skeleton of half-pipe?

[tdough]

I'm a poor college student and am want to build a mini half pipe as cheap as possible lol.
I have to make it an outdoor pipe so its going to be treated wood and the 2X4's are about $1.70 cheaper than the 2X6.
can i use 2X4's for framing instead of 2X6's or with that make it too weak?

[ROYAL]

You can, but may need a few more closer together in the radius of your ramp. That's also recommend when using 2x6. That's where all the weight and down force hits the hardest and will get bouncy and lose speed if you do not have enough ribbing closer together.