ON my new bike I noticed the loud clunking I've been experiencing seems to be a slipping chain that keeps falling off the largest rear cog when shifting. I've experimented with pushing in the derailer a tad and it seems to allow the chain to stay on. Supposedly when I left the shop after buying this they set it up to make sure all was ok but apparently they didn't do such a hot job as the shfiting has been difficult and clunky. what do i need to look at to ensure total shifting capability on the rear shifting. BTW I assume that between from and back gears I can run largest to largest ..yes? This should offer the easiest peddling?