Hi, my employer just bought a used road bike, the chain slips in almost any gear. I have seen the chain slip over the sprocket under load. I am thinking the sprocket and chain should be replaced. The bike has downtube shifters, with the flip of a lever it is friction or index shifting. I have counted 5 clicks with the index, that would tell me a six speed cassette is needed, is that correct? It has an 8speed cassette on it now....... Will this rear wheel accept a six speed cassette? If not, can the 8 speed cassette be used with the friction shift? This bike was purchased from a guy who sells lots of used bikes, so it may be a bit of a parts bike. Thanks for any help.