Bend a felloe by steaming the blank in a box fed by boiling water for about 1 hour per 2.5 cm of thickness, then clamping it to a curved form within 30-60 seconds of taking it out, and leaving it on the form for at least 24 hours to cool and dry. A 5 cm thick felloe therefore needs about 2 hours in the steam, and the form must be built with a little extra curve to allow for spring-back.
Part of Wheelwright Craft
Steam bending softens a felloe blank with hot steam so it can be clamped to a curved form and keep its shape once it dries.
The Bend Has To Be Finished Before The Wood Cools
The carrier gives the sequence: build a steam box connected to a boiling water source, steam the blank for approximately 1 hour per 2.5 cm of thickness, remove it quickly, clamp it to a form matching the wheel’s radius, and let it cool and dry on the form for at least 24 hours. Steam Bending adds that the whole bend must be completed within 30-60 seconds of removal, before the wood cools and stiffens, and that spring-back should be allowed for by overbending 5-10%.
Worked Example: A 60-Degree Felloe
Take a 5 cm deep ash felloe (assumed) on a wheel of 55 cm inner radius (assumed, 60 cm outer less the 5 cm depth), covering 60° (a 12-spoke wheel; Felloe Construction).
- Steaming time: 5 cm / 2.5 cm × 1 hour = 2 hours.
- Sagitta of the target curve (inner face): 55 × (1 - cos 30°) = 55 × 0.134 = 7.4 cm over a 55 cm chord.
- Overbend by 5-10%, applied to the depth of the curve: 7.4 × 1.05 = 7.8 cm up to 7.4 × 1.10 = 8.1 cm. Build the form to the larger curve.
- Radius against thickness: 55 cm / 5 cm = 11. The steam-bending article advises a metal strap on the outside of the bend when the radius is under 15 times the thickness (here 15 × 5 = 75 cm), so this bend wants a strap with end blocks.
Sources differ on steaming time
Spoked Wheels gives 30-60 minutes per centimetre of thickness, which for 5 cm is 2.5-5 hours, against 2 hours here. The carrier and Steam Bending agree on 1 hour per 2.5 cm, which is the figure used here. Steam longer rather than shorter if the wood resists; Steam Bending notes an extra 15-20 minutes does no harm.
What You Need
- Ash blanks, green or air-dried, straight-grained and knot-free
- A steam box with a vent and a drain, and a boiler, as in Steam Bending
- A bending form — a curved template matching the wheel, a little over-curved for spring-back
- A metal strap and end blocks for the outside of the bend
- Clamps or wedges, enough for every 20 cm of the felloe, ready before the box is opened
- Heavy leather gloves
Method: Steam, Bend, Hold
Step 1: Heat the box Bring the water to a rolling boil and let steam flow through the box until it vents freely.
Step 2: Steam the blank Load the blank on supports so the steam surrounds it. Steam for the time calculated above.
Step 3: Bend at once With the form and clamps ready, take the blank out, put the strap on its outer face, lay it on the form and clamp from the middle outward in one continuous motion. Work in less than a minute.
Step 4: Let it set Leave it on the form for at least 24 hours. Longer is better, because it will spring back a little.
Step 5: Check Release it and test the curve against the pattern. If it has sprung back too much, make the next form tighter.
Step 6: Drill After drying, drill for the spoke tenons at the correct positions.
Common Mistakes
- Under-steaming — the wood cracks on the outside of the bend because the middle is still stiff.
- Slow bending — the felloe stiffens before it is on the form and splits.
- No strap on a tight bend — the outer face tears.
- Forgetting spring-back — the felloe opens up and the circle comes out too large.
Safety
Steam and hot wood scald
Open the steam box with the door turned away from your face; steam at 100 °C scalds on contact. The steam box must vent, as a sealed box builds pressure and can burst. Never let the boiler run dry, wear gloves, and keep helpers clear of the swing of a springing felloe.
What’s Next
- Learn next: Jointing & Doweling — joining the bent felloes
- Combine with: Lamination and Bent Lamination — an alternative way of making a curved rim section