It seems there has been a disturbing number of crashes recently involving ATB's equipped with front disc brakes. Apparently, the design of these bikes (the front forks, actually) can result in the quick release (QR) becoming loose due to the action of the brake, and actually cause the front wheel to be ejected from the dropouts, with obviously disastrous results! Since the same forces act upon a nutted axle, the same may apply to these as well. If you have a bike equipped with front discs, please read on.

The problem is due to the placement of the disc brake calipers in relationship to the dropouts, as well as the angle of the dropout. To better understand this, take a bicycle wheel and, holding it by the axle, spin it. Now grab the tire. Notice how the axle tries to pull itself out of your hand? That's because when you apply this stopping "force" to the edge of the wheel it's axis of rotation, that is, the center everything wants to rotate around changes. In this case, from the axle to the spot where you grabbed the tire.

In fact, if you were to loosen your grip of the axle at the same time you grab the tire, the wheel would do just that. Pull out of your hand and rotate around the spot where you grabbed the wheel. Or at least it would try to.

Now that you understand the forces acting on the wheel, lets apply it to a disc brake. Most, if not all bicycle disc brakes are mounted near the axle. Worse, they're mounted behind the forks.

When the calipers grab onto the disc, again, the axis of rotation tries to move from the axle to the point where the brake pads grab. If you were to draw a circle centered around this point of contact in the direction the wheel is rotating, passing it "through" the axle, you would see that it is trying to force the axle down, towards the open end of the drop out. Not exactly, but close enough.

Now if your QR is properly tightened, it may not be strong enough to wrench the axle out of the dropouts. Especially if you have the newer forks equipped with "lawyer lips' (those round depressions that the nuts or quick release ends fit into). Not at first anyways. But as soon as you release the brakes, your mass (weight) as well as the bike's immediately forces the axle back into the dropouts.

So your axle is constantly being shoved down out of, then up into the dropouts. Constantly conflicting forces such as these have a tendency to loosen threaded fasteners. I.E. nuts and bolts. And despite the quick release's apparent cam action method of holding a wheel, it is still basically that: A nut and bolt. And eventually it will loosen to the point that it is no longer able to hold back the axle when the disc brake tries to pull it out.

Lawyer lips may delay this, possibly even prevent it if the rider realizes what's happening and take preventative measures. But lawyer lips or no, a loose wheel is bad news under any circumstances.

So why doesn't this happen with regular rim brakes? Again, it's because of their location in relation to the dropouts. Draw a similar diagram, this time place the axis of rotation on your rim brake pads. When you draw the circle you'll find that the force is now try to push the axle rearwards, into the back side of the dropout. since its already there this back and forth shoving around never takes place. At least not enough to seriously loosen the QR skewer.

Now there is an easy way to fix this design flaw. In fact, there are probably many ways. Two possibilities are:

1. Move the disc brake calipers from behind the forks to the front. This places the contact point, and therefore the "axis of rotation" above the dropouts, just like rim brakes.
2. Change the dropout so that the wheel drops out in a forward direction. This should work even with the calipers mounted behind the forks since again, the force acting on the axle will be driving it against the dropout wall, instead of the opening.

So does this mean you should stop riding your ATB and send the forks back to the manufacturer for repair or replacement?

Unfortunately you can't, not yet. They haven't started to incorporate any design changes yas of this writing. Some manufacturers are "looking into the problem", but you know how slow they can be to change something they think is "tried and true". Especially if it requires redesigning or retooling (which = $). So short of buying a new wheel and front brake set, there's nothing you can do right now.

However, if you do wish to continue riding your bike this way, realize that there is an additional risk in doing so!

In the mean time, there are some things you can do to at least reduce the chances of your wheels from being ejected from your forks:

1. Don't file off the "lawyer lips"! They are there for a reason. If you file them off, you are deliberatly compromising the safety of your machine! Besides. If you do, and your wheel does come off, you won't have a leg to stand on in court (excuse the pun).
2. Always inspect the wheels before each ride, Actually, this is good advice with any bike, regardless of what kind of brakes it has.
• Is the axle in the right place? Meaning is it all the way in the dropout?
• Are the nuts tight? or the QR properly adjusted? The lever should meet resistance halfway through it's swing, meaning when it's sticking straight out from the forks, parallel to the axle. A little before is O.K. After is not.
3. Check them periodically during your ride. Especially after a downhill or any time after a lot of repeated braking. Look for the same things.

Actually check them, don't assume that just because they don't look loose, or don't ride funny that they are all right, or that "they'll be OK for the last few miles". Because if they do fail, they will do so catastrophically, Most likely when your braking hard. Either because you're going too fast and really need to slow down quick, or you need to avoid a crash.

Now your quick release may never loosen, your wheel may never fly out. This has not happened to every single ATB on (or off) the road, it may not happen to yours. Just be aware that the possibility does exist, that that it can happen, and if you do continue to ride your bike, take the appropriate precautions.