Fitting a 100mm BBS02 in a 73mm bottom bracket, spacers & Q factor

[Niner63]

Hi everyone

I’ve been advised, elsewhere, to fit a 100mm BBS02 into my Titus Silk Road 73mm BSA BB because of concerns that the 73mm BBS02 motor would fowl the flared chainstays

If this is effectively my only option, with this frame set, will there be a noticeable difference to the Q factor ?

Are there any other options that I could implement or consider ?

TIA

nb CYC and similar motors are not within my current budget or a Grin all axle motor.

 

[Chalo]

You can reduce the Q dimension of BBS02 by quite a bit if you substitute unflared cranks for the stock flared arms that come with the motor unit. They can be unicycle cranks if you want a tidy looking installation, or regular cranks with a right side spider (which won't interact with the Bafang chainring).

I would be somewhat concerned about chainline, which is already very bad on BBSxx systems and which caused the innermost 2 sprockets out of 8 or 9 to be unusable for me due to chain drop and instability. If you move the chainring outward by 13mm or more, that puts probably 3 more gears out of action.

A narrow-wide ring might help a little, and a Lekkie ring with increased offset would help a little more. But I would still expect to lose the use of several ratios (though you might not miss them so much once you have lots of help at the pedals).

I wouldn't put a mid drive on that nice a bike. There's no real benefit to it, and you stand the chance of defacing your expensive frame (or worse). Better to use a frame that's stiffer to resist the unanticipated forces, and less precious in case you crack the right side dropout from increased chain tension, or gouge the frame with a motor driven chain. The subtle advantages of a light supple pedal bike frame like that are completely lost when you add a ton of weight and motor power to it.

If you choose a frame that's a more obvious match for a mid drive, then you won't have to mess with absurd Q or unworkable chainline.

 

[Niner63]

You can reduce the Q dimension of BBS02 by quite a bit if you substitute unflared cranks for the stock flared arms that come with the motor unit. They can be unicycle cranks if you want a tidy looking installation, or regular cranks with a right side spider (which won't interact with the Bafang chainring).

I would be somewhat concerned about chainline, which is already very bad on BBSxx systems and which caused the innermost 2 sprockets out of 8 or 9 to be unusable for me due to chain drop and instability. If you move the chainring outward by 13mm or more, that puts probably 3 more gears out of action.

A narrow-wide ring might help a little, and a Lekkie ring with increased offset would help a little more. But I would still expect to lose the use of several ratios (though you might not miss them so much once you have lots of help at the pedals).

I wouldn't put a mid drive on that nice a bike. There's no real benefit to it, and you stand the chance of defacing your expensive frame (or worse). Better to use a frame that's stiffer to resist the unanticipated forces, and less precious in case you crack the right side dropout from increased chain tension, or gouge the frame with a motor driven chain. The subtle advantages of a light supple pedal bike frame like that are completely lost when you add a ton of weight and motor power to it.

If you choose a frame that's a more obvious match for a mid drive, then you won't have to mess with absurd Q or unworkable chainline.

Thank you for your reply.

Ti Silk Road project has been shelved.

This Ti bike wasn’t my first choice to convert, I have an old steel gravel bike that I’d considered converting, however the eccentric BB casing prevented a Mid dive conversion. I then considered a DD 750W - 1,500W hub but I, initially, shelved that thought after being advised that dropouts would require modification to fit a torque arm, something I didn’t want to do but I’m now reconsidering and or fitting a lower W rear hub or even possibly a front hub as a last resort.

 

[amberwolf]

dropouts would require modification to fit a torque arm

Put the torque arms on the inboard sides of the dropouts (in place of the washers normally used at the axle shoulders).

 

[Niner63]

Put the torque arms on the inboard sides of the dropouts (in place of the washers normally used at the axle shoulders).

Thank you, I think I’ll start another thread about converting my gravel bike, different hub motor options, controllers etc