Construction machines use either tracks or wheels to turn engine power into movement across the ground. The choice matters because soil, speed, load, and distance all affect how safely and efficiently a machine can work. Tracks spread weight over a large area, while wheels concentrate weight into smaller contact patches.
This is why a bulldozer may cross soft mud while a wheeled loader moves faster on a firm jobsite road.
Traction depends on friction, contact area, surface conditions, and how much weight presses the machine into the ground. Ground pressure is the machine weight divided by the area touching the ground, so larger contact area usually means less sinking. Tracks often give better grip and lower ground pressure, but they are slower and cost more to maintain.
Wheels are faster, easier to steer on hard surfaces, and better for road travel, but they can lose traction or sink on soft soil.
Key Facts
- Ground pressure = weight / contact area
- Tracks have a large contact area, so they usually produce lower ground pressure than wheels.
- Wheels have smaller contact patches, so they usually produce higher ground pressure on the soil.
- Traction force is limited by friction: Fmax = μN
- Lower ground pressure helps machines work on mud, sand, snow, and loose soil without sinking as much.
- Wheeled machines are usually faster and more efficient for travel on pavement, gravel roads, and compacted surfaces.
Vocabulary
- Ground pressure
- Ground pressure is the force a machine applies to the ground divided by the contact area touching the ground.
- Traction
- Traction is the grip that allows a machine to push, pull, climb, or move without slipping.
- Contact area
- Contact area is the total surface area of the tracks or tires pressing against the ground.
- Coefficient of friction
- The coefficient of friction is a number that describes how strongly two surfaces resist sliding against each other.
- Roading
- Roading is moving a construction machine over roads or long distances instead of working only on the jobsite.
Common Mistakes to Avoid
- Assuming heavier machines always sink more, which is wrong because sinking depends on both weight and contact area.
- Thinking tracks always have more traction than wheels, which is wrong because traction also depends on surface type, tread design, weight distribution, and friction.
- Using mass instead of weight in ground pressure calculations, which is wrong unless mass is converted to force using W = mg.
- Choosing wheels for every fast job, which is wrong because speed is only useful if the ground is firm enough to support the machine without slipping or rutting.
Practice Questions
- 1 A tracked bulldozer weighs 120000 N and has 6.0 m² of total track contact area. What is its ground pressure in N/m²?
- 2 A wheeled loader weighs 80000 N and has four tire contact patches of 0.20 m² each. What is its total contact area and ground pressure?
- 3 A construction site has soft wet clay in one area and a compacted gravel road in another. Explain which machine type, tracks or wheels, is better for each area and give two reasons.