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Discussion Starter · #1 ·
I'm dropping from a 42T to a 32T, is there any magic range in how many chainlinks I should pop for the new size before I just start guessing?

If not, what's the best way to figure it out?
 

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The general rule of thumb is thread it on the biggest ring and big cog without going through the derailleur and add 2 links.

I'm guessing you will be removing ~10 links (42-32) but check it first.
 

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Fat-tired Roadie
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5 links/10 rivets depending on how you count. Or just size it as described above. If you're not changing rear derailleur, you can also leave it alone. Because I'm lazy, and wouldn't want to create extra messing around for myself if I decided 32t is too low, I would probably not shorten my chain at all if I was just replacing my big ring with a bash and not changing anything else, at least until putting some miles on the new setup.
 

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Fat-tired Roadie
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No derailleur makes it a little more complicated, at least in terms of the math.

Luckily, you have a bike you can use as a measuring stick. You probably won't get a lot of choices about chain length once you've got your new chain ring mounted. What do you use to adjust tension?
 

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Discussion Starter · #8 ·
My bike has horizontal dropouts and those back-facing bolts that I assume are to fine-tune the rear tire forward and back? I don't exactly know how it works, but I assume someone knows what I'm referring to.
 

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Fat-tired Roadie
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The back-facing bolts are called "chain tugs." I don't know how they work either, honestly - my singlespeed used front-facing horizontal dropouts, and I found I didn't need any hub retention beyond the quick release. This was a commuter I rode in a fairly high ratio, so if someone says that a chain tug is necessary on a singlespeed MTB, I'll believe him.


BMX freewheel threaded on an old-fashioned road hub by Andrew183, on Flickr

OP, you might have mentioned it's a singlespeed in the first place. :p Those first several completely off-the-wall answers would probably not have been posted, but they make plenty of sense on a geared bike.

Anyway, how you size your chain, exactly, is going to depend a little on how permanent you see this drivetrain configuration being and how long your dropouts are. A lot of singlespeeders have a couple of different cogs they choose depending on conditions, and there are some funky drivetrain layouts out there too. In general, though, you need a chain length that lands your axle somewhere in the dropout. Then, you'll position your axle in the dropout so there's a little bit of tension on the chain but everything turns freely. Too much tension will make a spot with extra resistance, and too little will allow your chain to fall off. Singlespeed drivetrains can be surprisingly fiddly.

If you have multiple cogs you use, try sizing your chain to be just long enough for the new chainring and largest cog. Hopefully your dropouts are long enough to let you use your smallest cog without shortening the chain.

Pics, or it didn't happen. ;)
 

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Hijacking this thread if you don't mind.

I'm changing out my rear cassete from a 11-28 to a 12-36 and am changing from a short cage to a long cage and was wondering how much chain I might need to add.

Crank is a 34 x 50. FWIW this is my road bike but I'm doing a ride with 13,500' of climbing this summer and need the help.

Thanks,
 

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Fat-tired Roadie
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Awesome threadjack. You're not asking about a singlespeed or a mountain bike.

Just get a new chain, and start over on sizing. Chains wear as a unit, so adding links is a no-no. Chains are also cheaper than cassettes, so when replacing a cassette, unless it was putting on the new chain that revealed its wear, it often makes sense to get a new chain, to get the most service life from the cassette.
 

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AndrwSwitch said:
Awesome threadjack. You're not asking about a singlespeed or a mountain bike.

Just get a new chain, and start over on sizing. Chains wear as a unit, so adding links is a no-no. Chains are also cheaper than cassettes, so when replacing a cassette, unless it was putting on the new chain that revealed its wear, it often makes sense to get a new chain, to get the most service life from the cassette.
I know I needed the new chain, but I've never had to measue before becuase I always had the old chain as a reference. Was just looking for an approximate link count to get me close to where I should be. FWIW the chain on the bike is still in good shape so I'm going to keep it for going back and forth between the cassets and I don't need a 12-36 in my normal weekend rides, but the Climb to Kaiser is a killer of a ride for those in great shape let alone someone who seems never able to drop that last 20lbs.
 

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dciandrew said:
I know I needed the new chain, but I've never had to measue before becuase I always had the old chain as a reference. Was just looking for an approximate link count to get me close to where I should be. FWIW the chain on the bike is still in good shape so I'm going to keep it for going back and forth between the cassets and I don't need a 12-36 in my normal weekend rides, but the Climb to Kaiser is a killer of a ride for those in great shape let alone someone who seems never able to drop that last 20lbs.
To make that drivetrain work, you're going to need a pretty big rear derailleur. So start by looking up your derailleur capacity - should be on the Shimano or SRAM web site, depending on what brand it is. That refers to the difference between the fewest teeth and most teeth your drivetrain combinations can give you. So for a 50/34 crank and 12-36 cassette, the most-teeth combination is 86 and the fewest teeth combination is 46. You need a rear derailleur with a 40t (!) capacity. That's a lot - more than a typical road derailleur. So you may want to back down a little in the gear range you choose, or if you have a 9-speed road bike, you can put a 9-speed MTB derailleur of the appropriate brand on the back of your bike and you're good. Another problem is that depending on the construction of the cassette you buy and your freehub, cogs that big have been reported to chew up people's freehubs pretty badly.

The compatibility issues involved in making road and MTB stuff work together on a 10-speed setup are more complicated. I think a 9-speed MTB derailleur would work and a 10-speed wouldn't, but I'm happy enough with the existing rear derailleurs on my road bikes and haven't researched the issue. When I installed a cassette that maxed out the capacity of my rear derailleur, I found getting the chain length right to be a little finicky, so I don't recommend going over the spec.

If you have the right rear derailleur, sizing is just big-big +1 link. (A link, here, is both the outer plates and the inner plates. If you're using a SRAM chain, the power link counts as a pair of outer plates.) Shimano has a different method they recommend, but it often results in a slightly longer chain and if you're pushing it on the gear range that the derailleur can handle, I think it's better to go a little shorter.

I'd suggest that changing your drivetrain back and forth is more trouble than it's worth, especially with Shimano chains, or 10-speed SRAM. If the jumps on a 12-36 cassette are big enough to bother you, a triple crank and narrower cassette might be a better way to go. That will also be more forgiving of chain sizing issues - on a triple, if the small/small combination causes the chain to drag on the derailleur cage a little, it's no big deal. IMO, on a double, you should really get it right.

I'm doing the weight thing myself this season. 9 more pounds, and I'm at my goal weight. I actually race better a little lower than that, but it doesn't give me any sort of a margin before I start having health issues. Frustrating that we can't magically be at our peak performance weights without having to work on it. :p
 

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I'm going to use an long cage X9 in the back. Its rated for a 36 casset and all my drivetrain is Sram so that part shouldnt be a problem. They only time I'll use the 12-36 is for this event and its training. So switching once a year is not big deal.

So with that being said are you saying I would need 87 links?
 

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Fat-tired Roadie
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I don't know how long your chainstay is, so I really have no idea how many links you'll need. I'd guess closer to 114.

If you insist on figuring out the number rather than just sizing it to the bike, Park Tool is happy to help you.

https://www.parktool.com/blog/repair-help/chain-length-sizing

The mathematical methods are near the bottom of the page.
 

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Fat-tired Roadie
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That sucks.

What did you do?

You'd have to really muck it up in order not to make something workable with some more tinkering and removable links. (Actually, what kind of chain is it?)
 
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