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SRAM x.9 rear derailleur and 8 spd cassette?

672 Views 9 Replies 4 Participants Last post by davep, Jan 4, 2010

veritechy

Discussion Starter · Dec 27, 2009

I read a lot of posts saying that the spacing is different for 8spd vs. 9spd cassettes and that the shifting is not going to be smooth.

I tried mating a SRAM x.9 RD with a shimano HG 8 speed cassette and it shifts smooth as butter. Any downsides to this setup? Any chance of premature wear, etc? I'd say it shifts better than the old SRAM 980 cassette.

BTW, I did it because 8 speed seems to be enough gears for the cassette. Also it is only 220g vs. 290g for the 9 speed cassette.

Thanks in advance

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mtnbiker72

· #2 · Dec 27, 2009

What shifters are you using?

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veritechy

Discussion Starter · #3 · Dec 27, 2009

I'm using x.0 grip shifters. I set the limiting screw so that it never shifts into "1".

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boomforeal

· #4 · Jan 3, 2010

been wondering the same thing m'self, as i have a few spare 8 spd xtr cassettes i'd like to use. i've read in a couple of places that the sram 9 spd stuff is compatible with all 8 and 9 speed cassettes, but your is the first "ive done it and its working great" testimony i've found.

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davep

· #5 · Jan 3, 2010

There is no such thing as an 8 or 9 speed derailleur. The derailleur is driven (controlled) buy the shifter. The derailleur position, total throw, and index-to-index spacing are all determined by the SHIFTER, not the derailleur.

The shifter (needs to IMO) should match the cog spacing of the cog, 8 speed shifter for 8 speed cog, 9 speed shifter for 9 speed gog...as well as the cable ratio of the derailleur.

Running a 9 speed shifter (with a gear locked out as Veritechy's is) might work in a pinch, but it is FAR from 'correct'. Each and every shift will be .46mm too 'short'. In the furthest cog, with this set-up, your derailleur will be out of alignment with the cog by 3.22mm (with a cog to cog space of 4.8mm on an 8 speed cog, the derailleur is closer to alignment with cog 7 rather than cog 8....even though your shifter is at the 8th gear).

I guess it all comes down to your idea of 'working'.

Veritechy, have you verified that you are actually getting to cog 8 when shifted to that cog?

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boomforeal

· #6 · Jan 3, 2010

davep said:
Running a 9 speed shifter (with a gear locked out as Veritechy's is) might work in a pinch, but it is FAR from 'correct'. Each and every shift will be .46mm too 'short'. In the furthest cog, with this set-up, your derailleur will be out of alignment with the cog by 3.22mm (with a cog to cog space of 4.8mm on an 8 speed cog, the derailleur is closer to alignment with cog 7 rather than cog 8....even though your shifter is at the 8th gear).

whoa, those are some teeny tiny numbers. curious, did you use vernier or digital calipers to make your measurements, and have you verified that the instrument is properly calibrated?

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davep

· #7 · Jan 4, 2010

boomforeal said:
whoa, those are some teeny tiny numbers. curious, did you use vernier or digital calipers to make your measurements, and have you verified that the instrument is properly calibrated?

There is no need to measure anything...these numbers are all directly from the mfg specs for both 8 and 9 speed cassettes.

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boomforeal

· #8 · Jan 4, 2010

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veritechy

Discussion Starter · #9 · Jan 4, 2010

Davep,
Thanks for the explanation. I've read a lot of places that says it would shift crappy and that's why I asked. I also understand that the alignment is different. I was just so surprised that it works so good. I just went for a 12 mile ride and lots of shifting and climbing. It shifts smoothly through all 8 gears, even under power. It's even better than my old SRAM 980 cassette. And you are right, it's the shifter, not the derailleur that is 9 speed. The derailleur is a dumb component. Could it be the cassette I'm using? It's from about 1990 and I don't see anything like it from Shimano anymore. It's also only about 210g, which I also don't see anymore. Or maybe I'm just lucky for this one setup. I certainly won't try it again.

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davep

· #10 · Jan 4, 2010

boomforeal said:

You have a problem with mfg spec??? I dont get it. I did not just make this up, there are accepted standards for parts like this (cog pitch, cogset width, cassette carrier dimensions, hub OLD width, axle diameters, BB threads and widths, chainlines, brake mounts, headset diameters and depths, steertube diameters, etc, etc. A simple search will give you all of this information right from the standards documentation.

ALL 9 speed cassettes are 4.34mm cog to cog. It is a standard measurement and can easily be verified. Older 8 speed is 4.8mm cog to cog, again this is a well know standard in the industry.

The measurement is also the same mfg to mfg and cassette to cassete. This can be verified by the fact that ALL 9 speed parts no matter the mfg, are compatible.

BTW. yes I do have the tools to measure this to the stated precision if I felt so inclined, but again there is no need, as the standard is published, and if the parts varied significanlty from this standard, they simply would not work.

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