Cook Bros CBR? question

[Martin]

Just wondering why this massive (CBR?) crank has a piece cut out of the innersice of the driveside arm (see aluminium piece)?

Who know's or is it done by a previous owner. I have seen it on other CBR's also.

Cheers,
Martin

 

[one-eyed_jim]

Almost certainly for derailleur clearance. Some cranks have very little room between the outer chainring and the inner face of the crank. That can cause upshifting problems if the outer plate of the derailleur cage contacts the crank before the chain has climbed onto the outer ring.

That particular crank looks like a user modification to me, but as you say it also appears as a factory feature on some cranks. In extreme cases, with a negligent owner, the derailleur cage can wear away quite a lot of metal from the crank.

 

[pete_mcc]

Totally agree. Cooks cranks were basically derived from bmx cranks where they had no need for front derailleurs or big rings. Early front mechs had parallel plates and so didn't cause an issue but 7series xt had a bulge in the cage down the bottom and this always caught the crank, so people started to file their cranks' the cooks and cqp etc did it themselves. I have a set that were done by hand, one by machine and one by a mech rubbing ( in fact I filed a nos set of cooks just the other day.....)

 

[Martin]

That explains everything, why couldn't I think of it myself.

Thanks you all.

Cheers,
Martin