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.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??
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. - TFojf 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??
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.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.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
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.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.
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.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).
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.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.
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.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.
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.benInMA said:Have you done your own comparison? Are you a road only guy or do you ride a MTB as well?
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: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.
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: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 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.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.
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.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.