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Discussion Starter · #1 ·
So why are disc brakes bad on a road bike? My Cannondale Cycolcross works good and stops but why is it hard to find a replacement wheel set??
OJF
 

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Bent rims. It makes for rough stopping. I'm suprised that I didn't trash my rims last saturday when the club went over two sets of railroad tracks that had four inch gaps. The next day the rr crossing was closed for repairs, and today the crossing was smooth. The Mavic Open Pro rims seem to be in good shape.
 

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ojf said:
So why are disc brakes bad on a road bike? My Cannondale Cycolcross works good and stops but why is it hard to find a replacement wheel set??
OJF
On a road bike tire traction is mostly biggest limiting factor in braking, not braking power so discs aren't required. They do make sense considering braking in wet weather and rim wear though.

One german guy has had different approach to disc brakes on a roadbike though: http://www.smolik-velotech.de/bilder/hydraulikbremse/ (pictures)
http://www.smolik-velotech.de/news/smolik/user_archiv.php?mode=show&id=4 (Explanation in German)
- smaller discs for control
- dual disc setup to control fork twist
- dropouts can handle hot discs & brake saddle
It's a interesting concept but might be 5-10 years away from production?
 

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Here are a few considerations -

1) The main benefit of discs is use in wet, muddy conditions. A disc rotor will stay cleaner and drier than a rim. However, in rain alone, both the rim and rotor get wet and require several wheel revolutions for the brake pads to clear the water. Contrary to popular belief, discs require longer stopping distances when wet just like rim brakes. So, you are much more likely to encounter conditions that favor discs over rim brakes while mountain biking than road biking. Disc brakes will not offer any better braking performance in the dry.

2) Disc brakes require a dished front wheel which is inherently weaker than a non-dished wheel. Some consider this a bigger problem on a front wheel than the rear because of greater lateral loads on the front.

3) Disc brakes require reinforced forks which reduce ride quality.

4) Under heavy, prolonged braking, disc brakes are more likely to suffer heat-related failure than rim brakes. Failures seen during testing include fade, melted plastic components, fluid leaks (hydraulic systems), and warped rotors. On the other hand, when a rim brake fails due to heat, it is usually a tire blow-off which might be considered more dangerous than the disc failures. Large rotor discs and those with large fluid resevoirs are more immune to heat-related failures, but these are not common on road bikes.

5) Weight. This includes the extra weight of the brake plus the extra weight required in the frame and fork.

6) Cost.

7) Rotor drag.

Remember that rim brakes *are* disc brakes. The disc rotor is the rim, and the swept area of the brake is much larger than current disc brakes. This means greater potential braking force. There are advantages to discs over rim brakes. However, in most road applications, the disadvantages are greater.
 

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NeoRetroGrouch
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ojf said:
So why are disc brakes bad on a road bike? My Cannondale Cycolcross works good and stops but why is it hard to find a replacement wheel set??
OJF
Others may find them of no value, but for me, they are GREAT! If I'm coming to an intersection at speed, a car runs the stop sign and 20 feet is the difference between stopping and getting hit, it's disc brakes or death. And that's dry. Rim brakes (including cantis and Vs with Travel Agents) will not stop me. - TF
 

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TurboTurtle said:
Others may find them of no value, but for me, they are GREAT! If I'm coming to an intersection at speed, a car runs the stop sign and 20 feet is the difference between stopping and getting hit, it's disc brakes or death. And that's dry. Rim brakes (including cantis and Vs with Travel Agents) will not stop me. - TF
Wow. I've done emergency stops with side pull brakes that lifted the rear wheel from the pavement. Any more braking power would produce an endo.
 

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TurboTurtle said:
Others may find them of no value, but for me, they are GREAT! If I'm coming to an intersection at speed, a car runs the stop sign and 20 feet is the difference between stopping and getting hit, it's disc brakes or death. And that's dry. Rim brakes (including cantis and Vs with Travel Agents) will not stop me. - TF
The two times I I've tried a true full-on panic stop on my road bike I've done endos... and that was with the old single pivot calipers. I would have crashed anyway, but still... I've observed others do the same. No need for more stopping power than that.
 

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fmw said:
Wow. I've done emergency stops with side pull brakes that lifted the rear wheel from the pavement. Any more braking power would produce an endo.
With any rim brake on a road bike, my hands are not strong enough to lift the rear wheel on the flat. Maybe I automatically shift my weight back and you don't, or maybe my hands just aren't strong enough (or both). Whatever the reason, THEY DON"T STOP FOR ME, so the rest is irrelevant.

I also disagree (again) with this 'lift the rear wheel' as max stopping power. I can shift my weight back and slide both wheels (usually with dire consequences). The max braking power is just before the wheels (usually rear first) lets lose.

TF
 

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Skidding a front wheel

TurboTurtle said:
I also disagree (again) with this 'lift the rear wheel' as max stopping power. I can shift my weight back and slide both wheels (usually with dire consequences).
On clean dry pavement, this is extremely doubtful. Given the traction coefficient of bicycle tires (nearly 1.0) and the typical height of a cyclist's center of gravity, you'd have to get your center of gravity well behind the rear axle to skid the front wheel - a difficult feat with your hands on the handlebars and feet on the pedals.

Just as an experiment, several times I have tried to skid my front on clean dry pavement, either in my regular seated position, or with my weight as far back as I could. In all cases I have failed - the rear wheel started to rise up without any front wheel skidding.

About the only way to skid a front wheel on a road bike is on loose or slippery surfaces (sand or gravel, wet manhole covers or painted stripes). But in these cases, it is tire traction that limit braking, not the brakes them selves, and the braking rates that the skidding occurs in these situations is well below the maximum braking rate on clean dry pavement.

Here's Sheldon Brown's take on braking:

http://www.sheldonbrown.com/brakturn.html
 

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Maybe it's cause I MTB and also ride motorcycles but I think the day will come when we will have disk brakes that are better & lighter then our rim brakes right now.

I am not sure, it seems like there are some misconceptions about disk brakes... AFAICT Discs are:

- Way less likely to fade & overheat
- If they do overheat you do not blow out your tires
- Far better modulation
- Far better power (yes it is noticeable)
- Far far far better in bad weather

Go down an extended 10% grade on your road bike and you basically have to let the bike run and save the brakes for corner entrances and possible emergencies. Try dragging the brakes for 1 mile down a 10% grade just "because" you don't feel like bombing 40+ mph for whatever reason and you will just about melt your rims brakes if you don't blow out your tires. There are many, many disk brake systems already on the market which can easily handle that kind of duty.

Also on top of the bad weather performance, rim brakes get all crudded up in the bad weather, and they don't regain their stopping power until you stop to clean the pads & rims of all the goop. Disks really don't have this problem anywhere near as badly.

Another potential advantage IF disc brakes are developed and mature for road bikes is the rim should be able to be made lighter, reducing moment of inertia. Further you would have no issues with Carbon rims & finding brake pad material which works on carbon.

Another item that is nice (especially hydraulic discs, which also appear to be lighter since the fluid is lighter then cables) is that they are extremely low maintenance. The pads last forever and there is minimal adjustment required once the system is set up right.

If someone chooses to I can only imagine it is possible to build something interesting like a 4" rotor hydraulic brake system for road bikes which would beat our current rim brakes in all areas, especially power, reliability, durability, etc.. and weight could arguably be competitive given a good enough wheel/rim design.

There are certainly a lot of other things on road bikes which people are spending huge $$ on which are not nearly as much of a functional improvement as disk brakes *could* be. (They are certainly not there today)

I would really like a proper front disk for my fixie... it would allow me to go down big hills and control my speed without the fade issues of rim brakes. It would allow me to carry more weight for commuting, etc.. and still have the same margin of safety. I would definitely love having disk brakes on a touring bike.
 

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benInMA said:
Go down an extended 10% grade on your road bike and you basically have to let the bike run and save the brakes for corner entrances and possible emergencies. Try dragging the brakes for 1 mile down a 10% grade just "because" you don't feel like bombing 40+ mph for whatever reason and you will just about melt your rims brakes if you don't blow out your tires. There are many, many disk brake systems already on the market which can easily handle that kind of duty.
The only actual test data that I have seen does not support this view. In fact, it says quite the opposite. The testing was done with prolonged downhill braking on tandems. The disc brakes available at the time, several years ago, all failed at lower temperatures than the rim brakes. I believe that some of the downhill brakes currently available would fare better because they feature larger diameter, heavier rotors.

If someone chooses to I can only imagine it is possible to build something interesting like a 4" rotor hydraulic brake system for road bikes which would beat our current rim brakes in all areas, especially power, reliability, durability, etc.. and weight could arguably be competitive given a good enough wheel/rim design.
There are two factors which primarily affect brake performance. The first is the size of the swept area of the brake. The larger the area, the more braking power. Since rim brakes have a much larger diameter rotor than traditional discs, they have a greater swept area and, thus, the potential for greater braking power. If a small diameter rotor is to be effective, it must have a much wider brake track to increase the size of the swept area.

The second factor is heat. What you do when you brake is turn velocity into heat. The way to increase the heat capacity of a disc is to increase its mass. A heavier rotor can dissipate more heat without warping. You can use an alternative material, like a ceramic composite, to allow the rotor to get hotter without warping, but you will also have to make the other components more heat-resistant. In the tests mentioned above, plastic parts melted and fluid leaked under high temperatures. Increasing the amount of fluid used in the system increases its ability to tolerate heat, but it also increases weight. Other materials can be substituted for the plastic, but these would likely increase cost.

Yes, it is possible to design a disc brake system that would perform better than a rim brake for most road applications, but you would increase cost and/or weight. Given the current product offerings, most road cyclists are better served by rim brakes.
 

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Have you done your own comparison? Are you a road only guy or do you ride a MTB as well?

I recently went out and test rode the Shimano, Hayes, and Sram (Avid) disk brake setups in order to choose one.

I've had 105, Dura Ace, and Ultegra setups on my road bikes as well as most of the Shimano V-brake lineup, several sets of Cantilever brakes, and I've had the high end Avid V-brakes as well.

There is not even any comparison, the road brakes are not even in the same ballpark with the current disk setups. I regularly ride the same roads on each of my two bikes, my local twisty descent the road brakes are fading almost instantly and there is absolutely no way you could stay under the speed limit and descend slowly on the road bike, you basically have to run the bike and brake hard for each corner, allowing the bike to dissipate the heat between the corners. On the MTB you can put one finger on the rear brake alone and descend the whole road at 10-15mph with absolutely no sign of fade, and even if you heat up the rotor it cools in seconds, far faster then a rim can cool off. Using both forget about it, you're not even heating the rotors.

In terms of weight you can already build up a MTB drivetrain with 6" Hydraulics to be quite competitive with Dura Ace on weight.

I think the only reason we haven't seen it happen on the road is it's banned for racing. If it wasn't banned rim brakes would be history pretty quickly.
 

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benInMA said:
Have you done your own comparison? Are you a road only guy or do you ride a MTB as well?
Yes, I ride MTBs and motorcycles, too. Up until the end of 2004, I also owned and operated a full-line bicycle shop. So, I've had the opportunity to ride lots of bikes. It sure was interesting riding bikes with brand-new disks. With most of them, we could ride around the parking lot with both brake levers bottomed against the handlebars. They don't work so hot until they are broken in.

My own personal MTBs have not had disks, but I've ridden many with disks off-road. If I get a new MTB, it will probably have disks on it.
 

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benInMA said:
There is not even any comparison, the road brakes are not even in the same ballpark with the current disk setups. I regularly ride the same roads on each of my two bikes, my local twisty descent the road brakes are fading almost instantly and there is absolutely no way you could stay under the speed limit and descend slowly on the road bike, you basically have to run the bike and brake hard for each corner, allowing the bike to dissipate the heat between the corners. On the MTB you can put one finger on the rear brake alone and descend the whole road at 10-15mph with absolutely no sign of fade, and even if you heat up the rotor it cools in seconds, far faster then a rim can cool off. Using both forget about it, you're not even heating the rotors.
Does your username BenInMA indicate that you live in Massachusetts? I don't know what roads you've been riding, but I've been riding in Massachusetts for nearly twenty years, and I've never had a problem with braking on descents anywhere in Massachusetts (or New Hampshire or Vermont either, for that matter). My road brakes allow me to ride at any speed I choose, with no fade. I have no problem decelerating at the limits of braking (i.e. just below the rear wheel lift off point) when the need (or whim) arises. And I still use single pivot calipers. I don't know any other experienced riders around here who have any kind of probem, either. If you're having a problem with road brakes on descents, it sounds more like a grip strength or technique problem, not an equipment problem.



benInMA said:
In terms of weight you can already build up a MTB drivetrain with 6" Hydraulics to be quite competitive with Dura Ace on weight.
What do you mean by "quite competitive"? Comparing Dura-Ace with XTR, the levers and cables/hoses are nearly the same weight, but the Dura-Ace calipers at 314 grams are about 1/3 lb lighter than XTR disc calipers (208 gm/pair) and rotors (278 gm/pair). In addition, disc hubs are heavier - for example, DT Swiss 240s disc hubs weigh 427 gm/pair, whereas the equivalent non-disk hubs weigh 346 gm/pair. When you add it up, the disc brakes are about 1/2 lb. heavier. Why lug around that extra weight on a road bike, when rim brakes work just fine?


benInMA said:
I think the only reason we haven't seen it happen on the road is it's banned for racing. If it wasn't banned rim brakes would be history pretty quickly.
I really, seriously doubt that has any thing to do with - especially here in the US. Firstly, most cyclists don't race, so that only restricts a small portion of the population. And even then, disc brakes are only banned for UCI races - they are still perfectly legal for the vast majority or US road races (most races in the US are run under USCF or local organizations such as as BRAC, OBRA, etc.). So, the disc brake ban only affects maybe a couple of hundred of US based racers who do UCI races, and not the tens of thousands of other cyclists who race, and definitely not the other hundreds of thousands of cyclists who don't race.

Maybe road cyclists haven't embraced disc brakes because they are not fashionable - or maybe because they just aren't needed by most road riders.
 

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I'm curious. Are disc brakes (a fairly recent fad) necessary on mountain bikes? Or, are they just cool? It's a serious question. I think my cantilever braked mountain bike will stop faster than is safe, just like my road bike will with its side pull brakes.

Or perhaps the issue is the difference between riding in Colorado and riding in Indiana. We have steep hills but not long ones. I can't imagine a MTB rider in the Midwest actually needing disc brakes. But then what do I know. I'm just a roadie.
 

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fmw said:
I'm curious. Are disc brakes (a fairly recent fad) necessary on mountain bikes? Or, are they just cool? It's a serious question. I think my cantilever braked mountain bike will stop faster than is safe, just like my road bike will with its side pull brakes.

Or perhaps the issue is the difference between riding in Colorado and riding in Indiana. We have steep hills but not long ones. I can't imagine a MTB rider in the Midwest actually needing disc brakes. But then what do I know. I'm just a roadie.
The biggest advantage in my neck of the woods (north central Illinois) is mud clearance. The clay here sticks to everything. Rim brakes are easily packed with mud. Disks don't have that problem. For riding in dry conditions, there's not much difference to me.
 
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