Steer a four-wheel wagon by mounting the front axle on a circular wooden bolster, a fifth wheel, with a vertical king pin through its centre into the wagon frame, so the whole front axle swings around the pin when the driver pulls the reins or tiller. For a wagon with a 2 m wheelbase, a turning circle of 8-10 m implies roughly 24-30 degrees of front-axle lock, which the front wheels must clear, and the king pin must be secured against loss.
Part of Cart & Wagon Construction
A fifth-wheel steering pivot is a turntable under a wagon’s front end that lets the whole front axle swing around a central king pin.
Turning Means Swinging The Whole Axle
A two-wheel cart pivots on its axle and needs no steering. A four-wheel wagon needs one axle to turn. The simplest way, given in the parent article, is the bolster pivot (fifth wheel): the front axle assembly sits on a circular wooden disc, a vertical king pin runs through the centre of the disc and into the wagon frame, and the entire front axle rotates about that pin.
Four-Wheel Wagon builds the same arrangement in iron: a lower plate on the front axle assembly, an upper plate under the reach, a king pin of 25-35 mm through both, and a chain or strap to limit the rotation. It calls the king pin the most critical connection, because if it fails the front axle separates from the wagon.
How Much Lock Does A Given Turning Circle Need?
When the front axle swings through an angle δ from straight ahead, the wagon turns about a point on the line of the rear axle. The king pin, at the centre of the front axle, travels on a circle of radius R = L / sin δ, where L is the wheelbase, the distance from king pin to rear axle.
The parent article says that for a 2 m wheelbase a turning circle is about 8-10 m across. If that is the circle the king pin follows (an assumption; the article does not say what it measures), then R is 4-5 m:
| Circle diameter | R | sin δ = L / R | Lock δ |
|---|---|---|---|
| 8 m | 4 m | 2 / 4 = 0.50 | 30° |
| 10 m | 5 m | 2 / 5 = 0.40 | 24° |
So the front wheels need to be able to swing through about 24-30° each way without touching the body or the frame. The rear axle’s centre follows a smaller circle, of radius L / tan δ, which is 3.5 m at 30° and 4.5 m at 24°, so the rear wheels cut inside the front ones.
Smaller front wheels help, as the parent says, because they leave more room for the lock under the body. If the front wheels foul the body, the turn is limited by the clearance, not by the steering, and a lock stop (a chain or a block) must prevent the wheels from turning under the wagon.
What You Need
- A bolster disc, hardwood, large enough to be stable, or an iron plate pair (Frame & Bed Construction for the frame)
- A king pin, iron if possible, with a retaining pin or cotter at the bottom
- A stop chain or block to limit the turn
- A pole or tongue fixed to the front axle assembly, and reins or a tiller
- Grease for the turning faces (Lubrication & Wear)
Method: Build And Test The Pivot
Step 1: Mount the lower bolster on the front axle assembly, centred between the front wheels.
Step 2: Mount the upper bolster on the underside of the frame, so the two faces rest on each other.
Step 3: Drill both for the king pin on a single line. A pin through misaligned holes binds.
Step 4: Fit the pin and secure it at the bottom with a retaining pin, so it cannot lift out or drop.
Step 5: Fit the stop so the front wheels stop at least a finger’s width short of the body at full lock.
Step 6: Test. Turn through full lock both ways with the wagon empty, on level ground. The pivot should move smoothly and the wheels should clear everything.
Step 7: Load test at half load, then full load, before the wagon goes on a slope.
What The King Pin Carries
The vertical load goes through the bolster faces, not the pin. The pin carries the horizontal pull of the animals and any lifting of the front axle. Take an assumed pull of two horses at 80 kgf each (Terrain Assessment uses 80 kgf per horse): 2 × 80 × 9.81 = 1,570 N, on a 25 mm pin, shear area π × 25² / 4 = 491 mm². Stress is 1,570 / 491 = 3.2 MPa. The pin is loaded lightly in steady pulling; the danger is loss, wear of the holes, and a snatch when a wheel drops into a hole and the axle assembly tries to lift.
Common Mistakes
- No stop — at full lock the wheels turn under the wagon body and jam or tip it.
- A king pin with no retainer — it works out on a rough road and the front axle is lost.
- Dry faces on the bolster — they grind and bind.
- Misaligned holes — the pivot sticks and the animals pull the wagon sideways.
Safety
Loss of steering on a slope
A wagon whose front axle jams or separates cannot be steered, and a loaded wagon is a crushing hazard on any slope. Keep a sprag or chock to hand, never stand in front of the front wheels or beside the pole, and do not take a new pivot down a hill until it has run at full load on the flat.
What’s Next
- Learn next: Wagon Bed & Side Boards — the body on this frame
- Combine with: Four-Wheel Wagon — the complete wagon with brakes