MechBase Rolling Chassis

Step 1: Baseplate

Set down the 32×32 baseplate on your work surface. Orient it so the longer side runs left to right in front of you.

Step 2: Main Frame Rails

Take two 2730 bricks (1×10 with holes). These form the two long parallel rails of the chassis frame.

Rail A (rear): Place one brick horizontally near the rear third of the baseplate. The Technic holes should face upward. Position it roughly centered left-to-right on the baseplate.

Rail B (front): Place the second brick parallel to Rail A, near the front third of the baseplate. The two rails should be about 8 studs (64mm) apart center-to-center. Visually check: there should be about 8 rows of baseplate studs between them.

Press both bricks firmly onto the baseplate studs.

Step 3: Cross Braces

Take two more 2730 bricks (1×10). These connect the two rails at each end, forming a rigid rectangular frame.

Rear cross brace: Place one brick perpendicular to the long rails at the rear end. It should span between Rail A and Rail B — one end resting on Rail A, the other on Rail B. The brick is exactly the right length to bridge the 8-stud gap between the rails with slight overlap on each side.

Front cross brace: Place the second brick the same way at the front end, spanning between the rails.

Insert 2780 pins (with friction ridges) at each corner where a cross brace meets a long rail. Push each pin down through the overlapping Technic holes so the friction ridges grip both bricks together. Use 4 pins total — one at each corner.

Step 4: Axle Mount Brackets

Take four 3701 bricks (1×4 with holes). These form the axle mount brackets that extend outward from each rail at both ends.

Rear mounts (2 bricks): At each end of the rear cross brace, place a 3701 brick perpendicular to the long rail, with one stud overlapping the rail. The brick extends outward from the frame edge — the outermost hole will serve as the axle bearing point.

Front mounts (2 bricks): Repeat at the front cross brace — one 3701 extending outward from each rail end.

Lock each mount brick to its rail with a 2780 pin through the overlapping hole. Use 4 pins total.

Step 5: Motor Mount Brackets

Take the two 3894 bricks (1×6 with holes). These provide motor mounting positions alongside the rear axle.

Place each brick parallel to the long rails, positioned just outside the rear axle mounts. They serve as anchor points for zip-tie or tape motor mounting. Each 3894 runs alongside the rear half of a long rail, starting about 2 studs forward from the rear cross brace.

Lock each with a 2780 pin where it overlaps the long rail. Use 2 pins.

Step 6: Rear Drive Axles

Take the two 3737 axles (10L, 8mm diameter, ~80mm long).

Left rear axle: Slide one axle horizontally through the left side of the frame. Thread it through the outermost hole of the rear-left 3701 mount brick, across the frame gap, and through the outermost hole of the rear-right 3701 mount brick. Center the axle so it extends about 8mm past each mount brick for wheel clearance.

Lock it in place: Insert a 32062 (2L Notched) into each mount brick's hole adjacent to where the main axle passes through. The notch should face UP — this acts as a shoulder that prevents the axle from sliding out sideways.

Right side (if building a dual-axle setup): Repeat for a second axle position. For a single rear axle, skip to Step 7.

Step 7: Front Caster Axle

Take one 32073 axle (5L, 8mm diameter, ~40mm long). This is shorter than the rear axle — that's intentional since the front is passive.

Slide it through the front axle mount bricks the same way as the rear axle. Center it so it extends about 4mm past each mount brick.

Lock with two 32062 (2L Notched) axles in the adjacent holes, notches facing UP.

Step 8: Drivetrain — Gear Mounting

How the drivetrain works: Each TT motor has a 3mm output shaft. A 12T pinion gear (69778) presses directly onto that 3mm shaft. The pinion meshes with a 36T double-bevel gear (32498) mounted on a separate 3mm axle. The 36T gear turns at 1/3 the speed of the pinion — this is a simple gear reduction, 3:1 ratio.

Important: The 8mm frame axles (Step 6) are structural only. The gears have 3mm bores and mount on 3mm shafts. Here's the layout for each side:

Motor 3mm shaft → 12T pinion (69778) meshes with → 36T gear (32498) on 3mm axle → wheel

Install the 12T pinions: Press one 69778 onto each TT motor's 3mm output shaft. The gear should sit flush against the motor gearbox housing. Teeth point outward, away from the motor body.

Install the 36T wheel gears: Each 32498 mounts on a 3mm axle. The double-bevel design means teeth on both faces — orient it so one face meshes with the corresponding 12T pinion. The center distance between the 12T pinion and 36T gear must be exactly 3 studs (24mm) — both are Group B gears so they mesh correctly at this spacing.

Mounting the 3mm wheel-drive axles: The 36T gear sits on a 3mm axle that runs through a hole in the frame mount bricks. Position each 3mm axle so the 36T gear meshes smoothly with its 12T pinion. The axle extends past the gear for wheel mounting.

Step 9: Wheel Installation

Rear wheels (drive side):

On each 3mm wheel-drive axle (the one carrying the 36T gear):

a) Slide one 4265c (Bush ½ Smooth) onto the axle past the 36T gear. The bush acts as a spacer — it prevents the wheel from rubbing against the gear teeth.

b) Press one 56908 wheel onto the axle past the bushing. The wheel has a 3mm bore that grips the 3mm axle. The wheel already includes its racing tire.

Order from inside out: 36T gear → 4265c bush → 56908 wheel

Front wheels (caster side):

On the 8mm front axle (32073):

a) The 56908 wheel has a 3mm center bore. Since the front axle is 8mm, you'll need an adapter — see note below.

Step 10: Motor Mounting

Each TT Micro Motor (37mm) specifications:

• Diameter: 37mm (~4.6 Technic studs)
• Output: 3mm round shaft, protruding ~8mm from gearbox
• Voltage: 3-6V DC nominal (run at 7.4V from LiPo via Adafruit HAT)
• Shaft height from motor base: ~15mm (~2 brick heights)

Positioning: Mount each motor so its output shaft (with the 12T pinion already pressed on) meshes with the corresponding 36T wheel gear. The motor shaft center must be exactly 3 studs (24mm) from the 36T gear center.

Mounting method: Use the 3894 motor mount brackets as anchor points. Secure each motor with zip ties or double-sided tape. The motor body sits alongside the frame rail, with the output shaft reaching across to engage the pinion.

Verify: motor shaft height aligns with the 12T pinion teeth, and the pinion meshes smoothly with the 36T gear.

Step 11: Electronics Platform

The Raspberry Pi 4 and Adafruit Dual Motor HAT need a stable, flat mounting area. Build this in the front-center of the chassis, forward of the front axle mounts.

Mounting: The Pi 4 measures 85×56mm (~10×7 studs). Use zip ties to secure it to the baseplate in the front area. The Adafruit Dual Motor HAT stacks directly on top of the Pi 4's GPIO header.

Wire routing:

• Motor A wires → Left TT motor (2 wires: power + ground)
• Motor B wires → Right TT motor (2 wires: power + ground)
• Route wires along the inside of the frame rails, between the rails
• Wire length needed: ~20cm per motor (measure before cutting)

Power: Connect the 7.4V LiPo battery to the Adafruit HAT power input. The HAT supplies regulated 5V to the Pi 4 via GPIO. Secure the battery in a rear corner, away from spinning wheels.

Step 12: Final Verification

1. All 16 pins (2780) inserted — frame is rigid with no wobble.
2. Rear axle(s) rotate 360° without obstruction.
3. Front axle rotates 360° without obstruction.
4. Both 12T pinions mesh with their respective 36T wheel gears at 3-stud center distance.
5. All 4 wheels spin freely and do not touch the frame.
6. Bushings (4265c) between each 36T gear and wheel on the drive side.
7. TT motor shafts engage 12T pinions at correct height.
8. Motor wires reach from Adafruit HAT to both motors.
9. LiPo battery connected and secured, away from wheels.
10. Pi 4 boots and Adafruit HAT is recognized (i2cdetect shows 0x60).