Gears - Top gears not working - BBS02

maxitwist

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Hello, Ive posted here before and everyone was very helpful, so thank for that.

I have a trek 7x which i fitted with a bbs02 with a gear sensor.
However, The 2 highest gears will not stay in gear, and will continually slip down, cause the engine to briefly shut off, and then regear itself.

I am not sure if the issue here is a chainline one or if the chain is not long enough and thats why.
The original chainwheel was 32T and the one Im using now is 36T, and its the original chain im using.

Looking for advice on what the problem likely is and a resource/tutorial for how to fix it.
Thanking you.
 
You mean the two lowest gears, largest rear sprockets I think?

If that's what you are saying it sounds like your chain is likely too short. Also with the 36T front chainring you get worse chain alignment than with the stock 44+ chainrings because it is a straight sprocket where as the 44 and larger offsets the chain for better alignment. We have two ebikes with the BBS02 kits and 36T front chainrings. One has an eight speed cassette and the other a seven speed and the eight speed doesn't like to be in the lowest gear. The chain in just a few rides wore the black off the side of the chainring teeth.


Put it in the lowest gear (largest rear sprocket) and look at how far your derailleur is pulled forward it should not be pulled all the way forward it should still have some forward movement and a good amount of chain slack when you move it forward.
 
Too small of front Chainring for BBS02 from what I understand. Improper offset. Seems that a 42T or 44T was the smallest one could go and retain proper chainline. Any movement of your shift cable will activate your shift sensor causing the motor to cut power as it is designed to do. I have a Trek 7.2 with a BBS02. Good possibility that the chain is too short also.
 
I had a similar problem after installing a BBS02. Your chainring is probably out of line with the derailleur.

On many Bafang installs the motor will push the chainring out further than its original position, thus forcing the chainring out of line with the gears. I tried a Lekkie chainring, which has a pie pan shape that draws the chain line inboard several millimeters. That helped, but didn't completely solve the problem. I ended up doing a complete derailleur adjustment. That did the trick; my bike shifts beautifully now and the gears stay put. It's not that hard.

I think that's probably a necessary step in any of these conversions.
Here's a great how-to video on the subject:
How to Adjust a Rear Derailleur
 
You mean the two lowest gears, largest rear sprockets I think?

If that's what you are saying it sounds like your chain is likely too short. Also with the 36T front chainring you get worse chain alignment than with the stock 44+ chainrings because it is a straight sprocket where as the 44 and larger offsets the chain for better alignment. We have two ebikes with the BBS02 kits and 36T front chainrings. One has an eight speed cassette and the other a seven speed and the eight speed doesn't like to be in the lowest gear. The chain in just a few rides wore the black off the side of the chainring teeth.


Put it in the lowest gear (largest rear sprocket) and look at how far your derailleur is pulled forward it should not be pulled all the way forward it should still have some forward movement and a good amount of chain slack when you move it forward.
No not the lowest gears, the highest gears you use when at top speed.
 
Hmmm well then I really don't know what you mean by they keep slipping down.

"The 2 highest gears will not stay in gear, and will continually slip down, cause the engine to briefly shut off, and then regear itself."

Your bike is shifting into a lower gear all by its self?

This makes no sense to me.
 
Yeah it wont stay in the highest gear, it keeps on going out of gear before recatching.
Would it be a chain alignment issue or does that also not make sense?
 
Yeah it wont stay in the highest gear, it keeps on going out of gear before recatching.
Would it be a chain alignment issue or does that also not make sense?
You need to adjust the rear derailleur. Check out the video I posted.
 
Could be heavily worn sprockets. The two smallest can wear out pretty quickly on an ebike. A bike drive train is designed for one human power, with a 750W motor you you have somewhere around three extra hp.
 
Shift sensor senses movement not tension. Likely you are running too slack of chain if it jumps cogs on the smaller cog sizes. Like M rider stated could be worn smaller cogs or not enough chain wrap on the small cogs. The slack chain would result in not enough chain wrap. Short chain would effect the larger, lower ratio, cog engagement. Changing to a 32 tooth from 36 tooth would not be any trouble if the chain length was cut for the 36 tooth. That small amount would be adjusted by the derailleur spring tension. If you are straight with the chain line at the middle of the cassette stack you are good on the chain line. 3/32 and narrower chains can make this angle with no problem. 11 and 12 tooth cogs are not good for longevity with e motors with lots of torque. 14 tooth cog with 36 tooth chain ring will haul ass if you let the bbs02 spin where it likes to be. You have to find a happy medium with the chain ring between high speed capacity builds or building a rig for torque. Idealistically we would have a transmission on the motor and have a single gear on the back. Think how motorcycles are built. The igh hub, multiple speeds perfect chain line, are as close as we can get to this with our e-bikes. Stay with it you will learn what works as we all went thru this mess when we first started building these things. After building for yrs. I am still learning. Good luck
 
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