Before a drag race the cars drive into a wet patch, spin the rear tyres in a cloud of smoke, then creep back to the line. It looks like showing off, but it is a procedure with a purpose. A burnout raises the temperature of the driven tyres and cleans their surface, and both change how much force the tyre can pass to the track. All figures below are invented examples for a generic rear-drive car, not data about any car in a game.
Grip depends on the tyre, not just the load
The most force a tyre can put into the road is F = μ × N, where N is the load pressing the tyre down and μ (mu) is the friction coefficient. Load comes from the car's weight. μ comes from the tyre and the surface, and it is not a constant.
A drag slick is made of soft rubber. Soft rubber deforms around the small bumps of the track surface, and that gripping action works best when the rubber is warm, because the compound is softer and stickier then. A cold tyre is stiffer and grips less. Too hot is also a problem, since the rubber can go greasy, but a cold tyre at the start line is the far more common case. So the aim of the burnout is to raise μ.
The actual coefficient depends on the compound, the track and the temperature, so the values below are illustrative. We use three: 0.8 for a cold, dirty tyre, 0.9 for a part-warmed one, and 1.0 for a warm, clean slick.
Where the heat comes from
A spinning tyre rubs against the road, and friction turns that rubbing into heat. The power going into the contact is the friction force multiplied by the sliding speed. Suppose the tyre pushes with 1,200 lb of force and the tread slides over the surface at 30 mph, which is 44 ft/s (30 × 5,280 ÷ 3,600). The power is 1,200 × 44 = 52,800 ft·lb/s. One horsepower is 550 ft·lb/s, so that is 52,800 ÷ 550 = 96 hp, nearly all of it becoming heat in a thin layer of tread. That is why the tyres smoke, and why a few seconds are enough to warm them.
Cleaning the tread
Tyres pick up dust, grit and small stones in the pits. Any layer of dirt between rubber and track acts like a lubricant, so μ is lower. Spinning the tyre scrubs the surface and throws the loose material off. At the same time the hot rubber sheds a thin film, and the tread that touches the track afterwards is fresh and tacky.
The water box
Many tracks provide a shallow wet area, the water box, in front of the staging lanes. The driver rolls the rear tyres through it, then does the burnout. The water has two uses, and a limit:
- It lets the tyres spin easily. Water lowers the friction, so the engine can spin the wheels without great effort and the tyres stay under control.
- It helps clean. It loosens dirt, which the spinning tyre then throws off.
- It is not the launch surface. Water is a poor thing to launch on, so the tyres must dry. The burnout and the roll back to the line throw most of it off.
What the extra grip is worth
The launch RPM post worked out a traction limit for an example car: 3,000 lb with 50% of the weight on the rear axle, a 100 inch wheelbase and a centre of gravity 20 inches high. The maximum acceleration is a = 0.5μ ÷ (1 − 0.2μ), in g. The 1.0 case gives the 2.44 s of that post, and 0.8 gives its 2.80 s. The 60-foot time follows from t = √(120 ÷ (a × 32.17)):
| Tyre state | μ | Max acceleration | Force at the road | 60-foot time |
|---|---|---|---|---|
| Cold, dirty | 0.8 | 0.476 g | 1,429 lb | 2.80 s |
| Part-warmed | 0.9 | 0.549 g | 1,646 lb | 2.61 s |
| Warm, clean | 1.0 | 0.625 g | 1,875 lb | 2.44 s |
Going from 0.8 to 1.0 raises the available force from 1,429 lb to 1,875 lb, 31% more, and takes 0.36 s off the 60-foot time. The force at the road is the car's weight (3,000 lb) times the acceleration in g, so 3,000 × 0.476 = 1,429 and 3,000 × 0.625 = 1,875.
Grip sets the launch, so a better tyre moves everything: the engine can use more torque before the tyres spin, and the best launch RPM rises with it.
That is the tie to the launch. The sweet spot in the launch RPM post sits just under the traction limit, and the limit is only as high as the tyre makes it. A driver who skips the burnout, or does a poor one, launches with a lower limit than the one they set the car up for. The same revs that were fine on a warm tyre spin a cold one.
What a burnout cannot do
- It has limits. Spinning too long overheats and wears the tread, and grip falls again.
- It does not change the load. N is still the weight on the driven tyres, as in the power-to-weight post.
- It shows on the slip. Look for a better 60-foot time, as the time slip post explains.
In the game
Pink Slip Kings is a 3D drag racing game in which you choose your launch RPM and balance power against tyre grip. Its listing does not mention burnouts or tyre temperature, so this post is real-world physics, not a description of the game. The listing does confirm race time slips and tuning to balance power against grip. The game is in closed testing.
Pink Slip Kings: Drag Racing is a 3D drag racing game with manual launch and shift controls, detailed tuning, a dyno, telemetry and race time slips, and Test & Tune and tuner sandbox modes. It plays offline, has no ads and no in-app purchases, and its in-game cash and cars cannot be bought or exchanged for real money. The game is currently in closed testing.