chain slap
 

I have a friend who rides a Trek Madone 5.2 with Ultegra compact. In some of his gear combination he gets chain slap. What I mean is that the chain will go slack and bounce off the chain stay. He has had the bike back to the shop at least twice for this problem. My thoughts are that his read Derailleur springs are getting weak an he will need to replace the RD. Anyone have any thoughts?

There is the possibility that the chain is a few links too long. Or that the RD hardware needs lubricated and isn't retensioning properly. I would move the chain to the big-big combo and check the slack that remains in the chain, if it's pretty taught then I'd lube up all the pivots on the RD.

Could be that the freehub body of his rear hub is binding a bit too.

Three possibilities.

1- chain is too long and on the small/small combination the RD cannot take up all the slack. This is obvious because the lower loop will sag visibly, or you can check by rotating the cage and see if there's room to increase tension. If there is, all is fine. (BTW RD springs don't get weaker over time less than eons). On some RDs you can adjust spring tension, but this lowers the efficiency of the system.

2- a sticky freehub is spooling the chain forward slackening the top loop. Probably not the case, because it causes slack problems far beyond chain slap, and you'd know.

3- chain is close to chainstay coming off smallest cassette sprockets. Consider the radius of the 11 or 12t sprocket and you'll see that the chain comes off pretty low compared to the height of the stay. With so little clearance it doesn't take much of a bump to raise the stay and slap the chain, so we shouldn't call it chain slap, but stay slap. There's no fix if you coast over bumpy roads in high gears, so that's why they make chainstay protectors. Either buy one, or improvise your own, and accept it as a normal characteristic.