Sidecar Motorcycle

Overview

A sidecar motorcycle (a "rig" or "outfit") is a motorcycle with a one-wheeled passenger car bolted rigidly to its side, making an asymmetric three-track vehicle. The pattern was industrialised by the 1939 BMW R71 and its Soviet descendant, the Ural, which is still built to this layout: an air-cooled boxer Boxer Twin Engine, a double-cradle steel Motorcycle Frame, and a Sidecar Chassis carrying a steel Sidecar Body. A rig cannot lean, so everything a motorcyclist knows about steering stops applying the moment the third wheel touches down.

How it steers

A solo motorcycle turns by countersteering and leaning; a rig turns by brute steering torque through the wide Handlebar, like a car with one front wheel. The physics are asymmetric. Accelerating, the unpowered sidecar wheel lags and the rig yaws toward the sidecar; braking, it yaws away. Right turns (with the sidecar on the right) can lift the sidecar wheel, "flying the chair", because the car's mass is inside the turn; left turns instead try to lift the motorcycle's rear wheel under hard braking. Riders learn to use throttle as a steering input.

Geometry tames this. At fitting, the Attachment Fittings struts are set so the two wheel tracks converge 10–20 mm over the wheelbase (toe-in, shimmed with the Alignment Shim Set) and the motorcycle leans 1–2 degrees away from the sidecar (lean-out, set by the turnbuckle Upper Mount Strut). Toe-in counters the sidecar's drag; lean-out counters road camber. The sidecar axle also sits 250–350 mm ahead of the rear axle ("lead") via the Trailing Arm, which keeps the rig from pivoting around the car under braking. A friction Steering Damper suppresses the low-speed shimmy the geometry can excite.

Powertrain

The Cylinder Pair puts two finned cylinders straight into the airstream, which matters because a loaded rig works the engine hard at low road speed. The 749 cc twin makes about 30 kW, modest, but delivered as flat torque through a 4-speed Gearbox whose reverse gear, engaged by the hand Reverse Lever, exists because nobody paddles a 365 kg outfit backwards uphill. Final drive is the enclosed Driveshaft to a 4.62:1 bevel set; shaft drive suits sidecar duty since the chain-stretching torque reversals of rig riding would wear an exposed chain quickly. Twin Carburetors feed one cylinder each.

Up front, the Leading-Link Fork replaces the telescopic fork most solos use. Sidecar steering and braking loads bend telescopic sliders and make the steering heavy; the leading link resists those loads in its pivot and is usually built with reduced trail for lighter steering effort.

Wheels and brakes

The defining quartermaster detail is wheel interchangeability: all three Spoked Wheel positions and the spare on the Spare Wheel Carrier take the same 19-inch wheel with the same flat-tread Road Tire, so one puncture never strands the rig. Tires are flat-profiled because the vehicle corners upright. Braking combines a 295 mm Front Disc Brake with two Drum Brakes; the sidecar drum is plumbed into the foot circuit of the Foot Controls so the rig decelerates in line instead of slewing toward the unbraked side.

The sidecar itself

The Body Tub is a pressed-steel boat-tail shell on rubber mounts, with a sprung Passenger Seat, folding Sidecar Windscreen, and a trunk in the tail. Its wheel is independently sprung by the Sidecar Shock on the trailing arm, so passenger comfort does not depend on the motorcycle's Rear Shock. A Tonneau Cover covers the cockpit when running empty, an empty sidecar is light enough to fly annoyingly early in right-handers, and many owners ballast it.

Attachment is deliberately reversible: Frame Clamps bolt around frame tubes, the Lower Mount Struts carry the weight, and the rig can in principle be unbolted back to a solo. The Electrical System system duplicates lighting on the car, the Sidecar Lamp Set mark its full 1.6 m width at night, fed from the engine-driven Alternator and a 12 V 12 V Battery.