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Shimano 105 - fitting lower gears?

Discussion in 'Bicycle Mechanics and Repairs' started by Norv, 21 Apr 2008.

  1. Norv

    Norv New Member

    Hi there,

    I recently bought a road bike kitted out with Shimano 105 drivetrain. My lowest gear is a 39x25, which i'm finding ok in general, but on rides with sustained 18-30% gradients, really hard work!

    I've thought about fitting a compact, but was told by a friend that compacts cannot be fitted to bikes with Shimano 105. Is this true, and are there other ways I can stick some lower gears on?

  2. It all comes down to total tooth difference.

    You can install a lower gear either by reducing the size of the chainring(s) or by increasing the size of your sprocket(s).

    I think the smallest stock chainring possible on a 105 is 38t so not much reduction to be gained there without swapping out the whole chainset for one with a smaller BCD, although companies like Middleburn might make a 37t to fit it's still an expensive way to achieve little improvement.

    Increasing the size of your largest sprocket is limited by the maximum sprocket capacity and the maximum tooth difference.

    Rear mech tooth difference capacity is determined by the length of the rear mech cage, ie how much chain slack it can accomodate. Work it out by subtracting the small chainring (39t) from the big chainring (53t?) and by subtracting the smallest sprocket (11t?) from the largest (25t) then adding the two figures together (=28t?). Select a new cassette which falls within the max and doesn't exceed the maximum sprocket size.

    Read the back of the mech, it'll tell you its capacity. You might require a longer mech with greater capacity to get the ratio you want.
  3. RedBike

    RedBike New Member

    Beside the road
    There's nothing specific about 105 that will stop you fitting a compact chainset. Worth just double checking your mechs will be capable with the LBS; but I doubt there will be a problem.
  4. robbarker

    robbarker Well-Known Member

    You can get a copact chainset but Shimano specifiy using a medium cage rear mech with them, and you probably have a short cage mech . A short will actually work with a compact, but it's not recommended.

    I used to run a compact but have now gone back to 53/39 and 12-27. The lowest gear is almost identical and the spread and useability is much better. My suggetsion would be to get a 12-27 cassette and see how you go with that.
  5. Norv

    Norv New Member

    Thanks for the advice.

    The reason I ask is that yesterday, I did a 100 mile sportive in North Cornwall. The 1 in 7 climbs literally came at you all day, which became a nightmare with a 39x25 lowest gear. I'm starting to think that fitting a 39x28 is the best option as I don't like the massive jump that you get between a compact and normal size big ring. Plus the fact, I very rarely do rides as harsh as yesterday's. On the other hand, will a 28 tooth sprocket really make any noticable difference?
  6. Monst

    Monst New Member

    The boonies
    Your mate is talking boll****s

    You can run a compact with both a short cage rear mech or a long cage rear mech with whaterver groupset you have - 105, Ultegra or whatever

    You can run a triple with a short cage or a long cage rear mech, but with a short cage you may not be able to get small, small. The 105 levers support double as well as triple.

    If you convert your normal 105 to compact you do not even have to change the chain. Just adust the indexing. You can even put whatever cassette youy want on - without adjusting the chain.
  7. Monst

    Monst New Member

    The boonies
    Sorry - Shimano's biggest road 10 speed
    sprocket is 27, not 28. (unless you bung on a mtb sprocket)
  8. RedBike

    RedBike New Member

    Beside the road
    You can only fit up to 27 with Shimano.

    I find having a closer ratio cassette makes up for the 'big' jump in changing from 34 to 50. You could always use 36/50. This will feel very similar in changing from 39 to 52.
  9. Is there a 10spd mtn bike cassette available now?
  10. Forgive me for editing your post.

    (1)It's rather more complex than that; If you spec a triple with a short cage mech you run the risk of losing the slack chain between the wheel and stay, with potentially disasterous consequences. I've seen it happen. Very Bad Idea.

    (2)Wrong, if the chain was properly installed for the original set up it'll be too long for smaller rings. Not by much I grant you, but it will be too long.

    (3)Huh? Why?

    (4)May I refer the right honourable member to an edited version of an answer I gave a few moments ago. Wrong, if the chain was properly installed for the original set up it'll be too short for larger sprockets. Not by much I grant you, but it will be too short.

    You cannot use a mountain bike cassette with a short cage road mech and expect anything but rubbish shifting. Road mechs are designed to closely follow the shallow profile of road cassettes as they move across.

    Install a close ratio road mech with a 12-34 and you'll have to crank the *b* adjust screw all the way in order to clear the big sprocket. The effect of doing this will be felt at the other end of the cassette, the jockey wheel will be so far away from the smallest sprocket that it will have real trouble moving the chain.
  11. Monst

    Monst New Member

    The boonies
    Sorry M

    You do not have to change/adjust the chain.

    This is my bike on a compact last year, with short cage rear mech..

    This is my bike with long cage rear mech and added triple (and blingy white bar tape) for this year. As I said, no need to change the chain!

    No shifting problems, no chain slap, no problems whatsoever with running the chain with either chainset or with a short or long rear mech.

    I use a long cage on the triple, my mate does not on his triple combo, but he is more mechanically sensative! To use his short cage with his triple he did take off a couple of links, but changing my bike from a double with short cage to a compact with short cage to triple with long cage, I did not need to touch my chain.

    Both versions above are using the same 12/27 cassette.

    The front derallier adjustment needs fettling when you swap to a triple as you need to change it for a triple type. Because of the chain angle on (on the naughty small small combo) the compact, this occasionally needed a tweak on the rear mech on first install.

    The biggest cassette sprocket one could get away with is a 32 on a road bike, but you need a longer b screw.

    This is how you do it.

    Buy a single 32 sprocket.

    Take the 13 or 12 sprocket (they are loose) from your road cassette. Put it to one side.

    Put on the 32 sproket on the bike. Put the rest of the cassette on.

    Cassette now goes 32/13 (or 12)

    Take one Dura Ace b screw (they are longer) and adjust and away to go.

    Of course you could just use a triple (or a 9 speed mtb cassette with a nice light n blingy XT/XTR derrailier, with the adjuster done up to cope with 9 speeds for a 10 speed shifter)
  12. Sorry I sometimes come across as an arse because I think in terms of 'best practice', anything which doesn't fit my narrow constraints I consider to be bodging. If it works for you then great.
  13. Norv

    Norv New Member

    Thanks for all the detailed responses.

    If I go for the option of a compact, how do I know i'm buying the right chainring? Where can I go to buy one? It needs to fit ok to replace a Shimano 105 39T ring.
  14. RedBike

    RedBike New Member

    Beside the road
    You can't fit a smaller chainring to your existing cranks. Sadly you will need to replace the lot. You can fit any Shimano 10speed / Shimano compatible chainset.

    Something like this
  15. Monst

    Monst New Member

    The boonies
    Apologies. our rant seems to have taken over. Please just get a 105 compact and all will be fine. A 105 compact is double, short cage, long cage, triple compatible.

    Um, Parkers is the cheapest..........