I just recently acquired a TREK 560 Pro (1986) 531 frame and updated it to Shimano 9 Speed with SRAM 970 12-26 cassette. While tuning it up for the first ride I noticed that the chain ,when on the small chainwheel (39) and high cassette cog (12) rubs on the inner face of the large chainwheel. Uh oh, chain line problem I changed out the BB from 109.5 to 119 (just happened to have one on hand, thank you ebay). I still have the same result. It only does this in this gear combination so does anyone have a solution or am I SOL and will not just be able to use this gear combo?
It is a condition called "cross chaining" and unavoidable, same dealio with 53 and 26. Big/big and little/little are no-nos. Go put the 109.5 bottom bracket back on, it's the correct spec for a double.
Interesting. I'm the original owner of a 1978 Trek Tx700 as well. I just recently had the rear drop outs spread to 130mm(as I did on the 560) and upgraded the TX700to DA. The BB spindle on this one had always been 118mm. I changed the BB to 109.5mm as per DA specifications. It has worked out perfectly. I now have the original DA 600 crankset (after many hours at the buffing wheel) on the 560 with recent Ultegra chainwheels. It looks great--kinda retro. I have noticed that the chainstays are shorter on the 560 as opposed to the TX700. I'm sure that this has something to do with the crosschaining debacle. Do you still think I should put the Ultegra crank back on with the 109.5er BB, and what solutions are avail such as different drive sys ratios to counter this problem?
I don't ride in extreme gears, of course, but the fact it does it is annoying, kinda like getting bad reception from that stupid Country Western station that you know you will never, ever listen to but you just want to know why your receptions bad, thats all.
"Cross-gearing" should generally be avoided 'cause it accelerates the wear on the drivetrain components, just as a perfect chainline minimizes wear. Picture the drive-&-driven surfaces of the chainring & cog teeth.... they will wear longer if the force from the chain is applied directly against that surface, as opposed to applied at an angle against the inner or outer edge of the teeth. Likewise, the chain will last longest if driven in a straight line as opposed to an angle. That's why single speed & hub-geared drivetrains generally last longer & often require less maintenance (all else being equal) than derailleur-geared drivetrains. Generally, the middle chainring of a triple can be used with all rear cogs; the inner chainring should be limited to all but the smallest couple of cogs & the outer chainring limited to all but the largest two cogs. Similar on a double chainring setup.
Hence the references to "useable gears" in postings about derailleur gearing...
Cross gearing usually produces more noise, too.