Indexing Issues with SRAM Cassette and SunTour Shifters

[DoctorRad]

Bit of a funny one this, but bear with me.

I've been running SunTour indexed thumbshifters (and wishbones) on various bikes for about 25 years, and have always been able to get at least 95%+ accurate indexing, whether it was 6-, 7- or 8-speed.

Until last week that is. The set-up was DX rear mech, 8-speed XC Pro thumbshifters, SRAM chain and SRAM 11-32 cassette (latter two modern and brand new). The indexing just wouldn't work. The main spring on the mech wasn't in great shape, so I swapped it out for an XT 8-speed. No joy, I was stumped.

Then I realised the variable: every other cassette or freewheel I'd ever used with the thumbshifters before was either SunTour or Shimano. Could it really be the SRAM cassette?

It appears so. Just swapped out the cassette for an HG-51 8-speed and everything's working fine on a quick test ride. Will check with some proper rides at the weekend.

Anyone else ever had similar issues?

I can only think that the different ramp shape on the SRAM cassette doesn't play well with the small amount of misalignment across the block you get with the SunTour shifters.

 

[andyz]

Re:

Funny you should mention it!
I've just been using a setup with XC-pro 8-speed thumbies, M737 rear mech and new Sram PG850 cassette.

Mine won't work either, and I've just about given up with the Suntour thumbies and was going to try some modified Ultegra bar-end shifters instead.

I do have an M737 cassette I could try it with first.

I wonder if the sram's sprockets are a different thickness?

 

[DoctorRad]

Re:

Almost exactly what I was running on Sunday = awful shifting. I would have given up if I'd not had a quarter-century of success with the shifters!

According to the Sheldon Brown site, the cog thicknesses are the same for 8-speed (but not 9) so I can only think it's down to the ramp shape.

 

[FluffyChicken]

Re:

Have you had it working with other 11-32 8speed cassettes. It's a big jump and possibly the curve is just that little bit out for the shifter/mech positioning.

 

[andyz]

Re:

Mine's an 11-28, and it still doesn't work very well. I'll try the shimano cassette today.

 

[DoctorRad]

In the past I've run the following, in many cases for years with few issues:

• 7-speed 13-28 SunTour screw-on with 7-speed XCM wishbones
• 7-speed 13-34 Shimano cassette with 8-speed XC Pro thumbshifters
• 8-speed 11-32 Shimano cassette with 7-speed XCM wishbones (friction 8th gear)
• 8-speed 11-32 Shimano cassette with 8-speed XC Expert thumbshifters

Pretty much always used Sachs / SRAM chains, but never used a SRAM cassette before. They genuinely appear to be the weak link.

Another friend had a pair of XC Expert thumbshifters from me and couldn't get them to index either. He's a big fan of SRAM cassettes, so I will check with him at the weekend what he was using.

 

[hamster]

Weird. I use SRAM cassettes all the time, in preference to Shimano. They always shift slickly with no trouble. I even run them on the back of the tandems - 9 feet of gear cable usually brings out indexing problems!

 

[DoctorRad]

Ordinarily, I'd agree. But given that SunTour shifters use a mix of 7- and 8-speed gaps (5.0 and 4.8mm respectively), it appears that the slight misalignments may be too much for the SRAM cassettes.

The really odd thing was that the worst cog wasn't at either end, it was third from biggest...

 

[andyz]

Ordinarily, I'd agree. But given that SunTour shifters use a mix of 7- and 8-speed gaps (5.0 and 4.8mm respectively), it appears that the slight misalignments may be too much for the SRAM cassettes.

The really odd thing was that the worst cog wasn't at either end, it was third from biggest...

There must be something wrong as I have the exact same problem - when in 3rd from top the chain keeps trying to skip up to 2nd. Personally, I can't be doing with dodgy shifting, so will be changing the thumbies to shimano ones.

 

[DoctorRad]

Yeah, but the really odd thing is that swapping to a Shimano cassette - which should have exactly the same cog spacing and thickness - appears to solve the issues. I will be testing at the weekend, but assuming the Shimano cassette is okay, it must be down to the different tooth / ramp shapes on the cogs.