joe@vwvortex

It depends on how big a range you have in front. On my tandem it did - so I kept two different chains and swapped em with the cluster.

tandem rider

tandemnh wrote:Am I safe in assuming I would need to change out my shifters up front to accomodate the 10th gear?

Probably. Ten speed requires 10 speed shifters. Jtek's chart https://jtekengineering.com/shiftmate.htm
shows a 9 to 10 speed conversion but I don't know how well it works.

rdtompki

I've decided to go with a 12-27 cassette as our everyday configuration. That still gives us a 24-27 low gear which should be fine unless there is going to be extended climbing over 10%. Tried doing a bit of climbing yesterday using the 36-32 combination (avoiding the small chainring) and that felt pretty good for short stretches over 6%. The 24-27 is exactly the same low gear on my wife's single so no compromise there. Heck, maybe we'll not need the 11-32 cassette. We certainly wouldn't need the 11 very often with a 60t chainring!

Chris_W

If you have the choice, get a SRAM 12-26 instead of the 12-27. I find that the 21-24 jump in the 12-27 is too large, and this is avoided in the 12-26 (it is a 21-23 jump instead). For me, this slightly closer spacing is more important than the one extra tooth, but your priorities may be different.

Onegun

Huh? If your rear der wraps enough chain to use your smallest combo without sagging with a 34 tooth cassette, there will be zero difference with a 27 tooth.

joe@vwvortex

I'm going to assume you know how to determine proper chain length for you bike. That being said - a proper chain would be shorter on a 27 tooth cluster than a 34 and not run into any problem wrapping in ANY gear.

andr0id

Not necessarily. It depends completely on whose instructions you follow.

Shimano says to make the RD cage perpendicular to the ground on the big chain ring and smallest cassette. This gives correct function to the max capacity of the RD and requires no chain resizing for a bigger cassette as long as it is within the RD specs.

If you use the big ring + big cassette + 2 links rule, then what you say would be true, but I would prefer the Shimano method where I might be changing cassettes frequently.

TandemGeek

The question is, should you shorten your chain when you change the size of your largest rear sprocket.

Technically, the answer is YES when you're making changes

This is the simple mathematic formula that a lot of folks use to calculate chain length.
Length = 2 (C) + F/4 + R/4 + 1

(C) = chainstay length
F = front chain ring tooth count
R = largest rear sprocket tooth count

So, for a tandem with 16.5" rear stays and a 12x34t cassette and 53/42/32 chain rings you get
Length of 55.75 = (16.5 * 2 = 33) + (53 / 4 = 13.25) + (34 / 4 = 8.5) + 1
Therefore, you need a 111 - 112 link chain for this combination.

Now, if you switch to a 12 x 27t cassette you get...
Length of 54 = (16.5 * 2 = 33) + (53 / 4 = 13.25) + (34 / 4 = 6.75) + 1
You now need a chain that is basically 2" shorter, i.e., a 108 link chain.

That's the technical side of the story. Now, in reality on most tandems with triple chain rings you can't use the small-small combinations because the chain line is so screwed up by the 145mm - 160mm rear spacing that the rear chain will usually begin to rattle and chatter against the middle chain ring or rear derailleur's cage when you are in the third or fourth smallest sprocket, perhaps even the middle sprocket on some tandems. Therefore, unless you make a shifting error and drop into granny while your chain is sitting in the lower 1/2 of your cassette that extra 2" of slack chain that is created when you switch from a 34t to a 27t cassette won't come into play or become a problem in and of itself.

All that said, geeks like me and others will still prefer to have a chain that's sized to match the big/big chain ring and sprocket count because it gives you the shortest chain possible that is sized correctly for optimal drive train performance. It's not essential, but it's technically correct.

Now, hopefully the followers of this thread will have picked up on the most important aspect of chain sizing... and that is making sure that your chain is always long enough to wrap around the largest chain ring and largest rear sprocket. If you short-change your chain length and happen to end up cross-chained in that large-large combination you could find that your drive chain, rings and sprockets will become completely bound-up to the point where they won't turn. If it's bad enough, the only ways to undo it so that you can resume riding is to break the chain (assuming you have a chain tool) or to pull off the stokers right side / drive crank (assuming they have self-extracting crank bolts) and even that's not all that easy once the drive train is bound-up by a chain that's too short. Note: The last time this happen to someone on a group ride they were lucky because (a) they had self-extracting crank bolts and (b) I happened to double back to check on them and had both the knowledge and tools needed to undue the mess caused by the LBS that built the tandem using a chain that was too short.

So, while it's not essential to shorten your chain when you go to a somewhat smaller cassette, it IS essential that you either confirm your chain is long enough to support or lengthened to support a change to larger size cassette.

Onegun

Oh, I agree with that, but what's it got to do with the question? The thread is about changing cassettes for different rides, the specific question was "Will the chain NEED to be shortened...", and the correct answer is NO.

Onegun

That's what the question perhaps SHOULD have been. And in the case of "should you", the answer is "not a bad idea", and Joe's method of swapping out correct length chains with cassettes is a fast way to accomplish that.

Perhaps just a poor choice of a word here, but "slack" gives the reader the image of a chain that would be sagging just by going to the smaller cassette, which is not the case. Perhaps "extra" would be a better word.

With a chain length that will fit a 12-34, there is no difference between "dropping into granny while your chain is sitting in the lower 1/2 of your cassette" on a 12-34 OR a 12-27. If your derailleur was originally capable of wrapping that much chain and keeping it reasonably tensioned, it still will! Nothing's changed. You're just carrying around 2" of extra chain you won't need until you put the 34 back on.

joe@vwvortex

Using that method and wide range gearing - you won't be able to use all the cluster in the small chain ring. It just won't happen. Will the bike be ridable? Sure. Will you be able to use all the gears or almost all the gears - no.

joe@vwvortex

Sorry - I don't do things half assed - maybe if I did - I might of answered the question correctly

Onegun

Answering the question that was asked is not "doing things half assed".

joe@vwvortex

I answered the question - it was your logic that was half assed.........

If you want to keep harping on the word need - IMO you need to shorten your chain. Can you run a longer chain - sure. You can also ride a tandem which has misadjusted derailleurs, frayed cables, and worn out brake pads . According to your logic - there's no need to fix any of those problems as the tandem will still be ridable.

Onegun

If you cannot debate a point without resorting to personal attacks perhaps this is not the forum for you. I've passed the insult on to a moderator, and I'm done.

joe@vwvortex

merlinextraligh

It's interesting that each subforum has its own vibe.

Sometimes I have to remind myself that sarcastic remarks I would post in the racing subforum and would be viewed as par for the course there don't fit in in the tandem forum.