Chain length compromise ?

[Moby]

I've a Challenge Hurricane and have had to alter the boom length. That went well (measure 10 times - cut once !) but I've now had to reduce the chain length.

I've done this slowly, link at a time, but there seems to be a point where you have to compromise. I'm used to a df, where the derailleur keeps the chain in constant tension on both the power and return run. This doesn’t seem possible on the recumbent due to the large 60t large chain. As can be seen from the photos you are constrained by the large front/large rear combination. This means the small/small combination leads to the return chain sagging. Is this usual ? I assume a front ring smaller than 60t would improve the situation - is that the way to go ? Any advice appreciated.


http://i1268.photobucket.com/albums/jj577/kearnss/largefrontlargerear.jpg

http://i1268.photobucket.com/albums/jj577/kearnss/largefrontlargereardetail.jpg

http://i1268.photobucket.com/albums/jj577/kearnss/smallfrontsmallrear.jpg

http://i1268.photobucket.com/albums/jj577/kearnss/smallfrontsmallrearchainsag.jpg

http://i1268.photobucket.com/albums/jj577/kearnss/smallfrontmidrear.jpg

http://i1268.photobucket.com/albums/jj577/kearnss/smallfrontmidrearchainrub.jpg

 

[TheDoctor]

I think it's simply that the rear mech is struggling to wrap that much chain.
If you have the chain long enough to run large-large, then there's gonna be some sag in little-little.
Two solutions that spring to mind - shorten the chain and make damn sure you don't use large-large!! Or don't use little-little.

 

[machew]

Or go for a hub gear, there one that is a CVT that gives you in effect 18 gears.

 

[BlackPanther]

My Optima Barons chain looks pretty much the same if I put it on small/small. Having ridden road bikes though, I'm in the habit of never choosing such a setting. As long as big/big engages o.k. (which I also never use, but better safe than sorry) then I wouldn't worry about a saggy chain.

 

[byegad]

Yes that's fine, so long as you can run big/big it'll work OK.

 

[neil earley]

Chain big to big run chain through any rollers , but not through rear mech , hold chain ends together and add 2 links. puts you in the ball park zone, I have allways used this method on MTB so should work on bent! Would be worth checking out on dealer who supplied your bent.PS hub gear is a good idea.

 

[Mr Fitz]

Neil's measurement technique above should give you the optimum chain length, but whether or not you get chain slack also depends on the "capacity" of your rear derailleur. This is effectively the number of total teeth difference that it can accommodate. Most derailleurs should come with info that gives the maximum capacity. So if you have (say) a rear cassette of 28-11, that gives a range of 17 teeth. Now if you have (say) a double front chainset of 52-39 that gives another 13 teeth, making a total range of 30. If your derailleur capacity is less than this it cannot take up the slack on the small-small combo and still have room to allow the big-big combo. Result: either you accept not using these extreme gear combos, or you get a derailleur with a longer arm and sufficient capacity.

The fact that you have a big 60T front chain ring may be the reason for your issues, but it really depends not on the size of this big ring but the difference between it and the other chain ring(s).

Hope this helps.