GETTING A GRIP
1 To help you get up hill, your vehicle might apply the brakes, explains Ian Shaw
When we think of traction, we naturally think of the grip that the tyre can glean from the surface it is travelling on.
Dry Tarmac provides plenty, while it rapidly diminishes in the approximate order of wet road, grass, snow, mud and ice.
Specialist tyres are available for all, but we have all experienced the situation where one wheel has perfect grip while the other spins uselessly. The action of the differential – essential for cornering – is traction’s Achilles heel.
Mechanically locking across the differential is the oldest method, highly effective and still used in HGVs and some 4X4s, but adverse steering effects mean it’s limited to low speeds.
Alternatively, friction plates within the differential provide limited-slip or later ‘active’ differential designs. However, the majority of traction control systems (TCS) use the ABS wheel rotation sensors to detect a speed difference and automatically apply braking to the spinning wheel, just like the ‘fiddle’ brakes on an old Austin Seven trials car.
Vehicle manufacturers have various names for this technology. VW confusingly calls it EDL (Electronic Differential Lock), but it’s not a lock, merely braking individual wheels. Some call it TCS, Grip-Control, or A-TRAC.
No matter, they are all the same and as ABS demands you keep braking, such systems also demand you go against the natural instinct to lift off the accelerator when wheels spin.
Instead, hold or slightly increase power and let the system shuttle the brakes on and off.
Some systems also actively use the accelerator, if for instance too low a gear is selected on a slippery surface, causing sudden deceleration; in a similar manner to an off-road motorcyclist now pulling in the clutch, modern systems apply a little power and ABS brake simultaneously.
ESP has the answer
No, not extra-sensory perception – although it appears to think for itself – but Electronic Stability Program. Mercedes-Benz uses the term ESP, as do others, and the three-pointed star (along with Bosch) is credited with its invention. Others call it DSC (Dynamic Stability Control).
All do the same: detect when the vehicle is not following the dictates of the steering wheel (understeer) or rotating toward a spin (oversteer).
It then brakes one or more wheels individually, increase or reduces engine power, and in later systems, even adjusts steering lock angle, no matter whether the driver is hitting either pedal hard, or not at all. Having experienced the ‘giant’s hand’ effect – steering the vehicle beyond any normal notion of grip on a frozen-lake test facility slalom – it is hugely impressive.
Many light commercial vehicles add systems aimed at the vulnerability of the van to prevailing conditions. Various cross-wind stability systems have evolved, picking up on ESP and using the lane-keeping camera systems to detect drift or potentially dangerous instability in slab-sided vehicles. They can then use the stability control system via the brakes or apply steering assistance. The latter is limited to those vehicles with later-technology electric, rather than hydraulic, power steering to make fine adjustments.