Motofiches

Gloved hands grip bicycle handlebars while riding fast down a curving road

Steering — the piece to read first

Countersteering

Push the bar away from the corner and the machine falls in. The geometry is simple; why it feels backward is not.

Read the piece

Three layouts, three compromises

Engine as Layout

Close-up of a BMW Boxer motorcycle engine's finned cylinder head and cover badge

The flat twin

Cylinders in the airflow and mass carried low, at the cost of width and a torque reaction when you blip it.

Engine as Layout

A single-cylinder Ducati engine displayed on a stand against technical blueprint drawings

The L-twin

Narrow and long, which pushes the wheelbase out and puts the rear cylinder where the airflow is not.

Engine as Layout

Cutaway motorcycle engine display showing piston, cylinder, valves and timing gears

The inline four

Smooth and wide, and it sits high because the crank has to clear the ground.

Engine as Layout

All four engine pieces

Geometry

Trail is the distance behind the steering axis where the tyre actually touches the road. It is the reason the front wheel wants to point where you are going.

Rake and trail

The front end, and the numbers it moves

All twenty pieces

The telescopic fork

Almost universal and structurally poor: it bends under braking and its damping fights its own suspension duty.

Dive

The nose-down movement under braking is geometry changing while you are using it.

Alternatives that lost

Hub-centre and leading-link systems separate braking from suspension and keep failing commercially.

What a number hides

Static geometry is measured at rest; under braking the fork compresses and every figure changes.

Wheelbase

Long is stable and slow to turn, short is quick and nervous; almost every other decision is negotiated against it.

Weight distribution

Front-to-rear bias decides which end lets go first, and it changes with fuel load and a passenger.

An adult mechanic’s hands setting chain tension with a ruler against the swingarm

Chassis

Chain tension is a suspension setting

The chain pulls between two points that move relative to one another every time the swingarm travels. Set it tight at rest and the suspension has to fight it; set it slack and the drive snatches. The number on the adjuster is a chassis decision.

Three pieces on frames