BrickVista - Rover Wheels


	ROVER WHEEL FEATURES

During the early development of my YogiCub Rover it became quite obvious that some seriously large and hefty wheels were required to support the Rover’s Drive and Control Systems. LEGO® has some large wheel and tyre combos in its range but none really fitted the design requirement or sprit, of a ‘true’ Rover wheel.

What makes Rover wheels unique you ask, well, your typical Rover wheel is a cross between an off-road tyre and a cheese grater.

A Rover wheel has to perform across a wide range of terrain surfaces and environmental conditions: soft sand, scorching rocky plains, wet and muddy ground, and even polar ice caps.

NASA Microrovers share a common pedigree with all wheels being made from aluminum, Stainless Steel and other composites. The size of Rover wheels range from about 50mm to 250mm in diameter.

Design Brief
The operational nature of Rovers means that they must get by with just a single set of wheels - Unlike automobiles where you can change tyres at your local service station. Rover wheels must be light, durable, and perform whatever the terrain or weather condition.

As a result Rover wheels don’t, have separate tyres and hubs, but rather a single wheel made from aluminum, Stainless Steel and other composites, all stable in extreme environments - no inflated rubber here. Treads and cleats on the wheels provide traction and grip for climbing in soft sand and scrambling over rocks.

LEGO® wheel and tyre combos are for the most part ‘Earth centric’, in that each wheel is comprised of a matching hub and tyre – just like your family car. Most LEGO® Technic wheels have a diameter ranging between 30mm and 80mm (with a few notable exceptions sitting outside each extreme). I was looking for a design solution which was a little more in keeping with the sprit of real Rover wheels, but like always implemented through LEGO® .

The wheel design had to support the operational weight of my YogiCub Rover, while providing sufficient grip and traction over uneven terrain (The lounge room floor with complementary magazines). Oh, it had to look the part as well!

Design Solution
Wheel construction centered around two distinct building methods, each allowing for different wheel features to be included. I found that wheels could be classified as being constructed by using either a Solid or Spoked method.

Solid wheels are built around an existing LEGO® wheel, building up its outer diameter in layers. Wheels of this kind tend to have an upper limit of around 120mm in diameter (depending on tread construction), are robust, and highly configurable. On the other hand Spoked wheels are constructed from the axle up with solid ABS. Technic elements combined with standard LEGO® elements yields wheel diameters 100mm upwards. Building a spoked wheel is a little fiddlier than building an equivalent solid wheel (as they can spontaneously fall apart) but when all spokes are in place, the internal bracing makes them very ridged and more importantly light.

Solid wheels need a fairly large LEGO® wheel as a starter. The large Technic Motorcycle wheel and tyre combo is a great ‘seeding’ wheel. Often the tyre is left on and built over as it adsorbs the slight variations in circumference generated by an outer layer of LEGO® elements. In some cases this can be used to give your wheel some internal dampening.

Spoked wheels are truly LEGO® wheels. They are constructed entirely from LEGO® elements: Technic elements for bracing and standard LEGO® elements for tread and cleat detail. Due to the internal structure of a spoked wheel the smallest working diameter available is about 100mm with no theoretical upper limit, however practicality suggests a functional wheel diameter of about 200mm as being quite achievable.

For the immediate application to the YogiCub Rover I chose to adapt a design developed by Joe Nagata featured in his LEGO® Mindstorms Idea's Book. It was a simple solid wheel construction with a modified axle connection. It’s a simple wheel design but fitted the design criteria well. To learn more about YogiCub’s wheels see the sidebar.

However the notion of a related range of configurable wheels was worth perusing. More so due to the fact the YogiCub’s bigger brother was going to need some bigger shoes to get around in.

The spoked wheel building method offered the most versatility in wheel diameter, width and tread configuration.

	Construction centers around a spoked core from which the wheels outer perimeter is supported. The rather odd looking Technic propeller found a very suitable home here. The center axle holds three layers of supporting beams, each containing 6 spokes. The spokes are comprised of Axle joiners and 3 unit Axles (with stud) integrated to the outer wheel layer by 1 x 2 Technic bricks interlaced with 1 x 4 Hinge bricks. Intermittent Technic bricks housed a ½ Technic Pin. The exterior of the wheel is covered with 2 x 8 plates pressed onto the exposed Technic pin and axle studs. A large Technic Turntable is fitted to the reverse of the wheel to allow for fastening and drive supply. Motors, and or gearing can be mounted directly to the turntable.

The feature of this design is that it allows for various tread and cleat configurations while maintaining a common wheel core. Wheels with very different tread configurations can be made with relatively simple modifications.

The design of the spoke layers also allows for various width wheels – The wheels illustrated above a are 8-stud wide wheels, but 6 or even 10-stud wide wheels would be quite feasible to build without losing any structural integrity. By adding hinge brick elements to the wheel’s perimeter you can also increase the wheel’s diameter. Again the wheels illustrated above have an outer layer comprised of eight 1 x 4 hinge bricks yielding a diameter of approximately 120mm. The next size up, would use ten, the next twelve, fourteen and so on. The tread connection detail of each wheel type would be consistent, so your tread elements are reusable, however each wheel size would require a unique spoke configuration and design. However the system is scalable, Just imagine!

MER WHEELS

The Mars Exploration Rovers (MER) use a newer modified wheel design than what was used on the Sojourner Rover. At 26 centimeters in diameter (a little over ten inches), the aluminum wheels of the MER are nearly twice the size of those used on Sojourner and are missing the recognizable sharp cleats.

Chris Voorhees is a mobility engineer working for NASA and is responsible for the development of the MER wheels. He says of the new wheel design that “a big challenge was to be able to get enough traction to get through soil and over rocks of the Martian environment, but also to be able to get off of the lander without getting entangled in the deflated airbags". The new design is "basically like a paddlewheel that is machined onto the outside of the wheel, providing both safety and capability."

Learn more about the MER wheels at the NASA Mars Exploration Rover web site.

MER Wheel

WHEELS, TYRES AND TRACTION

Traction test Vehicle

The traction performance
of LEGO® ® wheels.

Philippe Hurbain has undertaken a detailed measurement campaign to find out the traction performance of LEGO® wheels. He says “During a friendly sumo competition we discussed the merits of various LEGO® wheels for traction power (for sumo this is rather pushing power!)” Philippe built a simple test vehicle to which various wheels and treads could be fitted to 4 or 6 driven shafts. Tests were conducted over five different surfaces.

Check out the results.

SUMO Competitions
Sumo rules and competitions

French Enthusiast LEGO® User Group Sumo Rules

WORKSHOP 3D LEGO® RCX Challenge Sumo Rules

LEGO® Robotics Group of Ann Arbor Sumo Rules

	John Barnes' latest Sumo rules as used at BrickFest 2003

	Sumo Rules \| .pdf \| 103kb

YogiCub has a medium width wheel made from 2 LEGO® Technic Motorcycle wheels & tyres with the addition of a Technic Tread wrapped around both tyre's perimeter.

Learn more about YogiCub's wheels here

Ever wondered how a Rover works? Check out Rover TechNotes to discover how a Rover operates and explores on distant worlds.

Learn more about Rovers here

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All photographic images of Rovers are supplied courtesy of NASA/JPL, which does not sponsor, authorize or endorse this site. Visit the official NASA website at http://www.nasa.gov.

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