The Brief

The vehicle must start at the foot of the left ramp, then climb it, drop into the water, traverse the water, exit the water, and finally drive down the other ramp. It must do this all unaided and within 45 seconds. The width of the course is 300mm at all points. The other specifications are that the vehicle is four-wheel drive, weighs less than 450g, an is powered by two AA batteries. The wheels are made from foam and the chassis is made from acrylic.

The design of the wheels, chassis, and gearbox is all free. 

Design Concepts

Wheels

In this task the vehicle's wheels serve three distinct purposes: pushing the vehicle across the land, propelling the vehicle across the water, and lifting the vehicle out of the water. 

When on land, a conventional round wheel is preferable, as the constant radius ensures constant traction and smooth movement. When on water, some geometry is needed to displace water and move the vehicle forward. When leaving the water, the wheel geometry needs to provide leverage on the lip of the ramp in order to lift the vehicle. 

As only the front wheels need to lift the vehicle from the water, Concept 4 was chosen for the front wheels and Concept 1 was chosen for the rear wheels. A wheel size of 140mm was chosen as a compromise between mass and buoyancy.

Chassis

As the vehicle's speed is low enough for aerodynamic effects to be negligible, and the chassis shouldn't come into contact with the water surface, the main effect its design has on the vehicle's performance is the wheelbase. The wheelbase is important because affects the likelihood of the vehicle grounding as it enters and leaves the water.

The diagram shows the situation where grounding can occur. Through geometric analysis, it was seen that, with the chosen wheel diameter of 140mm, and an assumed wheel submersion of 35mm, a wheelbase of length 200mm would prevent grounding. Triangular cutouts were made in the chassis to reduce mass while maintaining stiffness. The width of the chassis is determined by the size of the motor and necessary gearing. A 50mm wide chassis was used.  

Gearbox

As speed was the primary objective and the motor and power source were not changeable, the choice of gear ratio was very important. The minimum viable torque, and therefore the maximum viable speed was found by considering the forces on one wheel as it ascended the ramp.

Using the known components to estimate the vehicle weight, and by examining the motor's specifications, it was found that a gear ratio of 367 was necessary. A ratio of 625 was targeted in order to allow for acceleration, friction losses in the gearbox and transmission, and the effects of non-round wheels.

Final Product

The acrylic chassis was manufactured using a laser cutter. The wheels were cut out of blocks of foam using a hot wire foam cutter and an Mdf laser-cut stencil.

The vehicle completed the course without aid and lifted itself from the water easily. However, it was relatively slow when moving up and down the ramps. This was likely due to an overly conservative gear ratio. Given the opportunity to repeat the project, the design for the front wheel would be revised to smoothen the ride, the gear ratio would be lessened, and for the rear wheel, the grooves would be widened to improve speed through the water. 

For more details, get in touch at Michaelsvanidze0@gmail.com.