Nearly every new performance and sporting car that’s sold today comes standard with electronic systems that have a major impact on how the car drives. Whether it’s traction control, stability control, ABS, variable power steering weight, or active four-wheel drive torque spilt, all help dictate the way you can drive the car.
In a rear-wheel-drive car, for example, power oversteer is out of reach if you have traction control. In a front-wheel-drive with stability control, traditional front-wheel-drive handling techniques – like powering through into understeer then getting the tail out with a quick throttle lift – can no longer be done, because the stability control intervenes. A four-wheel-drive car with a strong electronically-controlled rear-wheel drive bias has to be driven like a rear-wheel drive – rather than like a constant 50:50 four-wheel drive car.
In short, all these systems take away a lot of driver flexibility – often, they simply spoil the fun. So that’s it then, we’re stuck with a boring future of driving cars in the way the car companies say we should drive them....
Just because you buy a car with soft suspension, it doesn’t mean that you can’t fit firmer springs and dampers, does it? And just because you have a traction control system that intervenes too early, it also doesn’t mean that you need to live with it. Do-it-yourself modification of active car handling systems is here now – you can get a far better driving outcome. Altering these electronic systems can make a radical difference to the way a car drives, and often the mods can be done very cheaply.
Over the next year we plan to carry out some major modifications of these electronic handling systems. But don’t worry, you don’t have to wait a year for some of the results - those you can do right now! Specifically, if you have a car with stability control you can modify how much wheelspin – and so how sideways – you can get.
But first, let’s look at the car systems and how they work. Without this knowledge there’s simply no way that you’re going to be able to make effective modifications.
Traction control systems use wheel speed sensors (shared with the ABS) to determine if one or more of the driven wheels is rotating fast than the undriven wheels. So in a rear-wheel-drive, if either of the rear wheels is spinning faster than either of the front wheels, the Electronic Control Unit (ECU) determines that traction is being lost. Engine power is then reduced and/or the spinning wheel is braked.
However, that very basic overview misses a lot of important subtleties. Some traction control systems do not measure the speed of all the wheels – those that do are called ‘four channel’ systems. Three channel systems group a pair of wheels, measuring the speed of only the fastest. This is easily done in a rear-wheel-drive car that doesn’t have an LSD by measuring tailshaft speed – if one wheel is spinning, the faster rotation of the tailshaft will indicate this.
Another point to think about is that every time the car turns a corner, all the wheels follow paths of different lengths and so rotate at slightly different speeds. The ECU needs to take this variation into account, as it also needs to when the tyres have different diameters. (All tyres have some differences in circumference, let alone those which are more worn than others.) In addition, the ECU will have different strategies when wheelspin occurs at low speed than when it occurs at high speed.
When wheelspin has been detected, the ECU can decrease engine power by:
Many cars will do a combination of these actions – for example retarding the timing and partly closing the throttle. Note that the car doesn’t require an electronic throttle to do the latter – there are systems (eg Holden V8) where a cable is used to manually close the throttle against your foot pressure. However, cars with electronic throttle normally do a smoother job of dropping engine power. Yet another approach is to use an electronically-controlled second throttle in series with the first, driver-controlled butterfly. Some Toyotas have used this approach.
Many cars also use the ABS to automatically brake the spinning wheel. This transfers drive-power to the other wheel on the same axle (or on some four-wheel drive cars with open diffs like the Holden Adventra, it can transfer drive to the other end of the car). Some manufacturers, like Volkswagen for example, call this traction control approach an ‘electronic diff lock’. At lower speeds you can usually hear when this individual wheel braking process is occurring because there is a slight pulsating buzz through the car as the brakes are rapidly applied then released.
It is very important that traction control not be confused with stability control (covered below). A traction control system is designed to stop wheelspin; that’s all. Of course, by doing this it will also help control handling, but that’s not its primary function. So how does a traction control system affect handling?
Take a front-wheel-drive car. Normally, a front-wheel-drive can be fairly easily pushed into power understeer – the front tyres have to both turn the car and also pull the car along. When the combination of these loads is too great, the car will want to go straight ahead – to understeer. If the traction control system limits the amount of power that can be put through the front tyres, there will be more grip left to turn the car, so understeer will be reduced. But if the cornering load becomes too great, the car will still understeer.
A rear-wheel-drive fitted with traction control will be much less likely to power oversteer when cornering – the tail won’t be able to be kicked out with the application of power. However, the car will still happily oversteer if the rear cornering load becomes too great when wheelspin isn’t occurring. For example, if you enter a slippery roundabout and get on the power, the traction control system will cut in to stop power oversteer, but it won’t prevent the car from sliding if when the wheelspin has been stopped, the lateral load on the rear tyres is so great that the car wants to keep on going sideways.
Stability control is a very different ballgame to traction control. Instead of comparing driven and undriven wheel speeds, stability control compares the direction that the car is heading with the direction that the steering wheel is pointing in.
The amount and direction of the steering lock is measured by a steering angle sensor. By watching the input of this sensor, the ECU always knows where the driver is intending that the car should head. If the driver has on a lot of left-hand steering lock – say a half-turn - the ECU knows that the car should be negotiating a fairly tight left-hand corner. In other words, the car should be rotating around its vertical axis in an anti-clockwise direction.
The direction and speed of rotation of the car around its vertical axis is measured by a yaw sensor. (One type of yaw sensor is pictured here.) If the yaw sensor indicates that the car is heading straight on (ie no yaw) and yet the driver has turned the steering wheel to the left or right, the car must be understeering. If the driver has only a small amount of right-hand lock and yet the car is yawing wildly in a clockwise direction, the rear end of the car is sliding.
There are two important points to come out of this. Firstly, the stability control system only knows where the driver is intending that the car go by the steering angle. This means that you should always keep actively steering (ie not freeze) when driving a sliding car equipped with stability control – whether the system is modified or standard. Secondly, the system to a large extent doesn’t care what else is going on with throttle, wheel speeds and all the rest – its primary interest is in actual car yaw versus what yaw should be occurring.
(Of course, how much correction is applied will depend on the overall car speed as well as the angle of yaw versus steering wheel angle. But here we’re dealing with the most important inputs.)
When the stability control system senses that understeer or oversteer is occurring, it can take several actions. The most important of these is braking individual wheels. This is not the same idea as braking individual wheels in a traction control system; instead the desired outcome is to pivot the car around the slowed wheel – to yaw it back on track, if you like.
When a vehicle is understeering (the front sliding wide), braking of the inside rear wheel will reduce the amount of understeer that occurs. For example, if the car is understeering around a right-hand bend, braking the right-hand rear wheel (while the other wheels continue to turn at their normal rate), causes the car to pivot around to the right. That's where you want the car to go when you're going around a right-hand bend, so the understeer is reduced.
On many cars equipped with stability control this understeer reduction can be clearly felt as the inside rear wheel is braked – the nose of the car pivots back on line in small, rapid steps. However, other cars brake both rear wheels to control understeer. This is so that there is a weight transfer to the front of the car, allowing the front tyres to grip better. (The front wheels aren’t braked because then the forces that they’re trying to transmit to the pavement would rise, giving less grip available for turning the car.)
When the vehicle is oversteering (the rear sliding out), the outside front wheel is braked to correct the slide. If the car is oversteering around a right-hand bend, this would be the front left-hand wheel. If you picture this wheel nearly stopped but the others continuing at normal speed, you can imagine that the car attempts to pivot around to the left, reducing the amount of oversteer
So the primary way in which stability control stops the car from under- or over-steering is by braking individual wheels to yaw the car back on track.
All cars that I have driven with stability control also incorporate traction control, but they are not the same system! My Lexus LS400 is an excellent example of this. When the traction control system is operating, a telltale light flashes on the dash. But when the stability control is operating, a rapid beeper goes off. With the car in standard form, and even when driven hard, the traction control operates perhaps 20 times as often as the stability control.
Traction Control and Stability Control
Let’s look at how traction control and stability control interrelate. You’re in a front-wheel-drive car and have entered a corner. You’re going for the traditional ‘slow-in, fast-out' approach that works well with so many cars. You get to the apex and start getting back on the power. The front tyres, overwhelmed with their combined torquing and turning duties, start to lose grip. This causes both their rotational speed to increase (especially the inner wheel in a non-LSD front-wheel drive) and their lateral (sideways) grip to decrease. The result of this combined outcome is that the front of the car starts to drift into understeer.
Sensing that the inside front wheel is spinning faster than a rear wheel (or perhaps a combined average of the rear wheel speeds), the traction control system starts to limit power. This reduces the tractive effort load on the front tyres so they grip better – the power understeer is reduced.
But this is one corner you’ve really stuffed up – realising that the corner now tightens, you reef on some extra lock. The front tyres, despite the fact that they’re no longer power-understeering, go back to simple old understeer... they can’t cope with the sheer cornering loads. Now the traction control system can do nothing – in fact it doesn’t see anything wrong because all four wheels are rotating at more or less the same rate...
But the stability control can see that you’ve put on extra steering lock that isn’t being responded to. It immediately starts to brake the inside rear wheel, causing the car to pivot (yaw) around that slowed wheel. So the nose starts to follow the steering angle you’ve asked for – and you stay on the road.
See how the action of the two systems is actually quite different? In many cars they’re integrated under the one banner but it’s very important to modification outcomes that you separate the two ideas.
Next week we’ll take a look at modifying just the traction control component of a stability control system.