On the Threshold
Autocrosser Alert! Here’s that Solo Line guy again. Read on at the peril of being exposed as a “Masher” or “Stomper.”
Last article I talked about slowing down to go fast. Now I feel a need to talk about a fast way to slow down.
Are there slow ways to slow down? Uh…yeah! It took me awhile to figure out that letting the engine slow the car didn’t yield quick times. It took awhile longer, but I finally decided that mashing on the brakes, locking all four wheels and sliding didn’t seem to slow the car down very quickly either. Once the brakes have the wheels locked, the brakes are done. The only things left to slow the car are wind resistance and the friction levels of the tire contact patches. So, whence came the REVELATION?
At a professional driving school I attended years ago, they spent much time talking about, and drilling us on, “threshold braking.” They told us: To achieve the maximum rate of deceleration, we have to keep the tires rolling. When a wheel locks, the tire slides, and we lose the power of the brakes on that corner of the car. We want to apply braking pressure up to the “threshold,” the point just short of locking the wheels. If we can brake at the threshold, the point just short of lock-up, we will achieve the maximum rate of deceleration for that set of tires on that car. We will slow down faster.
How do you tell when you are at the threshold? You listen and you feel. On asphalt, the tires will talk to you if you pay attention. Just short of that point of lock-up, the tires are actually turning a slight bit slower than the car is moving, and you can hear a “chirp-chirp-chirp” noise as the rubber starts to slip on the pavement. On concrete, it is harder to pick up the chirp, but if you have both hands on the wheel and your seat-of-the pants antenna tuned in (your butt in gear?), you can feel the rate of deceleration and you can feel and hear the difference when a wheel locks.
When a wheel locks, you should hear a dragging sound, similar to dragging your shoe across a thin layer of gravel on the pavement. If rear wheels lock, the car will often start to swap ends. If all wheels lock, the dragging sound becomes that great howling protest when the tires are calling, “Hey, Dummy, let go of the brakes! Please!”
If you hear or feel a wheel or wheels lock, back off the brake pedal very slightly, just enough to let the wheel(s) turn again. Usually, we all back off a bit too much, then have to squeeze back up to the threshold.
To brake at the threshold, you must keep your front wheels pointed straight ahead. You must brake in a straight line. If you are at the limit of adhesion, you cannot ask the tires to also turn the car. If you turn the steering wheel without releasing brake pressure, the front wheels will lock because you exceed the limit of the tires, and you will be plowing toward that outside row of pylons.
Most of us probably do most of the braking in the straight line then release the brake at the turn-in point as we start the turn. However, it is possible to: (1) carry your speed a tiny bit further; (2) start your braking that tiny bit later; (3) still be full on the brakes at the turn-in point, (4) release just enough brake pressure to allow the front tires to also turn the car, but keep enough brake pressure to continue to slow the car up to a point just before the apex of the turn; (5) then resume full throttle as you cross the apex. This is the process that the professional school calls “trail braking.” You are trailing off the brake (gently releasing pressure) as you turn in and as you get deeper into the turn. If a driver in a rear wheel drive car is really good at this, you will see his or her inside front wheel lock just before the apex as the weight of the car transfers off the inside front tire and onto the outside front tire. The wheel will lock only for an instant as the driver releases that last bit of brake pressure and squeezes on the throttle.  In front drive cars, the physics are a bit different. You frequently see the inside rear wheel lock early in the turn as the car “lifts its rear leg.â€?
Now, please, here is a key point: If you are a masher or a stomper, you will never experience threshold braking. Brake hydraulics are just too powerful. When you mash or stomp the brake pedal, you lock the wheels. You go right past that threshold like a shot. You must SQUEEZE your way to the threshold.  Move your foot quickly to the pedal and squeeze on the pressure. To find the limit, squeeze on the pressure until a wheel locks, then back off just enough that the wheel can turn. Soon, you will be able to squeeze to that point just short of lock-up. You will be at the threshold.
One more point: If you can teach yourself to do the braking with your left foot, you will further increase your efficiency because you will be able to squeeze on the brakes at the same time you ease off the throttle. You will eliminate that moment of high speed coasting and unsettling nose dive that occurs as the right foot moves from the throttle to the brake.
Yes, Virginia, all of the above neato stuff is supposed to be done for you automatically if you have anti-lock brakes. However, the few anti-lock brakes that I have tried seemed to release the wheels way too soon, way too much, and for too long a period of time. The cars were “free-wheeling.� I was pretty sure that I could have stopped shorter without the anti-lock. Anyway, some of us are still competing without anti-lock brakes.
O.K.! Now we are all going to ease and squeeze and slow down fast. So, what else? Well, how about some course analysis? Would there be any benefit to knowing where and how the course goes?
Next Solo Line, “Carnal Knowledge.”
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