*shipping not included *prices in $CAD

Race Logic Traction Control

Quick Links:

How It Works

The system works by monitoring the speed of all four wheels using the ABS system or specially fitted sensors. When wheelspin is detected the engine power is reduced, by cutting a single injector pulse or a spark, until grip is resumed. This occurs in a thousandth of a second, and appears to the driver as a slight miss-fire with no loss in acceleration.

Maximum acceleration is achieved by limiting the slip between the tire and the road. The point at which a tire is just beginning to slip against the road gives the maximum coefficient of friction value.

From the graph above it can be seen the maximum coefficient of friction () occurs at a slip between tire and road of 10% when dry, and around 5% when wet.

Maintaining this level of slip is inherently difficult, as the grip levels drop off significantly above these levels, meaning the balance between too much wheelspin and not enough power is very fine.

To drive the car and search for these levels of slip is very difficult, the moment the wheels start spinning too much (and how do you feel what is too much?) the power has to be reduced (by what amount?).

Top rally drivers have a good feeling for this limit, but they still tend to stay on the side of caution, and modulate the wheelspin between 10-20%, as this will still achieve 90% of the available traction. The closer to 5 or 10% slip, the higher the chance of reducing the power too much, and hindering acceleration, but also the closer you are to using 100% of the available traction.

The main reason for this is the response time of a human being. The fastest human reaction to a sense stimulus is 1/10th of a second, and the fastest acting throttle reacts in around the same time. This means there is a 2/10ths of a second lag between the wheel reaching a critical slip level, and the driver being able to change the amount of power being applied. This is why really good drivers tend to drive between 10 and 20% slip, to give a margin of safety should the tires suddenly find a little more grip, causing the wheel to stop spinning completely.

Less experienced drivers will tend to allow 20-30% or more slip, again to maintain wheelspin rather then let the car 'bog' down, thus limiting their grip levels to around 85% of their maximum.

With the advent of fast reacting electronics on cars, this problem has been tackled with Traction Control systems. In race cars, Traction Control Systems have two functions, number one is to maintain the precise level of slip that will give close to 100% of the available grip, and number two is to maintain stable cornering. These two functions are linked, but require slightly different approaches.

The speed of reaction of a race Traction Control System is critical in maintaining a precise level of slip. The electronics themselves can react within a thousandth of a second, but to remain effective the engine power has to be quickly, and precisely controlled.

In road cars Traction Control normally relies on two methods of reducing the speed of the spinning wheel, brake application and throttle intervention. Brake application is a very effective and quick way of reducing the speed of a spinning wheel (almost unusable in a race situation - more later) but the accompanying throttle intervention is mechanically slow, and will also only reduce the airflow, which takes some time to become effective. On a road car the Traction Control System plays a third role, one of safety, in this role the level of slip is reduced to zero, and held there. This results in a very stable car, but one which will not accelerate at it's maximum potential at all times.

Race Traction Control Systems rely on much more precise, and faster acting ways of reducing power. The first method is shutting off fuel to the engine, and the other is cutting out the spark. Both methods have exactly the same high speed modulation ability, but the spark cutting system will happen potentially one cycle earlier. The magnitude of difference in reaction times between spark cut and fuel cut is negligible compared with the difference between throttle actuation and spark/fuel cut. (See fuel cut and spark cut below)

The Traction Control System then comes down to the interaction between the information from the wheel speed sensors and the level of power reduction applied. A good system would be capable of maintaining a level of slip that is adjustable depending on conditions.

Many factors affect the ideal level of slip, wet / dry conditions, speed of the vehicle, lateral g-force (cornering), tire compound, tire pressures etc. Ideally the driver should be able to dial in a base level of slip that takes into account weather and tires, and the system should adjust automatically for speed of the vehicle and lateral g-force.

When cornering, the system should reduce the amount of slip available, to prevent lateral slip from occurring, and vary this amount depending on the speed of the vehicle. At high speed, low grip situations, this slip should be around 1-2% to maintain forward momentum, and at low speed high grip situations, this can be much higher.

Fuel Cut

The idea of cutting fuel to an engine sets alarm bells ringing in engine builders, as they all know of the potential disaster of a high revving race engine running lean. Running in a lean combustion mode will elevate in-cylinder temperatures very rapidly, the denser the air/fuel charge, the more heat the lean burn can generate. Therefore it is vital that a fuel cut system will not cause a lean burn.

The simplest way of preventing a lean burn is to remove more than 50% of the fuel from the pulsed delivery. A mixture will only ignite if the air/fuel ratio is within a tightly defined window, look at the efforts being put into making lean burn engines fire on very low air/fuel ratios (1:20 or more). Removing more than 50% of the fuel will cause an air fuel ratio of over 1:25 and will result in a complete miss-fire, with the unburned fuel passing out through the exhaust valve. Even if a high air/fuel ratio did manage to ignite, the energy available from the amount of petrol injected wouldn't be enough to elevate temperatures significantly. Of course the ideal system will remove 100% of the pulsed fuel delivery, allowing the cylinder to take a gulp of fresh air, and the in-cylinder temperature would remain virtually unaffected. Racelogic Traction Control operates in this manner - the complete injector pulse is removed so no possibility of lean burn can exist.

Prolonged fuel cut on one particular cylinder would cause scavenging of the petrol lining, the inlet tracts, and when the next full fuel pulse arrived, it would be partially reduced in quantity by the re-wetting of these tracts. Therefore it is often important to manage a rotation of the cylinder cutting to prevent this situation from occurring.

Spark Cut

Cutting the spark to an engine will stop any chances of a weak mixture occurring, but it carries it's own potential problems due to a large quantity of unburned fuel travelling through the cylinder and out of the exhaust. This petrol can remove some of the oil lining the inside of the cylinder, and pass it thorough the exhaust, again this only becomes a problem if the fuel to one particular cylinder is cut for an extended time. The best way to overcome this is to rotate the order in which the cylinders are cut.

The unburned fuel in the exhaust will have a catastrophic affect if there is a catalytic converter in the exhaust, as it will try to convert the unburned fuel to harmless elements, effectively burning the mixture. This causes the catalytic converter to heat up very rapidly, reaching temperatures in excess of 1000C, and possibly melting down completely. Thus prolonged spark cut is not recommended for catalytic equipped cars.

Quick Links: