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PostPosted: Tue Apr 11, 2017 9:27 pm 
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Joined: Fri May 15, 2015 11:26 pm
Posts: 14
Hi

I seem unable to get the chain length right on my steel Merckx Corsa from the early 90's and wondered if you guys might have some advice?

The set up is as follows:-

Dura Ace 7400 rear derailleur
52-39 chainrings
Sram 8 speed 11-28 cassette
8 speed chain.

If I set the chain length to be correct for the 52-28 combo it is then to slack in the 39-11 and 39-12 combo and catches on the bottom of the derailleur cage. I then remove a link and find it good in the 39-11 but it is then way to tight in the 52-28 with the rear derailleur stretched far to much and I can feel resistance when turning the cranks. The 7400 rear derailleur looks to be rated for 28t from what I can find on the web but I am starting to wonder if it is likely to be a chain wrap issue causing my problem.

I have thought about using a half link but these look to be more for use on single speed setups, would this be ok to use in my case or is there anything I might have missed?

I am aware that these are not combos I should be really be riding in (cross chaining) but would like it to run better than it currently does if I can. I also assume that it's better to run a chain slightly to long than one to short, correct?

I should also add that I have tried moving the rear wheel back and forth a little in the horizontal dropouts but this seemed to have minimal effect on the problem.

Many Thanks for your help

N


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PostPosted: Tue Apr 11, 2017 10:04 pm 
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Joined: Thu Feb 25, 2016 9:55 pm
Posts: 127
I would be suprised if that derailleur can handle that range. First a 52/39 has a wider range than the 52/42 which was the old standard. Secondly most cassettes used to start at 12 and not 11. A 12-21 was not an uncommon choice for flat to rolling terrain. 11-28 would have been close to unheard of and much wider than the derailleur was intended for.


Last edited by Kurets on Tue Apr 11, 2017 10:04 pm, edited 1 time in total.

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Posted: Tue Apr 11, 2017 10:04 pm 


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PostPosted: Tue Apr 11, 2017 10:05 pm 
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Joined: Sun Mar 16, 2014 3:43 pm
Posts: 817
These 7400 (01) derailleurs were never designed for large capacity freewheels.
Max Chain Wrap 28T
Max Cog Size 26

Chain Wrap = (large chainring – small chainring) + (large rear cog – small rear cog)



Your chainwrap is 30 and your largest cog 28T
Both exeed the limits


Tightening the B adjuster screw on the RD all the way will maybe help


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PostPosted: Tue Apr 11, 2017 10:16 pm 
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Joined: Fri May 15, 2015 11:26 pm
Posts: 14
Thank you both, Seems I have the chain wrap at 28t confused with cog size. I may have to look a 26t cassette then. Best regards N


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PostPosted: Tue Apr 11, 2017 10:22 pm 
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Joined: Fri Dec 11, 2009 10:38 pm
Posts: 1867
Location: Welland, Ontario
Just size so that it's safe in the big-big and don't cross-chain when in the small ring. Bad practice anyway.

Problem solved.

_________________
There's sometimes a buggy.
How many drivers does a buggy have?

One.

So let's just say I'm drivin' this buggy...
and if you fix your attitude you can ride along with me.

http://www.youtube.com/watch?v=GekiIMh4ZkM


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Posted: Tue Apr 11, 2017 10:22 pm 


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PostPosted: Wed Apr 12, 2017 8:16 am 
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Joined: Wed Jan 22, 2014 11:48 am
Posts: 2435
Location: Vienna Austria
Exactly, make sure nothing beaks in big-big and go ride.


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