rear mech choice



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Graeme Beech

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I am refurbishing a 1976 Raleigh Record Ace and need a new rear mech.

The chainwheels are 52/42 and the new cassette I'm buying is 13/30 - hub spacing problems have been
sorted out! Rear mech requirements are thus capacity 27, max sprocket 30.

Shimano rear mechs have capabilities as follows SGS type: capacity 43, max sprocket 34 GS type:
capacity 37, max sprocket 27 SS type: capacity 29, max sprocket 27

Thus I appear to need the SGS type by virtue of max sprocket size; however the SGS is a large
mountain bike mech which I would prefer to avoid.

Will I be able to use a GS type despite the apparent max sprocket , bearing in mind its large
capacity (37) compared with my requrements
(27).

Can anyone advise me?
 
P

Paul Southworth

Guest
In article <[email protected]>, Graeme Beech
<[email protected]> wrote:
>I am refurbishing a 1976 Raleigh Record Ace and need a new rear mech.
>
>The chainwheels are 52/42 and the new cassette I'm buying is 13/30 - hub spacing problems have been
>sorted out! Rear mech requirements are thus capacity 27, max sprocket 30.
>
>Shimano rear mechs have capabilities as follows SGS type: capacity 43, max sprocket 34 GS type:
>capacity 37, max sprocket 27 SS type: capacity 29, max sprocket 27
>
>Thus I appear to need the SGS type by virtue of max sprocket size; however the SGS is a large
>mountain bike mech which I would prefer to avoid.

It'll work fine, there's no strong reason to avoid it. Shimano MTB derailleurs are routinely used on
road bikes, every day. The MTB design is also superior for use on very wide spaced clusters due to
the angle of the parallelogram - your cluster has much bigger jumps than any that the SS/GS
derailleurs were designed to run on (12-27 being the largest Shimano road cluster).

>Will I be able to use a GS type despite the apparent max sprocket , bearing in mind its large
>capacity (37) compared with my requrements
>(27).

Probably/usually yes.

The GS type will often do 30t OK - since your front chainring difference is pretty small, you can
usually eke a little extra cog clearance in back by running a slightly shorter chain (ie, there's
a little extra chain tension when in low gear). Make sure it can still clear the big/big
combination though.

How well it works also depends on the shape of the derailleur tab on the frame.

--Paul
 
S

Sheldon Brown

Guest
Graeme Beech wrote:
> I am refurbishing a 1976 Raleigh Record Ace and need a new rear mech.
>
> The chainwheels are 52/42 and the new cassette I'm buying is 13/30 - hub spacing problems have
> been sorted out! Rear mech requirements are thus capacity 27, max sprocket 30.
>
> Shimano rear mechs have capabilities as follows SGS type: capacity 43, max sprocket 34 GS type:
> capacity 37, max sprocket 27 SS type: capacity 29, max sprocket 27
>
> Thus I appear to need the SGS type by virtue of max sprocket size; however the SGS is a large
> mountain bike mech which I would prefer to avoid.
>
> Will I be able to use a GS type despite the apparent max sprocket , bearing in mind its large
> capacity (37) compared with my requrements
> (27).
>
> Can anyone advise me?

The capacity ratings are highly conservative, and bear little relation to reality. See:
http://sheldonbrown.com/gloss_ca-m.html#capacity for a detailed explanation.

The setup you're describing will, in fact work with _any_ Shimano rear derailer, but I'd generally
opt for the more versatile SGS type.

Sheldon "Versatility" Brown +-----------------------------------------------------+
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| the surge may sweep, | the tempest's breath prevail. | --Lord Byron Child Harold's Pilgrimage |
+-----------------------------------------------------+ Harris Cyclery, West Newton, Massachusetts
Phone 617-244-9772 FAX 617-244-1041 http://harriscyclery.com Hard-to-find parts shipped Worldwide
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