Earlier this year I was given a 2011 Top Kart Flash with a Rotax Engine which I have rebuilt to use at practice days (I’ve been out of karting for 30years). I took the kart out once and noticed that all the other karts had big plastic rear bumpers and my kart just has the metal frame width type. I was wondering if it is a requirement to have this newer wider plastic type bumper. I was also just given another kart exactly the same as the one I have 
I think it depends on the rules of the race series. You see the metal bumpers all the time in Club racing, particularly 4-stroke.
However you never see them in uspks or skusa.
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Full-width bumpers are required in almost every major series in the US since about 2009. Metal or plastic will depend on the series, but most places require the CIK-rated rear plastic bumper.
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Thanks! I think I will convert them both to the plastic type.
I think full width bumpers are used as a safety device to reduce open wheels from coming in contact. The plastic bumpers also offer some degree of cushion that metal bumper do not. If you convert you will need brackets and possibly hardware to mount.
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