I've seen several pics on various science project sites, but none have any details ....
Step 1: Materials Needed
Sorry, forgot to include pics for tools ..
Small water/soda bottle (soda is more rigid, but harder to cut & drill)
4 bottle caps Bendable straw
BBQ skewer Party ballon
Drill & scissors
Step 2: Cut Opening for Ballon
Use utility knife, cut opening large enough for your finger access and ballon
Step 3: Cut Holes for Wheel & Axel
On opposite side when hole was cut, put some duct tape and mark 4 holes.
The position should be straight & close to the tape edge.
Drill from INSIDE out, then expand hole to 1/4 inches
Step 4: Drill Hole for Wheels
Cap hole diameter should be slightly smaller than the BBQ skewer. Mine was 1/32". Slip in bottle cap to bottom. It's suppose to be fairly tight fitting so wheel will not wobble.
Step 5: Mount the Wheels
The skewer axel should rotate freely, cut openings with scissors if needed. It's OK if you end with an oval shape. DON'T cut too much, else car will drag on the bottom
Step 6: Drill Hole for Ballon Mount
Tape a ballon to short end of straw tightly. You can inflate here & check for leaks.
Thread the longer end thru, then tape it securely.
Step 7: Testing
Car should ride very nicely with no drags. Now inflate & enjoy !