Chain ring too far from BB ... chain bends in low gear

chibi-sphere

10 mW
Joined
Dec 5, 2016
Messages
20
Hi,

I have a converted bike with a gospade motor. Between the BB and the chain ring/crank there's something about 1.5cm wide which I'm guessing is to the doo-hicky that actually turns the chain ring.
The problem is, in the lowest and second lowest gear the bike is noisy and it can be seen that in those gears the chain bends as it comes of the chain ring so that it can reach those cogs closest to the frame.

There's nothing that can be done about that is there? Looking at the bafang and tongsheng motors, it seems they all have that thingy attached to the chain ring.
 
chibi-sphere said:
Between the BB and the chain ring/crank there's something about 1.5cm wide which I'm guessing is to the doo-hicky that actually turns the chain ring.
Not sure what you mean? Can you attach a picture of the specific part you are seeing?

The problem is, in the lowest and second lowest gear the bike is noisy
Which specific part of the bike is noisy?


and it can be seen that in those gears the chain bends as it comes of the chain ring so that it can reach those cogs closest to the frame.
You mean it is bending outward coming off the bottom of the chainring as it goes back to the derailer and rear wheel sprockets?

What gear are you in on the rear?
 
ATTACH]



Here's a picture of a bafang. The motor is in the foreground, the part that drives the chain ring is in the background. That part is about 1cm wide and hence holds the chain ring out from the BB by about 1cm. When the bike is in the lowest gear the chain has to travel from the chainring to the cog right next to the stays. So it has to bend. This is a problem on analogue bikes too where the you can't use the big chain ring big cog but it doesn't matter because you have a small chain ring close to the BB and hence there is no great angle between the small chain ring big cog. But on an e-bike there is only an outer chain ring, in effect. I believe that Bosch, Shimano etc. Solve this problem by going through the bottom of the BB to drive the spindle attached to the chain ring, allowing the chain ring to be closer to the BB.
 
chibi-sphere said:



Here's a picture of a bafang. The motor is in the foreground, the part that drives the chain ring is in the background. That part is about 1cm wide and hence holds the chain ring out from the BB by about 1cm. When the bike is in the lowest gear the chain has to travel from the chainring to the cog right next to the stays. So it has to bend. This is a problem on analogue bikes too where the you can't use the big chain ring big cog but it doesn't matter because you have a small chain ring close to the BB and hence there is no great angle between the small chain ring big cog. But on an e-bike there is only an outer chain ring, in effect. I believe that Bosch, Shimano etc. Solve this problem by going through the bottom of the BB to drive the spindle attached to the chain ring, allowing the chain ring to be closer to the BB.

A pic of your actual setup would be more helpful, but if it's a chain alignment issue then most of those are solved either through spacers or offset chainrings
https://talesontwowheels.com/2020/05/03/mongoose-bbshd-update-musical-chainrings/
 
If you don't use all the gears on the rear cluster, you can take apart the stack, and move the gears you do use rightward, and the ones you don't, move left of all of those. Some stacks just loosely fit on the stack hub, and some of them are riveted together in a unit so you'd have to grind or drill the rivet heads to restack them, once you unscrew the smallest cog from the right end of the stack.
 
amberwolf said:
If you don't use all the gears on the rear cluster, you can take apart the stack, and move the gears you do use rightward, and the ones you don't, move left of all of those. Some stacks just loosely fit on the stack hub, and some of them are riveted together in a unit so you'd have to grind or drill the rivet heads to restack them, once you unscrew the smallest cog from the right end of the stack.

This is what I did to make my BBS02 behave better. I was able to make 6 out of 9 cooperate perfectly when in the outermost positions. The others I put in the inside positions and locked them out with the low limit screw.

Are you using the stock chainring between 42 teeth and 52 teeth in size? Anything smaller (except for the special Lekkie 40t when used with Lekkie's gear cover) will cause the chainline to be too far to the outside.

Is the gear housing making contact with the frame? If it touches the frame before the bottom bracket tightens up against the bottom bracket shell, then installing the unit can make the gear housing bend out of parallel with the motor, and exacerbate chainline problems.
 
Thanks @amberwolf and @chalo. That's a good idea. I don't really need the 2 smallest cogs on the cluster, I can just pedal faster. :)
I'll have a look and see if my cluster can be rearranged. I am using the standard chain ring that came with the conversion kit.
 
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