Choosing a rear derailleur w/ triple chainring

dxc45

New Member
Hi Folks!

New to cycling and bike maintenance and looking for some guidance: I am trying to replace the rear derailleur on my road bike. I have a triple chainring setup (52-42-30) and an 8 sprocket cassette (26-13). All the drivetrain components are Shimano. I am having the following problems. The 8 and 9-speed short cage derailleurs I have looked at have a maximum front chainring tooth differential of 16, but my setup has a difference of 22. Long cage derailleurs present a different problem. The 8 and 9-speed long cage derailleurs have a minimum largest sprocket if 28 teeth, but my largest sprocket is only 26 teeth.

In summary, short cage derailleurs are adequate for my cassette but unable to accommodate the differential on my front chainring. Long cage derailleurs are just barely unable to accommodate the differential between my chainrings but are not optimal for my cassette.

Does anyone have any thoughts on how to go about solving this problem? Am I making a mountain out of a molehill?

I’m a new cyclist and eager to learn. Any input would be greatly appreciated.

Thanks!!
 

raleighnut

Legendary Member
Location
On 3 Wheels
You need a GS or 'medium cage' mech
 

Ajax Bay

Veteran
Location
East Devon
trying to replace the rear derailleur on my road bike. I have a triple chainring setup (52-42-30) and an 8 sprocket cassette (26-13).
The 8 and 9-speed short cage derailleurs I have looked at have a maximum front chainring tooth differential of 16, but my setup has a difference of 22. Long cage derailleurs present a different problem. The 8 and 9-speed long cage derailleurs have a minimum largest sprocket if 28 teeth, but my largest sprocket is only 26 teeth.
I don't know what specifications you have been looking at, or what models of RD, but this is not an issue (except for the advice of @raleighnut above). Nearly all 'normal' RDs have two key parameters/limitations: 'wrap capacity' and max sprocket size. The wrap capacity is a sum of the largest ring/smallest ring delta (in your and my case 22t) and the largest sprocket/smallest sprocket delta (in your case 13t, in mine 17t). I have a Shimano RD (old 105 GS) but the general specs are a 'wrap capacity' of 39t and and max sprocket size of 28t (modern ones up to 32t).
The total 'wrap' capacity of a SS (short cage) Shimano derailleur is 35t and 39t for a GS (medium cage) derailleur.
Your 26t sprocket size is not an issue. For the best shifting it might be worth taking a turn off the B screw.
 

DCBassman

Going up hills, very slowly...
My Scott, as stock, had the same chainring and cassette setup as OPs, with a Sora GS mech. I'd be surprised, given the spec, that it isn't already a GS mech, and whatever the spec says, I've had it working on 11-34t cassette at 8-speed, and with a hanger extender, 12-36t 9-speed. Plainly, there's a need to adjust the B screw, but it did go into every gear combination without the chain being slack.
 
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