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


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So last ride, I went over some completely inconsequential bits of gravel on the road, and instantly my ACM showed an imbalance in the tire.

Riding now feels like going over a road made from concrete plates and the regular joint between them - there's a regular little bump like going over a small obstacle. It's definitely related to speed, once every tire rotation (I guess). At first I thought a pebble got stuck in the tire, that would have explained it perfectly. But there's nothing there!:efeed51798:

With more riding, it got a little worse, and now the bump feeling isn't just up-down, but it's also a sideways "kick". So the wheel works normally, but every tire rotation there's this strange "kick" to the wheel, like going over some destabilizing bump, like the wheel is low-key trying to throw you off like a bucking horse. Not sure how to better describe it.

Can it be the axle shims? But then why is it related to tire rotation (once per rotation)? There's no visible out-of-balance of the tire/motor/rim.

Help plz:efee8c29ce: Happy about any ideas:efee47c9c8:

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42 minutes ago, meepmeepmayer said:

So last ride, I went over some completely inconsequential bits of gravel on the road, and instantly my ACM showed an imbalance in the tire.

Riding now feels like going over a road made from concrete plates and the regular joint between them - there's a regular little bump like going over a small obstacle. It's definitely related to speed, once every tire rotation (I guess). At first I thought a pebble got stuck in the tire, that would have explained it perfectly. But there's nothing there!:efeed51798:

With more riding, it got a little worse, and now the bump feeling isn't just up-down, but it's also a sideways "kick". So the wheel works normally, but every tire rotation there's this strange "kick" to the wheel, like going over some destabilizing bump, like the wheel is low-key trying to throw you off like a bucking horse. Not sure how to better describe it.

Can it be the axle shims? But then why is it related to tire rotation (once per rotation)? There's no visible out-of-balance of the tire/motor/rim.

Help plz:efee8c29ce: Happy about any ideas:efee47c9c8:

Wow, I can't imagine what could be the issue, but we know these are simple devices, mechanically.

It's either the tire, motor rim, motor axle?, or perhaps the shell.

Have you done a lift test (where the wheel spins up and shuts down)? I'm not sure what that would tell you, but maybe something will come of it.

You will probably have to open the shell to probe more. You're just going to have to knuckle down and start removing screws :(

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Put the pedal supports on blocks so the tire is off the floor and you can control the speed by tilting the euc by hand. If you can still feel or hear the kick, you can exclude the tire and the rim.

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21 minutes ago, meepmeepmayer said:

Thank you everyone for your ideas. I was following them and doing a lot of testing and was going to write up the results, but then I just saw the culprit. Not sure why I didn't see it before, maybe it got bigger on the last test ride?

Anyways: the motor rim is bent and cracked :efee8c29ce: (Problem guessing prize goes to @Tbx Nicolas :efee8319ab:). No idea why this happened as I was only going on a normal road with a bit of inconsequential gravel on it. Maybe general fatigue?

Bend visible from the outside. I was hoping a little bit of hammering would be enough.

105022.jpg

Turns out, there's a crack inside:efee8c29ce:

105023.jpg

A pretty big one:efee565ab0:

105024.jpg

That means a new motor is needed? Any other ideas how to fix this? I have an old motor (probably ok but the cable had to be cut) but I don't guess one can change the rims only?

--

Interesting side note: Good news, my ACM (production date in April 2017) already has the extra metal backing plates for the axle, preventing worn out shims. This fix has been going on for very long already! My old ACM (Production date in Nov. 2016) doesn't have them yet.

--

You sure it's hardware related? Not just the standard unrelaxed legs wobble?

no wobbles on the other 5 wheels, so yeah, positive.

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15 minutes ago, meepmeepmayer said:

Hard to know what is the gravel path and what is the wheel. In the spirit of today's results, you might want to check the tire rim if you see/feel anything.

i will again. i was watching marty take apart his acm and i wonder if an axle bolt wasn’t tight, like a half a thread loose, maybe that would cause the wobble. i can turn the wheel around and the wobble will be less. just too lazy to take it apart.

Edited by novazeus
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Before you take it apart, take a really close look at the rim. I didn't see the dent the first time, may be hard to spot if the shell blocks your view. Good daylight + lie the wheel down and also feel with your fingers while you turn your wheel through them.

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30 minutes ago, meepmeepmayer said:

Well, stance can depend on the wheel, obviously. It sounds like you don't always get them on the S2, so it probably isn't a hardware thing?

i’ve got 2 s1’s which are identical to the s2 just slower, and they are smooth as glass, less noisy too. 

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1 hour ago, meepmeepmayer said:

Thank you everyone for your ideas. I was following them and doing a lot of testing and was going to write up the results, but then I just saw the culprit. Not sure why I didn't see it before, maybe it got bigger on the last test ride?

Anyways: the motor rim is bent and cracked :efee8c29ce: (Problem guessing prize goes to @Tbx Nicolas :efee8319ab:). No idea why this happened as I was only going on a normal road with a bit of inconsequential gravel on it. Maybe general fatigue?

Bend visible from the outside. I was hoping a little bit of hammering would be enough.

 

Turns out, there's a crack inside:efee8c29ce:

 

A pretty big one:efee565ab0:

 

That means a new motor is needed? Any other ideas how to fix this? I have an old motor (probably ok but the cable had to be cut) but I don't guess one can change the rims only?

--

Interesting side note: Good news, my ACM (production date in April 2017) already has the extra metal backing plates for the axle, preventing worn out shims. This fix has been going on for very long already! My old ACM (Production date in Nov. 2016) doesn't have them yet.

--

You sure it's hardware related? Not just the standard unrelaxed legs wobble?

Glad you discovered the problem. Very interesting to see the crack in the rim. Hard to believe it occurred as you described it - maybe it's a fatigue crack that developed over time.

Since you're going to have to replace the motor, maybe take this opportunity to convert it to an ACM2 :thumbup:

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1 minute ago, Marty Backe said:

Very interesting to see the crack in the rim. Hard to believe it occurred as you described it - maybe it's a fatigue crack that developed over time.

Pretty sure it got a bit bigger over the last (offroad) test rides I did, but the start was sudden and with no good reason.

2 minutes ago, Marty Backe said:

Since you're going to have to replace the motor, maybe take this opportunity to convert it to an ACM2 :thumbup:

Lemonade from lemons, right?:efee8319ab: But what if you are low on lemons (money) you can spend with good consicence? Probably still makes more sense than getting a new ACM1 motor...

--

Also, lesson learned AGAIN: have a second wheel. I miss having a backup!

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2 minutes ago, meepmeepmayer said:

Pretty sure it got a bit bigger over the last (offroad) test rides I did, but the start was sudden and with no good reason.

Lemonade from lemons, right?:efee8319ab: But what if you are low on lemons (money) you can spend with good consicence? Probably still makes more sense than getting a new ACM1 motor...

--

Also, lesson learned AGAIN: have a second wheel. I miss having a backup!

If you are going to fix the wheel, then the only additional cost is the new control board (and pedal arms). I think you would be very happy that you did so given everything that is great about the ACM2.

I really cannot imagine not have 6 backup wheels. I get queasy just thinking about it :cry2: 

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37 minutes ago, Marty Backe said:

If you are going to fix the wheel, then the only additional cost is the new control board (and pedal arms). I think you would be very happy that you did so given everything that is great about the ACM2.

Board, pedal arms, pedal arm backing plates, axle nuts, and whatever I forgot. Hope I can get all that. And I'd probably replace the shell, my old ones both have some broken screw holes, and I'm eyeing the white one:efeeec645d:... (and having better lights wouldnt be too bad as well).

39 minutes ago, Marty Backe said:

I really cannot imagine not have 6 backup wheels. I get queasy just thinking about it :cry2: 

:efee612b4b:

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What tire pressure you running?  The wheel must have experienced a pretty severe impact to deform and crack the aluminum like that.   While you wait for parts or that new ACM2/MSuper with Tesla motor :w00t2:, I would try taking a couple of vice grips with some leather pieces to bend it back into place.

Get some JB Weld original and seal that crack up.

Edited by Hunka Hunka Burning Love
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1 hour ago, meepmeepmayer said:

Board, pedal arms, pedal arm backing plates, axle nuts, and whatever I forgot. Hope I can get all that. And I'd probably replace the shell, my old ones both have some broken screw holes, and I'm eyeing the white one:efeeec645d:... (and having better lights wouldnt be too bad as well).

:efee612b4b:

BTW, when you buy a new motor, it comes with the axle spacers and nuts.

All you need is the pedal arms/steel plate and control board. Let's not make it sound worse than it is. I'm an optimist - you can do it :thumbup:

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43 minutes ago, Hunka Hunka Burning Love said:

What tire pressure you running?  The wheel must have experienced a pretty severe impact to deform and crack the aluminum like that.

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.

45 minutes ago, Hunka Hunka Burning Love said:

Get some JB Weld original and seal that crack up.

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

41 minutes ago, Marty Backe said:

BTW, when you buy a new motor, it comes with the axle spacers and nuts.

Axle spacers? Another part I didn't even know was there:efeeab781c:

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.

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1 minute ago, meepmeepmayer said:

For now, the plan is to hammer the thing back into shape and see what happens.

It has been discussed before that hammering on the rim can damaged the magnets.  Three people have used vice grips or large pliers to straighten rims.

They all said it was easy.

Instead of JB weld to seal the crack, a thin layer of RTV would probably work, with the added benefit of not scratching the inner tube.

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