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Old 12-15-09, 03:41 PM   #1
rtciv
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Electric-assist trike, front wheel keeps coming off

Hi everyone. My friend has some kind of Pashley trike which has had an electric assist system added to it. The type with the motor as the hub of the front wheel.
The front axles protude from either side of the hub/motor in the usual way, and bolt into normal dropouts.

The problem is that, no matter how tightly the nuts are done up, apparently the front wheel comes right out of the dropouts after just a matter of a few 100m, or a mile at most. As in, right out, and it lies twitching on the road.

I have attached the best pictures I could get... which aren't very good... but I wonder if you have any ideas? Apparently the trike has been taken to a bike shop and they tightened it on really well... but it still comes off.

Thanks



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Old 12-15-09, 03:56 PM   #2
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See how the inner washers are cocked? They aren't the right ones for the dropouts. The inner washer can be no bigger than the dropout flat. You're essentially trying to clamp something to an inclined plane and it just slides right off.
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Old 12-15-09, 04:01 PM   #3
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Valid. Hmm... I'm not sure why it IS at an angle, though. The dropout doesn't expand into the tubular part of the fork until long past the edge of the washer. I will re-examine the trike with that in mind, though.
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Old 12-15-09, 04:21 PM   #4
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Jeeze, that is the worst candidate for a front wheel electric assist. I bet the dropouts aren't very deep either + ultra rusted.
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Old 12-15-09, 04:25 PM   #5
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I gather the wheel isn't original to the trike?
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Old 12-16-09, 04:13 AM   #6
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That axle looks kinda chunky compared to a front wheel. Could it be that the axle isn't resting at the bottom of the dropouts, but is rather sitting balanced right at the opening? That would explain why the inside washer is sitting at an angle.
The first outside washer is also an axle retaining washer, the kind that you normally find on forks w/o "lawyer lips", or axle retaining tabs. That washer isn't merely round, there's a "claw" on it (pointing to the right in the pic). That claw is supposed to hook into a hole in the fork , drilled right above the dropout.
The hole should be on the outside face of the drop out, but may be on the inside face as well.

Also, there's little point in using a knurled nut against a regular washer.

On further consideration, that wheel will put torque on the axle. On the right side this torque will act as to tighten the axle bolt, but if the left side is threaded in the same direction, drive torque will act as to unscrew the axle bolt. There should be something else in place to deal with the torque. If the thing that I think is an axle retaining washer is shaped after the axle flats rather than having a circular hole, then this could be it. If it is, it makes it even more important that the washers with the claw is securely anchored and prevented from rotating.
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Old 12-16-09, 08:02 AM   #7
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I gather the wheel isn't original to the trike?
Correct. It's of a smaller diameter than the original, too (the brakes have been lowered).

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That axle looks kinda chunky compared to a front wheel. Could it be that the axle isn't resting at the bottom of the dropouts, but is rather sitting balanced right at the opening? That would explain why the inside washer is sitting at an angle.
The first outside washer is also an axle retaining washer, the kind that you normally find on forks w/o "lawyer lips", or axle retaining tabs. That washer isn't merely round, there's a "claw" on it (pointing to the right in the pic). That claw is supposed to hook into a hole in the fork , drilled right above the dropout.
The hole should be on the outside face of the drop out, but may be on the inside face as well.
Yea I'm not sure why that washer is on there. There are no slots in the dropout to receive the tab on it.

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Also, there's little point in using a knurled nut against a regular washer.

On further consideration, that wheel will put torque on the axle. On the right side this torque will act as to tighten the axle bolt, but if the left side is threaded in the same direction, drive torque will act as to unscrew the axle bolt. There should be something else in place to deal with the torque. If the thing that I think is an axle retaining washer is shaped after the axle flats rather than having a circular hole, then this could be it. If it is, it makes it even more important that the washers with the claw is securely anchored and prevented from rotating.
hmm, yes. I assume the washers came with the new electric wheel and perhaps were intended for a fork with the hole/slot for the axle-retaining washer. I'll see what I can find about electric-assist wheels in general.
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Old 12-16-09, 08:38 AM   #8
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That's a pretty flimsy looking dropout to subject to the torque of a hub motor. I've done a couple of front hub electric conversions, one for my wife, and I fabricated special mounting tabs. I'll take a photo and post it in a little while.
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Old 12-16-09, 10:13 AM   #9
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Yea I'm not sure why that washer is on there. There are no slots in the dropout to receive the tab on it.
That's a good indication that they came with the wheel then.

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hmm, yes. I assume the washers came with the new electric wheel and perhaps were intended for a fork with the hole/slot for the axle-retaining washer.
See if the center hole of that washer is round or if it's a snug fit against the flats of the axle. If it's a snug fit then that's your anti-rotation device as well. In that case it really needs to hook into the fork somehow to do its job.
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Old 12-16-09, 10:35 AM   #10
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Here is the dropout tabs. I made them from 1/8" plate steel. These forks have mounting holes fore and aft of the dropout, so I was able to make it really robust, but a single fender mounting hole behind the dropout would probably be adequate.




From your picture, it does not appear that you have anything to mount a re-enforcing tab to. I would seriously consider upgrading to a beefier fork. It's not really that expensive, and this is a critical safety issue.
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Old 12-16-09, 10:50 AM   #11
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I'm a little curious how the front wheel "comes off" while riding. Even if the bolts loosen the rider's weight should hold it in the dropouts until it hits a bump or something. Perhaps, as Dan suggests, the motor torque is rotating it out once it slips loose due to the washers. If so I think you have two problems to solve.


Nice looking mounting tabs, Dan.
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Old 12-16-09, 10:52 AM   #12
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I'm a little curious how the front wheel "comes off" while riding. Even if the bolts loosen the rider's weight should hold it in the dropouts until it hits a bump or something. Perhaps, as Dan suggests, the motor torque is rotating it out once it slips loose due to the washers. If so I think you have two problems to solve.


Nice looking mounting tabs, Dan.
Ever try rotating a axle nut when it doesn't want to grab into the position you're trying to tighten it onto? It slides. Like a mother****er.
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Old 12-16-09, 11:03 AM   #13
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Those dropouts are also very forward facing from the looks of it, like the trike has a very shallow head angle and the fork ends are angled for a steeper head tube.

It's no surprise that the wheel pulls itself out in short order. The only way I'd use that wheel on that tricycle is with a different fork with proper dropouts, and a much more robust way to attach the wheel.
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Old 12-16-09, 03:39 PM   #14
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Yet Another Trike Conversion

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Old 12-18-09, 10:01 AM   #15
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I bet the fork ends are rusty because the dropouts couldn't take the torque. The motor twists the wheel by pushing against the dropouts. The torque is much higher at the dropouts than the rim because of the much smaller diameter. The dropouts bent, which caused the paint to flake off.

Imagine if the axle was welded to the hub, then imagine grabbing the wheel by the tire and turning it. You would easily bend the fork's dropouts. That axle is what the motor pushes against and the dropouts can't take the twist. They have bent themselves into an angle which causes the axle to cam itself out of the dropout due to the torque.

I like Dan Burkhart's dropout tabs but a reaction arm would be even better, similar to a coaster brake arm. Consider what would happen if the braking torque of a coaster brake was only resisted by an axle with flats on it, instead of a reaction arm. Create a couple of reaction arms which fit the axle as closely as the "Burkhart tabs" but attach with a strap about halfway up the fork leg or more.

If you do build my reaction arm suggestions, recall that they have to be able to take torque in both directions because when you cut the throttle at speed the wheel exerts a braking effect, so you can't just use a sheet metal strap like a coaster brake would.
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