Threshold Braking – Trail Braking

An MLR Performance Driver ‘Track’ Day at Millbrook begins with an ABS braking exercise. Braking from high speed with the ABS activated allows the driver to brake and steer to avoid a hazard. ABS is a safety aid. It allows the wheel to remain rotating throughout the emergency braking and steering phase, as opposed to a locked wheel which will offer a very limited percentage of grip. A tyre has to be rotating to grip the road.

Threshold Braking

For the more track focused driver this is not the most efficient technique as by the nature of its operation, the ABS system increases the overall stopping distance. If we want to stop quickly and safely on circuit, we need to consider a more efficient technique known as Threshold Braking. Once you are proficient in braking with the ABS activated, this is your follow on exercise. Threshold Braking involves retarding your Evo to the tyres limit of adhesion prior to the ABS intervening, or for those earlier EVOs, the wheel locking.

Once mastered, Threshold Braking stops you in the shortest possible distance, reducing lap time & promoting a systematic approach to corner entry speed. Using a road braking technique on circuit can be fraught with problems – the result is often a trip to the gravel trap.

The questions that follow from the braking session are often related to Trail Braking. Let me try to remove the mystery and confusion regarding this subtle and often misunderstood technique.

What is Trail Braking?

The safest braking technique when slowing for a corner on the public highway or circuit is to brake in a straight line, returning to the gas pedal prior to turning into the corner. This technique ensures all the tyres available grip is focused on one job only: stopping. This has to be the most efficient way to stop or does it?

The answer has to be yes it is. For both road and track, it is a safe and efficient technique. However, as with most questions regarding driving technique, there is always an exception to the rule. The debate begins if you drive a mid or rear engine car on circuit. I should say at this point that I would not advise or recommend trail braking as a road technique. It is a grip limit technique, that when used correctly, has the potential to reduce your lap time and has no place or benefit if used on the highway.

Trail Braking involves braking to threshold a little nearer to the corner than a straight line brake, so that 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 versus braking effort is decided primarily by vehicle configuration, vehicle weight and the corner being driven. If we use a Lotus Elise as an example we can consider the benefits and potential pit falls.

Trail Braking optimises grip as you turn by using the braking effect to induce weight transfer to the loaded side of the vehicle, increasing grip and stability. The effects of braking in a straight line are obvious as we can watch the nose of the vehicle dive. Imagine the nose diving with steering input applied and all becomes clear, (I hope). In effect we are using the brake pedal to accelerate the transferred weight onto the loaded side of the vehicle, to the tyres optimum weight and slip angle.

A rear or mid engined car has the majority of its weight in the rear, this encourages understeer as you turn in. The vehicles weight distribution and rear wheel drive literally push the vehicle off line as you turn in. If you drive a Porsche, Ferrari or Lotus, or any vehicle with a similar configuration – when used correctly trail braking will assist turn in grip and stability. It needs careful exploration to get the technique right and is not a technique to practice if run off area is limited.

What’s all this talk about rear wheel drive and mid engines? This article is in Wastegate Chatter! Yes – back to front engine front wheel drive. For the Evo driver Trail Braking has a limited use. You already have all of the weight you need sitting over the front wheels to assist turn in, trail braking in front engine FWD configurations will invariably have the unwanted effect of promoting understeer.

This could feel like a long winded way of saying ‘you don’t need to trail brake in an Evo to post a fast lap time’.

Sometimes we need to know what not to do, to know we are doing the right thing.

Until next time

Regards

Colin

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