Any mechanical advantage to short cage RD?
 

I have a low mileage Ultegra 6500 long cage rear derailleur. I'm planning on using it with a double crankset (52-42) and a 11-25 cassette.

Obviously if you use a triple crank, you should use a long cage RD. But is there any mechanical issue with using a long cage with a double?

The only thing I can think of is the extra weight and drag from the longer derailer arm... :)

yeah that might cost you like .002 sec over a 40k TT. So what you gain in shaving your legs is lost on the RD :rolleyes:


Actually I guess that reply embodies the only disadvantage* I can think of: some weight weenie is bound to point and snicker at it.

*(assuming your chain is properly sized and so on)

Don't forget the extra wind resistance... It's like dragging a parachute behind you. :p

Well, that's what I thought. Which begs the question why there are RDs with supershort cages.

Long cage derailleurs will shift more slowly. They have short cage derailleurs for quicker, cleaner shifts. That's about it.

edit: and I'm not sure, but I imagine short cage derailleurs came first, and then they invented long cage when people started using larger cassettes for climbing gears on mountainbikes and touring bikes.

^^^ Because it looks nicer!

Yes, it will look a little odd. Especially since it's going on a bike I'm building up as a tri/tt bike. OTOH, I bet 99% of the people who look at it wouldn't notice.

I guess I'll skip the afternoon cookie break and save the 20 grams off the engine rather than the transmission. And save my money for razors.

You won't have any mechanical issues. Every derailleur is rated for a max cog size. But a long cage derailleur is the most versatile because it can do a wider cog set then a short cage can do. BUT a long cage derailleur on a cog set that a short cage can handle will shift just a tad slower-supposely, and be a tad heavier.

Did you measusre that some way or are you just guessing?

It's a fact; you have to overcome the increased inertia created by the extra mass of the longer cage and the additional chain links! :D

I've noticed some increase in chain slap over rough terrain. No biggy really, so long as you use a chainstay protector.

"It's a fact" means that you measured it some way. How'd you do that? Otherwise that business about increased inertia is just an unsubstantiated theory.

I was kidding, grouch.

Hmmm. Back in the 60's all of the derailleurs that I'm aware of were a relatively simple parallogram design that just moved straight in and out. They all had relatively short cages and were limited to maybe 28t rear cogs. By today's standards they sucked. Then, in the late 60's Suntour produced the venerable VGT, slant parallogram, long arm rear derailleur. It would handle up to a 34t rear cog and shift the smaller freewheels with no chatter. It was a huge improvement in it's day but definitely not for weight weenies - we're talking masses of metal.

I guess that would make your supposition right. Then again, since all decent derailleurs today use the slant parallogram design, I could argue the long arm derailleurs came first.

No.

Look at Shimano's specs for Ultegra RDs, compare long (GS) to short (SS). The max cog capacity is the same (27). What differs is the *takeup* capacity ("total capacity"), which has to do with what you're running up front. A long-cage version of a given RD type will not allow you to run larger cogs in the rear.

Yep, the biking industry owes a lot of gratitude to Suntour for that design...so they put them out of business as a way of saying "thanks"!

Just quoting some skilled mechanics, actually. In my own observations, a poorly adjusted derailleur is much slower than both a properly adjusted short cage AND a properly adjusted long cage. If there really is a difference between short cage and long cage, it's so small you would be able to blame cable stretch, cold weather, dirt in the components, or the rider farting as the culprit instead.

You're right, I was born in the 80s and forgot they had derailleurs before then :D

That's what I think too. If there is a difference between otherwise identical Shimano derailleurs in shifting crispness, I think that it's so small as to be lost in the kind of background issues that you indicated.

I have an Ultegra long cage on my tandem and an Ultegra short cage on my road bike and I can't tell a difference even with the long shift cable on the tandem.

To repeat what I was saying except in more detail; every derailleur is rated for a max cog size. This number reflects the largest size cog that the derailleur can shift onto without jamming. The manufacture's rating is based on an assumed derailleur hanger length. If the actual hanger is longer then the assumed length, the derailleur may work on a cog that is a few teeth larger then the rating. If the actual hanger length is shorter than the assumed length, then the derailleur may not even work on a cog that is equal to the maximum rated cog size. Ratings for these derailleurs can be determined either by the manufacture, Sutherlands Handbook, Bike'alog, or an actual test can be done.

So how does a cage length effect shifting, which was a question that came up. Shorter cage rear derailleurs take up less chain slack than a long cage rear derailleur. The chain is held more snugly, which can result in smoother, more precise shifting. However, longer cage rear derailleurs are capable of taking up more chain slack than a short cage rear derailleur, allowing you to run a wider range drivetrain than would be possible with a short cage rear derailleur. Racers might appreciate the lighter weight and slight shifting improvement a short cage rear derailleur offers. However, these benefits come with a tradeoff - because of the decreased capacity a short or medium cage rear derailleur offers, it may not be possible to safely use the small chainring in tandem with the smallest cogs. The derailleur simply can't take up enough slack in the chain.

Notice we went back to capacity in explaining the difference between a long and short cage, just as I have explained in the first paragraph. Also I'm trying to keep this focused on road bikes only.

Thus to re-explain my position; Rear derailleurs are available with three different cage lengths (i.e. the distance between the pulleys). The length is called the capacity, and is measured in teeth difference: add the numbers of teeth of the largest chainring and the largest cog and subtract the numbers of teeth of the smallest chainring and the smallest cog. You can exceed the specified capacity of a rear derailleur by a couple of teeth but not more, or the chain will become stuck in unpleasant ways. For standard road bikes, my feeling is that you should stick with the shortest (racing) cage and adjust your chainrings and cogs because longer cages reduce shifting accuracy and make the chain bounce more easily on rough road surfaces. Many high-end rear derailleurs are available only with short cages. But if you want versatility then the long cage is more the route to go if chain bounce will not be an issue and/or you need larger capacity cogs-more teeth.

actually, cyclotourists used derailleurs long before they were accepted in the peloton...

So instead the other companies should just lay down and die for principles?

true, but didn't they still use like 15-21 cassettes and 52/43 chainrings on the touring bikes back then?

I can't imagine you'd need a longer chain...

Interestingly, that's a Campy vs. Shimano thing.

Campy recommends using the little/little method for sizing the chain. If you do that it will give you the longest chain that will work with your derailleur and it will be longer for a long cage derailleur.

Shimano recommends the big/big method of sizing the chain. If you do that it will give you the shortest chain that will work with your particular gear combination and it will be the same regardless of derailleur cage length.