Before I go chasing this problem down and taking things apart unnecessarily I was hoping for a more definitive answer as to what it might be. (p.s. I did a search came up empty handed)

While coasting at realitively high speed, say 15-20 mph, my chain will slack at the bottom of the chain line. No bouncing and/or releasing, just slacked until I pedal again. Also, I know this is usually a question on this normally, not back pedaling either. I serviced the freehub about 2 months ago. Clean the drivetrain weekly usually.

So....my question. What is usually the culprit of this type of problem?


I am going to say it's the freehub. Curious of what you answer is.

The chain goes slack on the bottom run? A dragging freehub should cause slack on the upper run.

+1, If the upper loop slackens when coasting it's freehub related. Lower loop slacking is gravity vs. a light action RD cage spring, and meaningless. You don't notice it pedaling because you're not looking for it.

It is excessive slack. To the point of rubbing the tire and almost touching the ground.

And it only happens coasting? That's sort of a mystery.

Any freehub related problem would spool chain from the lower to upper loop, and that it doesn't only proves the freehub is working perfectly.

Pedaling or coasting, lower loop tension is purely a function of the idler cage spring tension, and in small sprocket combinations the limit of it's take up capacity. If it only is a problem with small sprockets (both front and rear) than your chain might be too long. If it happens all the time look at the cage tension.

No, intermittant (sp) issue. Ultegra 6700 short cage derailluer. Rolling along then it will, all of a sudden, do it. Slow and it goes away just as quickly as it came. Maybe its a bigger issue than I thought.

Pedaling or coasting? Think back because the difference is critical. When you coast the cranks hold the front end of the chain, and the hub would spool the other to the upper loop, except that a working freehub prevents that. So the chain is stationary and there's no transfer of tension.

Pedaling is very different. Driving force keeps the upper loop tight, and the crank is spooling chain to the lower. If everything is right it runs through the RD and returns to the upper loop, but if, for example, there's a stiff link it can bind in the RD cage, causing slack in the lower loop. It pops back when the RD cage runs out of travel and the chain is pulled through by the cassette, causing a quick rebound.

This is a serious problem, because if the link is stiff enough you can pull the RD around the cassette to the top. It's easy to fix by lubing the chain.

Definately a coast.

1. Chain Legnth. Put the chain on the small cr, small cog. If the chain is slack, chain may be too long.
2. No tension on der. In small/small combo, rotate der cage in CC direction, you should feel fairly strong spring tension.

Going to take it all apart tonight! Just gotta get home on it first.

It's a mystery, but I'd run the chain for a stiff link just in case (my first rule of complex diagnosis, eliminate the easiest/cheapest first).

Backpedal slowly while holding the idler forward a bit so the lower loop sags. Watch and feel for any link(s) that don't unspool smoothly and straighten out immediately as they exit. Go a few times around to be sure.

Next pull down the loop and release a few times and see if the RD cage snaps back cleanly every time.

It sure points to a sticking derailleur cage hinge or a broken spring.

UPDATE: Got to ride home in the light of day and see the problem more clearly. Turns out it is the "TOP" of the drive line. :? It all points to the freehub. Unfortunately didn't get to take anything apart last night. Will do so tonight.

Thanks to all for your help. Truely appreciated.