To shrink-fit an iron tire, heat it evenly until it has grown by more than the length it was forged short, drop it over the felloes, drive it down and quench it at once with water. For a 120 cm wheel with a tire 15 mm short, the hoop must be heated about 330 K above ambient just to fit and around 500 K to leave workable clearance, which puts it red-hot and well above the temperature at which wood chars, so the water, the team and the clear area must all be ready before the tire leaves the fire.
Part of Wheelwright Craft
Hot shrinking is the fitting of a heated iron hoop over a wooden wheel and the quenching that makes the hoop contract and grip.
The Arithmetic That Sets The Limits
This article works with one example throughout: a rim 120 cm across, so a circumference of π × 1.2 m = 3.770 m, and a tire of 5 × 1 cm bar. The constants below are standard handbook values recalled from memory, not measured here, and they are listed at the end so they can be checked.
How much hotter than the wood must the tire be?
Iron grows by α × length × temperature rise, with α ≈ 12 × 10⁻⁶ per kelvin (recalled). For this tire:
- Growth per kelvin = 3,770 mm × 12 × 10⁻⁶ = 0.0452 mm/K (4.52 mm per 100 K).
| Rise above ambient | Growth in circumference |
|---|---|
| 200 K | 9.0 mm |
| 300 K | 13.6 mm |
| 400 K | 18.1 mm |
| 500 K | 22.6 mm |
To just fit, the growth must equal the shortage, so the temperature rise is the shortage divided by 0.0452 mm/K:
| Tire shortage | Rise needed to just fit |
|---|---|
| 10 mm | 221 K |
| 15 mm | 332 K |
| 20 mm | 442 K |
Take a shortage of 15 mm and a rise of 500 K, about 520 °C, which Temperature Reading lists as a dark red visible in dim light: the growth is 22.6 mm, so the slack is 22.6 - 15 = 7.6 mm of circumference. As radial clearance that is 7.6 / (2π) = 1.2 mm all the way round. That is the whole margin: the tire must be driven down evenly in seconds, which is why the team is organised as it is.
Two articles, two sets of numbers
Metal Tires forges the tire 4-15 mm short, depending on wheel size, and says the hot tire expands “3-8 mm”. Those two cannot both hold for the same wheel, because a hoop 6-12 mm short that grows only 3-8 mm does not fit at all. The expansion should be treated as a result to be calculated from the temperature, as above, and the carrier’s 1-2 cm is the larger end of the range of shortages. Use a small model wheel to settle the figure for your own iron and wood.
As the tire cools, when does it grip?
With 7.6 mm of slack at 500 K, the tire contacts the wood once it has cooled by 7.6 / 0.0452 = 168 K, so while it is still about 330 K above ambient, which is far above the temperature at which wood chars (roughly 250-300 °C, recalled). So the tire sits on the wood and burns it unless the quench starts immediately.
What force does the shortage produce?
If the wood were perfectly rigid, the whole 15 mm shortage would have to be taken up by stretching the tire:
- Strain = 15 / 3,755 = 0.40%
- Stress = E × strain = 200,000 MPa × 0.00399 = about 800 MPa (E ≈ 200 GPa, recalled)
That is more than iron can carry, since wrought iron yields near 200-250 MPa (recalled). The tire can only stretch elastically by about 0.10-0.125% of 3,755 mm, 3.8-4.7 mm, before it yields. The other 10-11 mm of the 15 mm must therefore be taken up by the wood compressing and the joints closing.
At yield, the tension in the 5 × 1 cm bar (500 mm²) is 200-250 MPa × 500 mm² = 100-125 kN, and the pressure on the rim is the tension divided by the radius and width, 100,000 N / (0.6 m × 0.05 m) = 3.3 MPa up to 125,000 / 0.03 = 4.2 MPa. That is in the range at which oak or ash start to crush across the grain (roughly 5-10 MPa, recalled from the US Forest Products Laboratory’s Wood Handbook). It explains the rule in the carrier that a tire too tight crushes the felloes: the margin between a grip and a crushed rim is small, and set mostly by the shortage you forged.
How much steam?
The tire’s mass is 5 cm² × 377 cm × about 7.8 g/cm³ = 14.7 kg. At a specific heat near 0.48 kJ/(kg·K) (recalled) and 500 K above ambient it holds 14.7 × 0.48 × 500 = about 3,500 kJ. If all of that went into boiling water from 20 °C (334 kJ/kg to reach 100 °C plus about 2,257 kJ/kg to evaporate, together 2,591 kJ/kg), up to 3,500 / 2,591 = 1.4 kg of water would become steam, about 1.4 × 1.67 m³/kg = 2.3 m³ at atmospheric pressure. That is an upper bound, since not all the heat goes into evaporation, but it shows the scale of the cloud around the people who are pouring. Absorbing the same heat without boiling needs 3,500 / (4.18 × 20) = about 42 litres of water per tire for a 20 K rise, and more for the water that is splashed away.
What You Need
- A wheel, fully dry-fitted and checked, joints tight, spokes seated (Jointing & Doweling)
- A forged, tested tire (Forging the Tire)
- A tire-heating fire that surrounds the tire, or a forge (Forge Fuel)
- A firm, flat platform for the wheel, with the hub supported, felloe side up
- Tongs or hooks, one set per carrier, long enough for hands to be at arm’s length
- Water: a trough, buckets and ladles in quantity, filled and in reach before the fire is lit
- Hammers and levers to seat the tire
- Protective gear: leather gloves and apron, eye protection, closed boots, hair tied back
- A team: at least two experienced hands for a medium wheel, four for a large one (Metal Tires)
Method: The Sequence That Keeps People Clear
Step 1: Rehearse cold. Before lighting the fire, run through lifting the cold tire, carrying it to the wheel, setting it down and pouring, so that every person knows their place and the signal to start.
Step 2: Clear the area. Keep onlookers, children and animals well away from the fire and the platform, outside the reach of tongs and of a tire that might be dropped.
Step 3: Water first. Fill and place the water before the fire is lit. A tire in the fire with no water at hand is already late.
Step 4: Heat evenly. Heat the whole hoop to an even red heat (Metal Tires says medium red); judge the colour in shade. An unevenly heated tire distorts and does not fit.
Step 5: Lift on signal. On the call, the carriers lift together at opposite sides and carry the tire at a walk. Nobody runs and nobody holds it alone.
Step 6: Lower square. Lower the tire level onto the felloes from above and let it drop into place. Keep everybody’s feet and hands outside the tire’s ring while it moves.
Step 7: Seat and quench together. Drive the tire down evenly round the wheel with hammers and levers from several sides at once, while the water is poured on the joint between tire and felloes all the way round, not on one spot. A single stream cools one section first and pulls the tire out of true.
Step 8: Keep pouring. Keep pouring until the steam has died down and the tire no longer sizzles. The parent article says to quench “from all sides”.
Step 9: Cool and inspect. Leave the wheel where it is until the iron can be touched. Then inspect the tire, felloes and joints for gaps, cracks and scorched wood.
Common Mistakes
- Taking the tire out before the water is ready — the tire burns the wood while the crew improvises.
- Heating hotter than needed — more than about 500 K above ambient gives more slack, but also hotter iron on the wood and more steam.
- Pouring on one spot — uneven cooling distorts the hoop.
- Standing inside the ring or over the wheel — a tire that cocks or drops lands on feet and legs.
- Letting a jammed tire be forced by body weight — use levers and hammers from outside the ring.
Safety
Red-hot iron, scalding steam and a burning wheel
The tire is about 15 kg of iron at several hundred degrees above ambient, handled by hand-held tongs; it can be dropped, can jam and kick when partly on, and can ignite the felloes. Quenching it produces a cloud of steam up to about 2 m³, which scalds on contact. A tire that is too tight can crack the felloes with a bang and fling splinters. Work only with a trained team, with all water in place before the fire is lit, a cleared area and full protective clothing. If a burn occurs, cool it at once with cool running water and see Thermal Injury Care and Severity Assessment.
Constants recalled, not measured
The arithmetic is first-principles, but these inputs are recalled handbook values: expansion coefficient of iron about 12 × 10⁻⁶ per K; modulus about 200 GPa; yield about 200-250 MPa; density about 7.8 g/cm³; specific heat about 0.48 kJ/(kg·K); latent heat of water about 2,257 kJ/kg; steam about 1.67 m³/kg; wood charring around 250-300 °C; crushing of oak or ash across the grain about 5-10 MPa. Check them against a handbook before using the results as design numbers.
What’s Next
- Learn next: Metal Tires — another account of the fit, with a different allowance
- Combine with: Wheelwright Craft — the finished wheel and its repair