Bicycle Tutor

Hi, I'm new to cycling and just bought a 2nd hand road bike, I've tried to do as much to it as I can myself. I've fitted a rear derailer which was taken off so it would be a fixed gear. But now sometimes when pedaling the rear cog goes round but doesn't turn the rear wheel. It's probably really simple but can anyone help?

Do you know if you have a freewheel or a cassette? It sounds like the pawls in either your freewheel or freehub body are not catching, and may need replacing.

I don't know! I really need to get more knowledge! How do i tell the difference? Thanks

It's explained a little ways into the freewheel video, and in the image in the text below the video.

It's a freewheel! Most of the time it powers the rear wheel but sometimes seams to slip!! Any suggestions? Thanks for the help so far Alex.

What often happens with freewheels is the insides get dirty and the pawls start sticking (which is why they don't catch when you pedal forward). You can either get a good bike shop to flush it out and re-lube it, or get a new one. The flushing option isn't guaranteed to work and a new freewheel is probably less expensive.