voshchev

So, I would like to move to a Shimano Ultegra 9spd short cage derailler, but I'm not sure about the capacity. The derailler has a 29 teeth capacity. On my bike I'm running a super compact double (46-30) and a 12-27 cassette, thus--if I'm calculating correctly--I need a RD with a 31 teeth capacity. So, the question is whether I can use an ultegra short cage (they look SOOO much better than long cage models) with this set up?

DMF 

Yes, it will probably work fine. Shimano understates their capacity by a couple teeth.



I really don't understand this nonsense about a short cage "looking" better than a long cage. Maybe I need to smoke some more mattress stuffing.

HillRider

You may have to avoid using the small small cog-chainring combinations as the derailleur won't "wrap up" enough chain. Otherwise it should be ok.

I agree with DMF. What possible benefit do "looks" have?

well biked

I've got some REALLY nice old rear derailleurs, some of them from top of the line bikes from "back in the day." All of them have very short cages, only good for "racing" drivetrains. I look at them and think, "damn, that's a beautiful derailleur, and it shifts beatifully as well." But when I want to REALLY go for a ride, I get on one of my bikes with a hideously long cage rear derailleur, fully equipped with a "sissy" drivetrain," and I go ride. Very happily. I'm getting old, I'm built as oppositely to Michael Rasmusssen as one could possibly be (and damned thankful for it), and I have no delusions of grandeur when I'm riding a bicycle. Long cage derailleurs rock.............But to each their own-

Wordbiker

While I agree that choosing a short cage derailleur for looks or even weight savings is a bit over the edge, there is an advantage to them: Crisper shifting. The short cage gives more chain tension.

well biked

I would think whatever difference there is would be more attributable to the shorter cage simply "holding" less chain between the pulleys. Regardless, I think the difference in real world performance is so small, if a rider didn't know which model was back there he/she would never be able to tell the difference. In reality, it's more of a theoretical thing, I think-

voshchev

Hmm... I actually do think they look better and that there's nothing terribly wrong with wanting something to look nice. Having moved from a single speed to a geared bike, I prefer the triangular shape of the chain to the square look. Maybe I'm just too vain, but...
In any case, thanks for the useful comments.

HillRider

My bottle cages are Al or plastic, my cranks are all triples and long cage derailleurs defile all my bikes.

Hill (not a slave to fashion) Rider

ruppster

I have a CrossCheck with a triple and a Deore rear der. I think it is a Super Long Cage. The said bike also has Tiagra shifters, has only 9 gears on the cassette, and is heavy. I find that the aforementioned bike shifts maybe a wee bit less crisp than my all Ultegra Short cage bike, but I also wouldn't stake my life on telling the difference.
I don't think the bike is even allowed in the same forum as the OCP club.

DMF 

I've been riding both versions of the RD-6500 over the past month. I detected no difference in shifting. Are you claiming that the spring rates are different?

Wordbiker

No, I'm claiming that the spring rates are exactly the same. The leverage against the spring is what makes the difference, but it is probably more pronounced for mountain bikes. With the chain seeing so much abuse and pounding, increasing tension keeps the chain from jumping gears (at the cost of slightly more thumb pressure to execute a shift) and also gives more reliable and "crisper" shifts. The longer cage has more leverage to wind the same spring and gives less actual chain tension.

For downhill racing bikes with a single chainring, running the shorter derailleurs keeps the chain tighter as well as further up out of harm's way. Some riders switch to road derailleurs for this reason.

masiman

Crisper shifting is somewhat less important since indexing came along. Doing it in friction mode, you could feel the difference much more readily between ders.

DMF 

There are three different springs at work here. The first puts tension on the parallelogram (the bit that does the shifting and acts against the hand lever). The second puts tension on the upper arm (the dérailleur body rotating around the hanger bolt). The last puts tension on the lower arm. All it does is maintain slack tension. The dérailleur never "sees" that tension; it ends at the guide wheel.

The only tensions that matter in shifting are those on the parallelogram and on the chain between the cluster and the guide wheel. Both have the exact same lengths (and springs, I presume) in -SS, -GS, and -SGS versions.

Now, there is a slightly larger mass moving with the long cage version — greater lower arm mass (minimal) and more chain. That might cause a slightly slower autoshift (usually the upshift) (the other shift is manual), but it has nothing to do with the arm springs.

Wordbiker

DMF, you know I respect you man, and I agree with everything you said...except that the lower arm spring does have some bearing on shifting force required (and therefore can be felt). Maybe it's just a leap of faith, information that I have misinterpreted or just a need to feel like I know something about how a derailleur works and just have to express it...I still feel the same. Short of setting up a testbench with strain gauges and video documentation, this is my theorization behind my madness:

When a derailleur is required to make a shift, it has to overcome several factors. The first is the chain's tendency to "want" to stay on the cog. This is due to a balancing act between the B-knuckle and P-knuckle springs taking up chain slack and wrapping the chain around the cog enough to prevent it jumping due to pedaling torque. As long as those springs are relatively balanced, the chain stays wrapped around enough teeth that it stays in place...unless it is derailed to another cog. This requires side pressure, either by cable pull or the spring in the parallellogram, which is the second factor: Overcoming that spring on the upshift, and overcoming cable friction on the downshift.

The third factor combines the first and second, which is "lifting" the chain over the teeth of the cog it is engaged with to either a smaller or larger cog. In either direction, the chain must first pass over the tops of the cog teeth before engaging the valleys of the next cog. It is at this critical moment that the lower arm can be seen first moving one direction then the other as the spring in the P-knuckle is first overcome, then the teeth fully engage, then the slack is taken up. If any of these springs are either weaker or stronger, then shifts are slower/easier or faster/"crisper".

I know this is all academic, probably even if I did have proof, but it is there. I've played around with derailleurs a bit, even pulling them apart, drilling alternative P and B-knuckle spring anchor locations to give more pressure, and in the end...I can't prove a thing. Maybe it just makes me feel more "racy", but I always run the springs at maximum tensions, chains and cages as short as possible.

DMF 

As you may have guessed, my rhetorical style is to assert my position and let someone tear it down if they can. It's likely (certain?) that you have more experience with RD function that I do. So no appeals to authority here.

You do mention something that I didn't take into account, and that is the requirement for some of the slack to be taken up and/or released on the move to a different sprocket. But it seems to me that a lighter effective tension would aid that requirement in a downshift as slack is (mostly) released. Not sure it would hurt in an upshift. And not sure how significant the effect would be.

At any rate, my experience is that there is little difference. Yours is that there is noticeable difference. Testing (confirmation of either position) is difficult without a theory as to why it would be so. So let's ruminate for a while and see if we can come up with such a theory.

As a sidelight, we should also consider why the engineers didn't compensate arm length with spring tension. It's easy to justify with the first two springs - they remain common to both versions so only the lower arm differentiates them. But the lower arm spring (P-knuckle?) can be changed with the arm. So why don't they change it?

Retro Grouch 

If you hae to ask you'll never understand. I guess that's why they have both chocolate and vanilla.

Agent_Embryo

Not wanting to start a new thread for a similar question:

I'm building up a Surly LHT and want to use 9 spd Dura-Ace downtube shifters. So far my drivetrain component list is:

-9 speed cassette 11x34
-Shimano 105 double chainring
-Shimano Deore SH-RDM510A rear derailleur

Waiting to purchase a front derailleur until I get everything squared away. I know it's kind of a weird setup, maybe too strange. I've been waiting on answers from Shimano, but have yet to hear anything back yet. So my questions are:

1. Does anyone know if the Deore rear derailleur will index with the DT shifters?
2. If not, I'm fairly sure 105 will work and Ultegra should def. work. Will a GS rear derailleur have the capacity for 34teeth on the cassette?
3. What's the difference between Max rear and Max capacity?

Thanks!

well biked

1. Yes, perfectly.

2. Shimano "mountain" derailleurs (Deore, LX, XT, XTR, etc.) all have a listed max. large cog capacity of 34t, doesn't matter if they're GS or SGS.

3.Max large cog capacity is the maximum number of teeth on the largest cog that the derailleur will handle without having the upper pulley on the derailleur bump into the teeth on the cog.

The max. chain wrap capacity of a derailleur is how much chain slack the derailleur can effectively "wrap."

A rear derailleur has two basic functions, one is to move the chain from cog to cog, the other is to maintain adequate chain tension no matter what gear combo is being used at the time.

Your max. chain wrap requirement, for any given derailleur drivetrain, is figured as follows: largest chainring minus smallest chainring plus largest cog minus smallest cog.

Agent_Embryo

Thanks well_biked!