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advice needed.. 11-28 to 12-30 cassette

Andy SchmuckAndy Schmuck Posts: 3
edited July 2012 in Workshop
Looking for some second opinions... I'm going on a cycling trip in the Alps in a few weeks time. Was looking to give myself a slightly easier lower gear for climbing, without having to fork out for various parts. I'm running an 11-28 just now with a shimano 105 5700 rear mech, on a compact front. But I notice I can now get a 12-30 tiagra cassette and was wondering if I could get away with running this cassette on my current rear mech - which technically states a 28t is the max. However, I've noticed my b-screw can still be turned in quite some way, which I'm thinking would give it some more capacity?

However, when I asked the mechanic at my LBS about this the other day, he said it wouldn't work, the way the rear derailler and hanger would need to be angled would mean there was less contact with the chain on the cogs and the shifting would be poorer. I can accept the shifting may be slightly poorer.. but was thinking it wouldnt be so bad that I couldnt live with it, if it gave me that lower bottom gear. I also appreciate some people will say the difference between the 28t and 30t is minimal and not worth it.. but I'm thinking over a week of climbing in the Alps every day it may just make a difference, marginal gains and all that!! Plus it would just give me that bit more piece of mind if there was a lower gear to go into if need be.

So, would do people think, is my LBS just covering themselves and going by the book? Or would this change more than likely be OK?

Thanks in advance.

Posts

  • sungodsungod Posts: 13,777
    if you say exactly which rear mech you have, someone else may have direct experience of whether it'll work or not

    if you do put on a 12-30, you'll need a new chain too
    my bike - faster than god's and twice as shiny
  • bianchibluebianchiblue Posts: 344
    Aren't 5700 mechs 30T max?

    And you wont need a new chain, just make sure you keep out of big-ring and biggest top 2 or 3 sprockets, which is sensible practise anyway. A shorter chain is better - lighter, shiftes better and less slap as well as less risk of derailling on the small ring.

    The whole "put it in big-big and add 2 links" thing is a pure saftey measure for idiots who shift into the bigger sprockets when in the big ring. Stay out of the bigger sprockets in the big ring!! It's bad practise! Bad chainline and less efficiency too...
  • Its a shimano 105 RD5700 - only thing I'm not totally sure about is whether its an SS or GS. Doesnt seem to say on it?

    bianchiblue - according to shimano tech docs if it had been an RD5700-A then it would take a 30T max but mine is without the 'A', so only takes 28T, officially atleast. I would have thought there would be some tolerance built in with these things though, hence my question.

    I've also just read about chain wrap capacity - if I was running a 12-30 with a 50-34 at the front.. then I would need a capacity of 34? In which case I seem to be covered according to tech docs with current rear mech. If anyone has experience of this though would welcome your views...
  • John.TJohn.T Posts: 3,698
    It will work OK. Shimano seem to have quite a lot of tolerance, 2 extra teeth should be fine. Both SS and GS will take the same largest sprocket. The difference is how much extra chain the GS takes up for a triple. Just be careful with chain length and B screw setting. Run through all the gears on a stand to make sure you can get them all OK.
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