What do i need to do to change to a compact?

ukracer
ukracer Posts: 421
edited December 2008 in Workshop
Bit confused as i thought compact was just the double and not triple ring , but i know thats not the case.
I have a double but i know its not a compact so what parts would i need to change?
Heres a bit from the review on this site (if it helps)

Its the carrera virago


"The spec has a good mix of parts overall, made slightly awkward by a few odd choices. It’s underpinned by Shimano‘s well proven Ultegra shifters and short cage rear derailleur, a Truvativ Elita forged alloy crankset and Shimano 105 front mech.

The cranks are elegantly shaped and have the now-standard 53/39 ring combo, but it would have been nice to see 172.5mm cranks on this M/L model rather than the 170’s supplied"

Comments

  • John.T
    John.T Posts: 3,698
    All you need is a compact chainset and the correct BB for it. Lower the front mech to suit and maybe take a link out of the chain. You may not need this so try it first. You may be able to get a chainset to fit your existing BB but check that it uses the same axle length. Also worth seeing if you would prefer longer cranks.
  • akcc05
    akcc05 Posts: 336
    i'm more familiar with shimano chainsets but i'm pretty sure compact (e.g 34/50) and double (e.g. 39/52) chainrings are interchangeable on your truvativ crankarms so you can just buy a new set of chainrings, you just have to make sure you get the Bolt Circle Diameter (BCD) right, which i think is 130 in your case, but double check to be sure since it maybe different depending on model year. shimano double front derailleurs are compatible with both compact and double so that does not need to be changed. compact gearing should have no problem working with the short cage rear derailleur too, unless you run 34/11 (unlikely and not recommened) with a rediculously long chain.

    if you want to have longer crankarms as the review on here says (172.5 instead of 170) then you will just have to change the whole chainset, in which case i would recommend shimano (like the non-series R700 or the R600, do some research), shimano chainsets have always been reliable. hope this helps.

    if there is any mistakes in what i said above i'm sure other more experienced members will help to point out for you
  • Gragi
    Gragi Posts: 448
    http://www.sram.com/_media/pdf/en/truvativ/specs/specs.pdf

    This has the specifications set out.

    It looks like from this spec that you can get Truvativ compact rings for standard 130BCD, but not the Elita rings - it would have to be rouleur 2.2 team or noir road 2.2 team.
  • akcc05
    akcc05 Posts: 336
    Gragi wrote:
    http://www.sram.com/_media/pdf/en/truvativ/specs/specs.pdf

    This has the specifications set out.

    It looks like from this spec that you can get Truvativ compact rings for standard 130BCD, but not the Elita rings - it would have to be rouleur 2.2 team or noir road 2.2 team.

    i looked at that spec sheet as well, seems much easier to get the correct size aftermarket rings from manufacturers like TA and Stronglight. personally i have never seen aftermarket sram chain rings on sale.
  • John.T
    John.T Posts: 3,698
    Compact rings are 110mm BCD. The smallest you can get on 130 is 38. To go compact you must change the complete chainset.
  • robbarker
    robbarker Posts: 1,367
    You'll need a chainset and compatible bottom bracket - your existing one may be. Shimano say that the short cage Ultegra rear mech doesn't have the capacity for a compact 34/50, and in theory you need a GS cage Ultegra mech but in practice it will work if you don't take the mickey with small/small and big/big ring combinations.

    If you race though, you don't want a compact. I don't like them due to the big gap between the rings and the constant front changing. You're better off with 39/53 and 12-27 IME. If you need lower gears get a triple.
  • akcc05
    akcc05 Posts: 336
    John.T wrote:
    Compact rings are 110mm BCD. The smallest you can get on 130 is 38. To go compact you must change the complete chainset.

    i didn't know that, actually that is logical, thanks for pointing out. learn something everyday.
  • John.T
    John.T Posts: 3,698
    Shimano no longer say that you need a longer mech. They have not updated all their info sheets yet. A standard mech will take 50/34 and 12/27 with no problem. Care is needed with chain length but that is all.
    As regards racing. For road races in hilly country you may be right but I just change my cassette to a 12/23 for TTs. Even in the rather hilly Circuit of the Dales 50 I only used the 34 twice and have never needed it in any other ones. It has not been a problem at all. More than I can say about the legs.
  • alfablue
    alfablue Posts: 8,497
    Second that John T, I run 50/34, 12/27 on a Dura Ace short mech - runs perfectly.
  • robbarker
    robbarker Posts: 1,367
    John.T wrote:
    Shimano no longer say that you need a longer mech.

    Got a source for that?

    The tech docs specify a maximum 14 tooth difference at the front and 29t overall capacity for the short cage mech. 50/34 and 12/27 gives 16t and 31t respectively.
  • John.T
    John.T Posts: 3,698
    Not any more. When the RC700 crankset came out Shimano were a bit cagey about which mech to use but said it could be used with a short cage mech. Since the website has been updated any mention of this has gone but the specs now give 16 teeth front max. This gives 12/25 as the max. As Shimano are covering their backsides with this there is quite a lot of leaway.
    The mech will take a 27 (28 or even a bit larger) so as long as the chain is the correct length there is no problem with this as many folks on here have proved. I do consider 50/34 with 12/27 to be the max safe setup.
  • robbarker
    robbarker Posts: 1,367
    John.T wrote:
    Not any more.

    Where's your source for that statement?
    John.T wrote:
    When the RC700 crankset came out Shimano were a bit cagey about which mech to use but said it could be used with a short cage mech.

    The documenatation for the compact chainset doesn't say anything about which rear mech to use, as this depends on the parameters of the rear mech, not the chainset.

    The documentation for the short cage Ultegra rear mech (which the OP has) was clear then and hasn't changed.

    Here's the source doc so you can see for yourself:

    http://techdocs.shimano.com/media/techd ... 616226.pdf

    John.T wrote:
    Since the website has been updated any mention of this has gone but the specs now give 16 teeth front max. This gives 12/25 as the max.

    The 16t spec (or 14t in the case of the OP's mech) refers to the difference in size between the front chainwheels, 50/34 is a 16 teeth difference. The range on the back is not relevant to that, but is to overall capacity, so 12/27 at the back (15t difference) and 50/34 at the front (16t difference) gives a capacity of 31t and, again, this is beyond the recommmended range of the Ultegra ss rear mech.

    It's worth mentioning that some Shimano short cage mechs are specced to use with a compact, such as the latest Dura Ace, but most aren't. It's always worth checking the manufacturer's specs before buying drivetrain components. Yes, a lot of people run Ultegra ss mechs with 50/34 and get away with it, but bodges tend to bite back from time to time, and always dozens of miles from home in the rain!

    The other option is a 50/36 compact which overcomes the capacity problem and, to my mind, makes a lot more sense that a 50/34, but Shimano ones are difficult to source.
  • Mister W
    Mister W Posts: 791
    I think you've got the old Ultegra spec there. The current Ultegra short cage mech is listed as being able to cope with a 16 tooth difference on the front and a 29 tooth overall difference. Which means (I think) that you could run a compact 50/34 chainset with a 12/25 cassette but not a 12-27........ although Shimano kit generally has a bit of leeway so you might get away with a 27.
  • John.T
    John.T Posts: 3,698
    http://cycle.shimano-eu.com/publish/content/global_cycle/en/nl/index/products/road/ultegra/product.-code-RD-6600-SS.-type-rd_road.html
    Ultegra mech as OP. Now 16 tooth at front. I do not disagree that Shimano say max of 29 teeth. By their recommendations you can use 50/34 with 12/25 or 5036 with 12/27 As I said when they introduced the RC700 they gave out different signals. I first started using 50/34 with 12/27 in 2003 and have used it on 3 bikes over more than 15000 miles since. I would not consider it a bodge. The rear mech is not operating on a cog bigger than its design parameters. It is only being asked to take up 1 link more than Shimano's very conservative specs suggest it should. By running the chain as long as possible it can cope with this easily without being too tight on the big / big combination. At the end of the day you do not have to use it but based on 15000 miles and over 40 years as an engineer I am happy to do so and confident to say that set up right it is safe to use.
  • robbarker
    robbarker Posts: 1,367
    It's the same mech - their marketing people obviously don't speak to their technical people!