I just bought a new specialized stumpy FSR, and I notice the chain rattles on the bottom of the front derailleur, when im in the front small ring. I tried moving the derailleur down, but than it wouldnt shift correctly into the big ring.
What gives? It's an LX derailleur, are there others that are bigger and won't rattle when im in the small ring?
This is common....
for the FSR design under two conditions. The first is the bike is in the workstand with the suspension fully extended. The fully extended suspension pushes the angle of the seat stays beyond the designed working angle of the front derailleur. If it's happening while you are on the bike then you don't have the correct amount of sag in the rear (the second condition). Sag is the amount that the suspension settles into it's travel when you are seated on the bike. You should measure sag with the black o-ring that is on the exposed piston shaft and adjust it using air pressure in the shock. Recommended sag of the FSR is right at 10mm (about 20% of total shock stroke) when seated in the riding position. It can be a bit more or a bit less, but if it's too much less then the chain will rub the bottom of the front D cage when in certain gear combinations in the rear and the small ring up front. I've got on 05 FSR that does the same thing when the suspension is unweighted. But when I'm on the bike with the sag set correctly it's silent.
So, check your sag. I'm guessing that your running with to little. As you've discovered moving the derailleur clamp height down is not an option.
To check sag. Push the o-ring up against the wiper seal where the piston shaft enters the air canister. Carefully (without bouncing etc.) sit on the bike and assume the riding position. Next carefully get off the bike. Now measure from the o-ring to the wiper seal. That distance is your sag. To increase sag reduce the air pressure in the shock by 5psi and retake the measurement. Continue until you have at least 10mm of sag. Then go ride and see if you are still getting the chain rub. If you are there may be other issues.
Like I said, it's a common thing for the FSR design, but it has a benefit. When off the bike the seat stay angle is a bit much for the design parameters of the front D. But when seated with the suspension properly sagged the seat stay angle is close to ideal (like a hardtail) so the shifting performance is spot on if everything else is adjusted correctly.
"I do whatever my Rice Cripsies tell me to!"
interesting... i'll try and adjust my sag. The chain doesnt currently rub, but its mm's away from hitting, and does hit when i get off road.