back protection for the track

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It's a bit like opinions on helmets. Some fit some heads better than other heads, and all are pretty safe. Once it's on you forget it's there, unless you're throwing yourself all over the bike on a race track.

I'd go to a shop, look at a few, try some on, consider the price, and buy whatever you feel is right. Probably best to stick to the longer ones that reach down to the tail bone, than some of the "trendy" little ones that protect about 3 of your vertebrae!