estasnyc

Please give advice about choosing one of two bottom brackets for a frankenbike. Might using a double crankset with a bottom bracket intended for a triple crankset be a better choice in this special case?

I just picked up a 172.5mm Shimano Octalink V1 Splined FC-5502 double crankset for cheap on Craig's List to replace a 170mm double that I've pulled off of one road bike. This part is straight-forward and has already been done.

The next part involves taking the removed crankset and moving it down to my commuter bike (which currently has bottom bracket cups and a spindle that may, in turn, end up being transplanted into a friend's Peugot having a french bottom bracket - But that's another story).

What makes my commuter bike a frankenbike is that the dropouts have been stretched out to 135mm for an MTB wheel instead of 130mm intended for a road wheel. Otherwise, there is nothing all that remarkable, really.

Now, which of the only two bottom bracket choices should I take for this bike? One, having a 109.5mm spindle length, intended for the double or one, having a 118.5mm spindle length, meant for a triple?

It all comes down to the chainline. According to Shimano the proper chainline for this double crankset should be 43.5mm.

But, presuming everything is symmetrically further out on a 135mm OLD wheel vs 130mm OLD wheel, might the ideal chainline be 46mm for this frankenbike? ( 5.0/2 + 43.5 = 46.0 ) This would strictly be hypothetical because there is no bottom bracket available to allow this.

Would I be correct in presuming that the longer bottom bracket for a triple will have its spindle sticking further out equally on each side? ( (118.5 - 109.5)/2 = 4.5mm ?). Or are there asymmetric differences between the two kinds of bottom brackets because of differences between a double and a triple crankset?

Could I get a crankset chainline of 48mm ( 43.5 + 4.5 = 48.0 ) by putting the double crankset on the triple bottom bracket or will something be totally out of whack because of some asymmetry that I'm unaware of?

Another question might be would I want to do this if I could?

pierce

thats the designed chain line when used with the specified BB, which is correct for the total group shimano specified. really, the optimal chain line is driven by your rear hub and cassette. so, you adjust the original BB by twice the difference between the crank's design CL and the bike and rear hubs chainline.


a 135mm "mountain" hub+frame probably has a 48-50mm CL. so yeah, I think the longer BB5500 will work perfectly. too bad you can't do this with the new hollowtech stuff, the axle width is fixed to the crank


anyways, a few mm either way isn't /that/ big of a difference. if the double's chainline is a little smaller than the bike/hubs, then it will favor using the big ring in front with more gears in back, and only using the front small ring with the low gear half of the rear.

I've done much the same sort of thing but with square taper stuff....

hueyhoolihan

i would just put them both on and decide which one i liked better. you may find that one does not really work at all. if so, there will be no question as to which is best. good luck.

estasnyc

This is exactly what I've been hoping for. So I should be fine doing this?

estasnyc

I don't have any english-threaded BB-5500 of any length that I would need to try out. I don't want to buy both versions just to experiment. The one on my road bike is italian-threaded.

BUT, I was also given a Dura-Ace BB-7700 bottom bracket when I bought the 172.5mm crankset off of Craig's List. The catch is that the threads on the non-driveside cup appear to be damaged and the bearings that would belong inside are missing.

I already searched through old messages here to see if anyone recommended where to get replacement parts to rebuild the BB-7700 and found that people have already specifically asked about this but that nobody had the answer.

Now, if someone now reading this may know a source then please do speak up.

Anyway, it looks like using the 118.5mm triple version of the BB-5500 appears to be viable for my project so I probably will end up doing that.

mconlonx

Go with the longer BB. Chainline might not be perfect, but it should work well enough. Worst case, crank will be a bit on the outside of a perfect chainline, so you definitely want to avoid big/lowest cog - large chainring crosschaining, and maybe stay out of the next low gear or two as well.

HillRider

+1 on the 118.5 mm bb length. Your reasoning that spreading the dropouts to 135 mm has increased the "proper" chainline is correct. As pierce noted, chainline differences of a couple of mm are not critical.