Chain tension, rear derailleur position

hegyestomi · March 2016

Hi all,
I recently replaced my chainrings on the front (53/36QXL), rear cassette (11-28) and also got a new chain.
After fitting I wanted to shorten my chain but my old method of changing gear into large-large on the front and rear doesn't seem to work. In this setting the chain length looks just right (rear d. points to 6'o clock) however when I get down to small/small the rear derailleur isn't able to tension a chain at all.
I pulled the chain together by hand and it felt that taking out three links would give 5-7mm distance between the jockey wheels (rear d. is curled right up the back at the small/small combo).
When I change to large/large then pulling the links together the derailleur would be at around 130 degree (4'o clockish) and looked way overstretched.
I'm worried if I take the three links out I'll be pulling the rear d. right to the front which just doesn't feel right.
I'm aware I wouldnt used the gears in the two extreme positions mentioned above but I don't want to rip off my derailleur if I change into those gears mistakenly.
I attached the two pictures where you can see the chain tension in large/large and small/small combo.
https://www.dropbox.com/s/vshwwd50vkm6zp0/IMG_20160306_141619.jpg?dl=0
https://www.dropbox.com/s/b7sdnc7w3kme4n7/IMG_20160306_141516.jpg?dl=0
Please let me know if you have any suggestions or option, I'm happy to listen and learn.:)
Thanks
T

nicklouse · March 2016

You seem to have forgotten that with the large large you do not pass the chain through the mech. And then you add one full link.

You seem to have oval rings so the sizing must be done at the point where most chain is needed.

There are other ways of sizing the chain as well.

hegyestomi · March 2016

Thanks very much Nick, I looked up Shimano's recommendation and will see how many links I need to remove.
There is a method of threading the chain through the mechs and jockey wheels which I used before but it doesn't seem to be accurate enough here.
I just don't want to over stretch the rear mech or have a loose chain

964cup · March 2016

You've alluded to half the simple test - in small/small, the RD must still tension the chain, so that the lower chain run doesn't touch the jockey wheels. In big/big, the chain must still describe an "s" through the jockey wheels - if it's a straight line, the chain is too short.

k-dog · March 2016

Yeah, that's just too long.

You're using a slightly extreme combination - if that's a short rear derailleur it's outside the recommended limit.

You can go a little further than manufacturers recommend though - you just need to balance the chain not being too tight in big/big and being okay small/small. A bit of trial and error is required when you're pushing things - the standard methods don't really work.

hegyestomi · March 2016

Thanks all, four links shorter it seems to be good now. I took out one at the time
I set up the chain to be just a touch under the jockey rings in the small/small combination.
When changing into big/big the rear derailleur is almost straight although it still points slightly backward.
Short cage RD should be able to deal with the 28 rear cog: I think the real issue is the chainset being pro-compact, 53/36 is pretty wide range.
Thanks again for all the suggestions!

nicklouse · March 2016

your comment

Short cage RD should be able to deal with the 28 rear cog:

shows you are missing the point about capacity.

if the difference between the big and small cog son the cassette and the chainset is greater than the capacity of the rear mech you will have, either, gears you can not shift into or a chain that is slack.

some reading for you
http://www.sheldonbrown.com/gloss_ca-g.html#capacity

Chain tension, rear derailleur position

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