Advice please on derailleur capacity

[CanAmSteve]

Hi - I'm building up some old frames and while I've had great success so far using "matched" components, I'm a bit wary of a mix&match approach with older (1980s) components. I know I can always try-it-and-see, but one thing perplexes me, which is rear derailleurs being noted as compatible with double or triple front chainwheels.

Now, obviously a front derailleur has to have a greater swing for three chainrings than for two - although I suspect a triple derailleur can probably be "limited" to work on a double?

But how is a rear derailleur suitable for double or triple front rings only? It can't be tooth spread, since a compact can have as much range as a triple (in theory). I can't imagine it's chain angle. It could be related to its overall capacity, but with many "MTB" derailleurs able to shift 11-36 (and perfectly usable on a road bike) surely a sensible gear range can be accommodated? My guess is that the reference to a "double" rear derailleur means it has a more limited range (short cage) and that when used with a common set of rear cogs, can only be expected to work with a front double. But please educate me. Thanks!

 

[daugs]

as I understand a rear derailleur has 2 figures and may be marked being max capacity (ie biggest cog) eg 28 and total capacity (being combo of front and rear) of difference in rear cogs (max and min) plus same difference at front, so its about differences and chain length rather than double vs triple

 

[CanAmSteve]

Thanks for that - that's also what I understood - that if you have too great a chain length/cog range, the derailleur doesn't have the range to both allow all the cogs to be selected (i.e. bog/big) AND then take up all the slack at small/small. So I suspect the reference to "double" or "triple" in relation to older *rear* derailleurs was sort of a shorthand reference to capacity, what with short cages being the norm.

 

[bm0p700f]

It is unlikely that a short cage rear mech will work with a triple chainset unless you have a really close ratio cassette/freewheel and even then it will be tight.

 

[daugs]

It is unlikely that a short cage rear mech will work with a triple chainset unless you have a really close ratio cassette/freewheel and even then it will be tight.

I run a short cage on one of my marins, no problem, see link in my sig re close ratio/slicks

 

[hamster]

An MTB short is about the same as a road mid-cage, which helps you get away with it.

On my road bike I can just get away with a 12-23 and 30/39/48 with a Campag Chorus rear short cage mech - it's a bit rattly in big-big. Which illustrates bm0p700f's point nicely. You could add a few teeth with Shimano as their short cage mechs can swallow a larger tooth difference.

Essentially a long cage mech will work with double or triple, but you will have to suffer the hideous 20g weight penalty of the longer cage.

 

[mattsccm]

A Campag Super Record will just cope with a 12/28 and a 28/38/48 triple. It can't quiet deal with the 30 t option I tried as it fouls the teeth.

 

[CanAmSteve]

Thanks again for the info. I have some substantial hills if I go north or south from home, so I find I use my classic bikes more if I can extend the gearing range a bit. I don't mind throwing on a more modern derailleur as long as it doesn't spoil the look - I'm finding Microshift and Shimano 105s working pretty well. Freewheels are more an issue - most now seem to have a "Maga-range" low gear that (while it can be helpful) requires an MTB derailleur to accommodate.

 

[chambo34]

Suntour Vx will cope with 12-34 rear and triple on the front, extremely reliable and surprisingly light gears with a nice vintage/retro look.

 

[CanAmSteve]

Thanks for that - good to know as I have a VX on one bike. Will keep my eye open for others.