Bicycle Mechanics - Rear cassette

I'm awaiting an 8spd shifter and derailleur to come through the post. I'm currently running 7Spd. Can I simply put an 8Spd cassette on the wheel, or will the hub be too short?

What range would you recommend for a cassette? Mainly road commuting (up and over the North Downs), with a little light cross-country. The crankset is 28 - 48T.

Any particular cassettes - thinking of SRAM 5 or 7.

Thanks

Richard

I am assuming that this is a Shimano Freehub design.

If the freehub body (the grooved, ratcheting mechanism which bolts to the actual hub) is a 7-speed then no, an 8/9-speed cassette will not fit. However, you can change the freehub body to an 8/9 though you will have to adjust the dish of the wheel. You may need to change to a longer axle and adjust the washer/locknut configuration - I am not really sure about this.

Alternatively, you can run the 8-speed shifters on the 7-speed cassette and 'lock out' the 8th position of the shifter. It won't be perfect but it can be done with satisfactory results.

Since your smallest chainring is a 28 tooth, I would get a cassette with a 30 or 32 for the lowest gear. I'm sure either cassette will be fine. I'd probably go for the cheaper one.

Hi Richard,

Dunno if this is significant or not but... The 9-speed Shimano XT wheel I had built for the Yatesy fits perfectly, just like the XT 7-speed I had before. This kind of suggests that the overall width/axle length etc won't be a problem.

HTH :).

See
http://www.argonet.co.uk/highpath/cycle_/faqs.htm#modern


Successful indexing depends on the cog-to-cog distance, not on the number of cogs.
Shimano 7speed uses a 5mm distance, but 8 speed was reduced to 4.8, so the 8 sprockets take up roughly the same width.
You can't use an 8 speed index on a standard 7 speed cog setup.

You can possibly respace the 7 speed cluster to 4.8, using different spacers.
On my Campag 8 speed (This is 5mm, like Shimano 7spd) I use cogs made by an Italian company Marchisio. The individual cogs and spacers are available separately, so you can make any system compatable.

You need to check if the spline pattern has changed , it may have done.
The axle length depends on your frame dropouts, and cannot be changed without bending your frame to suit, which is a serious busness.

If you want to fit a new freehub unit, which is wider then the old one, then you need to squeeze the hub inboard a few mm. You do this by adjusting the spoke lengths (called re-dishing the wheel). The new freehub will then fit using the old axle, but the wheel will be significantly weaker.

A possible solution:
If you have a 126mm dropout spacing, and wish to go from 7 speed to 8 speed, it may be possible to use 9-speed cogs, just use 8 of them! I've seen this done before, but I'm not sure where there would be problems. Perhaps the lock-ring is a problem?

Originally posted by D*Alex
If you have a 126mm dropout spacing, and wish to go from 7 speed to 8 speed, it may be possible to use 9-speed cogs, just use 8 of them!

I think Sheldon endorses the trick as well, Alex.

ok ive broken down and I have to say this....the thread title is Rear Cassette.....as apposed to front cassette? hehehehehehehe:beer:

Thanks, I think replacing the wheel with something half decent might be my easiest option initially and get the old one sorted later.

I would go to a 28 or 32 tooth big cog (I run a 36 front to 28 rear combo and it works fine.) If you have serious hills you might want some lower gears but if you are running 28 -28 or 28-32, I would get off and walk up any hills needing lower than that.