I got a 2.4" Rubber Queen in there, and those tyres are massive! It rubbed when I cornered hard but that was because I was running it on a 717 rim.

Usually use 2.35 Maxxis tyres, but now run 2.1s as I only use it for fast sprints around local trails, not all dayers out in the mountains.

Ah ok I misunderstood thought you were talking about tyre width, well as the bikes in the lounge I've just measured and there is between 25-30mm clearance on the top and bottom braces from the middle of the tyre (2.1 FF – quite low volume)

Not sure on the extra circumference on a 650 but it's not a huge amount is it? Reckon you should be okay if they tyre is low volume.

Bought the 2009 ss model for my wifes birthday last month, bloody bargain for £389 from Wiggle http://www.wiggle.co.uk/p/cycle/7/Charge_Duster_SS_2009/5360039071/

With the ss duster can you convert it to gears with a new drop out or do you have to go for a alfine?