It will be fine as long as the rear derailleur cage is long enough.
Those are some serious climbing gears.
Huh? You doing this for a friend’s bike?
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The way that I choose chain length is 1-2 links tighter than slack on the small chainring small cog.
Put it in the small - small i.e. 34-27 and see if there is chain slack. When I went from a 53/39 to a 50-34 I needed to shorten the chain. Unless your chain was already pretty tight, I’d be surprised if you didn’t have to take out a few links.
-corin
Your old largest combo was 50/27. Your new largest combo is still 50/27. If your chain was sized right to begin with, you shouldn’t remove any links. Adding links is just going to cause problems in the new small/small combo.
Of course, you could just test the chain on the new combo and see if it works.
It’s probably OK but check it in small/small. You may need to remove a link.
How do you know if it’s too slack?
http://www.parktool.com/repair/readhowto.asp?id=26
But I have to ask you, Do compact cranks make much sense?
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does this mean you’re giving up on the ghetto compact you had going?
The front change is of no consequence on chain length, however, using a 12-27 and 50/34 puts you on the cusp, performance-wise, using a standard derailleur. If it doesn’t shift well, I’d suggest a long cage (triple rear d.) I use 56/39 front and found the long cage helped with 11-23 or 12-25 and was essential for 12-27.
Martin C,
You still need the same length chain for the 50/34 that you needed for the 50/39 for any same cassette. The “50” is what is determining your chain length. The “34” is determining how little chain or how short your chain needs to be. To take up this “slack” is the function of your rear derailleur cage. If this slack is too great for a short cage derailleur, then you may need a medium or long cage derailleur to pick up enough slack chain to keep the chain from rubbing on itself around the “jockey” pulleys on the derailleur when the chain is on the 34 tooth chainring and on the smaller cogs on your cassette.
With a 12-27 cassette you will end up with a larger amont of “slack chain” than you would with a 11-21 cassette for instance. I would guess that at the very least your will need a medium cage derailleur for this set-up. Now if your bike came with the 12-27 cassette from the manufacturer as the “sandard” cassette it is likely that you already have a medium cage derailleur. If you are goin to use a 12-32 or 12-34 cassette, then surely you will need the long cage derailleur.
Well said by Ben above. From Park Tool (link in my post above):
DERAILLEUR CAPACITY: The derailleur capacity refers to the derailleurs ability to take up chain slack as the derailleur shifts between different gear combinations. Some bicycles may have sprocket combinations that do not allow the derailleur to take up the chain slack. In this case, the gearing on the bike exceeds the capacity of the derailleur. If the derailleur capacity does not match the gear sizes on the bike, the chain may appear to fail either the “too long” test or the “too short” test. A chain length cannot be found that will pass both tests. In this case it is better to size a chain for too long rather than too short. Both the EXISTING CHAIN and EQUATION methods below will safely size a chain when a bike is violating the derailleur capacity. It may be necessary to avoid gear combinations that cause problems in pedaling or shifting on these bikes.
I have run 50/34 and 12/27 with regular Rear DR. In the length test (50/27), the chian is just long enough. In shortness test (34/12), the chain barely avoids rubbing. This is not an ideal setup (especially since it is on 650 wheels and acompanying short chainstays), but it works if I remember to avoid the extreme cross-chain positions. I have even run 50/34 with 12/28. Thus, I think you ought to be able to run 50/34 with 12/27.
Before you removed links did you do the big/big test (see Park Tool)? How much bend was there in the chain in the rear dr? If you started with a chain sized for 53 chainring, it could be a bit shorter with 50 chainring, but I’d guess 4 links is too much.
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here is yet another guide for how long a chain should be (that is, *if *you already have the right deraill. for your chainrings and cogset). it is significant because it is the recommendation of the folks that make drivetrains. anyway, zoom in and look closely at the diagram on ‘chain length’ for guidance:
yes, i am sure a long cage deraill. would solve your problems, and, yes, with a new long cage deraill. you would need a slightly longer chain.
unrelated, how did you edit out that little clip of the shimano technical doc.?
did you use adobe pagemaker or something like that?
“if I had not pulled a bonehead and had cut the chain with the rear derailler at 9 o’clock rather than 6 o’clock”
Like Ferris Bueller saying, if the Ferarri (or whatever it was), was not on jackstands running in reverse when I leaned on it. Hopefully, you are having some sucess.
By the way, your signature is a disappointment for someone that rides fixed-gear. I max out around 32 mph on my 42/15.
Just FYI- I use a standard ultegra 9 12-27 cog with a 50/34 and a
standard dérailleur. To answer your question as to whether or not its
slack. You should just make sure that you have tension in the rear
deraileur. To get more comfortable with it, you can shorten the chain
link by link, and just use a 2mm allen key as a temporary pin, once
you’ve established tension in the RD, you have your correct chain
length. Clearly you need to test Big-Big, but it should work with a
standard road RD.