Forge a tire by cutting a flat iron bar to the rim circumference, less the shrinkage allowance, plus pi times the bar’s thickness because the centre line of a bent bar is longer than its inner face, plus a 2-5 cm overlap for the weld; bend it into a circle, flux the scarfed ends with borax and forge-weld them at bright yellow-white heat. Test the weld by bending it and verify the finished hoop against the rim before it is ever heated for fitting.
Part of Wheelwright Craft
Forging a tire is cutting, bending and forge-welding a flat iron bar into a hoop whose inside circumference is a little less than the wheel rim.
The Hoop’s Length Is Set Before The Fire Is Lit
The bar must be cut once, and a hoop that comes out too long cannot be fixed by adding metal. Work it out on paper from the carrier’s rule: the tire is cut 1-2 cm shorter than the rim circumference, which is measured with a traveller.
Worked example: a rim of circumference 3,770 mm (a 120 cm wheel, assumed), shortage 15 mm (the middle of the 1-2 cm range), bar 10 mm thick (the carrier gives 5 × 1 cm).
| Step | Operation | Result |
|---|---|---|
| Hoop’s inside circumference | 3,770 - 15 | 3,755 mm |
| Centre-line length | 3,755 + π × 10 | 3,755 + 31 = 3,786 mm |
| Plus weld overlap (up to 50 mm) | 3,786 + 50 | 3,836 mm |
The π × thickness term comes from the fact that the centre line of a bar bent into a hoop sits half its thickness outside the inner face: the centre-line radius is r + t/2, so its circumference is 2π(r + t/2) = 2πr + πt. Metal Tires does not include that term. The overlap is a starting figure from that article, 2-5 cm; how much length a particular weld uses up is best found by welding one trial hoop from offcuts and measuring it.
The weight is worth knowing before you pick the bar up: 5 cm² × 377 cm × about 7.8 g/cm³ = about 14.7 kg. The density is a handbook figure for iron, recalled, not measured.
Bending Takes The Iron Past Its Elastic Limit
A bar 10 mm thick bent to a radius of about 600 mm has an outer-fibre strain of (t/2) / R = 5 / 600 = 0.83%. Iron yields at a strain near σ_y / E; with a yield stress of about 200-250 MPa and a modulus of about 200 GPa (both recalled handbook figures, not measured here), that is 0.10-0.125%. The bend is therefore six to eight times past yield, so the bar keeps its curve, and can be bent hot or cold by leverage over a post or a former. Bend hot if the bar is thick or the leverage short.
What You Need
- Flat iron bar, 5 × 1 cm for a cart wheel, long enough for the length above
- A forge that heats a bar end to bright yellow-white, an anvil, hammers and long tongs (Forge Setup, Forge Fuel)
- Borax flux, as Metal Tires specifies
- A bending post — an iron rod in the hardy hole or driven into the ground
- A traveller to check the finished hoop
Method: Bend, Scarf, Weld, True
Step 1: Cut the bar to the calculated length.
Step 2: Bend to a circle. Heat a section, lever it against the post, move along, and repeat until the bar forms a rough ring with the ends overlapping.
Step 3: Scarf the ends. Heat each end and forge it to a taper so that the two ends overlap into one thickness. Clean off scale.
Step 4: Weld. Flux the scarfs, bring both to a bright yellow-white heat close to sparkling, lay one on the other on the anvil and strike firmly and quickly. The weld is good only if the metal is clean and hot enough when struck.
Step 5: Test. Bend the weld slightly, as Metal Tires directs: a good weld bends, a cold shut (a seam that did not fuse) breaks. If it breaks, reheat and weld again, accepting that the hoop is now shorter.
Step 6: True the hoop. Heat and correct sections that are out of round. Roll the hoop on a flat surface; it should track without wobbling sideways. Then check its inside circumference against the calculated value before using it.
Common Mistakes
- Leaving out the π × thickness term — about 31 mm for a 10 mm bar, twice the whole shortage.
- A cold weld — a seam that looks joined and parts at the first hard bend or the first quench.
- Trusting a weld that was not tested — the weld is the tire’s weakest point and the first to crack under hard use.
- Welding the overlap too short — little metal to fuse; the weld pulls apart.
Safety
Forge welding throws sparks and carries heavy hot iron
At yellow-white heat the iron sprays scale and sparks, borax fumes irritate, and a 15 kg bar at forging heat is held with long tongs. Wear eye protection, leather gloves and apron, keep bystanders back, work in the open or well ventilated since a charcoal forge gives off carbon monoxide, and never carry the hoop alone. For burns, see Thermal Injury Care.
What’s Next
- Learn next: Hot Shrinking Process — fitting the tire you forged
- Combine with: Metalworking — forge-welding skills in general