Ansir

Hello all and thanks in advance, the help I have received thus far has been invaluable on my journey into road biking.

I have been chasing a certain ride on my bike and have yet to find it. I am thinking my tiagra rear derailleur may be the cause of some rough shifts that completely destroy my pace.

And now finally my question...

I have a 25 as my large cog in the rear and a 52 large cog mounted to a triple in the front. Can I use an ultegra short cage 6500 9 spd for my cassette, which is nine spd.

Any help would be appreciated and I will try to provide any info that could help.

Thanks again.

urbanknight

You want a long cage with almost any triple set up. Short cage derailleurs don't have enough chain wrap capacity.

jr59

Depends on how tall you are! (Sorry just a poor joke)

Ansir

Argh, I figured as much but just wanted to confirm. thanks.

krazygl00

There is a formula to determine how much chain take-up your derailleur must be capable of.

(A/2-B/2)+(C/2-D/2)=Z

where

A=Largest Rear Cog (teeth)
B=Smallest Rear Cog (teeth)
C=Largest Front Ring (teeth)
D=Smallest Front Ring (teeth)

Z=Derailleur Take-up (teeth)

Divide Z by 2 and you have the number of inches of take-up. Using this you should be able to measure your current set-up and assess whether you need long or short cage.

By the way, after plugging lots of numbers into this I've come to realize that the only gearing combos that really need long-cage are triples. Even compact gearing with larger granny-type sprockets in the rear still work with short-cage.

crhilton

You'll learn to adjust your power and timing of your shifts so they come out nicer eventually.

If you're mashing you'll want to up your cadence before you mess with anything. Mashing and shifting under full power can definitely make for some poor shifts.

urbanknight

I'm not so sure about that. I once had a rear derailleur with a max chain wrap of 27 and a 50/34 crank with an 11-28 cassette (do the math). I accidentally cross shifted once and pulled the rear derailleur apart. It was a good excuse to upgrade to 10 speed, though.

They do have a little wiggle room in their numbers, but go too far outside that (you can now get an 11-34 cassette to go with your 52/34 double if you wanted) and you will be sorry.

And a more accurate formula would be (A+C) - (B+D) = chain wrap.

krazygl00

Using this formula doesn't equate to accurately sizing your chain. You can still have adequate chain wrap/takeup in your derailleur but if you have sized the chain too small, you can ruin your rear derailleur if you go into large/large gearing.

That's incorrect, and it will give you a figure twice as large as the correct one. Remember that the chain is engaged by only half of each ring/sprocket, hence the /2.

urbanknight

I'm sorry, but it sounds to me that you have no idea what "chain wrap capacity" means. Manufacturers rate their derailleurs that way. You can use any equation you want, but you have no manufacturer spec to compare it with unless you use chain wrap capacity, which is a number that automatically accounts for your /2.

For example, my Shimano 105 derailleur has a capacity of 29. My chain wrap for my 50/34 11-28 is 33 (78-45), so I'm pushing it a little. I can tell that I'm near the actual limit because the chain almost slack in the 34/11 while the cage is almost fully extended in the 50/28 (not that you should use those combos anyway).

Another example: In the case where my derailleur exploded, the chain was completely slack and would skip if you shifted into the small/small combo, so it couldn't have been any shorter, but it was obviously too short for the big/big (actually next-to-big in my case).

The OP asked which derailleur he could use, not which chain length. What you're missing is that if you go too far beyond the chain wrap capacity of a derailleur, it will be impossible for a chain to be the correct length. It will either be too long for the small/small combo, or too short for the large/large combo.

krazygl00

To be completely honest, I wasn't aware of and had never used the manufacturer's capacity specification. I've always used that formula and hand-measurement of how much chain I can pull with the derailleur in question. This method has not failed me yet, and is no less accurate than your method.

merlinextraligh

^ But why would anyone care onther than to determine whether the derailleur they're planning on using has enough chain wrap capacity to work with their intneded gears, or vice versa? In which case, you'd want to look at the manufacturer's chain wrap specs.

urbanknight

As Merlin said, I don't see how you can use that method to determine whether you need a short or long cage.

Barrettscv 

This is correct, see: https://www.sheldonbrown.com/gloss_ca-g.html#capacity

knobster

I would think it's more an adjustment issue than anything. Check out the park tools site under the maintenance section and see how to adjust your front derailleur. I am currently running a 28/38/48, 12/27 on one of my bikes with a 9 speed Ultegra short cage rear derailleur and it works great. 48 isn't a 52 so that could be the biggest issue with the chain wrap as others are saying above.