Bicycle Mechanics - Bottom Bracket Spacers not working per Shimano instruction sheet. Curious.
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Finally getting around to building up the 88 trek 850 into a touring setup. Installing a new Shimano FC-M591 Deore Crank+BB and the spacer instructions don't work. Shimano instructions show (for 68mm shell) three spacers on right (2.5 + 0.7 + 1.8) and one on the left (2.5). All are required to fit the crank to the BB shell, but Shimano's recommendations throw my chainrings way outside where it should be. Even the arm distances from the chainstays are off (much closer on non-DS than DS). I'll rearrange the spacers until it works right, but I'm curious if others have seen this. Or maybe I shouldn't have read the directions at all.
It appears that "no spacers" on the DS and "all spacers" on the non-DS is the only thing that gets them close to equal in the BB shell, and the chainrings still seem too far outside. The protective plastic cylinder gets torqued and starts to warp if I eliminate any spacers. And "all spacers" on the left leaves it with not much bite on the threads. Looks like I may go freestyle on this and shorten the plastic cylinder, eliminate several spacers, and possibly abandon that tiny latch pin (if the non-DS arm pushes in too far). Anybody know something I'm completely missing here? Maybe weird leeway using (68/73) MTB crank on road frame (88 850 is 26" wheel mtb frame supposedly, but the 68mm bb shell and 130 rear drops makes it more like a road frame)
09-15-12, 11:31 AM
Well...I did want to ask, "Why did you decide to go with this type of crank and bottom bracket?" It would seem to me to be easiest to go with a square taper crankset and bottom bracket as you would have close to a dozen symmetrical spindle sizes to choose from, and for asymmetrical spindles, half a dozen sizes to choose from each with several degrees of variation in the asymmetry.
And then there's Phi Wood's bottom bracket....
Just asking, that's all.
09-15-12, 12:06 PM
but Shimano's recommendations throw my chainrings way outside where it should be.
These cranks position the chainrings further out than the classic cranks you might be accustomed to. The primary market is mountain bikes, secondarily trekking bikes.
For example, in the early '90s, I was accustomed to having the inner face of the granny ring somewhere near the plane of the BB shell's driveside face. Nowdays that's not the norm, not for Shimano's stuff anyway. If they did that, the tail of the FD would be hitting the tread on the rear tire on many bikes, especially in this era of 29er popularity. Hence the 50mm chainline.
If you want to fudge the positioning by one spacer, you should still have enough bite on the non-driveside. Myself, I'd use metal spacers that won't split if they're under a lot of torque, and install the cups quite tightly... you don't want issues when you're on tour.
I kinda like these newer external bearing setups --and I was assuming there was a element of universal fit. (and the 48-36-26 was the chainring set I was looking for). It looks like it won't fit even if I freestyle the fit. The spindle is a full 10mm longer than a road crank spindle. Non DS crank arm pushed to its limit bottoms out before I can fully take up the needed slack.
Back to the drawing board on cranks. I might use the tiagra road crank from the bike that will be donating its shifters (it's 50-39-26). Or I start from scratch.
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