But first, two quick facts:
- Locked wheels are not the most efficient method of slowing down (lots of tyre screech is bad)
- Tyres exert maximum braking force when slight wheel slip is occurring (some tyre screech is ok)
Is threshold braking still relevant in a world with ABS? In a word, yes. ABS is a reactive system - is detects the onset of a locking wheel and reduces the brake pressure automatically until the wheel has regained grip. When threshold braking, the driver attempts to maintain maximum deceleration without locking wheels, so if ABS has kicked in, then something has gone wrong. However, many modern ABS systems are react so quickly to changing conditions that it is becoming more difficult to improve on them, but the best drivers still can - and many track cars do not have ABS.
So, how does one learn this strange and wonderful technique?
First you need to become familiar with the point of wheel lock in your car, which means finding suitably safe area of privately owned tarmac to practice on. Brake hard in a straight line at different speeds (30mph is a good starting point) and get a feel for how the car reacts under rapid braking to the point of wheel lock. Don't stamp on the pedal, but rather apply push it firmly and progressively. Get the answers to the following questions clear in your own mind:
- How hard do you need to push the pedal for wheels to lock (or ABS to kick in)?
- What is the sensation just before the point of wheel lock?
- When wheels do start to lock up, what are the warning factors?
- Which wheels lock up first?
- How does the car behave when a wheel has locked?
- Is there any feedback from the brake pedal?
Once you have the answers to these questions, it's time for…
Now you're familiar with the sensation of braking hard enough to lock wheels in your car or activate the ABS. Remember, as soon as wheels lose traction with the tarmac you're no longer braking efficiently - but even the best drivers lock wheels from time to time. So, it's important to learn how to regain maximum friction as quickly as possible when this situation arises.
Accelerate to a slightly higher speed (say 50mph) and repeat the process of inducing wheel lock. Try to get to the point of lock as quickly as you can without upsetting the balance of the car. But rather than simply screeching to a halt, release the brake slightly to free up the locked wheel(s), then reapply at a slightly reduced pressure to try and maintain decent braking performance.
Get to the point when you can reliably reduce the brake pressure and reapply the brakes in as little time as possible. Some ABS systems may make this process difficult due to built in delays in the electronics, and it might be at this point when you decide threshold braking isn't for you after all!
Now you will have a good feel for the car, appreciate the sensation of heavy braking and understand the point of wheel lock, so it's time to do the perfect run. Accelerate back up to around 50mph and repeat the process of braking heavily, but this time try to come to a halt as quickly as you can without locking any wheels. Remember, a little bit of tyre screech is ok, as long as the wheels haven't locked.
If a wheel does stop rotating, use the 'release and reapply' technique to maintain control of the car.
Repeat as many times as you need to until you can brake reliably without locking wheels - give yourself a pat on the back and check your tyres for wear and tear before going back on the public roads.
Now all you need to know is how to do it in different track conditions!