In recent years, more and more bicycles have been sold with forkends that have holes rather than slots, and  “thru axles”, rather than quick release assemblies. Why is this? Let’s make a comparison and see why.

A quick release assembly compresses the dropouts against the hub locknuts and that is all it does. The axle of a quick-release hub extends into the dropouts when the wheel is  in place on the bicycle. When the quick release is properly tightened, the locknuts press hard enough against the inner faces of the dropouts that they resist forces that would try to move the hub in the dropouts. The axle does not resist the load on the hub: the locknuts do. More about quick releases is here.

Image: A Shutter Precision disc-brake front generator hub with a quick-release axle. The axle extends into the front fork's dropuut slots.

Traditional hub locknuts are serrated (have teeth) so they bite into the dropouts. If the quick release is tightened properly, these locknuts can support any normal load, even the chain tension from a tandem bicycle – except for the loading from a front disc brake.

A front disc brake exerts an extremely strong force tending to pull the wheel out of the dropout. Look at the brake from the left side, and see why this happens: the disc is rotating upward through the brake’s caliper, and resisting that upward motion, creating the downward force.   As the brake is applied and released, alternation of upward force of weight load and downward force from the brake can loosen a quick release and eventually allow the brake to tear the front wheel out of the fork.  The force from the brake can even bend and break the quick-release skewer when the dropouts have “lawyer lips” that (loosely) retain the wheel if the quick release has not been properly tightened. More about disc brakes is here, with a description of their problem with quick releases.

The disc brake problem is made worse because some boutique hubs have smooth faces pressing against  the dropouts, also because exposed-cam quick releases have become common in recent years. These quick releases do not tighten as securely as traditional enclosed-cam quick releases.

The thru axle has been put forward as a solution to these problems.The thru axle serves both as an axle and as a quick release. It inserts into the hole in one forkend, passes through the hub, and threads into the hole in the other forkend. No part of the hub extends into the holes in the forkends. That can’t be: the hub could be inserted only by prying the forkends apart! 

The thru axle may only thread in, or it may in addition have a cam assembly like a quick release to secure it. You may view a Global Cycling Network video about how to use a thru axle.

One advantage of a thru axle system is that a disc brake can’t pull the wheel out. Another is precise alignment, so the disc will sit inside the brake caliper without rubbing. 

Image: A similar hub for a thru axle. The axle is not part of the hub.

The bearings of a thru-axle hub rotate around a sleeve that encircles the thru axle. Weight mostly rests on the thru axle rather than being transferred to the forkends by the surfaces at the ends of this sleeve. The sleeve is kept from rotating by the compression of the thru axle. But importantly, that compression need not be as strong as with a quick release.

A thru axle system does have some important disadvantages.

As the thru axle is not attached to the hub, it can be misplaced or lost. And different bicycle manufacturers supply thru axles that are not compatible with one another. If you lose a thru axle, hunting down a replacement could be troublesome. So, when you remove the wheel, you should immediately replace the thru axle in the forkends, so you won’t lose it. With quick releases, on the other hand, the only important incompatibility is with different lengths.

Dropout alignment is critical with a thru axle. A steel fork or frame with slotted dropouts can be rebent and will work perfectly; a thru axle fork or frame must be manufactured to very tight tolerances and can’t be realigned – well, most such forks are carbon or aluminum and can’t be rebent anyway.

Image: End view of the hub made for a thru axle. The thru axle passes through the large hole in the sleeve. The hub bearing is behind the dark circle, which is the bearimg seal. The sleeve can rotate on the axle if it is not secured tightly.

 A wheel change is not as quick with a thru axle as with a quick release – but on the other hand, a quick release and disc brake may be even slower to install due to imprecision of rotor alignment. This is not to speak of the extra time it takes to remove and reinstall a quick-release wheel when a front fork has “lawyer lips.”

 That the compression of the forkends against the hub faces need not be as great with a thru axle as with a quick release has resulted in a bizarre problem with front generator hubs: torque from the generator can rotate the hub’s inner sleeve , and with it, the connection to the wires to the lamps. The wires wind up and break. Only SON brand hubs which connect through a special, electrically-insulated forkend are immune to this problem.

To sum up,  I advise that bicycles with disc brakes have thru axles, because they solve the safety issue; but thru axles bring problems of their own, and if you are a retro-grouch like me, you’ll stick with quick releases and rim brakes, or youll pay close attention to the tightness of quick releases on bicycles with disc brakes, indeed, I have two of them!

John S. Allen is CRW Safety Coordinator, a certified CyclingSavvy Instructor and League Cycling Instructor and author of Bicycling Street Smarts.