Replacement rear derailleur - short or medium

caedev

I need to replace the rear derailleur on my Specialized Allez but don't know whether I need a short or long cage. Don't have the bike with me at the moment and need to order asap so it arrives before the weekend. Just looking for a striaght swap for the one shown in the link below.

http://www.evanscycles.com/products/specialized/allez-sport-18-2011-road-bike-ec025027

Anyone able to help identify what I need?

caedev

And I'm assuming new Sora will work with my 2011 group?

http://www.wiggle.co.uk/shimano-sora-9-speed-rear-derailleur-1/

smoggysteve

I would go for medium. It will give you more scope for cassette range. A short may limit your lowest possible gear.

If you are using Sora and most likely a compact chainset medium is usually default anyway

Also, 2011 Sora was 8 speed so will not be compatible with the new 9 speed.

smoggysteve

Actually think it is 9 speed so probably will. my mistake

mr_evil

The two pertinent parameters for choosing a rear dérailleur are maximum sprocket size and total capacity (which is the difference in teeth between the front chainrings, plus the difference in teeth between the largest and smallest sprockets).

According to the linked specs, the largest sprocket on your bike is 25, and the required capacity is (25-12) + (50-34) = 29 teeth. A short cage will cope with that.

caedev

Forgot to mention I have an 11-28 SRAM cassette on it. I'm assuming Specialized would have put a short on if that's all that was required for the 12-25, in which case that should be fine as a replacement.

mr_evil

With an 11-28 cassette, the required capacity is 33 teeth, which exceeds the 31 of the short cage version. While it might still work, it might instead explode spectacularly, so you should get a medium cage instead.

caedev

Ok - so final question, if I get a medium cage will that mean my current chain is no good? Or will it be too short if it currently fits with a short cage?

mr_evil

The chain should still fit.

caedev

Thanks for the help. Looks to me like the specs on wiggle are incorrect though. Other sites suggest 11-28 should be fine with a max capacity of 37 for the short cage...

http://cycle.shimano-eu.com/publish/content/global_cycle/en/nl/index/components/road/sora/product.-code-RD-3500-SS.-type-.rd_road.html

arlowood

Mr Evil wrote:

With an 11-28 cassette, the required capacity is 33 teeth, which exceeds the 31 of the short cage version. While it might still work, it might instead explode spectacularly, so you should get a medium cage instead.

A bit melodramatic. Short cage will handle 11-28 no problem with a compact chainset. FYI I am running a Tiagra 12-30 cassette with a short cage derailleur. Have done this with no probs on my last 3 bike builds.

Get a short cage - it will be fine.

StillGoing

The medium cage gives you more options as already pointed out, so i'd always opt for the medium. Chain length and RD aren't an issue. Chain length can be estimated by wrapping it around the large chain ring and large sprocket without going through the RD and adding 2 links.