Rear derailleur touching spokes in lowest gear- really bad?
 

I just upgraded a Trek 1400 road bike from older an older 7-speed rear setup to 10-speed. I got a new wheel with a new 10-speed compatible shimano hub, new cassette, and new chain. Using the old derailleur, shifting is smooth and quick through all 10 cogs. However When I shift onto the largest cog going up hills, the arm of the derailleur just grazes the spokes- ting-a-ting-a-ting-a... If I put the bike on a stand and shift onto the large cog, it's fine- I presume it's my weight on the bike that flexes the spokes just enough to make this happen. I know that it can't be good, but since I only spend a few minutes of my daily 1.5 hour commute (couple steep hills) in that gear, I've kind of been ignoring it.. The derailleur appears to hang perfectly straight and not be bent.

The question is how bad a situation is this? Is it possible that since it was originally a 7-speed derailleur, the guide cage the chain travels through is not as skinny as one would be on a newer 10-speed derailleur?

Thanks for any thoughts..

I'm a little surprised that you're having this problem. Are you using the thin (1 mm) spacer needed at the back of the 10-speed cassette? A 10-speed cassette is narrower than an 8 or 9-speed cassette and needs that spacer when using an 8, 9, 10-speed compatible freehub.
Another possibility is to tighten the low limit screw on the derailleur to keep it from moving too far in. I would assume that you have already tried that.
I think a 10-speed derailleur has a narrower cage than your 7-speed, and of course that would be a possible fix.

Al

As someone who just caught my deR in the spokes of my favorite MTB today, I would say yes, it is very serious. (got a tiny twig in there and somehow it dragged it into the spokes, just as I started mashing up a hill.) Bad deal. Trashed the wheel, wrecked the der, and bent the hanger. Ugh. Don't be me. Me is very unhappy this afternoon.

jim

Absolutely that serious. Don't ride it like that! Even if it looks straight - have the hanger adjustment checked. And get a new RD. Want a nice Ultegra RD for $40 brand new? PM me*. Any who - you are risking destroying your wheel and more.

* I'm not selling these - but I'll give you the link.

I think the spacer may be my problem- it's an Ultegra CS6600 cassette and Ultegra FH6600 hub (8-9-10). So unless the cassette came with the 1mm spacer (it doens't look like it's in there now), that's it!

It sounds like it would be a good idea to get the spacer and get a 10sp derailleur for really good clearance.

Thanks Al!

The other thing I've come across is that older derailleurs decay at the joint between the body and the cage. If there's any play there, you're chain will pull it out of true. But I'd check the hanger before I got into replacing derailleurs.

In case anyone cares- turns out I did have the 1mm spacer on there already. The solution was a new 10 speed rear derailler- the cage on the new 10sp derailleur was over 5mm narrower than the 7speed one that was on there! I have a solid 5mm clearance between cage & spokes now, and shaved 70 grams in the process.