Shimano Ultegra tooth limits

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Milfred Cubicle
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Shimano Ultegra tooth limits

Postby Milfred Cubicle » 25 Jun 2014, 5:40pm

Does anyone know if a short cage Ultegra rear mech will handle a 12-30 cassette, with a 39-53 chainset? I know Shimano claim 27-28 to be the maximum, but is this using a compact? Besides, Shimano make all sorts of compatability claims, which often turn out to be twaddle! I'd rather hear it from someone who has made it work. Thanks.

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recordacefromnew
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Re: Shimano Ultegra tooth limits

Postby recordacefromnew » 25 Jun 2014, 11:26pm

There isn't a simple, certain answer to your question, because the answer not only depends on what vintage your Ultegra is, but also the geometry of your frame/hanger.

For the two relevant official "limits" here - capacity and max rear sprocket size, capacity is mainly dependent on the mech's cage length, and for the short cage versions the official capacity increased from the 29T for the 6500/6600 vintages to 33T for subsequent models. Since your plan capacity requirement is (53-39)+(30-12)=32T, you have nothing to worry about in terms of capacity if your mech is one of the latter. If not, you can still make an assessment based on your existing setup, making use of the fact that for every 4 extra teeth on the largest sprocket/chainring combined chain length increases, or an existing chain would effectively be shortened, by a pair of links. If you gather the chain by a pair of links when e.g. the largest sprocket is 26T currently you will find out if the existing chain is long enough for a new, larger 30T rear sprocket when the chain is on large front large back, and if not, you can see whether the necessarily longer chain (which you can "model" by breaking the chain temporarily if you have a quick link) will droop when on small front small back.

For the max rear sprocket limit, the official (generally conservative) figures are 28T/27T/28T/28T for the RD-6500/6600/6700/6800 respectively, but in practice the limit is also affected by the distance between the centre of the rear hub axle and the mech bolt on the hanger (which varies but Shimano specifies 24-26mm for road and 28-30mm for mtb), the tooth count of the top jockey wheel, and not to mention B screw setting and design conservatism. Unless your hanger length is quite a bit shorter than the 24mm mentioned, I believe the chance of your top jockey wheel clearing a 30T large rear sprocket is excellent. In any case you can also check using the existing setup by knowing that for every 1T increase in the current large rear sprocket the clearance between the top jockey wheel and the large sprocket reduces by 2mm, and if it looks like not enough, you will be amazed by what you can achieve by playing with the B screw (http://sheldonbrown.com/deakins/lowgears.html).

Mark1978
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Re: Shimano Ultegra tooth limits

Postby Mark1978 » 26 Jun 2014, 12:42pm

I run Ultegra 6700 with a compact and 12-30 cassette. Since Shimano manufactures Ultegra 12-30 cassettes it would be unusual if they didn't work with Ultegra groupsets!

Brucey
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Re: Shimano Ultegra tooth limits

Postby Brucey » 26 Jun 2014, 1:00pm

I've seen up to three different max sprocket sizes listed for the same shimano mech before now (all from 'official shimano sources' ), and it has been clear that in some instances the SI techdoc (normally the fount of all wisdom in such things) has been in error.

The most recent versions of Ultegra and 105 do appear to be designed for 30T sprockets (even if you can't find a piece of paper that says so) but how easily they will do this is still dependant on the length of the gear hanger.

cheers
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honesty
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Re: Shimano Ultegra tooth limits

Postby honesty » 26 Jun 2014, 1:29pm

Actually I think the latest versions are designed for 32...