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 Post subject: Rear Dérailleur Sizes
PostPosted: Sat Feb 23, 2013 10:33 pm 
Retro Guru
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Hi

I have an old 8 speed short cage Shimano 600 rear dérailleur and I would like to run a 12-28 cassette. Is this possible as my LBS believes that the most I can go to is a 26 and I would have to swap out my rear dérailleur.

Thought?


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PostPosted: Sat Feb 23, 2013 10:53 pm 
rBoTM Winner
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Location: West Dorset
Velobase would suggest that your LBS is incorrect, have a look at the link below for the early 8 speed Shimano 600 RD, it shows max chain wrap of 28 and max sprocket size of 28T.

http://velobase.com/ViewComponent.aspx? ... &AbsPos=33

Martin


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PostPosted: Sat Feb 23, 2013 10:57 pm 
Dirt Disciple

Joined: Sun Feb 12, 2012 1:55 pm
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Location: Blue Ridge mountains
I'm running a 12-27 nine speed cassette with a Shimano 600 tri color rear mech with no problems.


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PostPosted: Sun Feb 24, 2013 7:30 am 
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Thanks for the help. I am running tri stripe 600, and it would be nice to have a larger gear to get me up the hills !!!


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PostPosted: Sun Feb 24, 2013 4:39 pm 
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Might sound thick but with a mech you have cog size, but then you have max difference, is that between big on the front and big on the rear added together, then small and small added together. Minus small from big and that is the difference?


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PostPosted: Sun Feb 24, 2013 4:50 pm 
Old School Grand Master
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AFAIK Cog size is usually the largest rear cog you can have without the mech actually hitting it...... The difference is usually due to how much "slack chain" the mech can take up. Big ring-big cog versus small ring -small cog.

Short cage mechs can't soak up as much slack as a longer cage mech and with large differences the chain simply bounces off the chainstay or falls off.

Shaun


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PostPosted: Sun Feb 24, 2013 8:02 pm 
rBoTM Winner
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The technical blurb for a rear dérailleur will usually tell you the max sprocket size and the chain wrap. The chain wrap is calculated by subtracting the number of teeth on the smallest chain ring and smallest sprocket from the number of teeth on the largest chain ring and sprocket. SO if you had a bike running 12-28 on the rear and 52-42 it works out as -

(52+28)-(42+12) = 26

So the RD in question would cope with this just fine, but if you run smaller inner chain rings then you may struggle.


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PostPosted: Sun Feb 24, 2013 9:39 pm 
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I am running a 52/38 up front. The 42/25 was a killer, the 38 is better but I would just like something a little bigger


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PostPosted: Sun Feb 24, 2013 10:04 pm 
rBoTM Winner
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Doing the sums then -

(52+28)-(38+12) = 30

Your RD has a chain wrap of 28, so may struggle with the amount of chain it will have to tension. You may be better off changing the large chain ring to a 50T to achieve the magic number of 28.


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