alexon

Hello all,

Last year I bought a GT Avalanche MTB and fit an electric kit on it.

In order to do that, I had to file away 1mm from the front dropouts because the axle on the electric kit’s wheel was 10mm. That’s quite a common problem with ebikes, and most people are advised to just take a file to the dropouts (with some caveats). Well, in the picture below you can see what happened to my bike today due to the torque of the electric motor on the fork

Anyway, my question now is: should I try to fix the fork or will it be cheaper to just throw it away and get a new one? If I try to fix it, can I buy an old fork, take it apart, and use the lower legs on my fork? Will they fit or does each brand have different dimensions? My fork is a Rock Shox XC28.

Thanks a lot for your help!

Alex

FBinNY 

To start with, this is not repairable, or at least not repairable within practical economic limits.

But I suspect there's more going on here. Electric motors generate torque on the axle identical to the torque delivered to the rim (Newton's third law), so I would expect that the motor hub would have a reaction (or torque) arm to attach to the strut, similar to the reaction arms we see on coaster brake hubs. Without that, I suspect you're in line for a number of failures going forward.

Also, you have a disc brake, which also creates some reaction forces on the axle, which would tend to stress the rear lip of the dropout, leading to this kind of failure. I can't tell whether the motor or the brake was the issue here, but keep in mind that it could be either.

AnkleWork

Is one of the caveats to use a round file?

MichaelW

Apart from the additional torque and reduced metal, did you create any stress raisers when you filed, eg notches or sharp corners?
Get a 10mm fork and why do you need suspension?

jimc101

The only thing you can do, is to replace the lowers, although that will probably cost the same, or close enough as to be not worth doing to getting new forks.

fietsbob

I believe dropouts sizing for electric hub motors are so much bigger than standard 9mm axles

you may be well advised to replace steel dropouts in a steel fork too ... off the shelf bodges are asking for trouble..
and it arrived..

alexon

Thanks for all your comments and your help :-)

#4 - Yes, I used a flat file and I think I left a bit of a sharp corner.

It's clear to me now that I did a poor job of enlarging the dropouts, but given the number of people who attempt this “workaround” with electric kits, I think biking forums (at least others I’ve read) should stress the importance of installing a torque arm to compensate for the powerful torque delivered by these motors. Simply filing away leaves a problem waiting to happen (especially when a strong force is applied to the front lip, which is usually thinner and more likely to snap).

As for me, it seems clear that the next fork will have to be new (no repairs or part swapping) and have 10mm dropouts (although most models I’m seeing are 9mm).

Live and learn (if you’re lucky :-D).

Cheers,

Alex

FBinNY 

Here's a picture of what it should look like.


Note the torque arm, and the side flatted axle. Without a torque arm the axle will try to rotate and act like a cam forcing the dropout open with tremendous force. With a torque arm, the axle is stabilized and doesn't unduly stress the fork

The sharp edge you left probably didn't help, but wasn't the main issue. If you had a torque arm on, I doubt the file work would have made much difference.

ClarkinHawaii

I don't get this pic--where's the dropouts?

Jeff Wills

Underneath the torque arm.

FBinNY 

It's hiding behind the torque arm and axle nut.

seeker333

I think I'd look for a beefy replacement rigid steel fork with axle-to-crown distance close to the original sus fork (loaded).

FBinNY 

Let's be real. Whatever torque the motor could possibly generate will be much less than normal braking forces, not to mention the normal stresses of bouncing off rocks.

Given that these types of forks have served so well in mtb for decades, there's no reason to worry about them when a motor is used ----- as long as a torque arm keeps the axle from camming.

The OP's problem isn't the fork or the filing, or anything else, except the lack of a torque arm.

seeker333

The lack of a functional fork is OP's current main problem, regardless of cause. While I agree torque arm is good idea, debating cause of failure doesn't put him back on the road. Rigid steel forks are about as strong as they come and they're generally less costly than sus forks.

OP, take a look at Surly forks, they make plenty of disc models and can usually be had for ~ 100 bux if you are a good shopper.

FBinNY 

Of course the OP needs a new fork, that bridge was crossed long ago. But the discussion is still relevant because the OP want's to prevent a repeat of the problem, even if he didn't say so directly.

To that end, understanding why and how his fork failed is important. I'm suggesting a torque arm as an absolute necessity regardless of the type of fork. Others suggest he needs a stronger fork or thru axle and they may be right, but I disagree on that score.

IMO the OP can buy the fork of his choice and be fine as long as he uses a torque arm, or torque key on the axle (if they make them for this application, but that would need a beefier dropout) or he can buy the strongest fork in the world, and still have problems if he doesn't use a torque arm or key of some kind.

alexon

Looks like this is turning into a lively debate, but I’d hate to start a discussion when I think all of us are basically in agreement:

1) Choosing a steel fork with big dropouts is one solution (especially if no filing is involved :-D). The problem, in my case, is that losing the suspension would defeat the purpose of this bike and change its anatomy (ironically, having a suspension is what got me to buy this bike in the first place and get rid of my old MTB with a steel fork).

2) Regardless of the size and resistance of the chosen dropouts, it is clear that a torque arm should be a highly recommended (if not mandatory) accessory in every e-bike and kit and should be mentioned more often in forums (until yesterday, I think I heard about it once or twice). It is a pretty cheap accessory and can definitely prevent major accidents.

For those interested, I’ll tell you how my fork broke, exactly, because I think this is a problem particular to bikes with front motors.

I was leaving my house and ran into a buddy. He asked about the bike and I showed him how it works. At one point, I lifted the front wheel and pressed on the accelerator to start it. He was standing right in front of the bike and, as the wheel was turning quite fast, he suddenly did a thing I hadn’t anticipated. He clamped it with both hands to stop it and “see how powerful the engine was”. One nanosecond later, the wheel came off in his hand and he was standing where with a WTF expression on his face (and mine). He apologized profusely, but I realized quickly that his little stunt had probably saved me from a serious injury down the line.

Thinking about it, I concluded that what tore the front lip of the dropouts clean off the bike was the torque that suddenly pushed the wheel *away* from the bike as he clamped it.

Since I have been using this bike for months and not had a problem, I’m guessing that this type of force is rarely exerted on the wheel in normal (or even off-road) use. Up to now, when I used the brakes (even hard), the wheel was pulled *towards* the bike and the pressure was on the back lip, which is thicker and stronger. That’s basically the reason I still have most of my teeth in place…

Anyway, now I’ll try to find an inexpensive fork (with suspension and 10mm dropouts) AND a torque arm to get back on the road soon. And I’ll try to spread this little piece of information around the e-bike forums (feel free to do the same :-D).

Cheers,

Alex