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is it simply the torque of the bolts that stop a eccentric bb from eventually spinning to bottom center?
I'm about to convert my cx bike to single speed...just trying to get my head around it
Whether it’s an expanding wedge/set screw/clamshell BB shell type, the eccentric is locked in place. If you’re using a pressfit eccentric conversion, they will generally use a couple of beefy bolts tying the two half’s together and holding their place by friction against the BB shell faces due to the tension on the bolts.
Depends - the eccentric BB on the tandem has two grub screws (big 'uns at M10, but still grub screws) to stop it rotating. The one on the last tandem, the BB shell was split and had bolts to tighten it around the eccentric BB
for aftermarket I'm less sure, maybe they have a mechanism where there's some sort of expansion within the BB shell to stop it rotating?
It a press fit one for a singlespeed conversion
The two bolts tightened against the shell.
Similar to nbt my experience of eccentric BB s is on my tandem with 2 huge clamping bolts parallel to chain stays ...Google gives me pics of pressfit eccentric BB s with two huge bolts parallel to the BB shaft pulling the bearing shells in...relying on friction but quite a big interface and probably way better than a standard press fit
I've used the Problem Solvers pressfit eccentric for about 7 years, it's never moved.
is it simply the torque of the bolts that stop a eccentric bb from eventually spinning to bottom center?
If you look at the forces involved bottom dead center isn't where it wants to end up.
You obviously push down with approximately your bodyweight at peak.
But the chain is always pulling backward with about 4x that force (chainring and crank length dependant). So the BB wants to end up at about 8 o'clock (viewed from the drive side).
To counter that the best place to adjust it to is about 1-2 o'clock with the highest BB and longest chainstays or 7-8 o'clock (low BB, short chainstay). In those positions, the chain tension is pulling it back/up and your weight it pushing it down/forwards and the BB doesn't want to move.
So its basically the force of the bolts that stop it spinning in the shell? I just wondered even if they are super tight wouldnt the weight of a rider and the constant forces going through the pedals eventually force the bb to center bottom
no
Thanks...i think I'm convinced. Just hope it doesn't kill a carbon frame. The bb shell is all carbon but it has reinforcement in it (its not just a hole...more of a carbon tube with some drain holes