You should have 4 brackets. Use 2 brackets on the front tube and 2 on the rear, on on the side tubes near the rear. You just put one side of the bracket over the tube. I'll take a pic of mine tomorrow, so that you can see what I mean.
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You meet the nicest people on a Honda.
Something like that. The 2 at the front of the rack are OK, but I put the back 2 further back on the 2 side tubes, near to the back of the rack. It'll be stronger like that, and won't have any chance of slipping from side to side.
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You meet the nicest people on a Honda.
A little tip. Before you install your rack, you may want to paint it. I don't know if I had a bad one, or they're all like that, but it started rusting immediately after I put it on my bike. I've since given it a coat of paint, and it's fine now.
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You meet the nicest people on a Honda.