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Things to consider:

  • Roll Cage

  • Impact Force/Strength

  • Attachment requirements

  • Stiffness

  • Tube-Tube Attachment Type

  • Manufacturing Type

  • Price

Tube Profiles

  • Square

  • Cylinder

  • Rectangle (volumetrically inefficient)

  • I-Beam (unidirectional applications)

Tube Types

  • Hollow

  • Solid (too heavy)

Steel Beam Analysis

Square (10 in of 1x1x1/16in)

Round (10 in of 1in D x 1/16in t)

Benefits

  • Probably easier to mount things onto (brackets, fasteners, subsystem integration, etc.)

  • Moment of inertia (x/y) = 0.0345 (square) vs. 0.0203 (round)4

    • ~52% Stiffer as a beam (vertical/horizontal deflection)

      • for equivalent diameter

    • ~29% Stiffer in torsion

      • for equivalent diameter

  • Easier to clamp for welding or cutting3

  • Moment of inertia = 0.0203

    • ~27% lighter (~27% less volume of metal)

      • for equivalent diameter1

  • Complex curvature is easier to manufacture (roll cage)2

  • Better disperses the stresses, no real weak points from corners, consistent strength. This quality makes it easier to design with due to no orientation limits.

  • Cheaper/ft.

  1. When designing to optimize strength to weight ratio, making a tube diameter larger before making it thicker will have a higher strength increase and lower weight increase than making it thicker. Observation from site: Tube Size vs Strength | Rorty

  2. Tubing formability can be used to our advantage when designing the chassis. In areas where multiple, smaller square tubes would need to be welded together, a round tube could accomplish the curvature required in one constant bend.

3. This is an assumption made on personal experience, although I am sure jigs and clamps exist to bind both types of tubes very easily. I have seen the use of a hole saw on a drill press to make chassis member cutouts, and to hold the tube in place is what is being referred to above.

4. The stiffness claims and moments of inertia account for two beams compared at the same dimensional sizes. Pound-for-pound, a round tube out performs a square tube in strength and stiffness (but it would obviously have to be larger in some dimensions).

MSXVII Chassis Design Page:https://uwmidsun.atlassian.net/l/c/zfCy6M90

Recommendation for MSXV: Ideally a hybrid of both tubes could be used to maximize the benefits of both types. If a compromise of both cannot be made and it’s one or the other, asses the benefits of both based on the team’s goals/priorities, and the closest tube to achieving our goals should be picked (e.g. lightest = round, easiest to integrate with other subsystems = square).

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