going from stock 11-30 to 12-25 cassette ==> would you shorten the chain?

Discuss light weight issues concerning road bikes & parts.
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by Delorre

Hi, out of the box, my bike came with a 11-30 cassette. As I've been using 12-25 cassette's for years, I also put a 12-25 on the new bike. In theory, I should shorten the chain, I guess with 1 link, but would you do it? Will it harm the shifting if I keep the stock chain length? If I shorten it, I won't be able to use the "mtb" cassette anymore should I need it (didn't happen in year, but you never know...) As you can't completely cross chain to small-small without a hack, the smalest combo went from 36-13 to 36-14. The rear mech tension screw is set accordingly to avoid any slack in the chain but still as close as possible to the cassette (as per the manual). All advise is appreciated!

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by sungod

the range of the new cassette is within the range of the old one, riding 12 or 25 on the new one is no different from riding 12 or 25 on the new one

if the rear mech coped before irrespective of cross chaining, it'll cope after

having said that, i'd always set the chain length to be correct for the cassette/rings, a longer chain increases risk of a slack chain hitting the chainstay if you accidentally cross chain on the small ring, also this is ww, lighter is better

by Weenie

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by Calnago

I'd leave the chain length as is. The rear derailleur has plenty of capacity. Why limit what you might want to do in the future with the rear gearing. I presume this is a Shimano setup you're talking about due to the lockout of the crosschaining combos. If any adjustment need be made I might put the drive chain in the largest rear sprocket, and smallest chainring combo and adjust it so that the upper pulley is as close to the sprocket as you can get without any binding/clashing of teeth etc.

On my Campy setups, I like to make the chain as long as possible when in the smallest chainring and the smallest rear cog (I generally use an 11 tooth cog while doing this setup). When setup like this I can throw on any cassette from 11/23 to 11/29 or 12/29 or whatever, make a quick adjustment to the "H" screw (as I described above), and all good. Only when changing the chainring combinations up front would I consider altering the chain length. With say, a new 9100 Shimano setup, it can be awkward using this method since, as you say, the system as is out of the box won't let you set the chain in the small/small combo, so I might tend to just force the issue a bit for that adjustment. I also have set the chainlength the "Shimano" way, with the chain on Big/Big before running through the derailleurs, and adding a link or whatever it is. In my experiment, this method resulted in the same length of chain had the "Campy" method been used, as I tried them both to see. Also, the screw that adjusts the derailleur tension on the new 9100 rear derailleur really has a lot of range that it can adjust through, like way more than the campy H screw provides. In fact, once the chain is set, I could make it completely slack or not slack simply by playing with that screw alone.

Oh, and on closing the pins... I use a Campy chaintool for campy chains and a Shimano chaintool for Shimano chains. And not just because you need the peening function for a Campy chain. Moreso because the actual diameter of the pin in the Shimano tool is ever so slightly smaller than the Campy pin in the Campy chaintool. I once noticed how much tighter it seemed to push a Campy chaintool pin through a Shimano chain once, and I was a bit concerned that I might be "widening" the diameter of the Shimano chainplate holes at the most critical spot in the chain. So I measured the diameters of the pins and sure enough, they were slightly different. I've never had a chain break on me or a bike that I've worked on, and I don't want one to, ever.
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by zank

If it's an R9150/R9100/R8050/R8000 RD, I would shorten it to spec. The new derailleurs seem to be more finicky with chain length.
Last edited by zank on Mon Nov 27, 2017 3:08 am, edited 1 time in total.

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by TonyM

You have an excellent bike with your Scott Addict Premium with Dura Ace 9170! Great bike!
If you keep only the 12-25 I would personally change the chain. Really not that expensive and 100% perfect!
If you want to use your "MTB" cassette, then keep the current chain/ chain length.

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