Previous Years Research

Owned by
mapitzgr@uwaterloo.ca
Last updated: Apr 25, 2021
4 min read

4-Wheel (current) VS Rocker Bogie

6-Wheel Rocker Bogie Design

Pros:

      • less ground pressure
      • requires less friction
      • better cornering
      • better at handling uneven terrain
      • minimize drag and reduce lift
      • better safety (if one tire blows, less likely to lose control)
      • increased stopping power
      • less unsprung weight
      • lower profile of front wheels (more aerodynamic)
      • Climbs large obstacles (compared to wheel size) with all wheels in contact with floor (works at slow speeds)
      • Rocking of the rover body is the average of the side movements.
      • Less susceptible to side tipping
      • No springs needed
      • Equal force distribution on wheels.
      • Extensive documentation

Cons:

      • heavier
      • more complicated steering, drivetrain, suspension
      • Requires 1 motor per wheel
      • Unstable at high speeds (damage to arms, suspension speed) and less effective traction.

4-Wheel Design

Pros:

      • lighter
      • simpler design

Cons

      • (pretty much all the Rocker Bogie Pros) 


Specifics to Optimize (for either design):

    • Minimize Weight
    • Maximize torque and speed
    • Maximize Strength
    • Maximize ground clearance
    • Minimize Maintenance complexity

Resources for bogie and 4-wheel

Beach Tire Information

Tire Constraints

    • Off The Shelf beach tires
    • Tire unit cost under $100
    • Diameter between 8” - 12” (larger sizes make it difficult to pursue a 6 wheel design)
    • 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

Tire Requirements:

    • Cheaper is better
    • Lighter is better
    • Larger ID is preferred (provides larger hub motor options, but should not exceed 10” as it leaves too “little” tire width for suspension)
    • Larger maximum load is better (rover drop may result in rover landing on 1 wheel first)

Design Solution #1: Custom Wheel Hub replacing exterior / rotating component of hub motor:

    • Most design intensive solutions, but also may provide the best outcome as we have the most control over design. Also the cheapest if we don’t account for sponsored machining costs.
    • Design essentially replaces the rotating exterior hub of the hub motor  with a custom machined hub motor with matching inner diameter.
    • Permanent magnets glued to the exterior shell will need to be removed and secured to the custom hub identically to the off the shelf wheel hub.
    • This should be validated with a 3-d printed mock-up first, prior to machining.
    • 5.5 in Hub Motor source

Design Solution #2: Purchase a hub motor appropriate for a beach tire:

    • One particular hub motor appears to have the appropriate outer diameter for some balloon tires. Need to validate with actual tires and hub motor (or 3-d printed mock-up of hub motor)
    • Tireless Motor source

Design Solution #3: Purchase a hub motor / tire assembly:

    • Torque speed curve for 60V motor model. Given that torque is linearly proportional to voltage, a 24V would give around 12 N*m at stall current, which is sufficient for a 6-wheel rocker bogie drive train.
    • Integrated Tire Motor source


Google drive folder with a 8-page ish study on deformation of the beach tires

Carbon Fiber

Below point notes highlight key differences between Aluminum and Carbon Fiber qualitatively, according to this article.

    • CF composite has anisotropic material properties (harder analysis), aluminum does not
    • CF composites mechanical properties are heavily dependent on manufacturing methods (i.e. roll-wrap v.s. Epoxy infusion v.s. pultrusion)
    • CF composite has better strength to weight ratio than aluminum. For example, at 70% fiber 30% resin, a CF part is almost 55% weight of an identical aluminum part, while being more rigid and having ~60% more strength.
    • Under severe weight limitations, CF components can fulfill geometry which better resists certain loads (think I beams resisting bending), while aluminum cannot due to density
    • CF composite undergo brittle failure (fail due to normal stress) while aluminum plastically yields (fail due to shear stress)
    • CF composite has low density, making it easier to machine than aluminum (not applicable in our case because we want to minimize machining)
    • Instead of welding, CF would be bonded using epoxies that provide bonds comparable in strength to welding. Can glue without lots of equipment, can’t weld without a welder.
    • CF composite has 6 X times lower thermal expansion (negligible for the service temperatures we are dealing with) than aluminum. Important if things get hot, prevent stress due to geometry change in components.
    • CF composite has 40 X lower heat conductivity (not super relevant in our case) than aluminum
    • Normal CF composites (what we will be dealing with) do not have high service temperature (resin curing / baking related reasons), different for specialized CF composites. Consider alternatives if service temperature exceeds 70 Celsius.
    • CF composites and aluminum are both resistant to corrosion. (not difference but worth mentioning)
    • CF composites have reduced resistance to UV (UV does nothing to aluminum, while probably speed up break-down of resin in composite). Use UV resistant coating.
    • CF composites typically are harder to design with, cost more to produce and manufacture than aluminum

Extra Notes About CF Composites

    • Anisotropic material properties in composites result due to the orientation of carbon fibers within an epoxy matrix (epoxy (glue) is cured around fibers to form shapes)
    • Different strengths can result from layout (i.e. pultruded tubes are stiff length-wise, making it good against bending, while weak against crushing loads perpendicular to fiber directions) 
    • In some cases, harden steels, carbides  (i.e. tooling steel) has better strength to weight than carbon fiber, but it isn’t selected due to design geometry (something has to be of a certain thickness) or manufacturing limitations (hard to machine)
    • Carbon fiber offers 2 to 5 times (really depends on specific fiber layout, fiber - epoxy combination) more rigidity (resistance to bending moment) versus aluminum. Pultruded CF in uniaxial loading parallel to fibers can achieve 5 to 10 times more stiff compared to aluminum.
    • Specific strength measures stress/density at failure point, Specific stiffness measures elasticity(elastic modulus)/density.
    • Epoxy Resin Infusion: Uses vacuum to force fibers together, then fills void with epoxy to maximize fiber/resin ratio, maximizing strength.
    • Roll-wrap: Prepregged (infused with resin) sheets of CF are wrapped around a shaft, cured, and then the shaft is pulled out.
    • Pultruded: Rolls of continuous fibers extruded by a machine, infused with epoxy, and then cured and cut to length (think of extruded aluminum, similar workflow)

This is a cool video that shows the difference between steel’s ductile and CF composite’s brittle failure modes. Notice how under torsion, CF breaks pretty cleanly at 45 degrees while steel twists (doesn’t snap).

Examples of Fittings for Carbon Fiber

URC Teams that use Carbon Fiber and Rocker Bogie (2020 and maybe 2019 videos)