oh yeah..one more thing…have you thought about brakes yet?
Is your project for class just to build the frame and demonstrate your welding/bending prowess or is the desired end result to have a functioning racing kart?
Keep in mind, this MIGHT get expensive when you start to source parts to turn a roller into a runner. No one is trying to convince you NOT to do this, but rather we are hoping to help you with the REALITY of your project.
My suggestion is to shoot for the ‘yard kart’ style like you see at walmart. You’ll have a much better chance of having a running kart by the end of the school year.