Having a lot of problems with the (2nd) drive chain

[richj8990]

What I realized about these shorter drive chains as opposed to a normal 112-120 link chain is that measuring to the exact link is critical. They are often only about 1/4 of the length that a normal chain is. Which means that each link's length in a shorter drive chain is 4 times as important, especially when you only have a tensioner and no rear derailleur / spring to take up any excess slack. I made two drive chains: one has 31 links and one has 30. The problem is, and maybe I can solve this with a 46T chainring instead of 48T, is that I really need 30.5 links. But you can't do 30.5.

The one that has 30 is a bit tight so I took off the tensioner. It doesn't 'look' tight, there is still some play but when the motor starts up, that chain gets firm immediately to the point that it's pulling the motor over toward the right! I saw it when I was looking down, and then saw the chain come off (the chain has come off a dozen times in the last month but has yet to break). I swear I saw the motor and sprocket lean over to one side, temporarily bending the mounting brackets to the right, and I didn't even add that much throttle, maybe 300 watts at the most. Crazy. I can't even push or pull the motor by hand more than 1-2 mm around left or right, so it seems tight but if the chain is too tight it's overwhelming the brackets. This issue means I have to now use the very, very loose 31 tooth chain, and hope for the best. The chain looks super loose on the bottom. May need to add a 2nd tensioner on the lower end. You would not believe how much tension difference there is with just one link of difference, it's amazing. I keep needing to recount the links to even believe it myself.

This is the only and final problem with the conversion. But it's a bad one; obviously you don't want the drive chain to seize or fall off the rings. The space between the 48T chainring and 16T freewheel motor sprocket is fixed (unless the overly tight chain pulls them towards each other and then it pops off the rings). So maybe so back to 46T, then a 30 tooth chain would not be so tight. If it helps at all the 48T was narrow-wide and the previous 46T was ramped, but both had the chain fall off if it was too tight or too loose.

 

[Chalo]

Are you aware of half links? They're a customary way to reconcile sprockets and chain length on bikes with limited chain tension adjustment.

 

[richj8990]

No but that's interesting.

I ordered a 46T chainring...I think that should solve it. 30 links this time worked, gingerly. But the motor & sprocket shifted over enough to just barely be hit the right pedal which means I need a longer crank axle spindle. Between the 46T chainring and a wider spindle --- I think the superboost axle is about 15mm wider --- that should solve the problem. Both the stock kit right pedal and the new one are going to be offset about 10-15mm to the right, which for a normal bike would be an ergonomic annoyance. But for an e-bike I think it will be OK.

BTW I didn't know this before but superboosted cranksets are down to as low as $80...a real Shimano Deore crankset, with chainring. These are a really, really good option for people that need a wider crank axle spindle for a stand-alone mid-drive motor. Because they are the equivalent of an elongated square-taper spindle with more strength and efficiency because of the hollow axle and outside bearings. All of them that I saw are direct mount, which means I also buy a direct mount to 104 BCD adapter, and attempt to put a chainring on each side. I've done that with a normal (flush edged) chainring spider and it works fine, so I think with some spacers on the outside it will be OK for the adapter. Question is: will the cheap chainring bolts hold up to 500+ watts of power. They should. They better. Because superboosted double chainring cranksets are rare and not cheap. The inner ring is not even direct mount which is false advertising if you ask me. In addition, Shimano's double cranksets are now a direct mount (ramped) outside, normal older 4-bolt style inside, and if the smaller one is 58 BCD like it often is and not 64, you can't even use a 64 to 104 adapter. So it's either this single chainring crankset w/adapter or back to the elongated square taper set with rings way over on the right side, not a good balance for weight distribution on the axle.