Magic Pie Vector "tugging" once per revolution

[Zack]

Hi I'd like to have my account activated.

I have a brand new Magic Pie vector that makes a strange tugging vibration when under power or when using the regen brakes. This happens once per wheel revolution. I removed the controller and tested all three Hall sensors. They all seem to show either approximately 3.2 volts or no voltage whatsoever depending on the wheel orientation.

Interestingly, the tugging goes away once it gets into the higher rpms. It returns when you back off the throttle *but* is not as bad once you've returned from the higher rpm.

Does this sound like failed hall sensors? All 3 of them? the motor runs smoothly except for the tugging once per revolution.

[Bikemad]

Hi Zack andto the forum.

If it is only happening once per revolution it is not going to be hall sensor related.

Is it noticeable when you freewheel without applying power or braking?

I suspect it is most likely to be something physically touching (tyre against frame, wheel rim against brake block or brake disk against brake pad etc.).

Check that the wheel is correctly aligned and that it rotates freely by hand without any tight spots.

Alan
 

[Zack]

Thanks, Alan.

Further tinkering has me pretty convinced it is the bearings. It makes sense they are under the most load when under motor power or regen brakes. It for sure doesn't have the same kind of sound/vibration when I rotate it by hand, or even pedal the bike myself. I did notice an unpleasant sound when the wheel is moved super slow - motor or not. At this point I am figuring how to remove the side plates so I can get at, clean, and grease the bearings.

I made a video of the bad behavior:
https://www.youtube.com/watch?v=UtgLgYKTaoY

I am sad the bearings are having trouble so early in the wheel's life. It has maybe 100 miles on it. I hope this one was just a fluke that got a particularly small amount of grease at the factory.

Any advice about getting the plates off (without banging on the axle!) would be super helpful.

[Bikemad]

Hi Zack, I've just been watching (and listening to) your video and it sounds like something is definitely touching and physically rubbing (but it's obviously not the freewheel).

As there is no freewheel and the torque arm is not properly fitted, I presume the wheel is only temporarily fitted on the bike for demonstrating the noise, unless you have actually been riding it like that for the last 100 miles?   

I couldn't tell from the video, but if you have a brake disc fitted, make sure the screws are not too long and rubbing against the controller inside the cooling fan/brake disc mounting piece:



Did the noise start gradually during the last 100 miles and slowly become worse or did it suddenly start very recently?

If one of the bearings has excessive movement, it could allow the stator to touch against the magnets inside the hub.

Firmly grab the tyre and try to move the wheel by twisting side to side and also pulling/pushing to see if you can see any obvious signs of movement.
If there is obvious movement, try and determine which of the following is most pronounced:



When you first received your Magic Pie, was there any signs of damage on the box (around the axle area) where it may have been damaged in transit?

If the motor has been washed down with a hose pipe (or worse still a jet wash) it is possible for water to get into the bearings if they are metal shielded bearings instead of the rubber sealed type:



Removing the covers can be very difficult on the MP4 as the large bearing seems to be held in place with some form of bearing locking compound, and I found it easier to remove the complete stator/axle assembly and L/H side cover together as I could not get the left side cover (controller side) to come away from the bearing, or the bearing to come away from the stator.

Before you attempt to remove the stator, you must first free the cover from the hub as it is sealed all around the edge with silicon sealant making it very difficult to remove. I gently tapped a sharp scraper between the cover and the hub (working my the way around) to gradually prise the joint apart.

After I had loosened the joint between the cover and the hub I removed the stator assembly by laying the wheel down horizontally with the axle placed on a piece of plywood (to protect my floor) and then climbing onto the wheel to use my weight to push the wheel and hub towards the floor using both of my legs and one hand while I pulled on the axle/stator with the other hand.
An axle nut threaded onto the end you're pulling on makes it much easier to grip with a gloved hand.

Removing the whole stator assembly requires a lot of force to overcome the pull of the magnets, and you also need to be careful that it is kept as central as possible to avoid damaging the laminations of the stator by dragging it against the magnet as it is withdrawn.

Don't be tempted to put your fingertips under the edge of the cover to try and help pull it off, as the tremendous force from the magnets could really trap your fingers if something slips!

Alan
 

[Zack]

The motor was just mounted for testing without the torque arms secured. I've only ridden  it with them locked down tight

No disc brakes.

The first 20 miles or so I had knobby tires on the bike and didn't really notice the sound\vibrstion. I should say that it sounds and feels different with my weight on the bike. A lot less grindy. When I switched to the big hookworm tires I noticed the vibration but thought it was an improperly seated tire for a while. It wasn't till I had the bike upside down recently that I suspected some part of the motor. I really hoped it was related to the freewheel, but no such luck as you can see I removed it.

The shipping box seemed fine. Nothing obviously wrong. It came from China.

I'll mess around with it some more tomorrow.

[Zack]

Twisting and pulling the wheel in all directions didn't show any play at all. I used mongo strength. Tonight I'll probably try pulling the stator.

[Zack]

Got it open! I made a puller thing with hose clamps and wood that popped the cover right off. It doesn't seem to be the bearings. It appears that the stator is actually rubbing on one of the magnets. See attached images. I guess I could use the dremel and grind off part of the naughty magnet?


Here's a video from before I removed the stator:
http://youtu.be/gmLkwrU1zZI

[Bikemad]

Zack, as it was only catching in one position, there must be a high spot on the stator which is catching the lowest magnet.
It would be better to use a file to remove the high spot from the stator rather than grinding the fragile neodymium magnets, but make sure you keep the magnets well away from the stator until all of the filings have been thoroughly removed.
After you have filed down the high spot on the stator, make sure that you apply a protective coating of paint or perhaps apply some grease to the exposed surface of the metal to prevent it from rusting.

There appear to be obvious chips on several of the magnets and the broken particles will probably be stuck somewhere around the magnets. Make sure you remove all of these particles before you attempt to reassemble the motor assembly.
It also looks like one of the magnets may have moved slightly, in which case it is probably no longer bonded to the motor ring but is simply held in place by its own magnetism.


I would run some very thin CyanoAcrylate glue into the joint between the magnets to ensure it stays in place during use, and I would also suggest coating any visible chips on the magnets with glue to seal and protect the exposed areas.
Before you apply any glue you should remove the other side cover first to prevent it from being glued to the motor ring and allow the glue to dry completely before refitting the cover.

The thin CyanoAcrylate glue is also very handy for repairing separated laminations on the stator. I use a pair of mole grips to gently squeeze the laminations back together and then run some glue across the outer surface of the stator laminations.
The thin glue will very easily find its way between the laminations where it will soon dry, bonding them securely together.

I looked at your latest video and it's reassuring to see that the large bearing is fitted with rubber seals rather than the metal shields. It is not unusual for the stator to touch the magnets when it is turned without the outer cover as it needs the  cover's bearing to centre the axle properly, but it obviously shouldn't touch with the cover correctly located and bolted down fully.

Alan
 

[Zack]

Thanks for all the tips, Alan. It looks like the "loose" magnet you mentioned isn't actually loose, but wasn't glued down flush with the others. It feels notably raised compared to the other magnets when I run my finger across them. Since I see scuff marks on multiple places around the stator (and around the bad magnet), I think I should try removing, cleaning, and regluing that single magnet. I assume nail polish remover will work to clean the magnet after I pop it out with a hobby knife. Then I have some CyanoAcrylate glue on the way to put it back. Will be repairing a few separated laminations with it as well.

I will give the magnet ring a good cleaning before everything goes back together. I don't see much in terms of debris from chipped magnets inside. It looks like they assembled it with many already chipped magnets. There are visible chips on the other side of the magnets too - that face away from the stator. I don't imagine it hurts performance very much, but is this typical?

[Bikemad]

I don't recall seeing magnets with chips on both sides and this is what my own MP4 magnets look like:





I'm wondering whether the magnet that is standing proud has been glued on top of magnet fragments. If so, I'm not sure how easy it will be to remove the magnet without further damaging it, as might break if you try to lever it while it is still firmly bonded to the hub casting.

It presumably didn't make any noises when new so either something has moved or the bearings must have more movement in them now than they had from new.
If it were mine, I would leave the magnet undisturbed (but make sure it is glued in place properly and simply remove enough of the stator to provide the required clearance.

If you have never used thin CyanoAcrylate glue before you need to be very careful not to apply too much. It is extremely thin and the capillary action will cause it to travel a long way (and end up somewhere you don't want it) and it really does bond skin tissue in seconds!

If you need to fill a large gap between magnets etc. I use sprinkle baking powder into the gap and wipe away any surplus before applying the glue. The thin glue quickly soaks straight into the powder and then it sets incredibly hard.

Alan
 

[Zack]

At this point I have ground off some of the stator where it appeared to be rubbing most - and reassembled everything. No change in the symptoms.

I did notice this:
http://youtu.be/UNS_nVnTECk

The wheel seems to go untrue just briefly when it makes the noise. It does that when not under motor load as well. I'm back to suspecting some kind of bearing problem.

[Zack]

The Pie has been shelved for the time being. I am still not sure what's wrong and have just about finished a build with a MAC 10T that has my attention for now. Hopefully I can get the Pie smoothed out eventually.