Aluminum Fork Failure Mode?

Some can break at the crown, but most failures invlolving hubmotors happen at the fork.

1, Aluminum alloy will only bend a tiny bit before cracking and breaking in two.

2. Torque arms in my opinion do not prevent this, but they can save your bacon when a failure happens. Keeping the wheel on the bike, instead of it dropping off is crucial.

3. The reason they break is two possibilities, both related to the fit and the tightness of the nuts and washers.

The dropouts must be nice and flat, with no cups or other molded shapes that the washer spans over. Any such cups, often found on expensive bikes designed for quick release hubs must be filed away, or filled with a custom made washer, one with a large hole for the axle, and a small diameter to fill in the cup.

What happens is this, any space behind the washer will slowly allow the washer to deform. Sometimes this puts a spreading force on the dropouts, I have seen steel dropouts slowly spread open as the nut was tightened. On an aluminum fork, the forces there may only result in a small crack wich goes unnoticed. Later on, the crack fails and the dropouts fall apart. Better not be going 25 mph at the time. The other way they fail, is due to the rotational force of the flats of the axle on the dropout. Again caused by the slowly deforming washers, in this case the washer bending gives a bit of space to the nut, which after the washer deforms, is not tight anymore. With a loose nut, the hub spins, breaks the dropouts, cuts the wires, shorts the controller, AND you go to the emergency room....

Here is how to deal with it.

Get the torque arms, they can prevent the rotation if the nut gets loose, and keep the wheel on the bike.

Be sure the washers and nuts fit well on the dropouts. File away any lawyer lips on the fork, or file down the washer to fill the cup without spanning any air space. Tighten the nuts gently with a small handled wrench. Watch carefully for any movement, spreading dropouts or an axle wanting to squirt out as you tighten are red flag warnings something fits poorly. If you get that far, wait 15 minuites. Then tighten some more, gently but firmly, a bit tighter than before. This is not lug nut tight by any means, but still good and snug. I'm guessing about 20 foot pounds, not so tight as to be uncomfortable for your hand on the short wrench.

Now it's ready for a test, just out and up the street a few houses and back, no full throttle. Check the nuts again. Many times just a hair more tightening can be done since the washers will deform slightly even on a perfect fit fork. This may be because the nut has a slight dish in its face. Now you can ride safely, but I like to check the nuts one more time the next day. Once a hub has been fitted to a fork once, this process is much less involved if the hub is removed to fix a tire. This procedure is for the first time, when the washers aren't broken in. Do not tighten as tight as a car lug nut, you will strip an axle for sure if you do. Firm, tight, but not with the two foot wrench, or straining yourself too much.

I developed this process after spinning the hub and ruining a motor and controller on my first try at ebikes. Since then I have several thousand miles on hubs and steel dropouts with no spins, no failures, no loose nuts.

Plenty of people are running front hubs on aluminum forks with no problems. The smart ones have two torque arms, but personally I won't do it. But it dooms me to having cheap steel suspension forks that only fit cheap bikes with 1" headsets. This is fine for me for street use, but my dirt bike has a rear hubmotor, and decent front forks.