Sunday, June 14, 2009

Disc Brakes

... or is it Disk Brakes? Anyway, I've never understood the use of disc brakes on anything other than a mud-eating mountain, downhill, or (maybe) cyclocross bike. The brake-choking mud or dirt seems to me to be a requisite to justify it.

Why is this? The reason is simple, if you have an interest in amateur physics (and I do): Braking Torque. Think of it this way: if the braking is occurring at the hub (as it is with a disk brake), then all the braking force has to be transmitted from the hub, through the spokes, to the rim (where after all, the "rubber meets the road.")

This means that the spokes (which are the weakest link of the wheel) do at least twice as much work on a bike equipped with disk brakes. I haven't seen statistics for this, but I'd be willing to bet that spoke breakage is higher on bicycles equipped with disk brakes. I also wonder if there is a higher rate of failure of the seat stays (which absorb braking force), since the force applied by the disk brake is several times higher than a rim-caliper brake. (see illustration)

On a rim-brake setup, you can think of the entire wheel as the "disk". Braking force is less, but more importantly, the braking force is being applied near the rim, and the only loads that are carried by the spokes are for driving force (which are inevitable) and structural (keeping the wheel true and round).

As long as the wheel is in fact true, and you can properly keep adjusted your rim-caliper brakes (of whatever type), They are a physically and structurally superior solution. It does require that you keep your wheels true, but I'm sorry, if you have wobbly wheels, your riding quality is going to be pretty poor no matter what. I will grant that using the rim for braking means that the rim is eventually going to wear out and you'll have to replace the rim (rebuild the wheel), but we're talking tens of thousands of miles here.

Of course, bike manufacturers are loading up their offerings with disc brakes, even for bikes that will never see off-road use (and of course many so-called "mountain bikes" will always be on a paved trail.) Here's an offering from Raleigh, here's one from Trek, here's a Civia, and even REI has one. These are all touted as premium bikes in their categories. But bear in mind that the disk brakes, while superior in an automotive setting, don't convey that much superiority in a bike that won't be used on unpaved trails.

Give me old-school cantilevers any time. They require some hand strength, but they never fade, and boy are they reliable.

Update: The excellent "Cozy Beehive" blog has a report on a disc brake induced failure here.

5 comments:

Donny said...

Though this might be classified along with off-road use, I think on-road cycling in wet weather (rain, sleet, etc) might be better with disk brakes. In my experience, riding in rain means my rims get dirty and scratched to hell by the brakes. While cantilever have better stopping ability, in this case I'd prefer disk brakes because they're further away from the ground, so they don't get the same level of grime in them to destroy the rim.

ChipSeal said...

I now have 10,000 miles on my Kyserium wheels. They have remained true this entire distance, in spite of a couple hundred of those miles being on gravel roads.

I would estimate perhaps a third of the "life" from brake wear has been used up.

John Marr said...

These type of freebody diagrams are tricky. Actually, the load on the spokes comes from the torque the braking force exerts around the hub. Since this torque is identical for a given amount of braking, it makes no difference if it is applied at the rim or the rotor.

The shearing force will increase on the frame at the brake mounts. However, I would expect this load is small compared to that imposed hitting a good bump. I doubt the stays would need much in the way of additional strength.

I've always thought the big advantage of disc brakes for road bikes was avoiding hot rims and blown tubes on those long, twisty descents. I think they're an excellent idea for heavy mountain touring and mandatory for tandems.

Robert Anderson said...

John, I don't disagree with your hot rims comment, although I thought the heat was more of an issue with melting glue for sew-ups rather than blowing tubes (which I've not heard of). But, I do disagree with your torque analysis. While the torgue is identical (and in fact my diagrams support this directly), there's no way the load on the spokes is equal.

Huack said...

Funnily enough, disc brakes require significantly less effort from fingers to achieve same braking power. Single index finger is sufficient to endo you (and quite easily too). All to do with leverage ratios, disc brake pads translate a lot less for the same lever pull.