What comes to your mind when you think of a wheel? Rubber tire on a metal rim is probably the very image that pops up in your head at the thought of a wheel, isn’t it? But this article will show you different kinds of this locomotion type used in, mostly mobile, rovers. Read on :)

Fixed wheel

Fixed wheel, also known as a standard wheel, has one degree of freedom and can only rotate around its axis. The center of the wheel is attached to the robot's chassis. The angle between the robot chassis and the plane of the wheel is fixed. This type of wheel is commonly applied in wheeled mobile robots where the wheels are attached to the motors and used to drive and steer the rover.

Ball wheel

Ball wheels contain a spherical ball, typically metal or nylon one, placed in a holder. These balls have three degrees of freedom, giving them the ability to move in any direction and are usually used for balancing a robot. The drawback of this design is that these wheelsets tend to have high friction and, more importantly, the mechanisms needed to use them as drive wheels is incredibly complex and, as of right now, inefficient. So, ball wheels are typically used just to balance and support the robot. This solution is also not suitable for traversing dusty, uneven, and greasy surfaces.

Omni wheel

Omni wheels allow for multi-directional movement. They contain small discs (rollers) that are placed around the circumference and are perpendicular to the turning direction. It enables the wheel to be driven with full force but also to easily slide from side to side.

Mecanum wheel

Mecanum wheel, similarly to the omni wheel, is a multi-directional wheel that allows ground vehicles to move in any direction. It’s also known as the Swedish wheel or Ilon wheel, named after its inventor – Bengt Erland Ilon.

This type of wheel design is based on a tireless wheel with a series of external rubber rollers mounted diagonally around its rim. A mix of differential wheel motions allows for vehicle motion in almost any direction with any rotation, which is a significant advantage of this solution. Each Mecanum wheel is an independent, non-steering drive wheel with its own drive system, and as it rotates, it produces a propelling force perpendicular to the roller axle, which can be divided into a transverse and longitudinal component relative to the vehicle.

Wheels for planetary rovers

We’ve covered several types of wheels used in some mobile ground vehicles. But the wheel types don’t end here, at least not if we count space exploration ;). Weather and terrain conditions on Mars or the Moon are very different from those on Earth. Therefore, rovers built for space missions must be equipped with appropriate wheels that will prove themselves in these challenging conditions. Below, we present a few wheel designs for rovers intended for extra-terrestrial applications.

Lunokhod

The wheels of a Russian lunar rover Lunokhod had a diameter of 51 cm and a width of 20 cm. The relatively lightweight wheel construction consisted of three rims attached to the hub with spokes, covered with mesh on the outside. Each of the eight wheels had to bear nearly 100 kg of weight, while they were designed for several times greater loads.

Lunar Roving Vehicle

Lunar Roving Vehicle was a four-wheeled rover used on the surface of the Moon during the Apollo program missions. Its wheel structure was based on a rim made of aluminum. Attached to it was a tire made of galvanized mesh. Half of the outer running surface of the wheel was occupied by titanium chevrons, which acted as a tread. Inside the mesh tire, a titanium rim with a smaller diameter was mounted, which protected the structure from stronger impacts.

Aluminum block wheel

Several rovers that made their way to the Red Planet were designed with wheels made of a solid block of aluminum. This wheel design was used, for example, in the Sojourner rover, Spirit and Opportunity, Curiosity as well as Perseverance. In the case of the Curiosity rover, its wheels are made of aluminum, with cleats for traction and curved titanium spokes for springy support. The wheels consist of a very thin running surface, only 0.75 mm thick. It is covered with a chevron tread protruding 7.5 mm, which not only provides grip with the ground and side-slid resistance, but is also an element that strengthens the filigree external structure. The wheel is reinforced with three rims: two on the edges and an additional one in about one third of the width, to which six thicker spokes are attached.

Metal mesh shape memory alloy

NASA has developed a Martian rover wheel that uses mesh and “shape memory alloy” that can return to its original shape after being subjected to deformation. The alloy comprises titanium and nickel that bend and twist like other metals. What’s different about them is their atomic structure which tends to restore its original shape after the alloy is stressed, bent, or deformed. 

The arrangement of titanium and nickel atoms needs the least amount of energy out of any alternative arrangement, so the atoms naturally return to that arrangement. Stretching and bending the shape memory alloy requires energy, but when that energy is no longer used, the atoms fall back into their original positions.

See what’s beyond wheels

Now that you know some of the different wheel types used in rovers, check out other common locomotion types applied in mobile ground vehicles. And once you're at it, see the specification of Leo Rover's wheels.