The problem my friends and I have is that we can't know the coefficient of friction between the cardboard box and the floor. We are allowed to compete with up to five vehicles, so I think at least one vehicle is going to be super lightweight and designed to floor it (pun intended) with the cardboard sled carrying a fairly light load and just try and run as fast as possible. I'm trying to make my own vehicle one that moves relatively slow but pulls a lot of weight. Right now I'm intending to use some scooter wheels I found lying around - I think I recall they are 73mm in diameter. I decided to make my own car a bit less weight conservative, as we will need traction to pull a heavy load I expect it may actually need to be weighted down itself to pull a heavier load (>/= 2kg???)
As we are limited to 'standard' 9V batteries (the rectangular block type) I think we're pretty much stuck with the weight of typical alkaline batteries. For our lightest vehicle, the builder used carbon fiber arrow shafts (hollow but strong and rigid) and made a 4 sided frame, and plastic attachments to mount the axel shaft in. That one will be the fastest by far I hope.
The car I built has a wooden square frame with two sides mounted with down-facing aluminum L beams, to reduce friction with the axels. Everything is secured with steel bolts and hex nuts, and my axels are 5/16" thick stainless steel. I built it to be really sturdy in case we need to put a lot of weight on the frame for traction, but it's still not more than around 4-5 pounds by itself. Again, the motor we need to use is the same for every team, so there wasn't a way around that. I'll see if I can't show you some pictures of the frame of my car as-is.
Thanks for the assistance, mklrobo!
