03-12-05, 09:10 PM
(Cross posted to mechanics board).
I've been trying all day to make my chain stop rubbing. I'm new to this many gears, so my apologies for getting some of this terminology wrong. When I'm on the inner chainring in front (granny gear), the chain rubs the middle chainring (in front) when I'm on any of the smallest 2 or 3 cogs in back. I have the same problem when I'm on the middle chain ring in front (chain rubs the largest chain ring) and try to use the smallest cog in back. I hope that made sense. Is this a problem I should expect with 27 gears on a tandem, or is there something simple that I'm missing?
Thank you to everyone for creating such a great forum here. Has anybody else noticed that bikes have gotten more complicated in the last 15 years?
Bike setup: Santana Noventa (Tandem) Stowaway.
03-12-05, 10:47 PM
Is this a problem I should expect with 27 gears on a tandem, or is there something simple that I'm missing?
This is quite normal for Santana's tandems build from '92-on which use 160mm rear spacing and, to a lesser extent, it also happens with tandems that have 145mm rear spacing that use race gearing, e.g., 56t - 54t or larger chain rings with 46t - 42t rings. For example, our tandems have 54/44/30t rings and the 44t x smallest cog combo will always cause some chain rub on the 54t as Debbie is one of those folks who needs a very narrow axle (107mm).
However, I suspect that the granny ring interference problem has more to do with the chain dragging across the back end of the derailleur cage vs. the middle chain ring... which is quite normal. But, on the bright side, all is not lost in that the gear-inch combinations that are marginal -- middle ring & smallest 2 cogs / granny ring and smallest 3-4 cogs -- can usually be found in big ring / smaller cog & middle ring / larger cog combinations.
As you might suspect, since tandems use drive trains that are designed for road bikes with 135mm rear spacing, the wider the rear axles get the more you disrupt the chain line. Santana developed a special front derailleur clamp a few years back that works with bolt-on front derailleurs to help mitigate some of the problems associated with the sub-optimal chain lines, e.g., chain rub on big ring, difficult shifting into the big rig, etc... Well, I think what they actually did was to spec. a wider set of crank spindles which truly necessitated a modification of the front derailleur travel which, in turn, precipitated the special front derailleur clamp (photo below) that offsets the front derailleur an additional 6mm. If your Santana is '99 (perhaps '00) or newer then it should have one of these already. If it doesn't, Santana sells them for about $30; however, you'd also need to acquire a bolt-on FD.
Regardless, even with Santana's extra-wide FD clamp you'll most like find that not all 27 gear combintations are flawless or useable on a tandem: perhaps 24 at best and more likely 22 depending on your chain ring and cassette size. Interestingly enough, not all 24 gears were flawless or useable on 8 speed tandems with triple gearing either... The downside of this "fix" is that it can give stokers who are sensitive to wide "Q-Factor" (aka, tread or crank width) fits that manifest themselves as knee and or hip discomfort. Fortunately, most cyclists aren't all that sensitive to Q-Factor but, for the ones who are and didn't know it, often times stoking on a tandem with a wide Q-Factor will be a harsh wake-up call.
Bottom Line: Tandems are different and demand some subtle changes in equipment use; bad chain lines are just one of them.