Monster trucks are huge, heavy vehicles with giant tires, so turning them with only the front wheels would require a very large space. Steering both the front and rear axles lets the truck rotate more easily around a smaller turning circle. This is important in arenas, where drivers need to line up jumps, avoid obstacles, and recover quickly after stunts.
Four wheel steering also helps control a truck when the tires are bouncing, sliding, or digging into dirt.
Key Facts
- Opposite phase steering means the front and rear wheels turn in opposite directions to reduce turning radius.
- Same phase steering means the front and rear wheels turn in the same direction so the truck can move diagonally in crab mode.
- For a bicycle model with front steering only, R = L / tan(theta), where R is turning radius, L is wheelbase, and theta is steering angle.
- For ideal opposite phase four wheel steering with equal angles, R = L / (2 tan(theta)), so the turning radius is about half the front steering case.
- Turning torque at the tires comes from lateral friction, with F_friction <= mu N.
- Hydraulic steering systems use pressurized fluid to multiply force, described by P = F / A.
Vocabulary
- Four wheel steering
- A steering system in which both the front and rear wheels can change angle to control the vehicle direction.
- Opposite phase steering
- A steering mode where the front and rear wheels turn in opposite directions to make a tighter turn.
- Crab steering
- A steering mode where all wheels point in nearly the same direction so the vehicle moves sideways or diagonally.
- Turning radius
- The radius of the path followed by the vehicle as it turns around a center point.
- Hydraulic actuator
- A device that uses pressurized fluid to push or pull steering parts with large force.
Common Mistakes to Avoid
- Assuming the rear wheels always follow the front wheels is wrong because rear steering can actively change the truck path and rotation.
- Confusing crab steering with tighter turning is wrong because crab mode moves the truck diagonally instead of rotating it sharply around a center.
- Ignoring tire friction is wrong because the steering system can only control the truck if the tires can generate enough sideways force on the ground.
- Using front steering formulas for four wheel steering without adjustment is wrong because rear wheel angle changes the geometry and can greatly reduce the turning radius.
Practice Questions
- 1 A monster truck has a wheelbase of 3.6 m. With front steering only at 30 degrees, estimate the turning radius using R = L / tan(theta).
- 2 The same truck uses opposite phase four wheel steering with front and rear wheels both at 30 degrees. Estimate the turning radius using R = L / (2 tan(theta)), and compare it with front steering only.
- 3 Explain why a driver would choose crab steering instead of opposite phase steering when lining up with a ramp after landing at an angle.