Load Capacity
Part of Roads and Transport
How to calculate the maximum load a vehicle, animal, or road can safely carry.
Why This Matters
Overloading is one of the fastest ways to destroy a vehicle, injure a draft animal, and damage a road. A wheel that shatters under too much weight leaves your cargo stranded, possibly in a dangerous location far from help. A draft horse pushed beyond its sustainable pull capacity can suffer permanent tendon damage, removing a critical working animal from service. A soft road section that receives loads beyond its bearing capacity develops ruts and potholes that worsen exponentially with each subsequent vehicle.
Understanding load capacity is not theoretical — it is practical safety engineering. When your community depends on a single wagon and two draft animals to move grain from farms to storage, the ability to load that wagon correctly can be the difference between a successful harvest and a catastrophe.
Beyond safety, proper load calculations allow you to optimize efficiency. Underloading wastes draft animal energy and travel time. Knowing the true maximum allows you to approach it confidently without guesswork. Over generations, communities that understood these limits built better roads, bred better animals, and designed stronger vehicles — giving them decisive advantages over those that did not.
Animal Pull Capacity
The foundation of load calculations is the draft animal. Every vehicle load depends on what the animal can actually move.
Sustainable pull force by animal type:
| Animal | Body Weight | Sustained Pull (8 hr) | Peak Pull (short burst) | Walking Speed |
|---|---|---|---|---|
| Heavy horse (draft breed) | 700-900 kg | 90-110 kg | 250-350 kg | 5-6 km/h |
| Light horse (riding breed) | 400-550 kg | 50-70 kg | 150-200 kg | 6-8 km/h |
| Ox (mature bull) | 500-700 kg | 70-90 kg | 200-280 kg | 3-4 km/h |
| Donkey | 150-250 kg | 25-35 kg | 70-100 kg | 4-5 km/h |
| Mule | 350-550 kg | 60-80 kg | 170-230 kg | 5-6 km/h |
Key rule: A draft animal can sustain a pull force equal to approximately 10-15% of its body weight over a full working day. It can exert peak force of 30-50% of body weight for short distances (getting a stuck wagon moving, pulling up a slope).
Calculating vehicle load from pull capacity:
On level, compacted gravel road:
- Rolling resistance = approximately 3-5% of total vehicle + cargo weight
- If your horse can pull 100 kg sustained, and rolling resistance is 4%, then: Maximum load = 100 kg / 0.04 = 2,500 kg
In practice, reduce this by 20-30% for safety margin, road irregularities, and animal fatigue over a long journey.
Gradient Penalty
Grade is the most significant factor that reduces load capacity. Steep hills dramatically cut what a team can pull.
Load capacity reduction by gradient:
| Grade (%) | Rise per Meter | Capacity Reduction | Notes |
|---|---|---|---|
| 0-2% | 0-20 mm/m | None | Essentially flat |
| 2-5% | 20-50 mm/m | 30-40% reduction | Noticeable but manageable |
| 5-8% | 50-80 mm/m | 50-60% reduction | Hard work for the animal |
| 8-12% | 80-120 mm/m | 70-80% reduction | Near limit for loaded vehicles |
| >12% | >120 mm/m | Effectively impossible | Animals cannot maintain traction |
Simple rule of thumb: For every 1% of uphill grade, subtract approximately 10 kg of pulling capacity from the animal. A horse that pulls 100 kg on flat ground pulls only 60 kg on a 4% grade.
Planning Routes Around Grades
When planning a new road route, identify the steepest section. That section determines your maximum sustainable load for the entire route. A road that is mostly flat but has one 10% grade section effectively limits all traffic to 10%-grade capacity. Route around steep sections even if it adds distance — the increased load capacity often more than compensates.
Road Surface Factor
The road surface multiplies or divides your animal’s effective pull capacity:
| Surface Type | Rolling Resistance Factor | Effective Load (vs horse capable of 100 kg pull) |
|---|---|---|
| Paved stone or brick | 2-3% | 3,300-5,000 kg |
| Compacted gravel | 3-5% | 2,000-3,300 kg |
| Packed dirt (dry) | 5-8% | 1,250-2,000 kg |
| Loose gravel | 8-12% | 830-1,250 kg |
| Soft dirt (damp) | 12-20% | 500-830 kg |
| Mud or wet clay | 25-40% | 250-400 kg |
| Deep sand | 30-50% | 200-330 kg |
This table shows why road investment pays back so rapidly. Moving from a dirt track to compacted gravel can double or triple effective carrying capacity with no change in the vehicle or animal.
Vehicle Load Limits
The vehicle itself has structural limits independent of the animal.
Axle load limits for hand-built wooden vehicles:
| Axle Construction | Axle Diameter | Maximum Load per Axle |
|---|---|---|
| Hardwood, 6 cm diameter | 60 mm | 400-500 kg |
| Hardwood, 8 cm diameter | 80 mm | 700-900 kg |
| Hardwood, 10 cm diameter | 100 mm | 1,000-1,200 kg |
| Wrought iron | 40 mm | 800-1,000 kg |
| Wrought iron | 60 mm | 1,800-2,500 kg |
Wheel considerations:
- Wheel diameter affects load capacity. Larger wheels roll over obstacles more easily and distribute weight over a longer ground contact arc.
- Tire width directly affects ground pressure. A narrower tire concentrates load into a smaller footprint and sinks into soft ground. Minimum tire width for heavy loads: 5 cm. Heavy load wagons on soft ground should use 8-10 cm wide tires.
Ground bearing pressure formula:
Ground Pressure = Total Vehicle + Cargo Weight / Total Tire Contact Area
For soft soil, keep ground pressure below 100-150 kPa (roughly 1-1.5 kg/cm²). Exceeding this causes the wheels to sink and ruts to form.
Checking Your Load in Practice
Without a scale, estimating loads requires practical techniques.
Volume estimation method:
- Measure the cargo dimensions (length x width x height = volume in liters if you use dm)
- Multiply by the material’s bulk density:
| Material | Bulk Density (kg/liter) |
|---|---|
| Wheat grain | 0.75-0.80 |
| Barley grain | 0.60-0.65 |
| Firewood (split, stacked) | 0.35-0.45 |
| Dry hay (loose) | 0.05-0.08 |
| Sand (dry) | 1.4-1.6 |
| Gravel (dry) | 1.5-1.8 |
| Water | 1.0 |
| Salt | 1.0-1.2 |
| Bricks (stacked) | 1.4-1.6 |
Observational checks:
- Axle flex: Hardwood axles should show no visible sag. Any visible deflection means you are near or over the limit.
- Wheel sinkage: On firm ground, wheels should not sink more than 10-15 mm. Greater sinkage indicates excessive load for the road surface.
- Animal behavior: An animal that strains visibly on level ground, takes short steps, or shows labored breathing is overloaded. Reduce load by 20-30%.
- Spoke tension: Gently tap each wheel spoke with a knuckle. All spokes should ring at the same pitch. A dull thud indicates a loose or cracked spoke under stress.
Signs of Overloading
Watch for these danger signals: wheels wobbling side to side (loose or cracking spokes), visible axle sag, joints in the wagon frame opening up, unusual creaking or cracking sounds from the vehicle structure. Stop immediately and redistribute or remove cargo. A failure on the road is far more costly than taking two trips.
Load Distribution
Where you place cargo matters as much as how much you load.
Two-wheel cart: Position the heaviest items directly over the axle. The weight at the rear of the cart becomes shaft load on the animal. Ideal shaft load on the animal is 10-15 kg — enough to keep the shafts stable, not enough to burden the animal’s back.
Four-wheel wagon: Distribute weight evenly front to rear and side to side. A rear-heavy load lifts the front axle and reduces front wheel traction, making steering difficult and risking a tip-over on slopes. A side-heavy load risks tipping on cambered roads. Center of gravity should be low — stack heavy items on the bottom, lighter items on top.
Bulk liquids and grain: These shift in transit. Fill containers completely to prevent sloshing, or brace them tightly. A barrel of water (80 liters = 80 kg) that shifts to one side during a turn can tip a lightly built cart.
Multi-Animal Teams
Using multiple animals increases capacity but requires coordination.
Team capacity is not simply additive. Two horses do not pull exactly twice what one horse pulls, because of alignment, matching, and gait synchronization losses.
Practical multipliers:
| Team Size | Capacity Multiplier (vs single animal) |
|---|---|
| 1 animal | 1.0x |
| 2 animals (matched pair) | 1.8-1.9x |
| 3 animals | 2.5-2.7x |
| 4 animals | 3.2-3.5x |
| 6 animals | 4.5-5.0x |
Use matched pairs (similar size, strength, gait) for best efficiency. Mismatched animals result in the stronger animal pulling more, fatiguing faster, while the weaker one underperforms.
For very heavy loads (moving large stone, dragging timber), use the largest team you can assemble and coordinate them with voice commands and an experienced driver at the lead pair.