Threshold Braking

Publication: Total Magazine Article 2009

The last area covered by Colin Hoad dealt with a systematic approach to cornering, exploring how subtle adjustments in our technique can have big gains in speed and safety on circuit. He also explained how to interpret the audible and physical “Grip Limit” signals a vehicle communicates to us, as it approaches its limit of adhesion through a corner.

We are moving on now to arguably the most important subject in our circuit driving tool box: Braking. Let’s begin by drawing on our road driving experiences before heading on to circuit. Why? Because that is the map we have in our muscle memory for how to apply the brakes. We will be making small adjustments to your road technique so it is good practice to start with what we already know and build from there.

Just pause for a moment and focus on how you retard your vehicle when driving on the public highway, on approach to a hazard (we will use a corner for this example). Your initial application of the brake is smooth and light, you apply more pressure as the corner approaches, easing smoothly out of the pedal, before finally releasing the pedal and squeezing gently onto the gas to negotiate the corner. All very smooth and timely, we have in fact sectioned your braking into three parts. We can summarise by describing the technique as Feel, Firm & Feather – it is safe and systematic.

My motivation to compare circuit braking with road braking is borne from experience. When driving on circuit we need to modify the three F’s approach. Our objective is to brake efficiently to the limit of adhesion of the tyre, over the shortest distance – a technique known as Threshold Braking. Requiring practice and a little remapping of your muscle memory, if you are new to circuit driving the tendency is to brake as you do out on the public highway and not adjust your technique – this will predictably lead to problems.

As your speed increases it is imperative that you threshold brake. Ultimately you will be achieving good speed on the straights. Braking in a road style could see you out brake yourself, as you will not be braking to optimum G. Your braking technique needs to match your new found speed.

Let’s explore the need to consider your vehicles braking system. ABS if fitted will allow you to brake to maximum effect and simultaneously steer to avoid a hazard. From a technical aspect it is a very useful road safety aid, but it will extend your stopping distance. Via a wheel speed sensor, in an emergency braking situation, the ABS system recognises the wheel is about to lock and skid. Prior to wheel lock, the brake line pressure to that wheel is released and reapplied as the wheel accelerates. It happens many times in a fraction of a second and does the job you want in an emergency on the road – but the nature of its operation will add metres to your stopping distance if used on circuit. Not good if you’re chasing a lap time or trying to stay ahead of the pack.

If you do not have ABS fitted, locking a wheel under braking will require a little more effort from the driver. The wheel/wheels will simply remain locked until you release the brake pedal pressure. A locked wheel offers very little grip – if you are struggling in the braking zone to reduce enough speed to negotiate the turn you may have to consider Cadence Braking.

Cadence Braking requires a fast and robotic pumping on and off the brake pedal to allow the tyre to rotate and regain its grip on the road. How quickly you apply and release the brake pedal will depend on how much grip the tyre has with the road. If the surface offers low grip you will have to pump the pedal more slowly than a high grip surface, as you need to allow the locked wheel more time to spin up before you can reapply brake pedal pressure. Find a safe place to practice and explore how your vehicle behaves at its optimum under braking, and consider the need to know your vehicle before venturing onto a circuit.

Let’s now get to grips with the technique of Threshold Braking. To find true speed throughout a circuit lap we should be feeling comfortable braking as late as possible and over the shortest piece of tarmac. If braking from high speed, the inertia of the vehicle will reduce the potential to lock the wheels at the initial press of the brake pedal. We can therefore afford to work the brakes hard with big pedal effort. As the speed reduces, we need to consider reducing the pressure on the pedal. The inertia is reducing as the speed reduces. Keeping your foot nailed on the brake pedal at the same high pressure as the speed reduces will provoke the ABS, if fitted, to intervene or a wheel to lock, if not .

It is all about feel. The reduction of pressure can be achieved by gently curling your toes away from the pedal or perhaps a gentle easing of pressure on the pedal. Imagine your braking as a straight line on a graph just below the point the ABS will activate or a wheel locks. Your objective is linear high G stops at the threshold of grip, without any peaks and troughs in your graph line. Press too hard as the speed reduces and you will see a spike in your graph as the ABS kicks in, or the wheel locks. To correct this you will then need to ease off of the brake pedal which ultimately reduces vehicle balance and extends your stopping distance. Likewise braking with too little pedal effort will also extend your stopping distance.

How you come off of the brake pedal is also to be considered. Falling off of the brake pedal aggressively will transfer weight through the platform of the vehicle to the back wheels, inducing understeer as you turn in. Be fast and precise, not rough. Master the process and your car will feed you positive messages, feeling settled as you turn in towards the apex. It is a small yet significant adjustment to our road technique. Our three Fs for circuit now read Firm, Firm & Feather.

Travelling at 140mph will see you covering ground at 62 metres per second, so we should be driving to reference points. We have them for the key points in a corner, so let’s extend them to the braking zone. In order to add consistency and safety to a lap, we need to be systematic, almost robotic with our approach to the circuit. Find a reference point to brake to: a count down marker; marshalls post; the start of a rumble strip – something that will not move – and brake at the same point on approach to each corner.

If you feel you are braking too early go past the reference point by a metre at a time and then brake, until you find the optimum reference point. This is the safest way to build a fast lap reducing the potential of running out of road in the braking zone.

I have a very structured approach to braking techniques when coaching, there are no prizes for falling off so ease yourself up to the optimum lap, and make it an objective to master threshold braking every session.

It is worth remembering the technique of Threshold Braking is a moving target. As your track day unfolds you will experience many changes. The track will rubber in, your fuel load will change, your tyres, brakes and dampers will all reach their optimum operating temperature and beyond if you push too hard for too long. Accidents and spillages all add to the challenge as will changing weather conditions – so keep focused and work each braking zone systematically.

Trail braking is often a discussion point on a training day and a technique we teach when you have the basics cemented in your mind and you can drive consistently at speed safely. It involves braking to threshold a little nearer to the corner than if you were braking in a straight line (discussed in the last issue). So as you turn in you are easing out of the brake pedal and onto the gas. In effect you are braking with a degree of steering input applied. The percentage breakdown of steering input verses brake effort is decided primarily by vehicle configuration, vehicle weight and the corner being driven.

Trail breaking optimises grip and stability as you turn in by using the braking effect to induce weight transfer to the loaded side of the vehicle. BUT and it is a big BUT, I would not advise you to practice this technique without instruction. It is fraught with problems and requires guidance from a professional in the passenger seat.

Safe motoring until next time when we will be looking at the abbreviated world of driver aids, ESP, VDC, PSM ,DSC and anymore I can think of.

Regards

Colin Hoad

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