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Upgrade 7-speed to 8-speed

MettanMettan Posts: 2,103
edited December 2007 in Workshop
Thinking about upgrading my 7-speed budget Road-bike to an 8-speed setup - (I've sort of mentioned this before but am not sure of some of the details) - it's currently got an 8-speed Shimano STI front right lever (came with the bike- bit strange, because it's a "7-speed bike") - so am thinking about getting an:

8-speed Rear wheel - probably Shimano R550 clincher
8-speed Shimano Megarange 11-34 cassette (for lower gears & hills)
(possible need an 8-speed chain???)

Now - the bike currently has a 7-speed Long cage Shimano Altus MTB-ish rear deraileur, so it can already handle an 11-34 cassette - but what I'm unsure about is if I put on the 8-speed wheel and 8-speed cassette, will the 7-speed rear deraileur be compatible with that setup? or will I have to use say 7 of the rear sprockets instead of the 8 on the cassette ? - & any other advice would be great.



  • Seems a lot of money for not much gain unless your rear wheel is currently knackered.

    However you have the right list of parts - you could get away with a 7 speed chain (probably) but if getting a new cassette it is a good time to replace the chain anyway.

    I use a 7 speed deore derailleur on an 8 speed block so the range of adjustment should be OK (Depending on the derailleur hanger width)

    Note that road 8 speed and above rear wheels have an over lock nut dimension of 130, the 7 speed hubs had 126mm. This is how they got the extra cog in for 8 speed. (9 and 10 they just narrowed the cogs, chain and cog spacing). So you may need to sping the frame out to get the wheel in there
  • MettanMettan Posts: 2,103
    Thanks Alan - I'm hoping get out of it -

    Lighter/better wheels - "rolling weight"
    "More" sprocket teeth/cassette for Lower gearing - "hills"
    8 gears - "cadence"

    I'll have to look into the wheel width thing, rear dropout width etc - I've heard some 7's having 126 and some 130 - I'll have to measure mine.

    Thanks for the info.
  • maddog 2maddog 2 Posts: 8,114
    it will work fine

    7 and 8 speed chains are the same

    so the 7 speed mech will be fine on a 8 speed cassette

    the only issue is using an old chain on a new cassette - it may be better to fit a new chain while you're at it.
    Facts are meaningless, you can use facts to prove anything that's remotely true! - Homer
  • MettanMettan Posts: 2,103
    Thanks Maddog -

    Just measured the rear dropouts - 130mm :)
  • acorn_useracorn_user Posts: 1,137
    Is this a Saracen? You can get 7 speed cassettes in the range you are looking for. I have a 13-30 on the shelf for when my commuter needs a new cassette. There are some 8 speed freewheels around, made by Shimano, that you could use so that you could use the current wheels if they are for freewheels. Giant use them on their lower end road bikes (along with single pivot brakes...).
  • rrsodlrrsodl Posts: 486
    I would just change the cassette. I have a 7 speed bike and I upgraded the cassette the other day to a 13 - 30 cassette - For £16 and 15 minutes work.

    You fit a SRAM 12 - 32 7 speed cassette for £15 and a few minutes work.

  • MettanMettan Posts: 2,103
    Thanks all - the upgrades been done - the R550 seems heavier than hoped for although moving from a 7 to 8 speed cassette has put the weight up a little - was hoping for a "wow factor" (weight-wise), but am reasonably happy for the price. In the next few months, I'll get the R500 or R550 for the front - good prices at present for a good basic wheel.
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