The Physics of Apex Curbs in Modern Karting: Why "Flying Boards" Cost Lap Time
In casual motorsport paddock banter, launching a go-kart aggressively across a raised apex kerb is sometimes jokingly nicknamed using a flying board. However, as demonstrated by Moto2 star Diogo Moreira during the MotoGP Karting Challenge event, attempting to cut chicane distance by launching over high curbs yields immediate penalties rather than lap-time gains.
1. Solid Rear Axles and Differential-Free Dynamics
In passenger cars and formula cars, an open or limited-slip differential allows the outside driven wheel to travel further along a wider turn radius than the inside wheel without tire scrub. A competition kart features a continuous 30mm, 40mm, or 50mm hollow steel alloy rear axle locking both wheels in rigid synchronization.
To prevent severe understeer on corner turn-in, kart chassis rely on Kingpin Inclination and High Positive Caster (typically 12° to 18°). When the driver steers into an apex, the front wheels tilt, raising the outside front corner and lowering the inside front corner. This diagonal leverage unweights and lifts the inside rear tire 15mm to 35mm off the pavement.
2. What Happens During a "Flying Board" Curb Strike?
When a kart strikes a 55mm sausage kerb at 90+ km/h with high entry velocity, three destructive physical events occur sequentially:
- Transient Vertical Shock Wave: Without hydraulic dampers, the 180 kg system mass (kart plus driver) undergoes 4G to 6G instantaneous vertical acceleration. The tire sidewall compresses completely against the magnesium rim lip.
- Drive Axle Severance from Tarmac: Both rear wheels become airborne for 0.20 to 0.45 seconds. The 2-stroke or 4-stroke engine hits the electronic rev limiter instantly as rolling resistance drops to zero.
- Violent Kinetic Landing & Torsional Snap: When the drive tires slam back down on the asphalt, the wheels are spinning faster than the chassis road speed. This produces severe longitudinal tire scrub, shock torque through the chain drive, and sideways snap-oversteer that forces immediate counter-steering, destroying corner exit drive.
| Apex Attack Style | Curb Depth | Airborne Duration | Exit Speed (km/h) | Sector Time Delta |
|---|---|---|---|---|
| Optimal Apex Clip | 15% - 25% (edge only) | 0.00 s (tires loaded) | 86.0 km/h | 0.000 s (Baseline) |
| Corrugated Ripple Ride | 40% - 50% (flats) | 0.04 s (micro chatter) | 83.2 km/h | +0.095 s |
| Aggressive Sausage Hop | 65% - 75% | 0.22 s | 76.4 km/h | +0.245 s |
| Moreira "Flying Board" Launch | 85% - 100% | 0.34 s | 71.8 km/h | +0.382 s |
3. Chassis Tuning: Torsion Bars and Frame Compliance
Drivers often attempt to compensate for aggressive curbing by tuning their chassis stiffness:
- Front Torsion Bar Removal: Loosening or removing the front torsion bar allows the front bumper rails to flex independently. This softens initial curb impact but reduces front-end turn-in bite on smooth tarmac.
- Rear Ride Height: Lowering the rear axle bearing cassettes lowers the center of gravity (CoG). While it stabilizes the kart on high-speed sweeps, it reduces the weight transfer necessary to maintain inside wheel lift over curbs.
- Tire Pressure Calibration: Running lower hot tire pressures (e.g., 0.70 bar vs 0.90 bar) increases pneumatic damping across rumble strips, but risks rim damage when striking sharp bevel steps.
Frequently Asked Questions
Why does a kart lose lap time when hitting an aggressive apex curb?
Unlike touring cars or single-seaters with springs and dampers, racing karts rely solely on chassis tube flex and tire sidewalls for compliance. When the inside tires strike a raised apex kerb at speed, the vertical shock wave violently lifts the rear drive axle into the air. This instantly breaks the contact patch, causing engine rev spike, wheelspin, and zero longitudinal traction upon landing, severely degrading corner exit speed.
How does inside rear wheel lift affect kart cornering?
Because racing go-karts have a locked solid rear axle without a differential, both rear tires turn at the exact same rotational speed. To turn without understeering scrubbing, chassis designers engineer high front caster geometry (jacking effect). As the front wheels turn, the inside front dips and pivots the frame, unloading and lifting the inside rear tire off the track surface so the outside tire can trace the cornering arc cleanly.
What is the optimal apex curb engagement depth in competitive karting?
Optimal curb usage depends on kerb profile. Flat chamfered paint strips can be clipped up to 60-80% to shorten track radius. High sausage curbs or bevel steps, however, should rarely be mounted more than 10-25% of tire width. Exceeding this boundary induces airborne hang time, chassis bending, and a minimum 0.2 to 0.45 second sector delta penalty.
What chassis damage occurs from repeated curb hopping?
Repeated high-G impacts can crack the front cross-member welds, warp the hollow 50mm rear drive axle out of true (leading to brake disk rubbing and drive drag), bend the steering tie-rods, and permanently deform the frame's waist section, causing asymmetrical corner weights.