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?
the spacer is there for a very important reason - it stops the chain from over travelling past/behind the cassette and jamming against the spokes - an action that usually happens after cable strecth on new bikes.
the guard is, i think, also a spacer - ive taken mine off and replaced the cassette without the guard and afterwards i noticed that there was play from the cassette on the hub... if you replace the spacer guard and tighten the threaded cap youll notice that theres no play at all from the cassette on the hub - if theres play then the chain wont sit right on the cogs of the cassette as it spins - causing wear and tear on the cassette and at worst may snap the chain...
so if youve snapped it off - or taken it off - check to see if the cassette moves on the hub....if it then im thinking thats a real bad thing and you should look into replacing the spacer
It's not a spacer for the cassette. Wen you changed your cassette did you change from an 8 to a 7 speed? Go to your lbs and ask them for a cassette spacer.
It is called a pie plate and all it does is prevents the derailleur from going into your spokes, if your derailleur is adjusted correctly this should never happen unless you crash and it gets bent into your wheel. You don't need it if everything is adjusted and any good bike mechanic should take it off for you.