The freewheel on my bike was worn out and needed replacing. It was worn out by the bicycle chain, which I’ve not replaced after using it for more than 2 and a half years. Initially I thought only the bicycle chain needed replacing, as I could see the chain floating on the front chainring, likely due to the chain being elongated after years of usage and not having been taking care of properly. But after replacing the chain, the chain started to slip on the freewheel. It slipped so badly that I was effectively left with only one useable speed on that heavily worn out freewheel!So I went online to see what I need to get if I want to replace the freewheel myself. At the time I didn’t know the difference between freewheel and cassette. And I didn’t even know that my bike is still using the old threaded hub! Until my colleague told me that the cassette I bought would not fit the threaded hub I’ve got on my bike when he was about to show me how incredibly easy it is to replace a cassette. What a waste of everybody’s time, including my other nosy colleagues who would also like to know how to replace a cassette. :)So for anyone who hasn’t a clue what system they are using, it is definitely worth the effort to take off the rear wheel to find out first before diving straight into buying the parts. Although it is quite rewarding to have understood the difference through the hard way, as I also found out various bits of information about different bicycle parts along the way, I think it’s probably not necessary to go through the process of ordering the wrong parts. Having said that, this online bike store I used is really quite impressive. I returned the cassette on Friday in second class post, and the money was already refunded back into my account on Monday! It costed me about £2 to send the cassette back, but they offer free postage service, using first class Royal Mail, when I buy stuff from them. Makes me wonder how much does it actually cost them to send the stuff. Such cost efficient retailers.