This is another build for Trowbridge soap box derby. These my son and I built these for 1st Westbury Cub Scouts

Materials
220 litre plastic drum
4 bicycle wheels (14" - 20" are ideal)
A pile of timber
3 pieces 1metre long
4 pieces 75mm longer than the wheel diameter
2x M10 bolts and matching lock nuts
M8 threaded bar
An assortment of screws and smaller bolts
15mm x 3mm steel bar for track rods, and control arms
2x 15mm equal T plumbing fittings
One standard issue 9 year old cub scout for checking sizes

Step 1: The Start

Take your drum, and drill a hole on the side near the bottom big enough to take jigsaw or padsaw blade. This is so a the hole for the driver can be cut. It needs to be big enough to allow easy ingress and egress but small enough to keep the cub safe when they eventually crash it.

Step 2: Rear Wheels

For this stage you need two of the 1 metre long pieces of timber, Bike wheels are great for soap boxes provided:

they are not too large (less than 20" is ideal)
they often need to be supported on both sides

The two 1metre lengths feed across the width of the drum, these photos were taken when we were experimenting with layout so the final version was slightly different, but you do have to use what is now the rear of the cart to support the axle.

We also used our standard issue 9 yr old to check the cockpit size

Step 3: Front Axle

The front axle is made in a similar manner to my other soap box cart, two pieces of 1metre timber mounted to the front of the drum. The gap between them is equal to the height of the 15mm equal T pipe fitting. You need to mount this securely and centrally at the front, more stress is imparted as the loads feed into the drum and axle in a smaller area than at the rear.

The 15mm t is drilled to match the bike wheel axle, we were able to adjust the wheel axle so we could off set the wheel and allow more thread to be supported through the T piece.

The control arms mount on to the wheel axle, its made from the steel bar. The bar is heated so it can be bent and twisted.

Step 4: Steering

Steering is a simple Pitman Arm arrangement. the M8 threaded bar is used as the steering column. A hole is drilled at the bottom front of the drum and the bar threaded through. It is held in place with nuts and washers, the Pitman arm is made from the steel bar and is positioned using nuts, once in the right place these are welded to lock the arm in place. The steering wheel is made from more steel bar with a nut welded to it and again once in the right place welded into place (although you could use lock nuts). an old broom handle is screwed to the steering wheel bar to make it more comfortable to use. Steel bar is used to create the track rods.

Step 5: Brakes and Finishing

An old plastic chair was sacrificed and screwed into the cart so that the driver is a little more comfortable.
The brake is simply taken from an old bike and mounted on the rear axle frame and operated via a simple lever