falling off bikes since 1975

Following on from our Fight For Your Right To Die A Stupid Death article about fixie riders with more fashion sense than common sense, it’s ironic that David Wrath-Sharman has written a nice article on hub brakes for fixie site 63xc.com.

DW-S is the antithesis of this fashion over function set, and he across as a common sense practical rather than theoretical engineer. This is no bad thing.

63xc.com: A couple of builders like Matt Chester don’t like disk brakes, because of the stresses they build into the frame. How do hub brakes compare?

David Wrath-Sharman: Well, the big difference with a hub brake is the reaction arm. This is mounted on the axle at one end and the forks or frame at the other and takes all the braking loads. I designed my hubs with very long reaction arms which go right to the top of the forks or right up against the bottom bracket. That allows the fork to react exactly as it would with a cantilever setup.

My only niggle with this statement is that the reaction arm is applying forces to his cantilever bosses at about 90 degrees to a normal cantilever brake. This isn’t quite reacting ‘exactly as it would with a cantilever setup’ and would need to be borne in mind by frame/fork designers if reaction arms ever make a serious comeback.

This unexpected force direction was my major worry when mounting a big Hope disc to the back of the tandem using a reaction arm and one of the cantilever mounts. Braking forces with the cantilever brake are trying to push the brake mount into the seatstay. With the reaction arm they are trying to snap it off by pushing it upwards.

Even though times have moved on now and we have the IS and post mount systems reaction arms still aren’t a bad idea. They are still being played with in rear suspension in an attempt to counter brake-jack, and have been used on motorbikes to counter fork dive. I wouldn’t discount someone re-inventing that as an application.

63xc.com: Presumably, the longer the arm, the less stress in the frame?

DW-S: Yes. If you know what you’re doing, you can have a very light frame with hub brakes.

Initially I failed to see what David meant here. If I pull on a torque arm the force at the end is the same irrespective of the length of the arm. How does that reduce the stresses on the frame.

Will from 63xc.com pointed out that David is bringing up the issue of leverage. By placing the point at which the braking forces are transferred to the frame a long way from the hub the forces on the frame required to counter the braking torque are less.

With this in mind then what DW-S is saying makes perfect sense.

For a given braking decceleration and force - I normally assume a stoppie with all rider weight transferred to the front wheel - it can be shown that all other things being equal the forces transferred to the fork are larger with a smaller disc. This appears counter-intuitive to many people.

I think of it as holding a pencil between finger and thumb - equivalent to dropout/disk caliper close together - compared to holding one in your fist - dropout/disc caliper further apart - and trying to push the free end of the pencil. The free end represents the tyre on the ground.

When the pencil is held in your fist it’s easier to resist the pushing, and the forces at the top and bottom of your fist are smaller than those at the top and bottom of your finger. Clear as mud.

So, larger discs place less force on the fork, and by extrapolation, rim brakes even less. By using a torque arm to apply the braking loads to the fork/frame in a similar position to a rim brake DW-S reduces the frame/fork loading to about a third that of a 160mm disc brake.

Of course the smarmy types ask why we don’t just go back to using rim brakes and get rid of hub brakes. A quick look at the rim mounted disc brake on something like a Buell Firebolt XB12R should shut them up. Yes that really is a single disc on a 1200cc sportsbike, powerful enough to do stoppies, and with no torque loading on the spokes.

Back to DW-S and I’m sure someone used to do Polaris on a bike with a single sided fork that used one of his hubs. Currently drum brakes and DW-S appear to be out of fashion. Hopefully 63xc.com can spread a little common sense.