Metal Ramps

Ok I'm building some metal ramps and some people have showed some interest in me building them some. So my question is what do I have to do so I don't get sued?. I don't have any pics of it yet I finished one for myself took 2 days(10 hours) and works good for me but here are the specs.
20 feet long
12 feet high
45 degree angle
braces up and down every 2 feet on both sides and some slanted on the inside facing oppisite way of ramp.
4 feet wide.

Like to here your opinion on this please.
Matt

Thats for the replies guys, I'll try posting some pics as so as I can cause my first and only ramp is leaving the yard tommorow(sold $600 cdn) but I'm goin to try to build another before winter. What I'm thinking of trying out next is 12' high 24' long wit a 50 degree angle. But I should have pics up in 2-3 weeks.

The ramps have a metal frame of 1'' square tubing and the 3 archs for the take off i get bent to whatever angle and the take off its self is 1'' plywood. I built it so the wood slides between 1'' sheet metal at both ends top and bottom spaced 1 1/4'' apart so the wood slides in between then holes are drilled every 1' and a bolt is ran though it. In order for the wood to fit it must be bet so I lay a tarp down put wood on and fill it with water but holding the ends up to the water stays. the wood has to sit for 4-5 days for it to bend a little. Once the wood is on and the frame is painted(prevent rust), I stain the wood so it lasts longer. I haven't found anything gripy to put on it yet. But I have found it works good cause all you need is the momentum goin up it, but you still have to stay on the pipe.

So if I can't just get them to sign a wavier then what could I do to sell them but to cover my butt? I would like to hear anything cause you wouldn't beleave how many people would like one(different sizes), and not just motocross guys skaters and bmx guys.