How to solve chain derailment on BBSHD and BBS02

[paxtana]

Assuming the bike was running fine and you just installed the kit, derailment is probably a chainline issue.
For non-chainline related issues please see the section further below.

You have 4 different approaches if it is a chainline issue.

1. Use a different chainring that wraps around the motor housing
2. Rework the cassette
3. Switch to IGH
4. Adjust limit screw on derailleur to keep it out of these gears (and compensate for this by using a larger cassette)

1.
The chainline is how straight the chain is going from the rear to the chainring. Typically this problem presents itself when in the largest rear gear (low gear), if the chain is too angled in this gear the chain may be prone to falling off. Since each bike is slightly different this is common for a BBS build until the user finds what works best for them. A good first step to addressing the issue is a chainring with good offset, moving the teeth as far inbound as possible. Chainrings all have different offset, see this link for a comparison of the offset.

Stock steel chainring has decent offset, as do rings like the luna eclipse and lekkie. However you want to make sure not to move it so far over that it is touching the chainstay. The Eclipse can do this as it moves the chain further towards the bike than any other chainring, though you can use a chainring spacer or two to move it further enough back that it will clear the chainstay if need be. Here is a video how to do that. So long as you have enough chainring spacers the Eclipse is the best option. Click here for a visual comparison of chainrings, and here for a video (note in video the lekkie measurement is slightly off, it does have better offset than stock).

Any chainring that is larger than 42t can wrap around the secondary reduction housing to put the teeth further inbound. However just because it CAN, does not mean that all the >42t chainrings actually do. The most affordable chainrings that are 42t do not provide as much offset as the expensive ones. The chainring with the most offset is the Eclipse, followed by the Lekkie. Another great thing about the aftermarket chainrings is that unlike stock they are "Narrow Wide", meaning each tooth alternates between narrow and wide for a more exacting fit on the chain, this helps keep it on.

Much smaller chainrings (30 tooth) will be more problematic since it moves the chainline much further to the right, altering the offset in the wrong direction. This is unavoidable since the rings are so small they cannot wrap around the motor housing like the larger ones. Often the smallest chainrings may present some chainline problems unless this is addressed as described below.

Here's a visual aid for chainline differences:

Example of a Mighty Mini 30T chainring with no offset and resulting poor chainline. The Luna Mighty Mini 30T is a popular chainring but can be problematic without additional modifications due to completely flat design that does not offset the chain

2.
Alternately you can rework your cassette, this is where you rearrange the sprockets on a cassette so you keep the gear range but you use less gears to do so. You may need to buy a cheap cassette to mod it, this link goes over options on that. It's a lot easier than you might think, and it saves wear on drivetrain since you shift less.

3.
You can switch to an IGH. With this your chainline never changes no matter what gear you have it in. (Nexus 3, Alfine 8 and Sturmey Archer 3 can take BBSHD power)

4.
Generally speaking, adjusting the derailer limit screw is not ideal as it limits your gear range, but it is an easy option.

Non-chainline related possibilities:

• If you are using the stock chainring this actually has pretty good offset but it is not a narrow/wide chainring so it does not have a good tooth profile for gripping the links. You may want to consider upgrading to a different ring.
• You also want to check the chain itself:
  • Whether it is stretched and worn out (there are chain wear tools you can buy the check the chain).
  • Whether it is properly lubed
  • What speed it is: Ideally you want to use 10spd or lower. Higher speed chains are thinner and more problematic to sit well on the chainring at all angles when shifting gears
  • A chain may skip or derail if a link within that chain is binding up. You can put the chain on the stand and run it through the gears by hand or using the walk function on the display, if you see a particular link causing it you may need a new chain.
  • Whether it is properly sized
• You also want to check the chainring itself, see if there are any notches, bends, debris or anything else that might cause the chain to bind up on the ring or not properly mesh with the ring teeth.
• If you are only seeing problems on the rear cassette you probably need to tune the derailer. Any local bike shop should be able to do this. A gear sensor would also be advisable.
• A lot of this is a common part of bike maintenance but is especially more important when you are putting a lot of power through the drivetrain. The higher the power you are putting through it the more it is going to magnify any inconsistencies or improperly maintained parts. If still seeing issue, a local bike shop may be able to provide further insight on the cause.

Examples of damaged chainring teeth causing drivetrain issues:


Assuming the chain is new and the chainring and cassette look visually good you can put the bike on a stand and use the walk function or even rotate the cranks by hand to slowly run through the entire length of the chain and stop it at whatever point while it is in the middle of derailing.

Once you see it happening, stop so you can closely inspect it at that point. See if this is happening on the front, on the rear, and where. Maybe a link is binding up, maybe the chain is not centered on the tooth (in other words the chainline is not good), maybe there is damage on a particular tooth causing it to stick, etc. Once you see where the issue is originating from it will be easier to fix.

Assuming everything looks fine both on the chainline and the non-chainline possibilities, you may want to consider a chain guide. A chain guide might be as simple as leaving your front derailleur on, or using an N-Gear jump stop... Or something more complex.

If absolutely nothing helps, keep an eye on the cassette hub while using under load and see if the chain is actually derailing or it is an issue internal to the hub itself. Cheap hubs use a low number of pawls to engage and they could get mangled in there with a lot of torque so you might need a new freehub body or a better rear hub.

[max_volt]

I myself have a hard time determining if the chainline is straight or not. Concerning the 2 photos it looks like if they dropped to the smaller cogs the fat bike would be good and the 700c crooked.

[paxtana]

It's just the way the picture was taken, at an angle so I could get the most detail into the pic. In the lower pic it is 100% parallel with the middle cog on the cassette, ideally this is what you're aiming for.

But yes the fatbike would only be good in the gears furthest from the frame, that is a certainty. That bike would be a good candidate for reworking the cassette if sticking with the mighty mini chainring.

[carlos]

i run a standard steel sprocket on front then on rear i have a nexus 8 speed hub with a 22 tooth sprocket the other way round with the dish outwards, can run chain slack and never comes off, even down rough footpaths

[Robobiker]

DIY

Chain Guide. bbs 02. 750 W. 44 tooth stock chainring. 11-34 / 8 speed cassette. Used a bar end plug double zip tied to a perforation to knock out fender for chain

clearance

.

[ColinC]

Clever! Love it when folks like you homebrew creative solutions to problems with their builds--AND then share that with the rest of us so we can possibly benefit :)

[Dannyb]

I've been riding single speeds for about 10 years where chainline is more critical. Why don't you measure chainline like a single speed? Front measured from center of seat tube to center of chainring - for 73mm shell it would typically be 45-55mm depending on crankset. Then measure rear dropout spacing - frame with 135mm disc dropouts would have a center line of 65mm. Next measure how many mm from the inside of the right dropout to the center of your cassette or freewheel and subtract this number from 65. For instance a white industries freewheel on a surly hub is about 18mm from the right dropout, so subtract that from 65mm and you get approx 47mm - that means the front chainring needs to be about 47mm from the center of your seat tube. It would be cool if someone could give a measurement of the BBSHD motor with one of these common chainrings to get a good reference point for the number of mm to the center of the downtube on a 73mm shell. I'm trying to figure out if is possible to get this kind of chainline measurement, but everybody only seems to care about offset.

[paxtana]

That does not sound practical for several reasons. For example if the bike has an intrusive chainstay then whatever measurement you intended to be generic would be completely invalid. Similarly, if you are using a bottom bracket adapter to get bsa, and so on.

[Dshue]

I know that I have my chainline written down somewhere but with these mid drives all that does is give you a reference to aim for. And if you get lucky you might hit it. But for the most part you just try to get as close as you can. I have found that on the two builds I have done with a BBSO2 (one with a 68mm bb and 7sp freewheel and one with a 73 mm bb and 8 sp cassette)that as long as you can get the drive side to clear and not use any spacers then the stock Bafang 44t chainring will give an acceptable chainline for 7 and 8 speed rear cassette/freewheel.

[Retrorockit]

I think he has a good point. Chainstay and adapters are special considerations. But it wouldn't hurt to publish the chainline of The BBSxx with the OEM chainring on 68mm and 73mm BB shells. to have a starting point. With the popularity of IGH hubs this is much more important than on derailer bikes where it's changing all the time anyway. Shimano IGH are very fussy about this due to their inboard shift casette.

[ncmired]

Absolutely agree, Retro ...

Also, I've been meaning to mention a couple of minor issues with the "BBSHD Chainring offset guide". The first is the offset measurement doesn't finally subtract the chain centerline (slightly different for each chain ring). The second is that the 33.85 measurement for the motor doesn't take into consideration that the bb insets into the motor quite a bit - the motor case indents near the center.

Unfortunately for DIYers it's a calculation jigsaw puzzle of a sort. First, the BBS motor dimension numbers are lacking. Second, there's no offset numbers for the BBS02 chain rings at all that I've found.

Here's a stab at calculating chain lines for the BBSHD chain rings, for 68mm and 73mm BBs:

BBSHD Chain Ring to Chain Line Calculations
BBSHD final reduction gear case width to chain ring flange*:			33.85
Chain Ring Brand / Model	Chain Ring Offset	Approx. Chain Line* 68mm BB 73mm BB
Luna Eclipse 42T	-24.8	43.05	45.55
Luna Eclipse 48T	-24.8	43.05	45.55
Lekkie 42T	-20.4	47.45	49.95
Bafang 46T	-19.0	48.85	51.35
Lekkie 52T	-18.3	49.55	52.05
Luna Alloy 40T	-9.0	58.85	61.35
Luna Alloy 42T	-9.0	58.85	61.35
Luna Alloy 44T	-9.0	58.85	61.35
Luna Eclipse 40T	-8.0	59.85	62.35
Luna Mini 30T	-8.0	59.85	62.35
Luna One 36T	-8.0	59.85	62.35
Lekkie 28T	-7.2	60.65	63.15
Lekkie 36T	-7.2	60.65	63.15

[Retrorockit]

Thanks for producing that. I'm going to add links to some Sheldon Brown IGH charts, This should probably be an IGH sticky somewhere.

[Retrorockit]

Another suggestion for IGH chainline issues is to use narrow 8/9 speed chainrings, and narrow rear cogs so you can use the more flexible derailer 7/8/9 speed chains. These are designed to run out of line to a large extent. Not as strong but with proper lube and replacement schedule good enough for a BBSHD.

[Retrorockit]

Here's a link to Sheldon Browns IGH chainline chart.
https://www.sheldonbrown.com/chainli...rnal-gear.html
Typically you will need a large chain ring to get your chainline correct and then run a large rear cog to deal with th overdrive gearing the IGH usually provides.
The Sheldon brown gear calculator has IGH hubs and almost alll tire sizes in it. You can try several chainrings and rear cogs at once to see where your gearing will come out.

https://sheldonbrown.com/gear-calc.html

If you don't have any other preferance choose mph @ 90rpm in the gear units tab. Most riders can do that, and most who can do more are trained riders and will know what else they want.

[edllorca]

On a recommendation I bought a bbshd for a MTB tandem with a stock chainline of 50mm and once I got it the BBSHD chainline was so far off it is unuseable. On a 73mm shell I measured a chainline of 63.5mm. My chain pretty much cannot swing to the low end of the 12 speed cassette. Since it is a MTB tandem I cannot put a 48T in the front to fix the chainline.

[paxtana]

Maybe you should look at reworking the cassette, as mentioned on this page

[Retrorockit]

Could you post a photo of the driveline so we can see how the tandem is set up. It will help avoid wasting time on suggestions that won't work.
I have cut down BB shells with a BB facing tool to be narrower on one side. Had my LBS do it. Said it was a PITA but worked. I also removed the gasket from under the drive side cover and used silicone sealer and gained another mm that way. Potentially this will get you down to 59mm chainline. Theoretically you could take a whole 5mm off of one side for an offset 68mm shell, and with no gasket hit about 57mm. I did the BB mod on a 73mm Trek TSDZ2 IGH project, and the gasket mod on a 73mm gary Fisher BBSHD. Didn't need to do both at once.

[ncmired]

Yep, a picture, along with what chain ring (28 or 36). If 36, would you be willing to go Lekkie 40, with a 45-46mm chain line (before spacers)?

Or lop the BB shell down as Retro mentioned, if you have the room.

Otherwise, you can only make your lowest current gear ratio functional by losing gears, as Paxtana said.

[edllorca]

Thanks for your interest fellas. Here are some shots. bike is a Ventana Conquistador. 73mm shell but unshaveable as the suspension linkage is right there. I don't think I need spacers for the BBSHD to clear the swing arm but I might as it is certainly right there with no spare space. The drivetrain is SRAM eagle GX 1x12 10-50T (not my choice) and is not rebuildable. The chainring is a 30 T at 50mm off of centerline. The chain line cant be any less or it will rub on the current 3" tires though I may go to skinnier tires. I included three pics, 1 on the lowest cog 1 on the highest and 1 where the chain is parallel to the frame centerline more or less and it lands on the 6th cog. According to my measurements the stock ring was at 63.5mm. I have not found out what the smallest chainring I can buy that envelops the final drive housing but I know if I go too large it will strain the chain too much for steep climbs.

So that is the story for the drive chain. The timing chain would have to move to the right side but that chainline is so bad too I suspect it will derail often and would probably wear gears out quickly

My latest thought is to run twin BBSXX 350W which lets me runs the timing chain on the left but I am not sure about the drive chainline yet with this scheme.

[ncmired]

Oh my ... you were hoping to run a dual chainring on the right, too (because of the BBSHD motor housing interference)?

Tandem, full suspension, mid drive ... yikes is all I can say. 73mm stoker BB? I'm thinking 750 watt BBS02B is a much better starting point with a standard left side timing chain setup, with the Lekkie 42T (or better the 40T) on the motor. Will the BBSHD vendor trade?

Are you riding road, gravel or dirt, down in the parks south? 30T to 40T is 6 gear inches (approx. 17 to 23), or 2MPH (6 to 8), at a cadence the BBS02 should be able to handle.

[Retrorockit]

My thinking is get 2 chainrings going on that rear BB and put the BBSHD in the front BB. 40T is the smalllest inset chainring. But the one you add in the middle doesn't have to be the same size. Probably should be if both riders want the same cadence, or if timing is a thing with tandem riders.

[ncmired]

Dunno ... put a motor up front on a tandem, and hit the throttle - "Whoa Nelly!" goes the stoker. The frame also has a side-adjust eccentric BB on the front - another problem to tackle.