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I'm trying to troubleshoot poor shifting issues on a mountain bike that appears to have been built out of the spare parts box…

The current problem is "instability" on the rear derailer – the chain bounces around and is very hard to get to settle into gear. The original setup was a SunRace M40 derailer paired with SunRace twist grip shifters and a Shimano 8-speed cassette. The chain measures closer to 7-speed than 8-speed (0.285" a bit over 7.2 mm).

On the SunRace website the M40 derailer is described as being 7-speed. So I figured that I might have found the problem (7-speed indexing steps paired with an 8-speed cassette), so I tried fitting an NOS SRAM 5.0 8-speed twist grip shifter (which, according to the instructions that came with them, are Shimano compatible). However, after reading @Chris in AK's comments below, and failing to find any definitive marks on the shifter, I actually measured the cable travel using the information I found in this article on drive train compatibility and it looks like the SRAM grip shifters are ESP (1:1) as the cable movement is slightly more than 4 mm / click.

The good news is that the shifting seems to be more stable, the bad news is that it reliably shifts two cogs for each clink (e.g., I move the shifter from the 8 position to the 7 position and the chain moves to the #6 cog).

Any ideas about what could be going on?

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I would check the SRAM grip shift that you purchased. SRAM makes grift shifters for both their own derailleurs and Shimano's. The actuation ratio on those shifters is different. SRAM uses a 1:1 ratio for their high end stuff. Most of Shimano's is closer to 2:1. The SunRace website you linked to lists the M40 as 2:1, meaning you'll need a SRAM grip shifter with 2:1 actuation.

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  • OK, just checked the instructions that came with the shifters. They are SRAM 5.0 Centera (link is to Ebay, shifters were made in 2000, SRAM no longer features them on their site) and claim to be compatible with SRAM Plasma / Quarz / Neos / Centera & Shimano rear derailers.
    – dlu
    Jun 23, 2015 at 22:26
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    If they are ESP, then they are 1:1 actuation (SRAM's marketing name for it) and they will not work with a 2:1 derailleur. I would look at the shifter itself and not the possibly mixed up packing from the eBay sale.
    – Deleted User
    Jun 23, 2015 at 22:44
  • There are no ESP markings on the shifter and I had no luck finding anything on the web that would tell me if the shifter was ESP or not, but I did find this article on drive train compatibility, so I measured the cable pull and get very close to 4.0 mm per step – so I think they are ESP.
    – dlu
    Jun 24, 2015 at 18:02
  • An excellent way to determine actuation! Now at least you know what the problem is.
    – Deleted User
    Jun 24, 2015 at 18:07

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