Wheel Construction

WHEEL RESEARCH

Considerations:

OTS : no custom hubs, cheaper, faster for prototyping
Timeline/prototyping
Accuracy to what can be used on an actual Mars mission
Weight
Price point
Customizability/design uniqueness
What has been successful for other teams
Size (less than 16”, greater than 10”) for 6 wheel drivetrain
maintenance
coefficient of friction
maximum load
width/tire thickness
Internal diameter (ID)

Main Solutions (In depth)

Consideration/Topic	Entirely 3D Printed Wheels	beach tires with 3D printed treads	Solid Tread, flat free wheels
general/description	self explanatory, design entire wheel	design various attachable 3D printed tread patterns to the beach tires	most basic option used as is
Timeline/prototyping	may take many weeks to create from designing, sourcing filament, printing, prototyping to final product	can reliably test far faster - begin testing with just the beach tires and can add treads later	would need to alter hub to accommodate motor
Educational aspect	improve design skills opportunity to analyze various tread styles for the terrain analyze possible spoke styles/internal support for weight of rover as well deformability create/implement custom hubs	opportunity to analyze various tread styles for the terrain figure out a way to firmly attach treads to wheel Stanford - zip ties (prototyping) Cornell (2020)-adhesives -cyanoacrylates, epoxy?	altering hub
Accuracy to what can be used on an actual Mars mission	airless tires would be more accurate to what is possible on Mars missions	n/a	airless tires foam filled
Size options	entirely customizable (between 10" -15")	Wheeleez: diameter: 9.4" width: diameter: 11.8" width: 7.3" diameter: 16.5" width: McMaster Carr 8" diameter 10" width: 1 5/8"	10" 12" 13" 15" 16"
Weight	unsure, but would likely be heavier as it is made of solid material	WheelEEZ: 11.8" : 1.3 kg (2.9 lbs) McMaster-Carr 10": couldn't find	n/a
Price point	n/a	11.8": $89.00 10": $85.58	$75-160 13": $124.43
Customizability/design uniqueness	entirely custom allowing for custom wheel hubs and any spoke/tread design we may want would have the ability to expand on this for future years	tire is OTS but treads would/could be custom made and this would also allow us to prepare for a more advanced wheel design for a future rover possibility of a custom hub (hub that comes with wheels is completely removable	n/a
maintenance	cannot be fully sure until prototyped design could be less resistant to catching sand, wearing down etc.	unlikely, but sand could enter hubs/bearings - should choose a reliable design (Wheeleez has multiple parts such that the entire hub can be easily taken apart and cleaned/repaired/replaced) cuts can be repaired using a soldering iron possibility for 3D printing different interchangeable attachable treads to have better treads prepared for competition/less worn down/new treads
Coefficient of friction	0.38 (for hole wheel design)* 0.45 (for spike wheel design)*	WheelEEZ: 0.2-0.25 (website did not have specific values, against steel) McMaster-Carr: n/a	n/a
Maximum load	n/a	WheelEEZ: 55kg McMaster-Carr: 1.2kg	0.3kg
Other teams	Monash Ryerson Rams ITU	Stanford Cornell	n/a
OTS Options/purchase options	filament here Thermoplastic Polyurethane TPU (AGH Space Systems, ITU) Polyethylene terephthalate glycol PETG (ITU) black SemiFlex clear generic TPE90A white NinjaFlex (very soft, therefore harder to print)	WheelEEZ McMaster-Carr	McMaster-Carr https://www.mcmaster.com/22245T33/ https://www.mcmaster.com/rubber-tread-wheels/
Example photos	Monash: ITU:	WheelEEZ wheel: Cornell: hub: Custom hub (cornell):

Materials:

Material type	Description	Coefficient of Friction μ	Benefits	Drawbacks	Other Notes
Air Tires	beach tires		can purchase commercially cost is under $100	have to design within constraints of COTS wheel hubs	similarly to Stanford, nylon chains and grips can be added to the surface of the wheels for more traction 8-12" diameter for a 6 wheel design (from old research can adapt Can adapt to a 6” diameter hub motor Width exceeding 4” (by experience, can analyze for more detail, too narrow tires can significantly increase ground pressure) Any 2 tires in design must be able to handle entire rover load without rupturing
Rubber	ATV sand tires		tried and tested to be efficient in sand dunes	would have to emulate the design ourselves/cannot buy commercially because regular ATV sand tires are too big	can change pressure within tires, when tires are deflated to 10-20 psi, better for sandy terrain
Rubber	cast with molds	0.9 (on asphalt)	customizable can use with custom hubs	prototyping time that could be spent working elsewhere need to learn how to use molds and casting	refer to Ryerson Rams design
Metal	Aluminum (curiosity rover)	1.10-1.35 (on aluminum)	more similar to current tech used on Mars (air tires cannot be used due to Mars' atmosphere)	depending on thickness of wheel/design, similarly to Curiosity Rover, the aluminum wheels could experience some damage
	Titanium nickel alloy (webbed material used for bike tires created by The SMART Tire company)		has been approved by NASA would be more suitable for real Mars conditions	difficult and expensive to imitate deformations can occur if shape memory alloys are not used	https://www.syfy.com/syfywire/nasa-space-tires-never-go-flat-coming-to-your-bicycle

3D Printed (use thermoplastic elastomers)	white NinjaFlex (very soft, therefore harder to print)	0.69 (for smooth wheel design)*	customizability compliant wheel internal design (with soft thermoplastics) Can rapidly prototype various wheel designs custom hubs can be used which is easier for mounting motors on wheels With custom hub design as well, possibility of swapping tires FEA justified	printing/prototyping time that could be spent working elsewhere less reliable than commercially designed tires	*referenced this report which used small scale wheels https://www.sciencedirect.com/science/article/pii/S2212827117300793 (μ measured on aluminum) another material option is elastomer latex, but that is not commercially available
	clear generic TPE90A	0.45 (for spike wheel design)*
	black SemiFlex	0.38 (for hole wheel design)*

Interesting Tire Shapes (treads and spokes?)

Tire Shape Name	Photos	Extra notes
Humvee Airless Tire		in prototype stage for (U.S.) Army Research Lab by Resilient Technologies made of polymers benefit: would be more accurate to tech that can actually work on Mars drawback: have to ensure that nothing can get stuck within spokes
Michelin UPTIS airless tire		demo clips of it rolling over a nail and continuing no need to check air pressure cast a design like this or 3d print (with soft thermoplastics) a design like this? benefit: would be more accurate to tech that can actually work on Mars drawback: have to ensure that nothing can get stuck within spokes
Loopwheel		a bike tire design, but the "spokes" are interesting bc they have been designed to readjust and therefore change the shape of the tires for rougher terrain
Sand tires		specialized ATV/UTV rear tires: paddle like ridging to dig into sand front tires: completely smooth, few ridges for steering purposes, ridges raised along the center allow for bite to make more immediate turns-might not be good to implement these bc the rover won't be going as fast (don't need to glide).
SMART Tires		bicycle tire design still in prototyping phase-final product is planned to be encased in rubber & polyurethane

Wheel designs from other teams:

Team Name	Year and Info	Pictures
IMPULS:	2021: 300 mm wheels, wheels have not changed from 2019 design (as far as I can see). Their spokes and frame seem to be custom made (machined and acrylic?)
Ryerson Rams:	Previously (before 2019) used rigid wheels, too much force into suspension, not enough traction (stated in video) 2019: Prototyped highly deformable wheels, flexible 3d printing, casting rubber tires - final design seems to be casted rubber tires.
Ryerson Rams:	2021: 3D printed PETG hubs and casted polyurethane treads for flexibility, shock absorption, traction
Stanford	2019: included nylon chains, inflated wheels
	2020: Wheels are still inflated, more traction(not explained in SAR) but it seems a (rubber?) textured piece has been attached to the outer surface of the wheel Small clip in SAR shows one wheel with a different surface? (side note they mentioned a “Grass hopper suspension”, distributes weight away from lowest handing wheel to glide/avoid obstacles/handle rocky terrain)
PCz:	2021: 3d printed 14 inch wheels for shock absorption and traction. Their design has similarities with the Curiosity rover’s treads:
Monash (9th place):	2019: beach tires
Monash (9th place):	2021: 3D printed wheels, individually driven- frame and treads are all 3d printed.

Research from NASA's rovers:

Nasa’s perseverance rover has wheels made of aluminum, cleats for traction curved titanium spokes for springy support. (20.7” diameter)

Change in wheels from curiosity to perseverance:

Curiosity began experiencing wear along the treads (6.4 mm thick) of the tires faster than expected. Furthermore, the aluminum tires of curiosity were milled to 0.75 millimeters thick which was thin enough to easily experience damage along sharper terrain.

Spring Tire Design from Goodyear:

Uses shape memory alloys which allow it to deform and then return to its original shape. This design is likely unattainable for our rover due to the special material it uses, but it is still an interesting design.

NASA Spring Tire Design:

Main conclusion (for now): many high level teams switched from inflated tires to 3D printed tires for their customizability and (seemingly) to emulate more of NASA's designs.

PROTOTYPING INFORMATION

Tread Adhesives (for prototyping/removable)

Adhesive name	Description	Cost
Silicone Sealant/glue	can be used for glass, metals, plastics water resistant, flexible can be removed with exacto knife and acetone (for excess remaining) any cuts made by exacto knife on wheel can be fixed with soldering iron equipment: caulking gun	~$20 for 11oz container
APAO Based hot melts	non crystalline adhesives that stay flexible and tacky good adhesion long open times can be used for woodworking, automotive assembly remove with rubbing alcohol, hot gun, scraper equipment: hot melt gun (glorified glue gun)	https://www.hotmelt.com/products/high-performance-product-assembly-apao-hot-melt-300-second-open-time requires a quote?
rubber cement	can be removed with heat and force: heat gun and scraping good long term option, used for outdoors (bike tire repairs) acetone can be used to remove any excess remaining note: acetone supposedly beads up on/does not affect polyurethane equipment: brush for application does suggest scuffing up the area first for better adhesion	~$9 for 8oz container mcmaster carr: plastic cement https://www.mcmaster.com/adhesives/for-use-on~polyurethane-rubber/cement-for-plastic/
other testing option	stanford used zip ties to attach the treads to their wheels would only use for prototyping

Tread Styles

Style Name	Photo	Notes
Symmetric		thick groove patterns, wear down evenly over time
Asymmetric		move in a single direction, most common type of tire tread
Directional (also called unidirectional)		highest level of traction, control designed to roll in only one direction
Michigan Mars Rover Team		highest SAR Score scored well in 2019 competition greatest diameter along center of wheel angled ridges/small paddles
PCz - zig zag pattern		4th place for SAR video shows strong traction on snow contact about material type?
RoverX		additional rubber (??) strips added, horizontal lines 10th place SAR
Anveshak		8th place SAR horizontal ridges
ATV sand tires		paddle like ridging to dig into sand generally smoother than traditional (car) tires printing time may be very long bc they stick out more, they could be harder to attach/could fall off
off set paddle tires		reduced weight maintains turning ability printing time may be very long bc they stick out more, they could be harder to attach/could fall off