Wheel, cart and road

Each part of a wheel takes a different load, and traditionally each had its own wood:

• Hub (nave) — elm, whose interlocked grain resists splitting when the spokes are driven in.
• Spokes — riven oak, split along the grain, never sawn across it.
• Felloes — the rim segments — ash or elm.

All of it seasoned. Green timber shrinks and slackens every joint within a season. See woodworking.

Mortise the hub for the spokes and set the spokes at a slight outward angle so the wheel forms a shallow cone — the dish. On a cart, the axle ends are angled down so the lowest spoke stands vertical under the load, and the dish makes the wheel resist the sideways thrust when the cart sways, instead of collapsing.

Fit felloes to the spoke ends, each spanning two spokes, dowelled together. Check the wheel is round and runs true — spin it on an axle past a fixed pointer before the tyre goes on.

The shrink fit is the entire trick. Measure the wheel's circumference with a traveller — a small measuring wheel rolled round the rim. Forge-weld an iron hoop slightly smaller — about half a percent shorter than the wheel's circumference.

Heat the tyre evenly in a ring fire to a dull red, so it expands just enough to go on. Drop it over the wheel, hammer it down, and immediately quench with water before it burns the felloes. As it cools it clamps the whole wheel into compression — joints tight, rim solid.

An ungreased wooden axle burns out, and axle failure far from home strands the load. Grease the axle arm with tallow, or tallow mixed with tar, before every journey, and carry grease.

At every stop, lay a hand on each hub. A hub too hot to hold a hand on means too little grease or a binding axle — stop and fix it before it chars.

A cart on soft ground is worse than a sledge. A road fails through water, not traffic.

• Crown the surface — higher in the middle — so rain runs off to the sides.
• Ditch both sides, with culverts under the road where water must cross.
• Metal the surface in layers with graded broken stone — small, angular pieces about 20–40 mm, which lock together under traffic — to a total depth of about 25 cm on firm, drained ground.

Inspect and clear the ditches after every heavy rain.

Ruts form regardless. If every cart in the settlement shares one track width, the ruts help each vehicle rather than destroying it and the road. Pick one width and make every axle to it — measure centre to centre of the wheels.

Keep a wooden gauge for it with the smith and the wheelwright, and check every new axle against it. This is the settlement's first interchangeability standard.

An unimproved crossing is where carts and animals are lost.

• Ford: a place with a firm gravel or stone bed, gentle banks, and water no deeper than a horse's knee at normal flow. Pave it with stone. Mark the depth on a post, and do not cross above the mark — moving water above knee depth sweeps away animals and carts.
• Bridge: timber beams for short spans, or a masonry arch. Build it above the highest flood mark.

A boat or barge moves roughly an order of magnitude more per unit of effort than a cart. A horse that pulls about a tonne on a good road can tow tens of tonnes on a canal. Rivers and canals were the heavy freight of every pre-rail society.

Where a navigable river or a canal route exists, move bulk goods — grain, stone, timber, fuel — by water, and use carts for the last kilometres. See rail for what comes later.