Please help with some jargon-busting. What are “double-declutching” and “cadence braking” and “heel-and-toe” driving techniques? How are they done, what are they for, and are these skills that all drivers should be good at?
Evans
Advanced driving techniques add to your competence and control and can sometimes be useful. But in modern cars, they are rarely essential.
Double-declutching is a blast from the past. It had to be “normal” practice for changing gear before the invention of “synchromesh” and “helical” cogs in gearboxes. But it can still be useful if a modern manual gearbox becomes awkward (e.g. grinding noise or refusal to engage smoothly) through age or defect. It helps to mesh cogs to line up with and engage teeth with each other more smoothly.
The technique is to fully depress the clutch pedal once to move from a gear to neutral and then release the pedal momentarily before depressing it again to move from neutral to another gear. Then release the clutch again to resume powered driving. Although a modern gearbox can be used by moving from one gear to another without pause as the lever passes through neutral, a momentary delay while in neutral (combined with engine rev adjustment when changing down) makes the process even more seamless.
What this, or double-declutching, does is to help “synchronise” the rate at which the engine is turning (ranging between 700 and 6,000 rpm) and the rate at which the wheels are turning (between 0 and 2,000 rpm), in relation to the gear cogs (of different diameters) which transfer engine power to the drive wheels.
Helical gear cogs (which slide together rather than biting together) and synchromisers (which help cogs adjust to change) achieve that quite smoothly without much help from the driver. On old-fashioned gearboxes that had “square cut” gear cogs and no synchromesh, the process was so crude (and unsmooth) that the design was known as a “crash box”, which involved quite a lot of noise and lever-wrestling (and wear and tear) for the unpractised.
Double-declutching was a “manual” skill that helped harmonise the meshing process (what synchromesh now does automatically) as was simultaneous deftness on the accelerator pedal. At a steady speed, when you change down from fourth to third (a lower ratio) the engine will have to rev faster to synchronise with the wheels. Skilled double-declutching does that in combination with an interim pressure on the accelerator to get the engine revs to the level they will need to be at in third gear at that road speed…in advance.
Indeed, if the judgement of that change is good enough, manual gears can be changed even without using the clutch! There’s a get-out-of-jail-free card in that for clutch-failure emergencies.
This is perforce a much-simplified explanation. Gears and cogs are a major and complex branch of mechanical engineering that could fill a library, never mind a book. They are abundant in many parts of our lives, but most of us just file their work in a taken-for-granted box marked “magic”.
Cadence braking has also taken a leap towards “automatic” with the introduction of Anti-lock Braking Systems (ABS) which are designed to allow maximum braking effort…without causing the wheels to lock solid and skid.
Before mechanical ABS was developed, skilled drivers aimed for a similar effect by braking as hard as possible, but releasing pressure as soon as the wheels did start to lock and skid. In practice that was an on-and-partly-off process in a rapid pumping motion.
It aims to slow or stop the car in the shortest possible distance while retaining steerage and control. The somewhat counter-intuitive fact is that braking applied to the rotation of a wheel is more effective than locking the wheels so they skid over the road surface. This is especially true if the surface is wet or slippery. Also, when wheels are locked their ability to steer the car is forfeit. So if, while emergency braking, your car still hurtles towards a collision and won’t steer properly, don’t press even harder on the brakes. Release them so the wheels turn again to restore some steering control.
Sheet ice is an issue for ABS, because almost any braking effort will cause the wheels to lock, so the system allows almost no braking effort at all unless the tyres are studded. A car with really chunky winter treads would probably stop sooner by deliberately locking the wheels to rip into the surface to achieve deceleration.
Another exception is deep sand, where locking the wheels causes very rapid deceleration because instead of skidding the locked tyre digs itself into the surface, changing the problem from stopping (which happens very suddenly) into getting going again.
Heel-and-toe is a technique when a driver needs to apply the brakes (to slow down the wheels…and hence the car) while maintaining or increasing engine revs at the same time. The driver’s right heel remains on the accelerator, and the right toes are twisted anti-clockwise to operate the footbrake. The left foot remains available for simultaneous gear changing. Where gear changes are not necessary, a self-explanatory alternative to heel-and-toe is known as “left-foot braking”.
These techniques are widely used in high performance driving in motorsports. Though rarely shown on television, the driver’s feet have to move with the speed and deftness of a tap dancer, which is why drivers wear special shoes…lightweight, flexible, with a grippy sole, and a smooth transition from the upper to the sole so there is nothing to snag on the edges of pedals.
Heel and toe can be useful for ordinary motorists when conducting complex manoeuvres at low speed – like hill-starting when the handbrake doesn’t work.
