Chainring problems

Sorry if ive posted this in the wrong section.
My bike has been having some problems lately as the chain kept on coming off literally 3 times in the distance of a mile. So there is a problem. My chain tension was fine so spun the cranks and saw there was a bend in the chainring making the chain around it wobble every rotation. I've now taken off the chainring and it seems bent but not as bent as i remember so now im wondering if there is something wrong with the cranks. I have a Shimano Sora chainset btw seems pretty cheap. Wondering if I do have to replace what chainring can I get that wont bend and how to I measure the BCD thingy on my cranks. Im out now but will try and take pcitures when I get back. Any advice is welcomed

If it appeared more bent when on the crank then that would suggest the crank is not on straight (which is hard to do if done up tight!) or the arms of the spider are bent or the BB axle is bent.

When you spin the crank around without the chainring on do the arms spin round in a straight line when looking from top or do they 'wobble'

Maybe better off just replacing the whole lot as rings seem to make up a disporpotionate amount of the cost!

And while you're here. I have some cranks where the bolt hole is under the crank arm. How the f@%& do you tighten those buggers?

Oh, and Sheldon will tell you how to measure the BCD likeglue2.

kerley: Ive just tested the spider and it seems straight now, I think because it was so near the frame i thought it was wobbling but seems to be fine. Ive taken another look at the chainring as well and seems to be bent on either side so yea, thats me problem.

Wicksie: Hi im pretty new here who or what is Sheldon lol

http://www.sheldonbrown.com/gloss_bo-z.html#bcd

Sheldon Brown's website is the Bible of bike maintenance.

Wow that helps more than anything else. Thanks alot

Yup. S'all you need.

Btw what is 'PDC'?

Btw what is 'PDC'?

Did you mean PCD?
Same as BCD

http://www.sheldonbrown.com/gloss_p.html#pcd

Thanks again