Shifter compatability 8 with 7

[cyberknight]

I have read via google search that you can use a 8 speed shifter with a 7 speed freewheel assuming you set the low stops so that the shifter does not pull the mech into the wheel ?
i am currently about to start building a mtb cicra 1994 ?
EDIT
It has a 7 speed cassette, hub 135m mm but i only have a 8 speed shifter in stock.
tried to get the cassette of but its stuck
In an ideal world i would buy a new wheel so i can mount my spare 8 speed cassette etc but i am trying to do this with what i have.

 

[Mile195]

I don't know for certain, but thinking about it logically, the chain width on 7 and 8 speed bikes will be the same, therefore the sprocket spacing should be the same, and therefore the index points on the shifter should also be the same.

Don't take that as gospel though.

 

[fossyant]

It will work. 8 speed is 3.0mm spacing, 7 is 3.1mm.

 

[cyberknight]

Next question...
I have an 8 speed cassette , if i take the smallest sprocket off will it fit on the 7 speed hub (maybe with a spacer ?).

 

[RecordAceFromNew]

Next question...
I have an 8 speed cassette , if i take the smallest sprocket off will it fit on the 7 speed hub (maybe with a spacer ?).

You can see the answer here regarding transplants. Generally you wouldn't want to remove the smallest cog because it has a serrated outer face for meshing with the lock ring.

Regarding 8 speed shifter on 7 speed Shimano cassettes, the thing to know is there is an error of 0.2mm at the cassette per shift. It will therefore work best if you adjust cable tension by centering the rear mech on the middle sprocket (and accept an error of 0.6mm at the extreme sprockets).

 

[andrew_s]

It will work. 8 speed is 3.0mm spacing, 7 is 3.1mm.

Sprocket pitch is 5.0mm for 7-speed, and 4.8mm for 8-speed. This is how far the mech moves per click.
If the indexing is right in the middle of the cassette, the mech will move 0.6mm less far than it ought to have done at either end of the cassette.
That's within the normal tolerance, so it will work OK when everything is in good condition, but you are more likely to have poor shifting if the mech pivots are worn, the cables are getting sticky, or if the indexing is a bit out.

[edit] It seems that your 3.1/3.0 is the spacer thickness. The sprockets are a different thickness too. See here.

 

[cyberknight]

You can see the answer here regarding transplants. Generally you wouldn't want to remove the smallest cog because it has a serrated outer face for meshing with the lock ring.

Cheers ,never occurred to me to remove one of the middle cogs .

Thanks all for your help.

 

[cyberknight]

All sorted and it shift smootly
linky ..
http://www.cyclechat.net/threads/free-bike-build.125814/