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..