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smallest ring on a road triple?

maddog 2maddog 2 Posts: 8,114
edited December 2009 in Workshop
they come with 30t usually but I was wondering how low you can go and still work okay.

according to Sheldon, a 74mm BCD will go as low as 24t but that would give a jump of 15 teeth to a 39 middle ring, and an overall spread of 28 teeth (assuming a 24,39,52 chainset)

What can a road triple front mech handle?

is 28t okay

is 26 doable?

Anyone played around with this?
Facts are meaningless, you can use facts to prove anything that's remotely true! - Homer

Posts

  • AnonymousAnonymous Posts: 79,667
    You might be better off getting an MTB crankset...
  • maddog 2maddog 2 Posts: 8,114
    I thought about that but the frame runs a braze-on mech, so I'm stuck with a road FD. I wanted to stay with a road triple chainset to ensure the shifting was okay, but just tweak the bottom end a bit.

    I've run mtb chainsets before with road mechs and the shifting is hit and miss.
    Facts are meaningless, you can use facts to prove anything that's remotely true! - Homer
  • AnonymousAnonymous Posts: 79,667
    This looks rather cheap :shock:

    http://www.spacycles.co.uk/products.php ... 1816&rs=gb

    28t...

    26t here

    http://www.cyclesportsuk.co.uk/product_info.php?products_id=5299

    TA do a 24...

    http://branfordbike.com/product/ta-74-bcd-chainrings-533.htm

    I suspect that'd have to get a smaller middle and outer to enable reasonable shifting though!
  • Mike67Mike67 Posts: 585
    You might find your rear mech struggles to take up all that chain slack needed when dropping from your 52t ring.
    They have a range limit too depends on the model you have.

    Eg the XT long cage (SGS) rear mech quotes a max front teeth difference of 22t (with 23t difference at the rear)
    Tiagra rear mech quotes 22t difference at front with 37t difference max (guess that means 15t difference at the back with 22t difference at front)

    You can of course adjust these figures to suit your gearing set up but your 28t difference up front would be pushing things.
    Mike B

    Cannondale CAAD9
    Kinesis Pro 5 cross bike
    Lots of bits
  • If you want to run lower than a 28t you'd be better off using an e-type MTB front mech which may not work with the 50t ring, and may foul the braze on. It may also be easier to walk as you'll be going so slow...
    Running a 29/34/50 is a 21t difference, and should be ok with a long cage rear mech. Running a 29t front ring with a 12/28 rear (on a 700x23c tyre) gives you a bottom gear of 27.4inches. A 30t front ring with 12/28 equals 28.3inch bottom gear and will work better. Your top gear would still be 110.1inch gear so still enough for 30mph...

    Phew.
    jedster wrote:
    Just off to contemplate my own mortality and inevitable descent into decrepedness.
    FCN 3 or 4 on road depending on clothing
    FCN 8 off road because I'm too old to go racing around.
  • maddog 2maddog 2 Posts: 8,114
    cheers all. Good link NapD.

    It's not for me. It's for my father-in-law who needs a really low gear, for various reasons.

    I'm tempted to give a 28t ring a try, along with a 11-32 mtb cassette. I realise the jumps on a mtb cassette are fairly big but he needs a low low gear and my current thinking is that this would be a better solution than a mtb/touring chainset, for the reasons given at the beginning.
    Facts are meaningless, you can use facts to prove anything that's remotely true! - Homer
  • de_sistide_sisti Posts: 1,219
    All my bikes have Stronglight Impact triples with road f/r mechs. Ratios are:

    50/34/26...................(12-28 cassette)

    48/34/26...................(12-28 cassette)

    46/34/24...................(12-28 cassette)

    No problems shifting.
  • Mike67Mike67 Posts: 585
    maander wrote:
    All my bikes have Stronglight Impact triples with road f/r mechs. Ratios are:

    50/34/26...................(12-28 cassette)

    48/34/26...................(12-28 cassette)

    46/34/24...................(12-28 cassette)

    No problems shifting.

    Maander
    Your biggest 'capacity' in those three is 40t difference, just three more than Shimano quote for their rear road mech. There is no doubt some leeway in their figures.
    Do you run the full length of chain required for all your gears?
    I ask cos I run a bit shorter to accomodate my 38/53 up front and an 11/32 rear. Just have to remember not to cross chain it too much :D
    Before you ask, 38 for cyclocross and 53 for road use (can't be bothered to keep changing chainrings)

    Maddog 2 was/is proposing a potential 49t total difference which would stretch even a long cage MTB rear mech.
    Mike B

    Cannondale CAAD9
    Kinesis Pro 5 cross bike
    Lots of bits
  • andrew_sandrew_s Posts: 2,511
    I've been using 24 - 38 - 48 for years, on an audax bike with STI/ergo

    I always size the chain for as short as will change into big/big, as I ride at night and sometimes lose track of what gear I'm in.

    You need a road triple front mech, and you may find that the rear mech doesn't keep the chain tight in 24/12 or 24/14, or that the chain drags across the bottom end of the front mech cage if the mech is further up the seatpost for a bigger big ring. Neither are much of a problem if you only use the 24 with the bigger cogs.
  • de_sistide_sisti Posts: 1,219
    Mike67,

    I use Campag ergos & Campag long cage rear mechs if that makes any difference
    (Chorus, Centaur & Champ). I measure my chain by the big chainring/largest sproket
    plus an extra link method.

    Road triple front mechs used. No shifting or chain related problems experienced with my set-up.
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