Can I use 7-speed Sti shifters with an 8-speed SRAM cassette?

[swee'pea99]

I'm pretty sure SRAM and Shimano cassettes have identical spacing, and Sheldon says "7 and 8-speed systems use a relatively wide, and almost identical, spacing between sprockets", so I was wondering whether I could put my 7-speed Sti shifters and SRAM 8 speed block on my new n+1. Obviously it would mean setting it up to use only the bottom 7 speeds, but I wouldn't be worried about that.

 

[mcshroom]

I'm not sure. There's a 0.2mm difference in pitch. I think you'd struggle to index it correctly.

The numbers for the different cassette spacings are given here
http://www.ctc.org.uk/cyclists-library/components/transmission-gears/derailleur-gears/shimergo

 

[swee'pea99]

Thanks. I'm not good with tables 'n that, but now I'm even more baffled! Looking at that, it looks to me like the spacing for a Shimano 8-speed is 4.80 while that for a SRAM 8-speed is 7.45, which would certainly suggest incompatibility. Yet the generally reliable Sheldon says:

SRAM, SunRace, IRD: mostly compatible
Most cassette hubs are compatible with Shimano cassette cogs. SRAM cassettes and most Miche, IRD and SunRace cassettes use the same inter-sprocket spacing as Shimano,

What am I missing here?

 

[Milkfloat]

Thanks. I'm not good with tables 'n that, but now I'm even more baffled! Looking at that, it looks to me like the spacing for a Shimano 8-speed is 4.80 while that for a SRAM 8-speed is 7.45, which would certainly suggest incompatibility. Yet the generally reliable Sheldon says:

SRAM, SunRace, IRD: mostly compatible
Most cassette hubs are compatible with Shimano cassette cogs. SRAM cassettes and most Miche, IRD and SunRace cassettes use the same inter-sprocket spacing as Shimano,

What am I missing here?

That is saying that you can mix companies, so you can run a mix of Shimano and SRAM assuming they are both the same speed. I know that certain combinations of mismatched speeds will work (mainly on the front) but cannot comment on 7/8.

 

[swee'pea99]

Thanks. I've decided t play it safe, and bought a new 7-speed Shimano. There's another £8.65 I'll never see again.

 

[fossyant]

No point having an 8 speed cassette on there if you only have 7 sprockets. I've run 7 speed cassettes using 8 speed spacers with 7 speed shifters. I've got a huge stock of very shint polished alloy Dura Ace 8 speed spacers. 7 speed spacers are 3.1mm and 8 speed is 3.0. It makes no difference in reality to the indexing as the mech jockey wheels can take up the .3mm if you index to the middle sprockets.

 

[swee'pea99]

I wasn't aiming to run an 8 speed cassette with 7 sprockets (not sure how that would actually work!) ...just to combine 7-speed shifters with an 8-speed cassette (set up so the smallest sprocket was never used).