At MotoGP speeds, air resistance is one of the largest forces holding a bike back on a straight. A rider who follows closely behind another bike can enter the lead bike's wake, where the air is already disturbed and moving differently. This is called slipstreaming, and it can reduce the following bike's aerodynamic drag.
Even a small drag reduction can help the rider gain speed, close a gap, or prepare an overtake before braking.
Key Facts
- Aerodynamic drag can be modeled as Fd = 0.5ρCdAv^2.
- Power needed to overcome drag is P = Fd v, so drag power rises roughly with v^3.
- A slipstream is the lower pressure, turbulent wake behind a moving bike and rider.
- A following rider in the wake can reduce effective CdA and accelerate more easily.
- The tow is strongest on long straights when bikes are aligned and close together.
- Riders must leave the tow before braking or turning because turbulent air can reduce stability and cooling.
Vocabulary
- Slipstream
- The region of disturbed airflow behind a moving vehicle where a following rider may experience less aerodynamic drag.
- Tow
- The speed advantage gained when a rider follows another bike closely in its slipstream.
- Aerodynamic drag
- The resistive force from air that acts opposite the motion of a bike and increases strongly with speed.
- Wake
- The turbulent, lower energy airflow left behind an object moving through air.
- CdA
- The product of drag coefficient and frontal area, used to describe how strongly a rider and bike resist airflow.
Common Mistakes to Avoid
- Thinking the following bike is being pulled forward, which is wrong because the main effect is reduced drag rather than a forward pulling force.
- Assuming slipstream works equally well at all speeds, which is wrong because aerodynamic drag grows with v^2 and the benefit is much larger at high racing speeds.
- Forgetting the cubic power effect, which is wrong because the power needed to overcome drag rises roughly with v^3, making small speed increases very costly.
- Staying in the tow too long before a corner, which is wrong because turbulent air, reduced visibility, and higher closing speed can make braking and turning less safe.
Practice Questions
- 1 A MotoGP bike has ρ = 1.2 kg/m^3, CdA = 0.32 m^2, and speed v = 80 m/s. Use Fd = 0.5ρCdAv^2 to estimate the aerodynamic drag force.
- 2 A rider in a slipstream reduces effective CdA from 0.32 m^2 to 0.27 m^2 at 80 m/s with ρ = 1.2 kg/m^3. Calculate the drag force before and after entering the tow, then find the reduction in drag.
- 3 Explain why a rider may move out of the slipstream near the end of a straight even though the tow is helping them gain speed.