4x44X4 AdviceCar Advice

Traction Control is not the same as Stability Control!

20 Jun 2015

Robert Pepper 6

Traction control and electronic stability control (ESC) are related, but not the same and the terms are certainly not interchangeable.

What they are

Electronic Stability Control (ESC) – is a system to keep a vehicle going in the direction the driver wants it to go, which is usually on the road and rubber side down.  ESC can be thought of as anti-skid control, and it primarily corrects understeer and oversteer.  Understeer is when you run wide around an corner, needing to wind on more and more steering lock to no or little effect.  Oversteer is when the back of the car wants to overtake the front. 

ESC uses wheel speed sensors to determine how fast each wheel is travelling, then a lot more sensors such as roll, yaw, throttle position, steering wheel angle and more to figure out whether the car is going where the driver wants.  The on-board computers then apply the brakes to one or more of the four wheels to correct the vehicle’s direction. 

For example, if you’re understeering it may apply the brakes on the inside wheels and release those on the outside slightly, thereby tightening the vehicle’s turning radius. In an oversteer situation it will brake one or more of outside wheels. Generally, if the steering wheel is turned the car will try and respond to that direction, so here’s some advice – look where you want to go, and keep the steering wheel turned where you want to go. 

In extreme cases stability control can also cut the throttle, despite the accelerator being floored.  It also uses traction control (below) to help it do the job where appropriate, for example to prevent fishtailing in powerful rear-wheel-drive cars.

The official, generic name is ESC, or Electronic Stability Control (or sometimes ESP, Programme).  Manufacturers have all sorts of other names such as VDC, DSC, ASC…usually some combination of Dynamic, Control, Vehicle, Active, you get the idea.   It’s all the same thing, as there’s only a few automotive electronics manufacturers that resell to all brands.  However, the effort that each car manufacuter puts in to calibration and design varies quite considerably, so the electronics tend to be much more effective in the more expensive vehicles. 

ESC is a wonderful safety technology and if you are buying a new car it is mandated by law.  If you are safety concisous and buying an older car, buy one with ESC (and side airbags).

Traction control (ETC, Electronic Traction Control) can be thought of as wheelspin control.  It does not control vehicle direction directly.  It is ESC that controls vehicle direction, and ESC can if necessary use traction control to help.  Traction control is split into two types:

Engine traction control –  is a mechanism that will reduce engine torque if the driven wheels are both spinning, thereby regaining traction as madly spinning wheels mean you’re going nowhere. Say you’re at the lights on a wet day and make a quick getaway, both wheels spin…this is where engine traction control kicks in to say “nope, no point you having any more power you’re just wasting it”. Engine traction control doesn’t actually slap you, but no doubt that’s coming in the future.

Brake traction control – detects a single wheel on an axle spinning faster than its opposite partner.  The system then brakes the spinning wheel, sending torque (“drive”) to the other wheel.  The vehicle then continues moving.  Traction control is primarily used in offroad vehicles to maintain momentum over uneven terrain, although it can and will kick in on road if a wheel begins to spin. 

Brake traction control definitely does not place undue wear on the brakes as in owners noticing they are suddenly going through brakepads very quickly.  The system has checks to ensure it does not overheat and there are other components that typically give up before the brakes do.  The reason is that the energy required to slow a single spinning wheel which is spinning relatively slowly is quite small, much less than the energy required to slow the complete vehicle at speed.  That said, some vehicles – the Pajero is a notable offender here – do disable brake traction control if components overheat.  Tip for Pajero drivers is to use the rear locking differential (if fitted) on long, loose hill ascents.  On softroaders what tends to overheat is the central clutch, not the brakes itself.

Brake traction control does not slow you down either, as that spinning wheel was doing nothing for you anyway.  It actually helps maintain momentum.

 Typical operation

There are so many variations it is impossible to list them all, but in general:

• Roadcars have an ESC off switch which usually just de-sensitives ESC so it kicks in later.
• Sportscars have the same switch, but the deactivation is greater so ESC kicks in even later.   Sportscars are likely to have two, three or even more modes of ESC ranging from fully on, a sports mode with reduced sensitivity, and fully off.  Such modes may be manually selected, or there may be overall sports modes which change ESC activation points as well as change automatic gearshift patters and throttle sensitivity.  Typically the first ESC off mode is a short press of the button, and the second mode is a long (3 second) press.
• 4X4s have an ESC off switch which disables ESC.  It will also disable engine traction control, but leave brake traction control active – the distinction is important, as you do not want your throttle control restricted.  Typically the same thing happens when the centre diff is locked or 4WD is engaged, and ESC is almost always disabled in low range. 

ESC may be re-enabled once the engine is turned off and then on, or maybe once a certain speed is exceeded. 

Exactly what the ESC button does is very much vehicle-dependent.  Read the owners manual for details.  The skidmark icon on left button on in the image below is now a typical ESC-off button – older cars had the acronym such as “ASC” on the button.

 Using esc and etc In the real world

Public roads – leave it on.  There is no reason to turn it off, you won’t go faster unless you are driving irresponsibly quickly.  A growing problem is younger drivers learning in benign front-drive cars all with ESC, then jumping into the likes of the Toyota 86 and discovering that rear wheel drive can bite once you remove the electronic safety net.

Dirt roads – leave it on. You won’t go any faster with it off unless you are driving irresponsibly quickly.  ESC can be a real lifesaver on loose dirt.

Offroad – rocks, mud and the like.  Turn it off, as the car will naturally be slipping and sliding a bit, and if ESC kicks in you will have your momentum killed.  Look at the Pajero at the top of this page; no way that could work with ESC enabled, or engine traction control.  Brake traction control is your friend and will never do anything but help.  Engine traction control and ESC will get in the way, so they are disabled.

Track driving – you can leave it on, especially in sports mode.  If in sports mode it kicks in, then either it has saved your backside, or you were sideways enough to be slow anyway.  However, this does not apply to older or poorly calibrated systems which were too early and harsh to kick in.  But if you want to drift or do stunts like j-turns you’ll need it completely off.

Car modes

Many cars now have different modes, particuarly 4X4s and sportscars.  The modes correspond to different driving situations.  In the case of sportscars there might be a normal mode, sports mode and track mode.  In the case of 4X4s it might be terrains like mud, sand, grass/gravel/snow and a normal mode.  Even normal roadcars have an eco mode for when you feel like pretending you’re doing something environmentally useful.

These modes typically change automatic gearshift points, throttle sensitivity, torque delivery, and ESC/ETC calibration.  The differences are subtle, and if the manufacuter has actually done some work, useful, or if it’s just been slapped on as a marketing exercise, pointless.  The split is about 50/50 in my experience, and the more expensive the car, the better designed the electronics.

Other tips and tricks

• There are myriad additional sub-programmes using the same basic systems as ESC.  These include anti-roll control, corner brake control and more.  As usual, the marketing people belch out weird and wonderful names for all this wizardy.
• The electronic systems are much more effective in later vehicles, in the same way your mobile phone of 2015 is much better than your phone of 2010 despite them both being called ‘smartphones’.
• ESC can largely recover from skids before they even start, but good drivers never have ESC activate.  And if it does, the correct driver reaction will assist the tech to do its job.
• Some older vehicles have brake traction control but no stability control.  No vehicle has stability control without both forms of traction control.  Stability control is made up of traction control, ABS and a programme to determine whether the vehicle is understeering or oversteering and take appropriate action.  The wheel speed sensors are sometimes referred to as ABS sensors, but that’s not correct as while they are used for the ABS function they are also used for much more.
• When people talk about “switching traction control off” (Top Gear, anyone?) then what they usually mean is switching stability control off, which may or may not also switch traction control off.
• If you hook into a corner too quickly under brakes, traction control will do nothing at all for you.  Instead, ABS will stop the wheels locking so you can steer, and stability control will help you make it around the bend without dying.
• There is often only the one dash light to indicate both traction control and stability control.  It may flash when in operation…but you won’t always be able to tell if it is flashing because of one of the traction control programmes or stability control programmes is in operation.
• The laws of physics remain unchanged whether ESC is enabled or not.  All ESC does is maximise use of traction, and traction is a finite resource.  It is possible to get a car sideways with ESC enabled, as in the ongoing war between engineers building better systems and humanity building better fools, the humans remain ahead.
• ESC is very, very good at catching and preventing skids.  Switch it off, try the same thing again and you had better have a very deep reserve of driving talent.
• Electronics are here to stay, whether you like it or not!

 Amazingly a friend sent me this after he came across the same incident.  He says:

“I passed the upturned Jeep, only a couple of hours after it happened on the Great Central Highway- was very upsetting to see, full of kids stuff etc, blood on water containers. My forensics estimated that the driver had wandered to the left of road, panicked, over-corrected to the right, left front wheel dug in a little and the rest as they say is history. It turned out to be a relatively good story in the end, in that no-one was injured.”