Aerodynamic Influence Over Leading and Pursuing Motorcycles Equipped With Downforce-Generation Wings

This study uses numerical simulations to demonstrate that aerodynamic wings on a leading motorcycle generate turbulent wakes and coherent vortices that independently and significantly destabilize a pursuing motorcycle by altering lift and wheelie tendencies, suggesting that competition governing bodies should consider banning such downforce-generating appendices to improve safety.

The Gist

Imagine two motorcyclists racing side-by-side on a highway. The one in front is the "Leader," and the one chasing is the "Chaser." For decades, racers have known that if the Chaser tucks in right behind the Leader, they get a free ride. The Leader pushes the air out of the way, creating a low-pressure "bubble" or slipstream behind them. The Chaser sits in this bubble, facing less wind resistance, which makes them go faster and use less energy. It's like swimming in the wake of a large boat; the water is calmer, and you can glide effortlessly.

However, this paper investigates a modern twist: What happens when the Leader's motorcycle is equipped with wings (similar to the little wings on a race car) designed to push the bike down onto the track for better grip?

The Invisible Tug-of-War

The researchers used powerful computer simulations to see how these wings change the air for the Chaser. They discovered that the wings create two distinct "weather patterns" behind the Leader that act differently than the simple slipstream of a normal bike:

1. The Turbulent Wake (The Old Way): Even without wings, a bike leaves a messy, chaotic trail of air behind it. This is the classic slipstream that helps the Chaser go faster.
2. The Wingtip Vortices (The New Way): The wings on the Leader generate powerful, spinning tubes of air (vortices) that trail behind like the smoke from a jet engine. These are organized, strong, and persistent.

The "Updraft" vs. The "Downdraft"

The paper explains that the Chaser's experience depends entirely on where they are sitting relative to these spinning air tubes.

• The "Updraft" Trap (Danger Zone): If the Chaser is directly behind the Leader, aligned with the wings, they get hit by an "updraft." Think of it like standing under a giant fan blowing air up. This upward push lifts the front wheel of the Chaser's bike. In racing, lifting the front wheel is bad; it makes the bike feel light and unstable, and it reduces the grip needed to brake hard. The paper found that these wing-generated vortices can actually cancel out the helpful "slipstream" effect, making the Chaser feel like they are losing their brakes.
• The "Downdraft" Benefit (Safe Zone): If the Chaser moves slightly to the side (a lateral offset), they might catch the "downdraft" from the outer edge of the vortex. This is like standing under a fan blowing air down. This pushes the Chaser's front wheel firmly onto the road, increasing grip and stability. In this specific position, the wings of the Leader actually help the Chaser stay planted.

The "Ghost" Effect

One of the most surprising findings is that these wing-generated air patterns don't just disappear. They travel far downstream. Even if the Chaser is several bike-lengths behind, these organized vortices are still there, influencing the bike's behavior. They act like invisible hands that can either push the bike down (good for braking) or lift it up (bad for braking), regardless of how far apart the racers are.

The Real-World Consequence

The authors connect this to real racing disasters. They mention specific crashes where a rider tried to overtake another, but when they tried to brake, their front wheel lifted up, and they couldn't stop. The paper suggests this happened because the Chaser was riding in the "Updraft" zone created by the Leader's wings, which robbed them of the downforce (grip) they needed to slow down.

The Bottom Line

The paper concludes that while these wings make the Leader's bike faster and more stable, they create a "dangerous weather system" for anyone chasing them. The air behind a winged bike is no longer just a helpful slipstream; it's a complex mix of helpful and harmful forces that can suddenly make a bike unstable.

The authors suggest that race organizers might need to rethink the rules. They propose that perhaps the wings should be made smaller or removed entirely to make the racing safer, as the current design creates unpredictable aerodynamic traps for the riders chasing the pack. They argue that while the technology improves performance, it introduces a hidden risk that makes the sport more dangerous than it needs to be.

Technical Summary

Technical Summary: Aerodynamic Influence Over Leading and Pursuing Motorcycles Equipped With Downforce-Generation Wings

Problem Statement
The integration of aerodynamic appendages, specifically front wings designed to generate downforce, has become a standard feature in modern motorcycle racing (e.g., MotoGP and WSBK) to enhance braking performance and cornering stability. However, the aerodynamic interaction between a leading motorcycle equipped with these wings and a pursuing motorcycle remains poorly understood. While the "drafting" effect (drag reduction) of a standard turbulent wake is well-documented, the presence of coherent wingtip vortices generated by downforce devices introduces complex flow dynamics. The central problem addressed is how these specific flow structures—characterized by upwash and downwash velocity components—affect the stability, lift, drag, and pitching moments of a trailing motorcycle, potentially creating hazardous conditions during overtaking maneuvers or pack racing.

Methodology
The study employs Computational Fluid Dynamics (CFD) using the Reynolds-Averaged Navier-Stokes (RANS) approach to simulate the aerodynamic environment.

• Numerical Framework: Simulations were conducted using OpenFOAM (v2112) with the Finite Volume Method. The turbulence was modeled using the $k$-$\omega$ Shear Stress Transport (SST) model.
• Geometry: The base geometry is a Yamaha YZF-R1 production sports bike, modified to remove road-legal features (mirrors, signals) and include a tucked rider. A simplified wing profile (Selig-1223 airfoil) with a -40° angle of attack was added to the front fairing to simulate downforce-generating devices.
• Simulation Cases: Three primary scenarios were analyzed across a range of longitudinal (5 cm to 7 m) and lateral (0 cm to 60 cm) relative positions:
  1. Case 1: Both leading and trailing motorcycles are wingless.
  2. Case 2: The leading motorcycle has wings; the trailing motorcycle is wingless.
  3. Case 3: Both motorcycles are equipped with wings.
• Mesh and Convergence: Meshes ranged from 12 million to 25 million control volumes, with refinement boxes around the motorcycles to capture boundary layers and wake structures. Convergence was achieved after 1,000 to 2,000 iterations, with aerodynamic coefficients ($C_D$, $C_L$, $C_M$) averaged over the final half of the iterations.
• Operating Conditions: Simulations assumed incompressible flow at a velocity of 70 m/s (252 km/h), resulting in a Reynolds number of approximately $6.99 \times 10^6$.

Key Results
The study reveals that the aerodynamic influence of a wing-equipped leader is distinct from that of a standard blunt-body wake, primarily due to the persistence of coherent wingtip vortices.

• Case 1 (Wingless-Wingless): As expected, the trailing motorcycle experiences a reduction in drag, lift, and pitching moment due to the low-pressure turbulent wake. This reduction facilitates overtaking and improves stability by reducing wheelie tendencies. The effects diminish asymptotically as longitudinal distance increases and lateral offset grows.
• Case 2 (Winged Leader, Wingless Pursuer): The presence of the leading wing significantly alters the flow field.
  • Longitudinal Effects: At larger longitudinal distances, the trailing motorcycle experiences increased lift and pitching moment compared to the wingless scenario. This is attributed to the upwash velocity component convected downstream from the leading wing, which increases the effective angle of attack on the trailing bike, thereby increasing the tendency for the front wheel to lift (wheelie) and compromising stability.
  • Lateral Effects: The lateral position is critical. When the trailing bike is aligned with the wingtip vortex core, lift coefficients increase negatively (unfavorable). However, at specific lateral offsets (approx. 25 cm to 60 cm), the downwash component becomes dominant, reducing lift and wheelie tendencies, offering a stability benefit.
• Case 3 (Winged-Winged): When both bikes have wings, the trailing wing's performance is heavily influenced by the leader's wake.
  • Downforce Loss: The trailing wing experiences a significant reduction in downforce generation when in close proximity to the leader, particularly due to the upwash component. This loss of front tire load is identified as a primary cause for reduced braking capability in race conditions.
  • Drag Recovery: While drag is reduced due to drafting, the loss of downforce negates some performance benefits, creating a "dangerous performing condition" where the bike is faster but less stable and harder to brake.

Significance and Claims
The paper asserts that the coherent vortex pair generated by downforce wings influences the pursuing motorcycle independently of the turbulent wake and persists across all tested relative distances. This challenges the assumption that drafting is universally beneficial; in the context of winged motorcycles, the wake can actively degrade the trailing bike's stability and braking performance.

The authors highlight that the current design philosophy, which integrates wings into the overall vehicle package, creates a safety paradox: while wings improve the leading bike's performance, they introduce unpredictable and potentially hazardous aerodynamic conditions for the pursuer. Citing specific racing incidents (e.g., the 2023 Valencia race and the 2024 Australian Grand Prix crash), the paper suggests that the loss of aerodynamic downforce in the wake of a winged leader is a contributing factor to near-crash and crash scenarios.

Consequently, the study concludes that while a complete removal of wings might disrupt current vehicle dynamics, motorcycling governing bodies should consider a steady reduction of allowed wing geometry or their eventual elimination to improve safety conditions across all racing categories. The findings suggest that the current aerodynamic appendages, while beneficial for single-vehicle performance, compromise the safety margins of the racing pack.