Mysterious imbalance in my ACM??? What is it?

[meepmeepmayer]

Thanks! Not too worried about damaging the motor though, it's toast already. It just needs to maybe bandaid-live for a little. But trying the grips first can't hurt.

[Hunka Hunka Burning Love]

Thinking backe, I bet Marty is right about the crack fatigue.  We've seen this exact scenario several times before.  Maybe the motor supplier is making the rim thickness too thin, and when they curve the flat piece to form the circle the bend area gets stretched and thinned out even more resulting in micro-fractures of the alloy.  The outward inflation pressure added to repeated impact events from riding off-road forces the edge to split open at the weak spot allowing the side to bend out like that.

JB Weld usually is like a thick tar or syrup that sort of settles as it sets.  I think you can sand it smooth too if needed.  A toothpick or dental tool can scoot it into the crack.  Place some in right before you bend the rim back.  It takes 24 hours to set fully. I'm not sure RTV would add much strength to bond the crack together since it's a silicone?  It might give again?

[meepmeepmayer]

Hey the general fatigue thing was my idea I suspect it's just bad luck or bad material in a production batch (like those breaking pedals that looked like baked from sand and glue with traces of metal dust in it). Or the tire is a rim breaker? Had that impression too, with it conspiciously bending right next to that Kenda logo.

Shouldn't happen either way.

I'll try bending it back carefully and maybe some silicone.

[steve454]

6 minutes ago, Hunka Hunka Burning Love said:

JB Weld usually is like a thick tar or syrup that sort of settle as it sets.  I think you can sand it smooth too if needed.  I'm not RTV would add much strength to bond the crack together.  It might give again?

Good point, you're right the silicone RTV would not add much strength.  I was thinking the crack wouldn't get any bigger.

And yeah, while the JB Weld is wet it can be smoothed  with something, and sanded later if need be.

[meepmeepmayer]

Ok, thought it's some actual metal stuff, but it's epoxy. Epoxy (hard) sounds better than silicone (soft). Guess I'll try that. Thanks, guys!  Maybe it can even be a long term fix?

[meepmeepmayer]

8 minutes ago, meepmeepmayer said:

Ok, thought it's some actual metal stuff, but it's epoxy. Epoxy (hard) sounds better than silicone (soft). Guess I'll try that. Thanks, guys!  Maybe it can even be a long term fix?

In other words, I'm with Hunka on this one. Here you go, for your club.

[Marty Backe]

49 minutes ago, meepmeepmayer said:

The Kenda tire wants 2.5 bar which is less than the 3 the standard tire expects. I wouldn't know any impact, I barely even jump down curbs. Guess the crack was there before, and the bend happened more or less randomly.

Never heard of that but will check out. For now, the plan is to hammer the thing back into shape and see what happens.

Axle spacers? Another part I didn't even know was there

Does the board come installed on the aluminium heatsink? Because during disassembling that, you find you need a lot of parts - spacers (board on sink), these plastic sheets between the mosfets and the sink, etc.

The control board is fully self-contained.

[meepmeepmayer]

Sorry, now I'm having language problems. That means yes? You just need to screw the sink with board and everything in, and that's it (and some thermal paste)?

[Marty Backe]

4 minutes ago, meepmeepmayer said:

Sorry, now I'm having language problems. That means yes? You just need to screw the sink with board and everything in, and that's it (and some thermal paste)?

You remove the old board (one bolt that goes into the pedal hanger and 4 screws that go into the shell). You take the newly purchased board out of it's bag and put it in the wheel. The board/heat-sink is an integrated unit.

[meepmeepmayer]

Thank you!

[Tbx Nicolas]

Otherwise you could open the motor and exchange de rim with the old one. However it's a delicate operation where you can leave some fingers ( @Hansolo) and you will need some material to tear appart the motor. I've already "repaired" such a crack for someone that couldn't afford a new motor, you will need a good hammer, a piece of wood and some determination ! However the crack will stay there and could cut the inner tube: to solve this, I had no mercy but hammering the crack inside the rim to smooth it out. But we have to keep in mind that such a crack creates a weakness and could break over time...

Regarding the bend, in my opinion it happened on hitting a hole or a small curb with a consequent speed. I know so many people also that don't have enough pressure in their tire (I ride at 3bars), and by hitting curbs and holes bend the rim. Normally, when you create such distortion on a hole, you pinch the inner tube which instantly deflates. 

[Hansolo]

@Tbx Nicolas  pfft!

 

[meepmeepmayer]

@Tbx Nicolas Wow, great info, thanks! So it is (theoretically) possible to exchange the rim. Looks complicated, though.

Good idea with the curb. I hit one once, but I don't think that was it.

I'll try to bend it back and use the epoxy to "seal" the crack so it doesn't expand or cut the tube (don't think that will be a problem, though) and see how it holds up.

[meepmeepmayer]

edit: This post was premature, read the next one.

A little update on this (wow, almost a month already!):

• Finally managed to semi-fix the bent rim. After epoxying the crack inside the rim (which I'm pretty sure was absolutely pointless, what is it going to do?) I failed to bend the thing back with hammers (it was surprisingly stable and hammers give you little fine control).
  Went to a friend's house who has some more and better tools, and we used a bench vice to bend the thing back to normal as much as possible.
  But now there's an extra fissure that couldn't be prevented.
  Not sure if these cracks are a problem per se.
• It still has a wobble, but it is notably better. I also didn't sand back the epoxy yet, which might further reduce any imbalance.
• Did a test ride to see if it would bend again and the tube exploded (not sure if related to this, but probably no coincidence, or I pumped up the tire too much), so the jury's out on how long and good this will work.
• Will post some more pics when disassembling for the tube change and finding out what happened.

Summary:  If your rim is bent (and maybe cracked), it's unlikely bending it back will be a permanent solution. But as a temporary one, you can try, it's good enough and you lose nothing. In the end, you will want (need) a new motor/rim.

[meepmeepmayer]

Another (better update):

My ACM is wobble free now! Rode today until I remembered that it should wobble, but it does no longer. Turns out sanding back the epoxy did lot there to remove the imbalance (which I thought was from the not perfectly bent back rim itself).

The rim is bent a little too much (from overdoing it when bending it back in shape), which produced a fissure on the outside, too. But you don't feel anything. I'll epoxy the outside fissure, too (because why not) and that will be it!

So it looks like if you have a bent rim, if you can bend it back, that's enough! Just be careful doing that.

Will see how it holds up in the long term.

Slightly overbent rim and sanded epoxy.

Crack on the outside. You could hear when it happened, but then it was already too late.

[meepmeepmayer]

Final update:

Over 300km and some grueling offroad rides later, I can safely say the repair is a 100% success. I completely forgot there once was an imbalance. So if you have a bent rim, bend it back carefully, and that's it! ?