i recently took my bike in to a shop to have the back wheel retrued. the guy forgot to put back on the plastic guard that goes between the cassette and the hub (i think thast what its called ). what is the purpose of that piece of plastic and do i rly need it?
my real concern is that now the backside of the cassette is exposed, and that dirt and crap will get stuck in there. im wondering if having the guard on tehre will stop some of the dirt from getting in there
You don't need that plastic guard. Like it was stated, dirt will still get back behind the cassette. Having that on your bike is like still having the reflectors on the wheels and pedals of your bike. You're good to go without it.
It will prevent your chain from destroying your spokes and possibly hub if the rear der is out of adjustment. I say run without it but first do this: Shift into your granny ring and largest cog. Once in the gear push the rear shifter as if you were going to shift to a lower gear. Does the deraileur move in toward the spokes even farther and then settle back once you let off pressure on the shifter? If it didn't move any more after already being in gear then you know the low limit screw is making contact. Good start. If it did move farther toward the spokes the limit screw is way off and you're headed for disaster. Here's how to set that screw. Look where the chain leaves the deraileur pulley and meets the cog. Turn the low limit screw till the cog teeth to make contact with the very center of the chain rollers. Now the only way the chain can get in the spokes is if you smash the derailer on a rock.
P.S. All this is in the setup instructions that come with a new rear deraileur.
Get a lighter and a screwdriver, Heat the end of the screwdriver until it glow's red, melt through the plastic guard, Repeat until you can pull it off.