bike, maybe with a handful of fingers on the brake, on my mt bike I can lock it with 1 finger, that's the kind of power I want from my road brakes.

 

[rljones]

While marketing will certainly play a role in driving disc brakes on road bikes, a more practical drive may be the problem, to which Andrew alluded in his article, of carbon rims overheating.

My understanding is that present carbon rims are extremely sensitive to overheating with temperatures going to the several hundreds of degrees Celsius during hard, down hill braking. This high temperature is locally distributed to the tires (clinchers or glued surfaces). This results in increased tire pressure (one of those Gas Laws..). Shifting the heat to a replaceable disc brake would improve safety wtih respect to the tires/glue, avoiding more heat than is already generated by road friction.

Additionally, while the overall weight may not be much less, and could be slightly greater, with carbon rims/hubs/disc vs al rims/std rim brakes, the rotational weight should be lower. Decreased rotational weight is always desirable.

Thus, disc brakes, whether they truly brake any faster or better than rim brakes, may replace rim brakes as wheels evolve into lighter and less heat tolerant forms on higher end bikes. Following this change, the trickle down effect of technology would then lead to disc brakes appearing on most bikes, just as we now see carbon forks almost everywhere.

 

[Duckwah]

When they get hydraulic disk brakes for road bikes i will be the first one lining up for them!!

My rim brakes scare the **** out of me everytime i use them on long down hills because i'm very heavy and they just don't seem to stop me

this is no joke, last year i could smell burning rubber off my pads after a very long fast downhill (i'm talking several kms of a mountain descent)

 

[JoeRider]

There is definately a big difference in stoping vs. effort when it comes to disk brakes. It is hard to describe without riding something with them. It is very nice for off road conditions where you are on the brakes 2 out of 3 hours. For now I'll stay with disk on my MTB, but I haven't found a condition where I really need it on my Road bike.

JR

 

[lokstah]

Originally posted by hedgehog
I read on one of the magazines (I believe it was bicycling) that campy has a disc brake under development, don't know if it's just a rumour or not.

Might be a moot point; I think Campy has a full mountain group under development.

 

[lokstah]

Originally posted by Julian Radowsky
Lets think about this:
The braking surface of a road wheel rim IS A DISC. that gets pressure applied simultaneously from both sides by pads.

I am also pretty sure that because of the larger circumference and radius from the hub of the rim braking surface over a small hub mounted disc, that the stopping power of a standard road wheel rim surface, with good brake pads, is SUPERIOR to the stopping power of most, if not all hub mounted discs...

I agree with hedgehog; I believe this is incorrect (even though I've never used disc brakes), even if it's counter-intuitive at a glance. When they first became popular, I wondered the same thing -- since the braking surface is so much closer to the rotational hub of the wheel, doesn't it detract from the braking power of the disc brake that the discs constitute the radius they do? The answer is yes, of course, but that's apparently mitigated by the fact that clamping down on a hardened steel plate, and not your fragile rim, permits disc brake designers to impliment a leverage-intensive, vise-grip-like equation that exerts tremendous grabbing force at a fraction of the effort required with rim-brakes.

And they can, of course, because crushing or overheating the disc is hardly a concern. The mechanical freedom overrides the physical limitation of being mounted so near the rotational hub.

 

[Julian Radowsky]

lokstah and others: In my posting I did state that given the SAME applied braking force, rim brakes are more efficient, but as some other posters have mentioned, the forces applied to hub mounted disc brakes would crush or severely distort the rim - not good.

Another thought - would you want someone in front of you in the pack of 50+ riders to have 'sudden stopping power' disc brakes - methinks this would be inviting disaster - even if all of the pack had brakes like this, reaction time is still an issue!

 

[tomasbikes]

IMHO (being a roadie and a mechanical engnieer) the additional weight of a disk brake and the reinforcing required in the fork (for the caliper mounts and the braking forces) present the most serious obstacles to disk brakes on road bikes. This weight penalty is not as evident in mountain bikes, with their suspension forks and knobby tires. This would affect handling and "feel" substantially. And, why would you EVER want to lock up you front brake anyway?

Tom C.

 

[mikeo]

Looks like a Record Group Set is going to get even further beyond my economic reach!

 

[mikeo]

Originally posted by mikeo
Looks like a Record Group Set is going to get even further beyond my economic reach!

PS What about a Lefty on a road bike?