Bicycle Mechanics - Compatibility of drive train components
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08-17-12, 09:26 PM
I recently pulled an older mtb out of retirement and upgraded parts of the drivetrain, and am now unable to get it shifting smoothly no matter what I try. It is a 1996 Trek 6500 with Shimano LX crankset and LX front/rear deraillers (these were installed back in 2000, but we're not used much prior to retiring the bike shortly thereafter). Also installed Sachs grip shift at that time. Triple front, 8-speed rear.
The recent drivetrain upgrades include a SRAM 870 chain and SRAM 8-speed rear cassette. It will not shift properly (either double shift when downshifting, or skips gears, depending on how I adjust the barrel screw). Since it has been unused for the last decade, I am not sure if the problem relates to that, or due to incompatibility between drivetrain components. I have fiddled with it dozens of times over seversl rides with no luck...Now I am not sure what to try/replace next!
Note: I also replaced the cable and housing hoping that was the culprit, but it did not help the situation at all.
Any thoughts appreciated!
08-17-12, 09:31 PM
Could the rear of the bike gotten bumped within the past decade? If so, then there is a roughly 75% chance that this is the source of the problem. The rear derailleur hanger can get bent, causing the rear derailleur not to be parallel with the cogs, so your shifts will be off. Check this. Take it to a professional mechanic so that this can be checked. There is no feasible way for you to do this yourself, so you'll need the tools and the skills of the volunteers at your local bike co-op (if one exists near you) or the keen eye of a professional mechanic.
08-17-12, 09:59 PM
"...It will not shift properly (either double shift when downshifting, or skips gears, depending on how I adjust the barrel screw). Since it has been unused for the last decade, I am not sure if the problem relates to that, or..."
Pretty classic symptoms of DER cables needing to be cleaned & lubed.
08-17-12, 11:14 PM
With Bill here, soon as I read through the post, thought you need replacement cables. All the parts you mention will work together, I had a similar setup to your back around 96/97 for a while.
The hanger is always a favourite on here, but not sure you will have done too much damage to it if just in storage, think if you just replace the cables, the bike should shift fine again.
08-18-12, 03:51 AM
A few things to check out: 1) Not all Gripshifts are Shimano compatible; you didn't mention which model you had; 2) Even if the Gripshift is Shimano compatible, the shifting will be poor when paired with a Shimano rear D using the linear spring (not enough return force); 3) And a gummed out or kinked or rusted cable may not help matters, either. Luck.
08-18-12, 04:18 AM
A few things to check out: 1) Not all Gripshifts are Shimano compatible; you didn't mention which model you had;
BITD all original Gripshift was Shimano compatible, also, the OP said they had Sachs shifters, these were different to Gripshift, these were 100% Shimano compatiable, used them myself. It was only after the SRAM brand was introduced that SRAM designed a range which was incompatible with Shimano.
2) Even if the Gripshift is Shimano compatible, the shifting will be poor when paired with a Shimano rear D using the linear spring (not enough return force);
Wasn't a problem BITD, it all worked together fine.
3) And a gummed out or kinked or rusted cable may not help matters, either. Luck.
The cable is most likely the issue, from memory Sachs shifters didn't have the same issues with gumming up as Gripshift, and didn't require special lube
08-18-12, 06:04 AM
Thanks for the suggestions, all. I edited the original post, and am saying again here that I have already replaced the cable and housing and it did not improve the problem at all...
08-18-12, 12:09 PM
Make sure that all of the mechanisms are clean, well lubricated and move freely before you begin any adjustments. When replacing cables and housings it is essential to ensure that the housing ends are cut squarely and ground or filed flat and that the appropriate ferrules are installed and that all components are seated firmly. It is also critical to replace the lower loop of housing which goes to the derailleur with new housing of the correct length, again with proper ferrules.
After ensuring that the derailleur and hanger are straight, go through this procedure from start to finish without skipping any steps: http://www.parktool.com/blog/repair-help/rear-derailler-adjustments-derailleur Be aware that the adjustments need to be done in small increments; 1/4 turn or less may make the difference between lousy and perfect shifting. Be systematic, stop find out why and fix the problem before proceeding if any step cannot be done correctly as each step's success depends on the previous ones being correct.
08-18-12, 12:55 PM
If derailleur hanger and derailleur cage alignment check out OK, check the cable attachment at the rear derailleur - http://www.sheldonbrown.com/drivetrain-mixing.shtml#alternate
08-22-12, 02:43 AM
Yup, I read too fast and missed the Sachs part. however, I stand by the spring tension issue. Around '96, Shimano switched most of their rear derailleur parallelogram springs from the stronger torsion springs to the lighter tension linear coil springs. This adversely affected shifting in the smaller cog range and gave Gripshift a bad reputation for a time. A lot of aftermarket parts were sold to counter this, from higher tension springs to Bass Worms - remember those?
To check shifter/derailleur compatibility, you can hook the shifter up to the mounted derailleur via just the derailleur loop, and without the chain in the derailleur (remove the bottom pulley and rotate the back cage plate), see how it indexes under the cogs.
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