Moisture

My Nishiki came with a 42/52t front crankset and 13-28t freewheel. Short cage shimano light action derailleur. Im currently using a 34/50t front crankset.

I was toying with the idea of trying out one of my rear wheels which has a 9 speed 11-34 cassette on it.

I know that the short cage RD has a max cog capacity of 28t, but isn't there some wiggle room for how large of a cog it can reach? On top of that, going from 42t to 34t on the front chainring may account for enough of a difference in chain slack to clear the 34t cog on my other wheel?

base2 

34-11=23
50-34=16
16+23=39
If your derailleur will handle 39 teeth of chainwrap, you're golden.

It's best not to exceed any of the manufacturer recommendations, but by a tooth or 2 it often does not matter for various reasons. What Shimano is guarding against is a perfect storm of variables that *could* cause problems.

I don't know if I'd take a max 28 tooth large cog rated derailleur to 34 (6 tooth beyond specs,) but I've seen worse hold together for a long time. You might just try it & assuming you are careful on the test stand you will see if it'll work before you commit.

I'd feel more comfortable if it were a 12 or 13 or 14-34 cassette as the chain wrap spec added to the max cog spec wouldn't constrain whatever wiggle room you have to do so safely in the event of a "big-big" combination.

Retro Grouch 

Personally, I think the 34 tooth big cog is going to be the problem. I think you'll find the guide pulley is going to hit it.

HOWEVER it sounds to me like you already have a freewheel with the 34 tooth biggest cog. I wouldn't spend money to buy a freewheel on spec but, since you already own it, if it were my bike I'd bolt that freewheel up just to see if I could get it to work to my satisfaction. What do you have to lose? Jusrt don't go crazy testing the big/big combination until after you extend the chain.

Moisture

So if I look at it this way, does it make sense?

42+ 28 = 70t

34+34 = 68t

So technically, the chain has to wrap around less teeth than before, and it is simply a matter of the short cage clearing a 34t cog which is not designed for, but should otherwise be possible, correct?

I guess I'll get around to try out the new wheel this week and see what happens. I have my long cage Acera derailleur i can slap on if the current light action does indeed not clear the largest cassette cog.

base2 

I don't think you followed the math quite right.
It is:
From there, you check the max/min cog sizes on account of this relates to the angle the parallelogram is designed to move on. Assuming your derailleur will wrap the 39 tooth difference, you still might physically run the upper jockey wheel into the big cog.

You can mitigate the potential jockey wheel collision by setting the b-screw to hold the upper jockey wheel further away, but the result is the derailleur must move further to de-rail the chain to the next cog. The result is often slow, sluggish or inconsistant shifts; Especially with indexed shifting systems.




Seperate:
Sheldon Browns method of sizing for big/big + a full link pair will ensure the chain is always long enough to avoid ripping the derailleur & hanger off the bike, but the trade off is small/small may (will) have a lot of chain sag.

csport

If you are looking at the chain length it should be the big-big: 52+28=80, 50+34=84. Divide by two (half of the chainwheel/cog touches the chain) and you get 2 half-links difference.

The chainwrap base2 was writing about is the length of the RD sufficient for the properly sized chain to be able to shift into the big-big (50-34) and not be slack in the small-shall (34-11).

Edit: base2 responded as well while I was typing

woodcraft

No, the chain has to be long enough for the big/big so 50/34 has longer chain.

The short cage won't clear the 34 cog without a Roadlink (if that will fit your RD). With it, it will work fine except for

some slack in the small/small but that is not really used anyway.

Moisture

Even if it did clear the 34t cog, my understanding is that this would negatively impact the shifting performance anyways, correct?

I've read somewhere that short cage derailleurs offer better shifting performance, is this true?

My shimano light action RD seems to have stiffer springs in it(more positive shifting, less chain flopping)

whats better? Something like a newer shimano derailleur which allows the chain to flop around over bumps? Or something that keeps the chain more tight? Because I know that too tight isn't a good thing.

As for shifting performance, I find that keeping the chain and pivot points well oiled makes a dramatic difference.

When fitting my lightly used 9 speed chain onto the Nishiki, I noticed that the chain was wrapping over itself slightly in small/small. Removing one inner/outer link from the chain was just enough to stop the chain from wrapping over itself, but the derailleur was still pulled all the way back. Is the chain still too long?

csport

As you said, it may be too short. Check the big/big + 1 full link (1") method that people mentioned. If the chain is too short you will destroy your RD when you shift into the big/big.

See also this post: https://www.bikeforums.net/19651934-post3.html

base2 

If the rider shifts his rear dérailleur cage six mllllmeters, a normal separation between sprocket planes, the chain angle will be 8.9 degrees if the jockey pulley is 1 1/2 links from the sprocket (above). If the jockey pulley is 2 1/2 links from the sprocket, the same six-millimeter movement sideways creates a 5.4-degree angle.

Sort of. On shorter cage derailleurs the springs have better leverage on the shorter cage for firmer hold & higher tension on the chain. Or, worded conversely: On longer cage derailleurs the cage has more leverage on the spring so the tension is less. Think of a levers & fulcrum...Or a teeter-totter with 2 different sized people at either end.

Neither is really better or worse. Light shifting is considered better quality by the more discerning tastes of superior bike snobs. <sometimes me! > Heavier weight springs &/or shorter cages do help with avoiding chain jumps or drops. The modern solution to marrying these two opposite design objectives is to incorporate a clutch into a long cage that restricts cage movement unless there is a corrosponding change in cable tension. You'll find a clutch on Shimano Shadow derailleurs. A heavier spring &/or a clutch not necessary unless you find yourself dropping chains or experience random shifting on particularly rough terrain.

I leave the clutch on my equipped derailleurs dis-engaged unless I am actually on rough terrain or the mountain bike park where I think I may need it.

Nope. You did it right. As long as is practical with nothing wrapping over itself you are good enough. The caveat is big/big is determined by the total capacity of the derailleur. 39 teeth, or better in this case.

In big/big, you should have enough slack you can place 2 small screwdrivers between 2 links squeeze out a full link of slack before the derailleur stops or pulls straight. The idea is that you want enough slack that when the chain climbs over the top of the teeth in both big front & big rear at the same time the chain is still long enough to avoid damage or unnecessary force on the hanger should you accidently shift both at the same time.

I hope I've been helpful.

HTupolev

No. The clutch doesn't act like a person operating a clutch for a manual transmission in a car, it doesn't disengage based on shift actuation. It's just a simple one-way clutch: it rotates freely in one direction, but cannot rotate in the other direction. This way, the derailleur can wrap up chain freely, but does not want to release chain. Generally the pivot incorporates a friction mechanism, so that instead of locking up entirely in the chain-release direction (which would obviously make the derailleur unusable), it's simply somewhat resistant to motion in that direction.

One way to think about it is that it's a bit like a Simplex Retrofriction shifter. Or like a SunTour ratcheting shifter, although these use a ratchet instead of a one-way clutch to achieve the result.

Just on ones that are specifically Shadow+. "Shadow" without the plus just refers to Shimano's new derailleur geometries. The Shadow road models like the R7000, R8000, and R9100 rear derailleurs use a simple sprung pivot for the cage, no clutch.

base2 

Ah, I hadn't realized that the cable tension/release wasn't part of it. It makes better sense though the way you describe it. I had a much more complex mechanism of operation in my head. Not sure what I was thinking.

As far as Shadow vs Shadow Plus. That's a good catch. I almost typed the "+" but opted against it. Doh!

Always glad to learn.
Base2

Moisture

Thank you guys for all this information! When I did try shifting it into big/big, the derailleur still had some space to move forward, so I concluded that the chain isn't too short.

Phil_gretz

The other consideration that I don't think I saw mentioned yet is the amount of RD inward deflection needed to reach the innermost cog. That you can test empirically. Set it up without cabling and see if you can move the derailleur so that the guide pulley centers on the largest cog. If so, you can reach.

The Roadlink comment ^ above was spot on. This is an adapter that allows the RD fixing point to be lowered by 15mm or so (estimating, I have one in the parts bin, but haven't measured it.)

Moisture

So should I try to make my current derailleur work first, and then simply install my spare Acera long-cage is my back-up solution?

I really like the light action. Do you guys think its worth the switch? Some extra gearing >28 and <13 would be appreciated, but not totally necessary. The main reason which motivates me to stick with my current setup is rim width.

My current Damco rim measures at about 24.xmm wide. The Alex rim with the 9 speed cassette is i believe 18mm wide. Both are equipped with a 28mm tire.

I prefer a wider rim based on stability and handling when pushing the bike around turns, but I did enjoy the alex rim quite a lot when using it on my old Norco. What do you guys think?

Moisture

I just tried installing the rim. The rear derailleur just barely cleared the largest cog. It was skipping about slightly so I have the limit screw set to stop at the second largest cog. (30t). Will give it a try and see how it feels.

70sSanO

I run a RoadLink on2 bikes and they work fine. That said, I would never run one if a compatible quality derailleur was available to run the max cog/capacity without the RoadLink.

I’d just get a RD that fits your max cog/capacity.

John