SG90 Ackermann Steering Rover Chassis

This is a 170 mm rover that steers like a car instead of skid-steering like a tank. Two steering knuckles turn on M3 kingpins and ride on 624 bearings. Their steering arms are angled 17.6° inward, so both arm lines meet at the centre of the rear axle. That is the Ackermann condition: the inner wheel turns tighter than the outer one and the front tyres don't scrub in corners. One inverted SG90 or MG90S moves a single trapezoid tie bar through a pin-in-slot drive, so the horn radius doesn't need to be known. Two N20 gearmotors, clamped back to back under the rear of the deck, drive the rear wheels separately, so a controller can act as an electronic differential. The deck has a Raspberry Pi Zero pattern and zip-tie slots for a 2x18650 holder. It's meant for anyone who wants to learn proper steering geometry or run car-like path planning (pure pursuit, bicycle models) on real hardware.

All 8 printed designs lie flat on the bed with no supports. The knuckle is symmetric top to bottom, so both sides use the same print: flip one over for the other side. The front beam and the rear motor carrier bolt under the deck on the same 44 x 24 mm M3 rectangle. Every height is set to one Z plan (wheel axle at z = 30), so the deck sits level.

Assembly order:

1. Press two 624 bearings into each front wheel, one from each face. Heat-set an M3 insert into each rear wheel hub.
2. Push the SG90 up through the beam's servo pocket from below so its tabs sit against the beam underside, and fix it with M2 screws in the tab slots.
3. Bolt the beam under the front of the deck with 4x M3x16 screws and nuts on the I1 rectangle. The servo body goes up through the deck cut-out.
4. Hang one knuckle under each beam ear on an M3x25 kingpin with a washer above and below and a nylock nut. Snug the nut until the knuckle swings freely with no play. Flip the second knuckle upside down so its axle points outward.
5. Drop an M4 nut into each knuckle's nut slot. Mount each front wheel on an M4x30 through a washer, the wheel and a second washer, and thread it into the nut.
6. Centre the servo (1500 µs), fit the double-arm horn pointing forward, and open the horn's outermost hole to 3 mm. Bolt the tie bar under both knuckle arms with M3x25 and nylocks. Drop the M3x20 horn pin through the horn into the tie bar's centre slot and lock it with two jam nuts.
7. Lay both N20 motors in the carrier channel with their shafts pointing outward. Close the cap, then clamp cap, carrier and deck together with 4x M3x30 and nuts on top of the deck.
8. Push the rear wheels onto the D-shafts and tighten the grub screws against the flats. Stretch rubber bands into the tyre grooves, then zip-tie the battery holder and screw the Pi Zero onto its standoffs.

Tuning: set straight-ahead with servo trim, not with the linkage. Limit steering to about ±25° in software. Past that the tie bar pin leaves the useful part of its slot. If the horn pin sits higher than the tie bar, stack M3 washers or nuts on the pin as a spacer rather than bending the linkage. The rear track (about 95 mm) is narrower than the front (130 mm) because it depends on the N20 length, which varies between suppliers. The Ackermann angle assumes the 120 mm wheelbase, so keep it.