Help solve a chain drop problem

[JackElliott]

This is my first electric bike build. I am using a '90s Nishiki mountain bike frame with short horizontal dropouts, a Bafang BBS02 mid drive motor with a Lekkie Bling Ring, and a Shimano Nexus 3-speed hub on the rear, with stock 19T sprocket, flipped over so that the driveline is good.

Under power, the chain pops off the sprocket.

The chain has been cut to the shortest length it can be, but even with the axle of the hub as deep in the dropout as it will go, there is some slop in the chain. Pretty saggy. Cutting out another link in the chain will shorten it by 1'', which is too short to let the rear wheel axle seat into the dropout. There is room in the dropout to lengthen it, and that might be my next step.

But before I modify the dropout, I could use some wisdom.

 

[tommie]

similar here, BBS02, Alfine 8, i had to use a tensioner to take up any slack, not expensive, worth a try.

 

[WoodlandHills]

I have the same dropouts with a Nexus 3 and I used one of these: http://surlybikes.com/parts/drivetrain/singleator It can be installed to either push up against the chain or to pull it down tight.

If you need a mount use this: http://www.niagaracycle.com/categories/problem-solvers-chain-tensioner-with-derailleur-hanger?gclid=CLb16v3n1c0CFQYuaQodb3UOxA

 

[JackElliott]

Hm. What I'm dealing with here is the "Horizontal dropout, short" as shown at http://www.sheldonbrown.com/fixed-conversion.html, about halfway down. But I cut off the derailer hanger, perhaps foolishly, thinking I would not need it.

With an angle grinder I was able to elongate the dropout so it more closely resembles the "Without hanger" photo, and that gives me room to tension the chain enough so it no longer drops. But as the chain stretches, I'm going to have trouble.

 

[Rassy]

For many chains you can get a "half-link" so that you only have to shorten the chain 1/2".

When I set up my sister with a BBS02 and a Nexus 8 and vertical dropouts I didn't install any tensioner. Even a fairly loose chain stays on, and it is a cheap FS Walmart bike which originally had a 7 speed deraileur, so I figure the alignment is very true.

Edit: I looked at your Sheldon Brown link and one of his recommendations was to use a half link.

 

[spinningmagnets]

Research "chain guide" too...

 

[DanGT86]

Double check your chainline to make sure its very straight. A little bit of misalignment goes a long way with the torque of a motor involved. Also your small tooth count bling ring is going to provide tons of chain tension so you may be flexing the frame causing a misalignment. My 80s steel roadbike will make unauthorized shifts if I stand up and crank really hard from a dead stop due to frame flex. If I only had one speed it would result in the chain popping off. If you find out that it is the frame flexing you could adjust your chainline a little to compensate for the direction it wants to go under hard load.

 

[tommie]

For many chains you can get a "half-link" so that you only have to shorten the chain 1/2".

didn`t think of the half-link at the time, so went for a tensioner, only concern is how does the IGH cope with the torque of the BBS02?

 

[JackElliott]

Thanks, all.

@Tommie and Woodlandhills: A chain tensioner is probably in my future. As is me continually asking myself why I cut off the derailer hanger from the frame before painting it. This, I think, is going to be one of those things that I will end up regretting. I can use an add-on derailer hanger, but stacking more stuff under the drive-side axle nut isn't going to make things sturdier there.

@Rassie: Thanks for tip about half-links. But I don't think they will work with a chainring that is narrow/wide, like the Bling Ring, as it requires alternating wide and narrow links and a half-link will switch things out of step every other chain revolution.

@spinningmagnets: My chain drops are at the hub sprocket, and the chain guides I've see are for the chainwheel.

@DanGT86: Once I flipped the sprocket on the IGH over to move it a smidge closer to the bike centerline the chainline is quite good. Yes, the smaller Bling Ring chainwheel is going to trade torque for speed, but speed is not as important to me as torque, so we'll see how well the frame handles the force. That's the fun part: testing things to see which break first.

@Tommie (again): This BBS02 is set up with a Gearsensor to briefly cut power when the shift cable moves. To prevent breaking the IGH. It says here . . . .

This little issue aside, the bike goes real good. Gonna take it out for a longer cruise today. I may want to sling a net under it to catch parts that fall off.

 

[JackElliott]

This chainstay-mount chain tensioner might be my hero.

http://www.jensonusa.com/DMN-Chain-Tensioner-Chainstay-Mount

 

[m4k3r]

Hey guys, new here but thought I'd throw in what I've experience. I've been running dave's tangent on my Tracer for about a month now. First I had a tensioner on when I installed the drive, had one chain drop in a week (chain fell to the inside). Then I took the tensioner off just out of curiosity and dropped the chain every ride (all to the outside). Well, here's my trick I stole from GMBN on YT so credit to them. Cut a 2" piece of black rubber hose, drill 4 holes in the top to slide zip-ties through and strap it to your chain stay....done and done! (Just make sure you line up the chain properly before securing the zip-ties, that's one thing GMBN didn't mention)

Here's the youtube vid i stole it from https://youtu.be/pgBOKgcMidk?t=120. Hope this helps and see you guys around! (now back to lurking)

P.s. I used a 2" piece of black flexible pvc since that's what I had laying around and seems to be working great!

 

[JackElliott]

How long before the chain chews :lol: through a ziptie and you hear the sound of a chunk of PVC pipe get sucked into your derailer? :lol:

 

[m4k3r]

How long before the chain chews :lol: through a ziptie and you hear the sound of a chunk of PVC pipe get sucked into your derailer? :lol:

Not sure but will update if anytime soon. Put a few hundred miles through it already and no signs of wear yet (other than slight deformation where the chain rests on the flexible PVC). The chain doesn't touch or even come close to the zip . Zip ties are on top attached to the chain stay while the chain rests on the hose.

 

[JackElliott]

Very cool, m4k3r!

But I don't think the tube will take up slack in my chain, I don't have a derailer and your bike does, so you have the chain wrap in the derailer to take up chain slack. That tube thing looks good to control the chain from flopping around but it's not a tensioner.

I took the bike with the axle pulled as far back in the newly-elongated dropout slots for a 10-mile hilly ride today (maiden voyage) and it was a real pleasure. But the chain is now slack, and I'm running out of dropout slot to pull any more tension on the chain.

 

[amberwolf]

You can make your own tensioner out of a brake arm and a jockey wheel off an old derailer:
https://endless-sphere.com/forums/viewtopic.php?f=28&t=13704

Unfortunately Aussiejester's pics are no longer on the web, and he didn't upload them to the forum as attachments so they are lost, but there are pics of my version down the page a bit.

This one
https://endless-sphere.com/forums/viewtopic.php?f=28&t=9342&p=193357&hilit=tensioner#p193204
you could build the same way.

Or this one
https://endless-sphere.com/forums/viewtopic.php?f=28&t=9342&p=193357&hilit=tensioner#p193196
in a different way

 

[JackElliott]

Thanks amberwolf.

Because I shortsightedly ground off the derailer hanger from this frame I'll have to use a bolt-on derailer hanger to use any chain tensioner, DIY or store-bought. The more stuff I stack on the drive-side of the wheel axle, the more chance that the hub could get yanked forward under motor torque. Only one way to find out! A shop here in town has a Surly Singleator for only $30 (US) so I'll give it a try.

It also looks like if I increase the cog from the stock 19T to a 20T, I might be able to make it all work without a tensioner.