Weight transfer and the friction circle.

What weight transfer is

Brian Beckman's The Physics of Racing, a series he began in 1991, opens with this topic, and Allen Berg Racing Schools republishes that chapter on its own site as teaching material. Beckman says balancing a car means managing weight transfer with the throttle, brakes and steering:

• Braking moves load toward the front tires.
• Accelerating moves load toward the rear tires.
• Cornering moves load toward the outside tires.

The reason, in Beckman's explanation, is that the car's inertia acts through its center of gravity while the tires push at ground level. The higher the center of gravity, the more load moves, so a lower center of gravity means less transfer. His worked example:

Because load changes which tires have the most grip, Beckman says braking can bring on oversteer and acceleration can bring on understeer. Blayze coach Dion von Moltke sums it up as "where the weight is, is where the grip is." How that plays out in car balance is on understeer and oversteer.

Why transfer costs grip overall

HP Academy explains tire load sensitivity: a tire's grip rises with load, but not in proportion. The tires that gain load gain less grip than the unloaded tires lose, so any weight transfer reduces the car's total grip. HP Academy also says springs and anti-roll bars change how load is shared between the tires, and dampers change how quickly it moves, but neither changes the total amount that moves.

Beckman (Part 2) adds that a tire at its limit will slide if weight transfer takes load off it, and that a sliding tire can grip again if the force on it is reduced or load is moved back onto it. He describes tire noise as a rough guide: a squeak near the limit, a squeal at it, a squall over it. He also generalizes that older and harder tires, low-profile tires and slicks tend to let go more suddenly than new soft tires, taller sidewalls and DOT tires, while stressing that every tire should be judged on its own.

The friction circle

Beckman calls this the traction budget (Part 7). A tire's maximum force is limited to its coefficient of adhesion times the load on it, which means the most acceleration it can support, measured in g, is roughly the same whatever the car weighs.

The key point is how braking and cornering combine. Beckman says they do not simply add; they combine by the Pythagorean formula, so any mix of braking and cornering has to fit inside a circle of available traction. Wikipedia makes the same point about trail braking: braking and cornering share the tire's limited grip, and trailing the brake into a corner splits it between the two.

Beckman draws a clear line between racing and road driving here. On track the aim is to stay as close as possible to the edge of the circle. On the street, he says, you should stay well inside it so there is always grip in reserve. Track techniques belong on a track.

Not quite a circle

Beckman notes that for a whole car the budget is often egg-shaped rather than round, for example flattened at the top where acceleration is limited. Adam Brouillard of Paradigm Shift describes most cars' G-G plots (the friction circle as a data logger records it) as a circle with part of the top cut off: the less acceleration a car has compared with its grip, the more is missing. Brouillard uses this to explain why powerful cars want later apexes (see the racing line).

He also cautions against reading too much into the plot. A well-rounded G-G plot shows good car control, but on its own it does not prove good corner entry, because what matters is being at the right point on the circle at the right point on the track. Reading data is covered on data and video.

Why coaches stress smoothness

In Part 14, "Why Smoothness?", Beckman models the suspension and tires as damped oscillators. Jerky inputs feed energy into them that the car cannot use, upsetting it and costing traction, while gradual, rounded inputs match the way the car naturally responds. He describes every steering movement as having a gradual start and a gradual stop, and names two exceptions where a quick input is right: a quick steering correction when the front is already sliding, and a quick throttle blip to match revs on a downshift (see heel-toe). HP Academy reaches the same conclusion from load sensitivity: smooth inputs limit weight transfer and keep total grip as high as possible.

Using weight transfer on purpose

Coaches describe several ways drivers move load deliberately. These are principles from the named sources, to be worked on with an instructor:

• Entry oversteer. HP Academy suggests that a car oversteering on entry can be helped by braking slightly earlier and releasing more of the brake before turn-in, so the rear carries more load.
• Tightening the line. HP Academy says a slight throttle lift in a long corner moves load forward and tightens the line. Driver61 describes a brief lift in slow hairpins to rotate the car toward the exit so full throttle can come earlier.
• Maintenance throttle. Ross Bentley defines it as the throttle needed to hold a constant speed, more in corners and uphill, and says it keeps the car balanced front to rear, which he claims gives more total grip.

HP Academy also notes that a wet track limits how hard a car can accelerate, and so limits weight transfer, and that softer, more compliant setups are often used in the wet (see wet weather). The tire side of the story is on how grip works.