Thru-axles are extra-long hub axles that fit directly through the hub-cartridge bearings and screw or clamp directly into the dropouts; the axle must be fully removed before you can take off the wheel. Front thru-axles are generally 15mm or 20mm in diameter. They stiffen the fork against lateral and twisting flex, and they offer a higher degree of safety against the wheel falling out of the fork than a quick-release skewer does. In particular, the closed ends of the fork dropouts prevent the braking force applied to the hub rotor from pushing the axle out of the dropouts, even if the securing system is loose.
Variations in Design
A fork with a thru-axle will offer improved tracking and steering as well as smoother up-and-down action than one with a quick-release skewer. Thru-axle systems vary—there are bolt-on and quick-release versions—but they share some common traits. The axle comes as a part of the fork, not as a part of the wheel. All 15mm thru-axle hubs have the same (15mm) inside diameter (ID) of their bearings and the same position of the disc-brake rotor relative to the caliper mounting tabs. Ditto for 20mm or 24mm hubs.
Traditional thru-axles generally resemble a long bolt that clamps into the fork’s dropouts. The head is too large to pass through the hub bearings and the dropouts; it snugs up against the drive-side fork dropout. The ends are usually round, but they may also be hex-shaped. A clamp-in thru-axle usually has some sort of bolt system on the opposite end from the head to draw the ends toward each other, but some of these axles are threaded and simply screw into the opposite dropout. Pinch bolts (or a pair of quick-release levers) tighten the dropouts around the ends of the axle once it is fully installed.
Quick-release thru-axles have a lever on the end to release them. Except on a Manitou fork, you can use the lever as a handle to screw the axle out of or into the opposite dropout. When you flip the lever closed, it either expands the axle inside its through-hole in the dropout to secure it, or squeezes the dropouts against the hub-axle ends. A quick-release thru-axle can be as quick to use as a quick-release skewer and a standard hub.
Removing a Thru-Axle
To remove a quick-release thru-axle, on all forks except Manitou, flip open the lever fully. If there is a cutout in the ring under the lever, engage the lever into that cutout (in the fully open position) to ensure that the lever unscrews the axle, rather than spinning freely. Rotate the lever counterclockwise to unscrew the axle, and pull the axle out.
To remove a Manitou QR15 HexLock Thru Axle, flip open the lever fully (it will go from pointed straight up to pointed straight down) and rotate it counterclockwise 90 degrees (a quarter turn) so that it is pointed straight forward; an internal spring will cause the lever to pop out a few millimeters. Grasp the lever and pull the axle straight out.
To remove a traditional draw-bolt thru-axle, loosen whatever clamp bolts or levers are securing the bolt head, and unscrew the draw bolt. Loosen the pinch bolts to free the axle head, and pull the axle out to that side.
To remove a threaded, clamp-in thru-axle, loosen all of the clamp bolts on both dropouts. Unscrew the entire axle and pull it out.
Installing a Thru-Axle
To install a thru-axle, slide the wheel into the fork (sans axle), aligning the rotor between the pads of the disc-brake caliper. If the dropouts have little inboard lips that sit on the hub ends, everything will be lined up to insert the axle; if not, you’ll have to carefully hold the bike up as you line up the hub and dropouts to install the axle. Push the axle through from the side with the bigger hole in the dropout.
On a QR15 axle other than a Manitou, tighten the axle into the opposite fork leg using the quick-release lever as a handle; if there is a cutout in the ring under the lever, flip it open fully and twist until the lever engages the cutout so that the axle rotates along with the lever. Periodically flip the lever over and check for tightness; keep screwing the axle in until the lever takes a firm push to flip it into the tightened position. When properly adjusted, the lever should not only leave a momentary imprint in the palm of your hand to indicate proper tension, but it should also be pointed up, parallel to the fork leg.
If proper tension and the lever being pointed up do not occur simultaneously, then you will need to adjust the thread. To do so, remove the fixing screw and triangular washer securing the notched thread-adjustment ring. Unscrew the axle a few turns and then push inward on the lever end to pop the thread-adjustment ring out of its notches. Turn the ring the appropriate direction to loosen or tighten the adjustment of the QR lever as needed so that it is at optimal tension when it is in the straight-up position. Replace the triangular washer and fixing screw. Check that the lever and axle are tight and that the hub is secure in the dropouts.
A Manitou QR15 HexLock thru-axle has a hex shape on either end of the axle that mates with the same shape inside of the dropouts, and a key on the end opposite the lever to engage the opposite dropout. Orient the axle so that the laser-etched instructions along its length face up, toward the handlebar. With the lever flipped open (down), slide the axle straight in from the same side you pulled it out. While pushing inward to compress the spring and engage the key in the opposite dropout, rotate the lever 90 degrees (from pointed straight ahead to pointed straight down). Flip the lever over to the closed position to secure the wheel in the dropouts.
If it requires insufficient pressure to push the lever over to its closed position (it didn’t give you the satisfaction of providing enough resistance to temporarily indent and squeeze the blood out of the palm of your hand), flip the lever open (down) again, unscrew (counterclockwise) the adjustment ring under the lever a bit, and flip the lever back up to see if it is now tight enough. Similarly, tighten (clockwise) the adjustment ring with the lever open if it is too hard to flip the lever up to the closed position.
For a screw-in thru-axle, with or without clamps on the dropouts, tighten the axle with a hex key or Torx key of the appropriate size. If the fork has pinch bolts on the dropouts, tighten the bolts on the threaded end first. Push down on the handlebar a few times to compress the fork and settle the head end, and then tighten the bolts that pinch the dropout on the head end. On most traditional (usually 20mm) thru-axles, tighten the draw bolt from outside the dropout into the thin end of the axle. You may need to snug up a clamping bolt on the head end a bit so that the axle does not spin when you tighten the draw bolt. Once the axle is seated, tighten all of the clamping bolts on both dropouts.
Adapted from Zinn & the Art of Mountain Bike Maintenance, 6th edition, by Lennard Zinn with permission of VeloPress.