In preparation for a tour in the mountains next summer, I'm wanting to get a good granny gear. I have a 48x36x24 triple up front, a 9 speed Deore M591 rear derailleur. The RD will take 34t cassette cog, but has a chain wrap of 45, meaning technically I would need to go with an 11-32 cassette. My question: is chain wrap an absolute, or does it just mean I can't cross chain big to big or little to little, something I wouldn't do anyway? In other words, can I get by with a 34t in back because the RD will accept that, while fudging a bit on chain wrap?

you could break the RD or the chain if you accidentally cross-chain. So wouldn't recommend that hack.

Only if you insist in using a chain that's too short for the actual cog/ring sizes. Hint- When sizing a chain try one link (pair) longer first. If all works well then you're good and have a little bit extra for some on tour repair. Andy

really shouldn't be a problem. RD specs are lawyerly conservative, can usually take several teeth more than listed. to know for sure, put it on the stand and see if the combination works.

if it seems just a little too much slack to take up, put a 46T up front and shorten the chain. or you could set your limit screw to lock out the 11T that you probably won't be needing anyways.

I used to use a 52/42/24 crankset and 11/32 cassette. When I was on the 24T chainring, the rear derailleur did not have enough cage to take up the slack when I was on the two smallest sprockets. But that was badly cross chained, should not use those gears anyway. Thus, I used that system for about 10 years without any problem. There were a few times that I accidentally got into one of those gears and then the odd noise out of the back (chain rubbing on part of the derailleur cage) alerted me that I was in a bad gear.

I think you should go for it. And just avoid the most cross chained gears.


But I had enough links in the chain that if I accidentally shifted onto the big and big combination, there was enough chain slack to avoid damaging anything. The OP should do that too.

OP here. Thanks, everyone. Think I'll go for the 34t cassette. For measuring chain length I used to do the big and big, add two links. Now I do small and small, make sure the RD cage doesn't fold back on itself. Six one, half-a-dozen the other probably.

I do as Andy suggests and keep enough chain that a clumsy shift to big-big isn't a big deal. And I have no issues with excess chain slack in teh small-small combo. First, on my ramped and pinned 9-speed cranksets, I cannot even use that gear because the middle chainring will pick up the chain. On my 7-speeds and all my setups in the past, small-small, even with so much chain slack it was nearly dragging on the chainstay, mattered little. (I only use the smallest available cog and inner ring when tough hills lever out temporarily, rather than doing a pair of double shifts.)

So,in short, I have exceeded capacity many times with no issues. But I have walked home from undersized chains after a big-big shift.

The cage folding back on itself - a new, interesting noise but the odds are everything will work just fine. (Can be hard on the chainstay paint if you ease up on pedaling over rough roads.)

Ben

Do what you want, but I think you should do it the way you used to, then no damage occurs if you accidentally shift into the big and big gear. But, if you plan to use a chain that is too short on a bike tour, it would be a good idea to bring a few spare links and a second quick link in case you find you had a disaster.

Several years ago I was out doing a century ride with a local bike club. There was a small crowd standing around a bike on the side of the road. I stopped to ask if they needed a wrench or something. The bike owner said it was the strangest thing, both front and rear derailleurs broke at exactly teh same time. I said that was impossible and got off my bike to look. Then I tried to backpedal, and the crank would not turn. And that is when I noticed it was on the big and big, rear cage was pulled really tight on the chain. I told him to pop the wheel out and put it in a different gear to put it together, and avoid that gear until he could get to the mechanic that made his chain too short. He had a ***** of a time even getting the wheel out because it was jammed so tight, but he got it out. It appeared to shift ok, so he got rolling again. I did not see him for the rest of the day, so I have no clue if anything else was damaged or not.

But if he bent the cage, the cage could have gone into the spokes later, the deraileur hanger could have snapped off, all kinds of things could have gone quite badly.

I also rode a bike for over 10 years where in the small small, the chain would rub a bit on the derailleur, but as the other person said, it aint no big deal, you hear the noise and it reminds you not to be in small small (which realistically, one hardly ever does, or should not if you are anywhere aware of what you are doing shifting wise).

and yes, I have always used the big big + two links, not running through the rear der. to get chain length right on a new chain before taking out the excess chain length.

Accidentally shortened my chain too much on a 52/42/32 x 14-32 setup. @27.5-102 gear inches
In the big x big combo things get stuck but fortunately don't destroy anything (yet). Once I replace the 32t for a 26-28T chainring I will probably buy a new chain... provided they come in sizes that fit.

Generally one sizes a chain to the situation, not hopes for the right size to be what's in the box. One more reason to be a good customer of your LBS as they often have the extra links for next to nothing (or free if you bought the chain from them) if the new/boxed chain is too short. Andy

+1.

I do that for my long chainstay, old stump jumper ,
always need more links than boxed chains provide.