Fork flex vs. spoked-wheel flex vs. brake type
 

I've got a disc-braked CX bike and I've always been slightly concerned about how flexible the front fork feels fore-aft compared to my rim-braked road bike. My perception of flex was based on the standard "hold the front brake and rock the bike forward and back" which feels dramatically less-rigid on the CX bike. I had attributed this behavior to having a more flexible fork on the CX bike.

But as I thought about this more, I realized that perhaps my rigidity test is really more related to the type of brakes on the bike. When you hold the front brake of a disc-braked bike and push/pull the bike fore/aft, the braking force has to be transferred from the hub to the tire contact patch through the spokes whereas on a rim-braked bike, the braking force is transferred through the rim. With rim brakes spoke deflection is still involved since you're pushing on the bike fore-aft at the hub, but this fore-aft motion is working on the spokes in pure tension rather than winding up the spokes torsionally.

Anybody else notice what I'm talking about? Does it make sense?

If so, then it would seem that disc brakes are subject to some "windup" forces when in use that aren't there in rim brakes. And while I really like the feel of disc brakes in terms of stopping power, reliability, performance in the wet, etc. they don't feel as direct and rigid as caliper brakes to me.

- Mark

I have always found that excessive fork flex can be seen from the side by a riding buddy, or from the top by the rider, under hard braking. Simply take the bike for a ride and check if you can see the fork flexing under hard braking. I have seen forks flex a scary amount and not fail, but I have never seen a fork snap.

For kicks, I stuck a stick in the spokes near the rim to simulate rim braking on my disc-braked bike. It somewhat reduces the flex, but not dramatically. Slighting down the fork, I can definitely see it flex fore/aft a fair amount. This is one of those situations where it is difficult to visualize the internal forces involved. I guess I need to get out pencil/paper and start drawing free-body diagrams.

- Mark

Indeed they are. Basically a disc brake wheel will see the same kind of forces as a rear wheel does.

Seems odd to me as I've always read that disc forks need to be stronger to withstand the twisting force of the brake, but then again strength and rigidity aren't necessarily the same thing.

A rim brake fork will se a load evenly distributed between fork crown and both legs, applied by the drop outs. The disc brake fork will see a bending force applied between drop out and the top of the caliper mount. Like in this post http://forums.mtbr.com/showpost.php?...8&postcount=56