7/8 speed conversion

[Andy in Germany]

I'm changing my bike from a 7 speed to 8 speed rear cassette. On evidence thus far during the test ride, it seems you can't run an 8 Speed cassette on a 7 speed mech: changing from 5 to 4 is intermittent and it doesn't go into 1 at all. I this down to the mech or should I look elsewhere for the problem?

Before someone says I should just get a 9 speed setup like a sensible person, I'm using an Xtracycle load carrying bike and I need the extra chain strength.

 

[Tim Hall]

Are you using indexed shifters? At 7 and 8 speed level, the mech is essentially dumb, all the indexing is in the shifter, combined with the spacing between the sprockets on the cassette.

(As an aside I'll put a small bet that the tensile strength of a 9 speed chain compared with an 8 speed is neither here nor there)

 

[Yellow Saddle]

Are you using indexed shifters? At 7 and 8 speed level, the mech is essentially dumb, all the indexing is in the shifter, combined with the spacing between the sprockets on the cassette.

(As an aside I'll put a small bet that the tensile strength of a 9 speed chain compared with an 8 speed is neither here nor there)

I ain't betting against you on that one.

 

[Hardrock93]

Are you using indexed shifters? At 7 and 8 speed level, the mech is essentially dumb, all the indexing is in the shifter, combined with the spacing between the sprockets on the cassette.

Tim, for riders of low tech bikes, like me, could you explain in what way mechs for 9 speeds and above are smarter? I just asumed all (cable operated) mechs moved according to the cable pull from the indexed shifter.

 

[Tim Hall]

Tim, for riders of low tech bikes, like me, could you explain in what way mechs for 9 speeds and above are smarter? I just asumed all (cable operated) mechs moved according to the cable pull from the indexed shifter.

I'm pretty sure a 9 speed and above mech is also as dumb as a rock, so, again, all the magic is a mix of the clicks on the shifter and the cassette spacing. However, I have a feeling that an 11 speed Shimano mech has a different pull ratio than the lower speed Shimano offerings. But as I can't be sure I thought I'd err on the cautious side and limit my answer to 7 and 8 speed.

 

[Hardrock93]

I'm pretty sure a 9 speed and above mech is also as dumb as a rock, so, again, all the magic is a mix of the clicks on the shifter and the cassette spacing. However, I have a feeling that an 11 speed Shimano mech has a different pull ratio than the lower speed Shimano offerings. But as I can't be sure I thought I'd err on the cautious side and limit my answer to 7 and 8 speed.

Ah, thanks. I misunderstood your post and thought you were implying there was something sophisticated going on with 9 speed and above mechs!

 

[Andy in Germany]

Hello. People answer quickly here

Sorry, yes indexed shifters. So in theory the lower and upper limits should be the same on a seven or eight speed shifter then?

The mech is a little elderly though so I may need to replace it. I have at least two near mint mechs hidden away that I salvaged from scrap bikes so that shouldn't be any trouble.

I've been told before that a 9 speed chain is narrower but 'of better quality materials' so it should be no trouble on a load carrying bike, but I'm illogically unconvinced, partly because I know what abuse the chains get when I ride. Mind you I also use a ten year old design of cell phone...

 

[Yellow Saddle]

No, the limits will be different. If the spacing between the sprockets are the same in 7 and 8, then it means that the 8-sprocket cassette is wider and the limits therefore have to be wider. Remove the chain and undo the cable, redo the limits and set up the gears from scratch.

 

[Yellow Saddle]

Suntour's last stand was a 7-speed indexed down-tube system. It wasn't possible to upgrade this using Shimano or Campagnolo shifters because the pull ratios are different. But as long as you have same-same up front and on the derailer, then you are OK, it will work.
Everything remained compatible until Shimano's 10-speed Dyna-sys mountain groups came around. Then you again could not mix and match, which means you cannot use a road shifter with a MTB derailer and vice versa.

 

[Ajax Bay]

yes indexed shifters. So in theory the lower and upper limits should be the same on a seven or eight speed shifter then?

@Andy in Germany What indexed shifter are you using? And it's a seven speed one; yes? @Yellow Saddle has answered the 'limits' question.

 

[Andy in Germany]

Thanks again for the replies. My shifter is a reconditioned Shimano 8 speed rapidfire unit. I changed this some considerable time ago thinking I could change the cassette the next time the chain wore out. It worked well, but 'jumped' in the middle: the upper and lower 7 speed gears were in place but it took a couple of clicks to get from 5 to 4. I generally assumed this was because the spacing was a bit different but of course it could be diferent AND wider.

What doesn't help, is that the gear cable is almost as long as a tandem to reach the rear of the Xtracycle. I've posted a picture taken last time I took the panniers off and washed it: you can see the extra length and corners the cable has to manage.



Will have a look when I get to work tomorrow.

 

[Tim Hall]

Beating @screenman (I think) to the call, but you might want to look at how straight the mech hanger is. Admittedly in 8 speed land it's not as critical as the more sprockety cassettes, but worth a shout.

 

[si_c]

If you are using an 8 speed shifter, with 8 speed cassette, then you should be ok. As has been said before, it sounds like the limits are off, which basically means you need to re-index the gears properly. The length of cable shouldn't be an issue with 8speed if it wasn't with 7, and the derailleur should have sufficient movement to account for the extra sprocket. I used a 6 speed derailleur with an 8 speed cassette for a short time without too much difficulty (albeit with friction shifters, not indexed).

 

[Prometheus]

Andy you are worse that that Colinj feller, but nice to hear from you again.
What's the strange beastie like to ride?
Because in theory everybody should be touring on one, as yet I’ve not seen one on the road.

PS and it's time you had a bright NEW shiny Android thing in your back pocket.

 

[Andy in Germany]

Hello again... a friend and I had a good look at the thing in our nice warm workshop today and it is running a lot smoother. Still not perfect but at least I can use all the gears, sometimes when I want them. The limiters were a bit too tight and the cable needed another good hard tug to get the lower gear to work. It still jumps a bit arount the 4/5 mark, but I'll try again when I'm thoroughly annoyed by that.

@Prometheus: Thanks... I think. Riding the Xtracycle is a dream, when the gears aren't placing up. Today was my first day after getting used to the normal shaped 'Commuter Bike' and the difference in ride because of the length is remarkable. I'm suprised more people don't tour them, although for long trips without support I think some people may be a bit leery of the non-standard rear end.

If you are using an 8 speed shifter, with 8 speed cassette, then you should be ok. As has been said before, it sounds like the limits are off, which basically means you need to re-index the gears properly. The length of cable shouldn't be an issue with 8speed if it wasn't with 7, and the derailleur should have sufficient movement to account for the extra sprocket. I used a 6 speed derailleur with an 8 speed cassette for a short time without too much difficulty (albeit with friction shifters, not indexed).

The length of cable means you have less leeway to get the indexing right, especially as it has to go through all manner of bends to get to the rear mech, and there's that bit more to stretch. It's a slight niggle with a longtail such as this. I should use a tandem cable really, but I discovered after a previous outbreak of poor organisation that a regular cable fits -just.