How grip works.

Grip is the force a tyre can pass to the road before it slides. Much of what this page says comes from two kinds of source: Wikipedia's vehicle dynamics articles, which cite engineering texts such as Pacejka's Tire and Vehicle Dynamics, Gillespie's Fundamentals of Vehicle Dynamics and Milliken and Milliken's Race Car Vehicle Dynamics, and Brian Beckman's The Physics of Racing, a series he began in 1991, whose weight-transfer chapter Allen Berg Racing Schools republishes as teaching material. Racing schools teach these ideas in the classroom; Skip Barber, for example, lists weight transfer and balancing braking, steering and throttle among its one-day school topics. This page is the theory, not driving advice.

Slip angle: a tyre grips by twisting

A cornering tyre does not travel exactly where it points. Wikipedia's slip angle article, citing Pacejka, defines slip angle as the angle between the direction the wheel points and the direction it actually moves. The rubber of the tread and the carcass deforms to make that angle, and the deformation is what produces the cornering force.

The same article describes how the force builds. Over the first few degrees of slip angle, cornering force rises roughly in a straight line. Beyond that it keeps rising, more slowly, up to a peak, and then falls away. Past the peak, more slip angle means less grip, not more.

Inside the contact patch, Wikipedia explains, tread elements stick to the road while the wheel moves sideways, so they bend further as they travel back through the patch. That puts the centre of the force behind the centre of the patch (the pneumatic trail), which creates a self-aligning torque that tries to straighten the wheel.

The friction circle: one budget, shared

A tyre cannot give full braking grip and full cornering grip at the same moment. Wikipedia's circle of forces article (also called the traction circle or friction ellipse), citing Pacejka and Wong, describes a maximum total horizontal force for the tyre, which is split between cornering and braking or accelerating. How large that maximum is depends on the tyre's design and condition, its temperature, the road surface and the vertical load on it.

Beckman calls this the traction budget. In Part 7 of his series he explains that cornering and braking (or accelerating) forces do not simply add: they combine like the sides of a right-angled triangle, so the resulting force must stay inside a circle. Because a tyre's maximum force is its coefficient of adhesion times its load, he notes, the maximum acceleration it supports in g is roughly independent of the car's mass.

A whole car's budget is not a perfect circle. Beckman says it is often egg-shaped, flattened at the top where the engine cannot match the tyres' grip. Coach Adam Brouillard of Paradigm Shift Racing describes most cars' plots of cornering against forward acceleration (G-G plots) the same way: a circle with part of the top cut off, and the weaker the car's acceleration relative to its grip, the more is missing.

Beckman also draws the line between racing and the road: racing aims to use the budget right up to the edge, while street driving should stay well inside it to keep a reserve. Driving at the edge belongs on a track, under instruction.

Load sensitivity: more weight, less grip per kilogram

Pressing a tyre harder into the road gives it more grip, but not in proportion. Wikipedia's tyre load sensitivity article explains that the friction coefficient falls as vertical load rises, so maximum sideways force grows with load at a diminishing rate. It cites data from Milliken and Milliken in which the peak ratio of lateral to vertical force falls from 1.10 at 900 lbf of load to 0.97 at 1,800 lbf.

HP Academy draws the consequence: when load moves from one tyre to another, the loaded tyre gains less grip than the unloaded tyre loses, so any weight transfer reduces the car's total grip.

Weight transfer moves the load around

Wikipedia's weight transfer article, citing Gillespie, explains that the load on each wheel changes whenever the car accelerates, brakes or corners, because the tyre forces act at road level while the car's mass sits higher up. It happens even in a car with no suspension at all. Load moves rearward under acceleration, forward under braking and to the outside wheels in a corner.

How much moves depends on the car. Longitudinal transfer grows with mass, acceleration and centre-of-mass height and shrinks with a longer wheelbase; sideways transfer depends on track width in the same way. Lowering the centre of mass reduces transfer in every direction.

Beckman gives a worked example: a 3,200 lb car with a 20 inch centre of gravity, a 100 inch wheelbase and an even front-to-rear split, braking at 1 g, moves 640 lb onto the front, leaving 2,240 lb at the front and 960 lb at the rear. He adds that braking can therefore push a car towards oversteer and acceleration towards understeer, because the shift changes which end has more grip.

HP Academy says springs and anti-roll bars change how load is shared between the tyres, and dampers change how quickly it moves, but neither changes the total amount transferred. Beckman's Part 14 argues for smooth inputs: modelling the suspension and tyres as damped oscillators, he says abrupt inputs upset the car and cost traction. Driving technique built on this is covered in weight transfer.

Understeer and oversteer

Which end of the car runs out of grip first decides its balance. Wikipedia notes that the engineering definitions in SAE J670 and ISO 8855 use the understeer gradient: positive is understeer, negative is oversteer, zero is neutral. At the limit, understeer means the front tyres saturate first and the car runs wide; IndyCar's glossary uses the American terms "push" or "plow". Oversteer means the rears saturate first and the car rotates towards the inside, which IndyCar calls "loose". More is on understeer and oversteer.

Downforce: aerodynamic grip

IndyCar defines downforce as the vertical pressure its aerodynamic design creates to press the car onto the track, improving grip and cornering. Wikipedia's downforce article, citing Katz, separates this aerodynamic grip, which comes from extra tyre load, from mechanical grip, which comes from mass, tyres and suspension. Downforce rises with the square of speed and usually costs drag.

IndyCar shows the trade-off in practice. It runs its highest-downforce package with larger wings on road and street courses, a balanced package on short ovals, and its lowest-downforce, lowest-drag package on superspeedways, where it says speeds often exceed 230 mph.

Ground effect is another route. Wikipedia describes shaping the underbody so air speeds up and pressure drops beneath the car, aiming for downforce with less drag than wings. It notes that Formula 1 made flat undersides mandatory from 1983 after ground-effect cornering speeds became dangerous, and that ground effect returned to F1 in 2022, bringing renewed "porpoising" problems.

The tyre itself

Compound changes the budget too. IndyCar runs Firestone tyres and, on road and street courses, a harder primary tyre with black sidewalls and a softer alternate with red sidewalls that grips more but wears faster; drivers must use both in each race. Its glossary describes "marbles" as bits of worn rubber that collect off the racing line and reduce grip there.

For club drivers, a Maxxis tyre engineer interviewed by Grassroots Motorsports in 2021 (in a sponsored article) said R-compound tyres have shallower tread, stiffer casings and quick-warming compounds that grip hard but wear faster, with a narrow temperature window and poor wet and cold grip. In the magazine's single autocross test they lapped in 27.703 seconds against 28.114 seconds on 200-treadwear tyres. Beckman adds a broader generalisation: older, harder and low-profile tyres and slicks tend to let go more suddenly than new, soft, higher-profile and DOT tyres, though he stresses each tyre should be judged on its own.

What we left out

We did not give a typical slip angle at which tyres reach peak grip, because the figures we found rest on a single unconfirmed Wikipedia passage. We also left out a worked example of how much total grip a given amount of cornering load transfer costs, for the same reason. The physics here relies largely on Wikipedia as a secondary source; we did not obtain a primary engineering text.