Dakar rally vehicles travel at high speed over rocks, sand ruts, jumps, and sharp washboard bumps. Their suspension must keep the tires on the ground while protecting the chassis and driver from large impacts. A bypass shock absorber helps solve this problem by changing its damping behavior depending on where the piston is in the shock body.
This gives soft damping for small bumps and firm damping when the wheel moves through a large part of its travel.
Inside the shock, oil is forced through valves, piston ports, and external bypass tubes as the wheel moves up and down. Near the middle of the stroke, open bypass paths let oil flow around the piston more easily, so the shock feels softer and allows smooth wheel motion. Near the end of the stroke, those bypass paths close off, forcing oil through more restrictive valves and greatly increasing damping force.
Engineers tune tube positions, valve stiffness, oil viscosity, and gas pressure to control how the vehicle responds in different terrain.
Key Facts
- Damping force often increases with piston speed: Fd ≈ c v, where c is damping coefficient and v is piston velocity.
- Bypass tubes create alternate oil paths around the piston during selected parts of the shock stroke.
- Small bumps usually use the middle travel zone, where open bypass paths reduce damping force.
- Large impacts push the piston into end zones, where fewer bypass paths are open and damping becomes firmer.
- Kinetic energy absorbed by the suspension is Ek = 1/2 m v^2, so faster impacts require much more energy control.
- Shock absorbers convert mechanical motion energy into thermal energy in the oil and shock body.
Vocabulary
- Damping
- Damping is the resistance force that slows suspension motion and helps control bouncing.
- Bypass tube
- A bypass tube is an external oil passage that lets fluid flow around the shock piston during part of its travel.
- Piston
- The piston is the moving internal part of the shock that pushes oil through valves and ports.
- Compression stroke
- The compression stroke is the motion that occurs when the wheel moves upward and the shock shortens.
- Rebound stroke
- The rebound stroke is the motion that occurs when the wheel moves downward and the shock extends.
Common Mistakes to Avoid
- Assuming a shock absorber holds the vehicle up, this is wrong because springs support the vehicle weight while shocks control the speed of suspension motion.
- Treating damping as the same at all positions, this is wrong because a bypass shock changes oil flow paths depending on piston position.
- Ignoring piston speed, this is wrong because damping force usually grows as the piston moves faster through the oil.
- Thinking softer damping is always better, this is wrong because soft damping over a large impact can allow bottoming and damage the vehicle.
Practice Questions
- 1 A shock has an effective damping coefficient of 900 N s/m in a soft bypass zone. If the piston speed is 0.40 m/s, estimate the damping force using Fd = c v.
- 2 During a large impact, the damping coefficient rises to 3200 N s/m and the piston speed is 0.75 m/s. Estimate the damping force and compare it with a soft-zone force of 900 N s/m at the same speed.
- 3 Explain why a Dakar rally vehicle benefits from soft damping near the middle of suspension travel but firm damping near the end of travel.