I wanted to copy a build I saw on kinetics:
https://www.kinetics-online.co.uk/folding-bikes/brompton/cableless-brompton/?unapproved=20963&moderation-hash=6e29df56c279f7513761d078a46bca94#comment-20963
Using a basic A-line to start, I plan to get the following hub:
Sturmey Archer S2C Kick-Shift Hub Kit 36h 116mm Silver with Coaster Brake
́My question:
Kinetics mentioned a special tensioner is needed to allow the coaster brake to work. Can anyone elaborate on why and if it would be possible to have bike shop do it? Not sure if I can order it.
My end goal is to not have any visible cables on my bike and this looks to be my best bet while retaining a back brake and front.
Thanks, I hope to share results once I make it happen!
by SirBaconz
1 Comment
So coaster brakes typically do not work on bikes with chain tensioners or derailleurs. When you pedal to move, the part of the chain between the top of the chainring and the top of the gear on the wheel is in tension. Coaster brakes are operated by pedalling backwards, this tensions the chain on the other side of the chainring & gear.
On a non folding hub gear bike, the chain goes directly from chainring to wheel gear on both sides in a neat loop, and backpedalling as described above is fine. However on the Brompton, because the distance between the BB and the wheel essentially changes between being folded and unfolded, the tensioner is a necessary part of the drivetrain. If you backpedalled against a stuck wheel on a Brompton with the tensioner, the tension in the chain would push against the spring in the tensioner, which is much weaker, and the tensioner would just move instead of the backpedalling affecting the hub gear. It would feel like having incredibly stretchy brake cables.
I don’t think a beefier spring in the tensioner would solve this, to be stiff enough to work it would make folding a nightmare. Maybe a tensioner that can be locked in place once the bike is unfolded? I don’t know what that would look like. And don’t forget to unlock it before you fold again!