alathIN

Is there any disadvantage to having a medium cage rear derailleur when you only rarely need it?

I have a short cage now,and either use 12-25 or 12-28 cassettes. This works for 99% of my uses.
I have a one time hilly event for which I'm getting a 11-32 cassette which will require the medium cage RD.

Is there any downside to just leaving the MCRD on there afterwards?

This is a TT/Tri bike if that matters.

rmfnla

Try the new cassette with the existing RD before changing anything; it might work as is...

Andrew R Stewart 

The main disadvantage is when your team mates see it and give you s..t for being a tourist, not a racer. Really there might be a slight bit of shifting reactiveness loss with the long cage but I highly doubt most riders would notice that.


As an aside I never like using a product at it's maximum capacity as the standard practice. So a short cage der with a 28T cassette is such, the der is at its limit. I much prefer to use a product in the middle of its range when possible. Andy.

fietsbob

you can run a bigger difference in the tooth counts between the big ans small chainrings ..


with No difference, 1 chainring I ran a 14~34t freewheel using a short cage RD..

this one

the motion is straight across, as shown, or angled more , for wider range gear combo.

they bolted on the ends to make that switchable.. (perhaps sneaking around Sun Tour's patents?)




...

shelbyfv

Since you are riding the TT/Tri bike, I think it's safe to assume you are racing. If you are racing, every detail counts. You aren't going to leave your heavy 32T cassette on when you don't need it either. Get two chains with quick links and change the whole enchilada as needed.

alathIN

Thanks for the feedback.

SRAM says 28t is the max for my short cage, 32 seems over the top. The only thing worse than lugging an unnecessarily heavy RD through a race would be to have a bike suddenly become quirky about shifting during a race. Although admittedly it would be weird if it was OK in practice then suddenly got balky in a race.

shelbyfv, you have a point.

Barrettscv 

A Shimano 12-30 cassette should work with your short cage derailleur and give you a deep enough gearing.

spectastic

the medium cage will hang lower to the ground, and increase the chances for bending the hanger, probably not a huge concern for most people. I just crash often enough that the hanger always hangs on the back of my mind.

the longer cage requires greater leverage for the spring to exert the same amount of tension. less tension means slightly looser chain. don't know how much worse the shifting is. been a while since I had one of those.

another factor to consider is that tension on the chain keeps the chain on the gears. so when you're hitting a bump or something, the longer cage will bounce around more, while the short cage will stay more solid.

I'd say go for a compact crankset and stick with a short cage. they'll cost around the same I think (new crankset vs new derailleur + cassette). compact is also pretty versatile, so you can pretty much set it and forget it, not worrying about switching back.

Andrew R Stewart 

FB_ IIRC SunTour's patents ran out, and in a wink of time Shimano adapted the slant parallelogram. I did like the adjustable slant of that Campy. Andy

bradtx

alathIN, First try the cassette, w/new chain, that you want to use. It's possible that the worst will be chain on chain interference when using the small-small combo. If more gears are affected by interference, then look for a longer cage RD.

These days I run more bikes with triples than doubles and the length of the RD's cage is a moot point WRT shifting performance.

Brad

revchuck 

I have two bikes set up with 5800 105. One has a short cage RD, the other a medium cage. I keep a 12-25 cassette on one and an 11-25 cassette on the other most of the time, changing to an 11-32 on the bike with the medium cage RD only when I need to. There's little-to-no perceptible difference in shifting performance between them.

Weight is not a big issue on a TT bike, and the additional grams are more than offset by the four tooth difference when climbing. Aero may be an issue; I'm unclear as to how much given that the air back there is so dirty already.

migrantwing

@alathIN

A short cage derailleur does work with a 30t cassette on my bike, as I have done exactly that. 32t MAY be pushing it, it all depends on a few factors: Someone once told me that for every additional tooth on a cassette, you need 2mm extra space between the lower jockey wheel and the largest cog. So, 28 to 32 is 4 teeth x 2mm = 8mm. Put on the 28t, wind the derailleur B screw all the way in and then measure the distance between the lower jockey wheel teeth and the largest cassette teeth. If there's a little more than 8mm, then the larger cassette SHOULD work with the short cage. You will need a longer chain though, by around two full links. As with most things, it's more of a 'will it, won't it' depending on what type of bike you have, how low the derailleur hanger sits, chainstay length and other factors.

The other thing you can do is try a RoadLink from wolftoothcomponents.com or emergency derailleur hanger to make the derailleur sit lower than it is at present to give you the extra gap you need.

EDIT** And don't run big/big if you go 30t or 32t. You shouldn't ride in that combination anyway, but even more so if you up the capacity of your cassette. If you NEVER run that combination, you may also get away with not changing your chain.

bulldog1935

I think I'd stick to a 30t max cog and run the short cage