gear trouble

[PJ79LIZARD]

Hi I have a sarcen tour with double chainset 53/39 and 7 speed altus rear set, Is it possible to put an 8 speed freewheel on the back seeing the sti shifters say 8 speed even though I only have 7 at the moment. or will I have to get an 8 speed rear derallier and a bigger freewheel hub to accomadate the larger freewheel. As I am looking to get a 8 speed freewheel which has a larger ring to give me a little help on the larger hills I seem to be encoutering more frequently as my rides are getting longer. Any help on this will be very appreciated cheers. Paul

 

[Big John]

If you have 8 speed STI's I reckon your halfway there. You'll not only need to change the freewheel to an 8 but you'll need to change the axle length. You're going to make everything wider so you'll need to make sure it fits between the dropouts. I'm no expert but I think you need an 8 speed rear mech too but if it was me I'd try the 7 first and see what happens. When you start mixing and matching you can get some strange results.

 

[PJ79LIZARD]

Hi thanks for your reply, I was looking at the rear mech last night, The shifters move the rear mech 8 times, I don't quite understand why the chain doesn't come off the last sprocket because when it's in 7 gear, the mech does move another notch along, so I'm thinking maybe it will work with an 8 speed freewheel if it was fitted, As I have seen some rear mechs for sale stating 7/8 speed compatible which I dont understand. but like you say I will have to change the freewheel hub if that is not going to be wide enough. Seems a lot of messing about to get an extra 2 gears, But it would be worth it when I hit a big hill, which seems quite frequent living in the west midlands. cheers for your thoughts

 

[TheDoctor]

An 8 speed cassette won't fit on a 7 speed hub - it's physically wider, so you'll need to replace the freehub body.
If all you want is a slightly larger spread of gears, I'd just get a wider ratio cassette.

 

[PJ79LIZARD]

I have been thinking about that to I currently have a 14-28 on it, but have seen some 12-32, would I have to make the chain longer or would it make no difference? it has a long caged rear mech on it.

 

[RedBike]

Something is wrong here. From memory a Shimano 7 speed freewheel has the sprockets spaced a 5mm intervals. Whereas an 8 speed Shimano cassette (There is no such thing as an 8 speed freewheel) has the sprockets spaced at 4.8mm

This means you can just about swap one for the other and get them to work. Although you'll always be 3/4 of a mm out at best on the indexing on your highest/lowest gears. Not alot but it is enough to cause poor shifting / excess noise.

Without knowing exactly what shifters / rear mech you have it's hard to advise. But in theory you could fit a MTB rear mech (Deore are great and cheap) and then use a MTB cassette. You would also need to change the chain. It's never a good idea to use a worn chain on a new cassette.
A standard road rear mech has a limit of 27teeth. Tthey can sometime be forced to accept slightly larger cassettes but there's often problems in doing so.

btw, you can't fit a cassette onto a wheel that takes a freehub and vice versa.