Lateral g on the Skidpad: A Number With Conditions

Lateral acceleration on a skidpad is v² / (r · g), so the answer depends as much on the circle radius as on the car. A car pulling 1.0 g on a 100 m radius circle is travelling at 112.8 km/h. On a 60 m circle the same 1.0 g requires only 87.3 km/h, at which speed almost no aerodynamic downforce exists. Two published lateral g figures from different circles are not comparable.

Why radius decides everything

A car on a skidpad is in steady state, so lateral acceleration equals v² / r. Rearranged, the speed needed for a given lateral g on a given radius is v = √(a · r). A larger circle means more speed for the same lateral acceleration, and more speed means more aerodynamic downforce, which means more grip, which allows more lateral acceleration.

Look along the 1.00 g column. Achieving it demands 61.8 km/h on a small circle and 159.5 km/h on a large one. At the second speed a car with any aerodynamic package is carrying real downforce, and at the first it is carrying essentially none. A large radius skidpad therefore flatters an aerodynamic car and a small one does not, using exactly the same units.

The other conditions that move it

• Direction. A skidpad figure is usually the average of clockwise and anticlockwise laps, because road camber, cross wind and even fuel distribution make the two directions differ. A single direction figure has the same problem as a single direction top speed run.
• Surface. Prepared, rubbered asphalt returns a higher figure than an ordinary surface, and the difference exceeds most car to car gaps.
• Tyre and temperature. Friction runs from 0.7 to 1.0 for street tyres against 1.5 to 2.0 for slicks, and a summer compound below about 10 °C surface temperature loses 15 to 25 per cent. The tyre matters more than the chassis.
• Tyre wear during the test. Sustained cornering heats one shoulder of each tyre continuously, so a long test degrades what it is measuring.

What the figure does and does not tell you

A skidpad number measures steady state grip and nothing else. It says nothing about transient response, which is how quickly the car changes direction and is what a driver actually feels, and nothing about balance, since a car can reach a high figure while understeering heavily. It is a measurement of the tyres, the mass and the vertical load, plus whatever downforce exists at the resulting speed.

Load sensitivity puts a further ceiling on it. Because a tyre's friction coefficient falls as vertical load rises, weight transfer during cornering costs grip: an axle carrying 10,000 N split evenly produces more force than the same axle split 3,000 and 7,000. That is why a lower centre of gravity and a wider track raise a skidpad figure without any change to the tyres.

Reading a published figure

Ask for the radius first, then the direction convention, then the tyre. With those three, two figures can be compared. Without the radius, a lateral g number is a measurement whose units are incomplete, and a difference of a few hundredths between two cars tested on different pads means nothing at all.

Questions readers ask

How is lateral g measured on a skidpad?

By driving a constant radius circle at the highest sustainable speed and calculating v² / (r · g). The radius is therefore part of the measurement: 1.0 g needs 112.8 km/h on a 100 m circle and only 87.3 km/h on a 60 m one.

Why does the circle radius matter?

Because a larger circle requires more speed for the same lateral acceleration, and more speed means more aerodynamic downforce. A large radius pad flatters an aerodynamic car while a small one measures almost pure mechanical grip.

What is a good lateral g figure?

Around 0.9 g for an ordinary car on street tyres and 1.1 to 1.2 g for a performance car, though the radius has to be stated for the comparison to hold. Figures well above that require track compounds or aerodynamic downforce.

Why is the figure averaged over both directions?

Because road camber, cross wind and fuel distribution make clockwise and anticlockwise laps differ. Averaging removes those, in the same way a two way average removes wind and gradient from a top speed record.

Does a high skidpad figure mean the car handles well?

Not necessarily. It measures steady state grip only, and a car can post a high figure while understeering heavily. It says nothing about transient response, which is how quickly the car changes direction and is what a driver actually notices.

Sources

• Autosport technical forum, tyre coefficient of friction, on friction values and on load sensitivity during cornering.
• Vehicle Physics Pro, tyre model documentation, on lateral force generation and how it responds to vertical load.
• Tire friction overview, on the relationship between longitudinal and lateral peak friction.

Speeds are calculated as v = √(a · r) with g = 9.81 m/s², and assume steady state cornering with no aerodynamic contribution.