The leg section of the B9 robot consists of two vertical pillar-like tubes connecting the tread base to the torso. On the original prop, these were formed metal cylinders with surface detail added as separate appliqué pieces. For replicas, the leg section is typically one of the first major structural elements built after the tread base, since it establishes the robot’s height and the mounting points for the torso above.

Original Dimensions

The B9 leg section dimensions (approximate, based on reference photography and builder documentation):

  • Diameter: approximately 8–9 inches
  • Height: approximately 14–16 inches (varies by season — the suit was modified between seasons 1 and 3)
  • Surface: ribbed or ridged detailing running horizontally around the tube

Dimensions vary slightly between different builders’ measurements because no official blueprints have been publicly released. Cross-referencing multiple reference images is the best approach — compare against the actor’s known height to establish scale.

Construction Approaches

Approach 1: Fiberglass Layup

The traditional method for one-off and small-run builds.

Positive form construction:

  1. Build a cylindrical form to the leg diameter from cardboard, foam, or a purchased concrete form tube
  2. Add the ribbed surface detail to the form — carved foam strips, applied cord, or carved into a foam skin wrapped around the tube
  3. Wax the form thoroughly for release
  4. Apply polyester or epoxy fiberglass in 2–3 layers using woven cloth or mat
  5. Allow to cure, then cut the form out from the inside (score and collapse the cardboard or dissolve foam with acetone if using foam)

The result is a rigid hollow fiberglass tube with positive surface detail. Finish with body filler, primer, and paint.

Approach 2: Cast Urethane Over a Core

For builders who prefer casting over laminating:

  1. Build the leg form as above, then use it as a master to create a silicone mold
  2. Cast the leg shell in rigid urethane resin (Smooth-Cast 65D or Task 3 for impact resistance)
  3. For a hollow shell, use rotational casting technique (see advanced casting)
  4. Bond the cast shell to a PVC or aluminum pipe structural core

This approach is better suited for producing multiple identical legs (for multiple builders or replacement parts).

Approach 3: Carved Foam

The fastest approach but least durable:

  1. Cut large foam blocks or sheets to leg shape
  2. Carve or cut the ribbed surface detail
  3. Coat with FlexCoat, Plasti-Dip, or a glass-filled epoxy to harden the surface
  4. Prime and paint

Foam legs work for display builds but won’t hold up to regular handling or convention transport without additional hardening.

Surface Detail

The leg section has horizontal ribbing around the tube. Reference photos show approximately 6–8 ribs of varying widths, with the topmost rib being the widest.

Adding ribs to fiberglass:

  • Wrap rope or foam cord around the form before fiberglassing — the fiberglass captures the profile
  • Glue foam strips to the cured leg shell and cover with a skim coat of body filler

Adding ribs to cast urethane:

  • Incorporate the rib profile into the mold

Whichever method, the rib profiles should be smooth and consistent. Uneven ribs are visually obvious on the finished robot.

Structural Core

If the robot will be walked in or moved regularly, the leg section carries significant load. Build the structural core from:

  • Schedule 40 PVC pipe — Available at any hardware store, lightweight and easy to cut. Sufficient for static display builds.
  • Aluminum tube or pipe — Stronger than PVC, more expensive. Appropriate for walk-in builds or robots that get transported frequently.
  • Square steel tubing — Most rigid option. Used in full walk-in builds where the frame carries the weight of the person inside.

The structural core runs through the center of the leg shell and connects to the tread frame below and a mounting plate for the torso above. Get the alignment of both cores exactly vertical and parallel — a twisted torso on the finished robot is very difficult to correct later.

Attachment Points

Lower attachment (to tread): The leg cores bolt to mounting plates welded or bolted to the tread frame. Use heavy flanges with multiple bolts — this joint carries the full weight of everything above it.

Upper attachment (to torso): The torso mounts on a plate or bracket at the top of each leg core. The torso needs to rotate (on the original, the torso spins), so the leg-to-torso connection typically incorporates a lazy Susan bearing or turntable bearing.

Finishing

After structural assembly:

  1. Fill any surface imperfections with automotive body filler (Bondo)
  2. Sand progressively from 80-grit through 220-grit
  3. Prime with automotive high-build primer
  4. Block sand the primer for a perfectly smooth surface
  5. Spray the final color — the B9’s leg sections are typically painted in a silver-grey color (the original used automotive metallic paint)

For detailed guidance on the sections above and below the legs, see the torso construction guide and the B9 construction overview.

Frequently Asked Questions

What are the approximate dimensions of the B9 robot leg section? Based on reference photography and builder documentation, the leg section is approximately 8–9 inches in diameter and 14–16 inches tall, though dimensions vary slightly between seasons. No official blueprints have been publicly released, so builders cross-reference multiple reference images against the actor’s known height to establish accurate scale.

What are the three main construction approaches for B9 leg sections? The three approaches are fiberglass layup over a cylindrical form, cast urethane over a structural core, and carved foam. Fiberglass is the traditional method for one-off builds; cast urethane suits builders producing multiple identical legs from a mold; and carved foam is the fastest but least durable option, suitable only for static display.

How is the ribbed surface detail added to the B9 leg section? Reference photos show approximately 6–8 ribs of varying widths with the topmost rib being the widest. For fiberglass legs, rope or foam cord is wrapped around the form before laminating so the fiberglass captures the profile. For cast urethane, the rib profile is incorporated directly into the production mold.

What structural core materials are recommended for B9 leg sections? Schedule 40 PVC pipe is recommended for static display builds as it is lightweight and inexpensive. Aluminum tube or pipe offers more strength for frequently transported robots, and square steel tubing is the most rigid option used in full walk-in builds where the frame must carry the weight of a person inside.

How is the leg section attached to the tread and torso? The lower attachment bolts the leg cores to mounting plates on the tread frame using heavy flanges with multiple bolts, since this joint carries the full weight of everything above. The upper torso attachment typically incorporates a lazy Susan or turntable bearing to allow the torso to rotate, which is a key feature of the original B9 prop.

What finishing process is recommended for B9 leg sections? The article recommends filling surface imperfections with automotive body filler, sanding progressively from 80-grit through 220-grit, then applying automotive high-build primer and block sanding before the final color coat. The original B9 leg sections were painted in a silver-grey automotive metallic paint.