I was very close to pulling the trigger on the Woosh BBTS/DWG22C kit but I'm getting myself tied in knots in regards to gear ratios and chainline. I understand chainline isn't so critical on an assisted bike, but I'm struggling to understand how some of these numbers are workable.
Currently waiting on some answers from Woosh, here's what I don't know:
- Whether I can use the BBTS with my current cranks (Middleburn R01 W/ RX7 Narrow Wide 32t chainring). They said I could in an email but I saw someone from Woosh say that it wasn't possible using 3rd part cranks with the BBTS on another thread here.
- What Chainline is achieved using the stock cracks with the BBTS
They've told me that the spindle on the BBts is 144mm, currently I get 50mm chainline with the Middleburns on a 113mm BB, so looking at c.65mm using the BBTS which just sounds crazy. Will have to wait and see what they say the stock cranks chainline is. At 50mm my biggest sprocket is a touch noisy but perfectly usable, at 65mm I don't imagine the lowest few gears will be usable, which may not be a problem as I understand grinding up 20% inclines is not great for a hub gear anyway.
I built the drivetrain with gravel/mountain biking in mind: 32t chainring and 11-42 10 speed cassette. Now I'm planning to use the bike for commuting and child transport. It'll be roughly 10miles 4 days a week with an elevation gain on the way home with gradients between 1 and 5%. My best guess is that with assist the gearing will generally be too low, though the Sheldon brown calculator says in top gear I'll be hitting 20mph @80rpn so I guess I'll not be outrun by the motor. Worst case I won't be using the low gears at all.
The other limiting factor is my chainstays clearance. They flare out steeply from the BB above roughly 34t I'm going to have to bring the chainline out beyond 50mm anyway. Something like a 48t would probably have to be 55mm or more to completely clear the chainstays as this will be beyond the diameter of the indent in the chainstay.
In this way the 144mm spindle of the BBTS might not be so bad. If I
can use the Middleburns and I did want to bring the chainline back in a bit they do an offset spider designed for use on their external BB cranks. Can't find exact details but I think it's offset about 8mm inboard as , probably bringing be too about 55mm. If I get the double 130bcd spider this gives a bit more leeway either side.
This is all just speculation. I just need to get on and order the kit really and see how it works out then make adjustments to suit. Just writing this out for my own purposes more than anything.