Adult Soapbox Kart

Creating a full-suspension soapbox kart capable of highway-speed travel from raw parts & materials is a monumental undertaking.

• My kart in its current state represents a total time investment of around 300 hours over the course of a year and the current configuration uses approximately $600 worth of parts and materials.
• Think of this as doing your part to stimulate the economy and keep yourself off the couch. The sheer breadth of tools required to build it will leave you proud of your hard work as you roll down nearby hills in your super-cool soapbox kart.

Before starting to build karts 2 years ago, I had no experience in metalworking.

Prep

• You may want to jump into this project with wild, reckless abandon and that's ok. but you may also be the kind of person to plan everything out.
• What I recommend is to make drawings (detailed ones) of your frame and suspension components, make cut lists for all your materials so you know how much to order (50% more than you think you may conceivably want), and share your designs with other people who might be able to guide you

I recommend a moleskine and the book "How to Make Your Car Handle" While it's an old book, it is readable and well-suited to this project.

Seat intro and mock-up

• Of all the parts to make first, I chose the seat. Not the frame, the wheelbase, suspension parts, etc. the seat. The thinking was that it would give me something to sit in while I test-fitted everything around me.
• The way I made a seat was as a single sheet of thin metal that was cut and riveted to form a 3-dimensional object. It should be concave, and if you want it to have convex bends in it you must use more than one piece of metal.
• To work out the basic shape for the seat, start with a piece of paper. Draw the centerline of the seat and trace an approximate outline of your body; that will be the back/bottom. From there, draw lines perpendicular to your body outline toward the edge of the paper. Cut along these lines and that's what will allow the center portion to bend to your body.

Actual Seat

• For the real seat, do the same process as the mock-up, but with sheet metal instead of paper and rivets instead of tape. Clamps are handy to hold things in place while you drill holes for the rivets

  • If the edges are too sharp, feel free to attach upholstry to your seat; I used some black and red cloth-backed pleather from a fabric store.

    • I held it on by riveting on some extra scraps of metal on top of the fabric.
    • My upholstered seat has held up to 8 months of sun, rain, and snow so far...

Materials Selection

• You may make your soapbox kart out of anything you want! Steel, aluminum, fiberglass, carbon fiber, wood, anything!
• Make sure that you make it strong enough to keep you as protected as you need to be for your intended use

  • we have a wooden kart that was made for more traditional quarter-mile, straight, head-to-head soapbox racing but nobody has been brave enough to try it on the open road.
• Also consider ease of construction/modification. For frames, metals are easier to start working with and modify than composites like fiberglass.

  • Expect to have to modify your kart. My kart has gone through several changes as I've built it so it needs to be modifiable.
• My kart was made with 1" and 1-1/4" black pipe, plus 1/2" and 3/4" EMT from Lowe's. Both are steel and thus can be MIG welded with ease

  • A kart intended for really rough hills or high-speed races should be made to be more sturdy than lightweight. I would suggest using larger sizes of EMT or switching it out for equivalent sizes of black pipe

straight frame members are pretty simple

• Cut them to the proper length either straight with a chop saw or, to make welding easier, fish-mouthed with a tubing notcher or milling machine.
• If you are making fish mouths, make sure to double-check your intended length.

Bending Frame Members

• Frame members made out of EMT or other lightweight tubing can be easily bent using a hand bender from home depot
• Heavy pieces that need to be bent require a hydraulic bender

Pieces I bent using the bender: the upper frame rails, the rear shoulder harness crossmember, and the upper crossmembers that hold the steering shaft bearings.

Only one piece on this kart required bending in a hydraulic bender: the rollbar

• Rather than just make one tight bend on the rollbar, I opted to go for one central bend and two side bends. It didn't turn out nearly as symmetric as I had hoped, but my subsequent attempts have been better