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Short or Medium cage? 52/38 with 12/25

daniel_bdaniel_b Posts: 9,647
edited January 2015 in Commuting chat
So the last part I believe I need to buy for the TT bike is a rear derailleur.
I will be running a 52/38 with a 12-25

So looking online around the place it appears you add the two big cogs, add the two smallest cogs, and subtract one from the other.
(52+25)-(38+12) = 77-50 = 27
So looking online it would suggest the short cage will run up to a cog difference of 28 maximum, so this should work, but of course if at some point I want to put a 27 or 28 (The sum would then equal 30) on the rear, then it might not do the job.

So would I be safest to go for a medium cage?
I assume a medium will be fine with the current setup.

The PBK descriptions seem to not agree with my figures above though......
Short cage:
http://www.probikekit.co.uk/bicycle-derailleurs/shimano-105-5701-bicycle-rear-derailleur/10769447.html
Medium cage:
http://www.probikekit.co.uk/bicycle-derailleurs/shimano-105-5700-gs-bicycle-rear-derailleur/10769457.html#customer-reviews

The Total Capacity it refers too is the result of my sum?
On there it reckons the difference is up to 33 for the rear derailleur, so it should in theory handle up to a 31 cog on the rear - not that I would get one.

CRC says something different:
Maximum low sprocket: 30T (short) / 32T (med) with double chainset
Maximum front difference: 16T, total capacity 34T (short) / 22T, total capacity 40T (med)
Weight: 221g (short), 232g (med)

That would lead me to believe a short cage will be fine?

Cheers

Dan
Felt F70 05 (Turbo)
Marin Palisades Trail 91 and 06
Scott CR1 SL 12
Cannondale Synapse Adventure 15 & 16 Di2
Scott Foil 18

Posts

  • timothywtimothyw Posts: 2,482
    If it were me, I'm pretty sure I'd get a medium cage to be on the safe side (and you seem to think there is a possibility that you might put a bigger cassette on the back anyway).

    One tip I can offer (because it isn't mine but Graeme Obree's) is that you may want an even larger big ring on the front (eg 55 tooth or more) in order that you can take advantage of a straighter chainline and so marginally greater efficiency - which is to say pedalling in 55-14 is more efficient than 51-13 as the chainline is straighter (not a particularly good example but you get the idea).

    Of course, this may depend on a flat TT course and an Obree like ability to smash the hell out of the big ring for an hour.
  • daniel_bdaniel_b Posts: 9,647
    TimothyW wrote:
    If it were me, I'm pretty sure I'd get a medium cage to be on the safe side (and you seem to think there is a possibility that you might put a bigger cassette on the back anyway).

    One tip I can offer (because it isn't mine but Graeme Obree's) is that you may want an even larger big ring on the front (eg 55 tooth or more) in order that you can take advantage of a straighter chainline and so marginally greater efficiency - which is to say pedalling in 55-14 is more efficient than 51-13 as the chainline is straighter (not a particularly good example but you get the idea).

    Of course, this may depend on a flat TT course and an Obree like ability to smash the hell out of the big ring for an hour.

    Interesting, cheers Timothy, the bigger chainring on the front may be something for the future, as I have only just bought the 52/38, so it sounds like the Medium cage is the way forwards - thanks for the reply :D
    Felt F70 05 (Turbo)
    Marin Palisades Trail 91 and 06
    Scott CR1 SL 12
    Cannondale Synapse Adventure 15 & 16 Di2
    Scott Foil 18
  • You can calculate this knowing the max teeth capacity of the derailleur in question... I'm pretty sure a short mech is plenty
  • TheStoneTheStone Posts: 2,291
    I've run a 6600 short mech on 50-34 12-32, so I'm fairly sure you'll be fine.
    Just make sure you keep the chain long enough for big-big.
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