Publication: Total Magazine Article 2009
Last issue Colin Hoad of CAT Driver Training covered Threshold and ABS braking techniques, discussing how small adjustments to your road braking technique can pay big dividends once out on circuit.
Over the last three issues we have focused our learning in the core skills required to drive safely and quickly on circuit. Now I think we should spend some time discussing active driver aids, or as our senior instructor Nick Arnold calls them, TLA’s (Three Letter Acronyms). We know there is a button on the dash – we can turn the system off, but should I? What is really happening underneath me when it is intervening? And what is the relevance of a stability programme when driving on circuit?
These are the questions I am frequently asked on training days, hence we include a session to explore the whys and wherefores of this technological insurance policy on our track training courses. From a safety aspect stability programmes are becoming a main stream feature. Manufacturers have paid for the technology and we can now find them fitted as standard, not a costly extra. If your current car doesn’t have one of these systems in place, when you trade up to your next track day weapon, the chances are it will have one fitted.
ESP, VDC, PSM and DCS are acronyms manufacturers use to describe their electronic stability systems. Depending on whose brand name you are driving the title may vary. In reality they all work in a very similar way. This article is designed to give you an overview of their operation – for detailed information regarding the system fitted to your vehicle I suggest you read your handbook. You will find subtle differences to system features depending on the vehicle you drive.
How does it work? Sensors fitted to your vehicle relay key information to a control unit that computes the attitude of your vehicle through a corner and in emergency situations. If you are driving beyond the vehicles limit of adhesion or make a sudden input to the controls (as in an accident avoidance scenario), the system will intervene, reducing engine power and braking each wheel individually to keep you on the road.
Wheel sensors communicate individual wheel speeds to the control unit. If you are under steering the front wheels are travelling more quickly than the rear wheels, and vice versa for an over steer scenario. If you are sliding your vehicle gracefully in a four wheel drift through a turn with equal lateral acceleration, the system will be working. A clever device known as a Yaw Sensor bolted to the tunnel down by the handbrake measures lateral acceleration, calculating vehicle attitude and degrees of slip. Steering wheel and throttle pedal sensors complete the package.
When new to track days and driving on circuit, my advice is to leave the system on. It is an insurance policy, designed to help you get out of trouble if you get it wrong. Let me qualify that statement. The system cannot defy physics – if you enter a 50 mph corner at 75 mph you are still going to end up in the gravel trap. If you enter the same corner at 55/56/57/58 MPH the system will do its best to keep you on the tarmac. Don’t take liberties and do drive to the rules. The system will intervene, giving you a gentle warning you are over driving.
How do you know when the system has helped you? Engine power will decrease even though your foot is on the gas, coupled with a feeling the vehicle is braking. The vehicle may give an involuntary shuffle and fidget on the road as if you had adjusted the steering. You may also hear the audible pumping of the ABS pump as the brakes pulse through the recovery cycle. As a rule I find the more expensive the car, the more seamless the intervention.
While we still have the system switched on, let’s clarify a technical point and one that is often over looked. The system needs data to work with before it will intervene. If you drove through a corner knowing you are running out of road but gave no reaction on the controls, the system has been given no information to recognize that there is a problem. As your inputs are steady state it will therefore not intervene. You will leave the road and perhaps complain the system is not working.
If instead, in the same corner you recognize your mistake and make an input on the steering to adjust your line, the yaw sensor picks up the sudden change in lateral acceleration. The wheel speed sensors register the slide and as if by magic the system jumps in and helps you. So stay focused and don’t freeze behind the wheel!!
Now having dialled into how the system works, you are beginning to drive with confidence. Your lap time is reducing whilst your speed is increasing. At what point should you consider turning off the system? That is a BIG question and perhaps the answer is never. If you like the feeling of having a silent partner working with you on circuit, no problem leave the system switched on. There are no prizes for falling off and the whole point of participating in track days is to enjoy yourself. An electronic safety net brings peace of mind and less stress, so more brain space to use and concentrate on driving technique: win/win.
But, and there often is one, if you are driving consistently at or towards the vehicles limit of adhesion with the road (grip limit), with the system switched on, it can cause problems.
The system is designed to work a few percentage points before grip limit is achieved. Some manufacturers give you a dial to adjust the amount of slip you require. A BMW M3 CSL for example, has the facility to provide the driver with 15% of slip so you can enjoy the freedom of under and oversteer as well as optimum lap times. But the system will still help collect a major mistake if you make one.
Programmed to prohibit the vehicle achieving true grip limit your brakes are now working and taking on heat each corner and you may not even be aware the system is operating. So you will always be under the optimum level of grip your driving and your vehicle can achieve as the system will be gently working away through each corner – even if your inputs are steady state and ultra smooth.
The smoother you are the less you feel the systems intervention. Your first hint things are not all they should be is when your brake pedal gets longer and longer, travelling further towards the floor before retardation takes effect. When driving on circuit, allow your vehicle a rest period between sessions. Your brakes will quickly overheat if you pound around lap after lap without consideration for the mechanical components and their operation.
When competing, if left on the system will compromise your lap time. To produce an optimum lap the system will require turning off. That is a statement, not advice. Always look to practice and experiment in a safe controlled environment and take the time to understand your track day machine.
The snow has disappeared and spring is on its way. Have you booked your first track day of the season yet? Don’t forget the GTROC and the MLR offer discounted track days and discounts for new members so don’t miss out. Until next time keep safe.
CAT Driver Training Ltd.
Tel. 01234 757633
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