opening a Bafang CST... and putting it back!

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quamau   1 W

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opening a Bafang CST... and putting it back!

Post by quamau » Dec 30 2017 6:34pm

Hi there,

I need help with a Bafang CST which I just opened to substitute the nylon gears. After opening, and learning that the motor was in perfect condition but for one split gear and another one with a damaged teeth, I ordered replacement gears, swapped them, mounted everything back, and... I have got a problem.

After screwing down the plate, the wheel does not spin freely. To give you an idea, the forward drag is comparable (if not a tad higher) that the drag I get when I counter-rotate the wheel, which engages the motor.

The fact that this drag appears only when the cover plate is screwed down fully makes me think that I made a mistake with shims or washers. Unfortunately I did not take pictures while opening...

Is there anybody out there that could tutor me in making sure that everything has been mounted correctly?

Thanks in advance for your time

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motomech   1.21 GW

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Re: opening a Bafang CST... and putting it back!

Post by motomech » Dec 30 2017 7:15pm

Did you count the number of teeth on the replacement gears? I believe the gear ratios are particular to the CST and the gears from the other BPM's will not work.
Motomech '11 Motobecane Fathom team, frt. mounted, recycled SWX02 from a Jump rental, 14S (52V nom.) Turnigy LiPoly, PSW Power 9-FET 20A controller. Mean Well HLG-320H-54A, Schwalbe Marathon's, 27 to 28 MPH.

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fechter   100 GW

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Re: opening a Bafang CST... and putting it back!

Post by fechter » Dec 30 2017 7:36pm

I think you'll have to take it apart again and look for where it's rubbing and look for spacers, washers, etc. that might go in a different spot.
"One test is worth a thousand opinions"

quamau   1 W

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Re: opening a Bafang CST... and putting it back!

Post by quamau » Dec 31 2017 4:49am

Number of teeth is OK: I believe it's 42, anyway everything runs very smoothly once the motor is disassembled, or until the cap not not tightened.

Opening it up again... it's obviously what I have to do, and I did it already. However, there is not much to misplace that I don't know what to change. There is no visible wear, but I did not run the bike on the road yet, only a little of hand spinning.

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fechter   100 GW

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Re: opening a Bafang CST... and putting it back!

Post by fechter » Dec 31 2017 12:03pm

Sometimes a bearing can shift a little when pulling things apart and may need some force to get back in place. Like squeeze the housing around the axle to make sure the bearings are fully seated.
"One test is worth a thousand opinions"

quamau   1 W

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Re: opening a Bafang CST... and putting it back!

Post by quamau » Jan 01 2018 6:56am

Thank you fecther, this is a thoughtful suggestion, I will take it apart once more with your words in mind.

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