Stop Brake
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Emergency braking techniques
In defensive driving courses I have encountered various techniques of emergency braking, which were taught down to the innocent trainees. The Trainees, on their behalf, were usually terrified of this particular drill, because of what I call "brake phobia" -- a fear, sometimes nourished by the instructors themselves, that under hard braking, the car would do unbelieveable things: flip, swerve, spin, break or slide endlessly.
The main concept was to avoid locking up the wheels, and trying to stay at the threshold between rolling wheels and locked wheels. There were different methods: pumping, releasing and re-applying, squizzing, pressing and releasing.
1. Cadance braking
This is a traditional method, which is originaly a practice of older car, which had oldschool drum-brakes with unboosted master cylinders, that had to be "pumped". Even than, the efficiency of such a manouver, even if perfected, was not always very good. With modern brakes, however, it's useless.
First off, cadance braking will almost never make the braking distance shorter. On the contrary, it will increase it dramatically. In a real life scenario, if we add up speed, reaction time, perception time, road conditions and car conditions (which are all far from perfect in the real world), we end up stopping even further. Drivers who pump will also hesitate, start off braking weak, increase pressure, and only than resort to rapid pumping. Such an action, however, will be very inefficient, because the drivers usually pump mindlessly, every time coming off of the brakes completly, and than re-applying, without even reaching the point of threshold.
Cadance braking is sometimes taught as a method of "evasive" braking, which alledgly enables aviodance. But, in reality, such a manouver is not always possible, and if it is, the success of an evasive manouver depends on speed more than anything. Without wiping off a lot of speed, which is practically impossible with pumping, an evasive action will not work. Additionally, constant and voilent weight transfers will unsettle the car, and additional steering inputs can spin the car round. If the driver manages to lock-up the wheels, he probably won't manage to release them to enable steering to take effect. This will make the braking distance even longer.
Also, when pumping, what to do with the clutch and gears? De-Clutching might result in a lock-up of the driving wheels, re-engaging can create engine drag. Not engaging at all will shut down the engine, again disabling steering. In an automatic gearbox, each time you let off the brakes the car revs up a bit.
This practice also won't give you good "communication with the driver behind". If he won't notice the screatch of your tires, he probably won't notice flashing brake lights. In general, being rear-ended is better than being front-ended, because of crumple zones, and because both cars are traveling in the same direction.
Mechanically, pumping makes more wear on the brake pads, shocks the car, and wears the tire all around (in contrary to locking-up wheels, which leads to wear in one specific spot on the wheel).
2. Progressive braking
This is a slightly better practice. This time, the driver is squizzing the brake pedal down progressively, up to the point of the threshold, and he than maintains it by constant adjustments to the brakes. The problem with this practice is that it is hard to achieve accuratly, and will usually make you stop only slightly shorter than pumping would.
Another sort of progressive braking is "hesitation". This is the natural response of any driver. As wierd as it may sound, drivers don't naturally stamp on the brakes. Because normal driving involves a very small brake application, and because of fear from hard braking, drivers would usually press the pedal relatively weak to begin with, see if it helps. When they see it's no good, they will increase braking pressure, and usually won't even get to the point of threshold. I have seen this in many courses: drivers, some quite experinced and with good performances in other drills, come up to a car not of theirs, in an empty track, with an instructor, after they were told to brake hard. They mentally prepared themselves, went off, and still squizzed on the pedal.
On the road, especially near crashes, tire marks can be seen. These marks usually appear only a few metres before the crash. Meaning that the driver delibratly or naturally tried to brake progressively, thus applying too little pressure too late.
Again, this practice won't nessacarily provide better steerability or control, because of weight transfers, and the risk of locking up only the front wheels -- both things unsettle the car.
3. Regressive braking
This is a much more efficient practice, this time, the pedal is depressed up to the threshold or even up to wheel-lock-up, and than a bit of pressure is released to keep the car braking at the threshold. This is easier, but the problem is that driver usually apply too little pressure to begin with, about 60%.
4. Threshold braking
Even perfect threshold braking is not nessecarily ideal in all cases. In a combination with steering or cornering, it can unsettle the car. It will increase distances on soft surfaces, and it is extremlly hard to maintain, especially when the road is slippery. In a scenario of split-grip, when two wheels are on a different surface, it is not possible to hold all four wheels at the threshold, so any attempt to do so will lock the wheels on the more slippery surface.
5. Reconmended: Stamp braking
So, all of the sophisticated methods made us stop in a significantly larger distance, and without too much control. What to do, than?
Well, the answer is quite simple: stamp that brake pedal, as hard as possible as quickly as possible. It's a simple equasion: maximum braking + minimum time = car stopps. Yes, in cars without ABS this will cause the wheels to lock-up, but actually, locked-up wheels still stop you quite quickly, even on the wet, or downhill (or both). The tires usually stay in a nice shape.
And what about car control? Well, the car is skidding, but it isn't out of control. A good stamp on the brakes means that all four wheels lock-up (unlike locking only the front two), so they are now operating like four quite large patches of rubber, which are being pressed down on the road by an enormous force and pushed straight ahead by an even greater force. Where would it go other than straight? Yes, road conditions can be far from perfect, but the effect on the car will be minimal. What usually happens in highspeed braking in the wet, is that the nose of the car steps in an angle to where it was pointing at first, but the car as a whole keeps on sliding head-on, regardless of where the front is pointing.
In ABS commericals, none-ABS cars are made to stop further away and in a great angle in relation to the original direction of the front. This is usually done by playing around with tire pressures or by driving the car on a skidpan, where two wheels are actually on an ice-like material. This causes the car to violently spin-out, but even so, it still keeps on travelling straight. If we look at the Bosch demonstration, we see that in the first experiment, testing raw braking distances on the dry, the none-ABS car stopps further away and is pointing 45 degrees sideways. However, such a difference in stopping distances is not noticeable in any other experiment, even those done on wet roadways. On the skid-pan, both cars stop pitched forward and in the same distance, although the none-ABS car spinned four times.
Actually, the reaction of a skidding car to split grip might be far less dramatic than this. One must bear in mind, that the difference between ice and wet asphalt is enormous. In real life, differences in grip are much smaller. Take for an example a situation where a distracted driver, or a driver that was cut-off, has found himself with two wheels on one side of the car on a gravel or grassy shoulder of a two-lane roadway. The usuall scenario is panic, progressive braking, sharp steering, and round it goes. Ask a defensive driving instructor and he will tell you not to brake hard or to steer sharply, but to get the car stablized, and than gently recovering from the shoulder. However, what if there's an obstacle on the shoulder? This is a very possible scenario, usually not covered by such courses. The natural response is to steer sharply, out of fear to what might befall the car if you were to brake on the shoulder.
However, if you are in such a trickey situation, and you stamp the pedal, the car will, in 90% of the cases, stay perfectly balanced. An ABS-equipped car will always stay pitched forward. A standart none-ABS will only step out slightly towards the roadway in higher speeds (above some 40mph), and will still keep up with it's original lane of travel.
The only "real" problem is steerability. A car on locked wheels is practically un-steerable. As the front wheels lock-up, the wheel will shake slightly, and will than feel very light. If you try and turn it, you will find that it has no effect on where the car is going. Ask an instructor and he will also tell you that turning the wheel while the wheels are locked, will increase the braking distance. But, in real life, releasing the lock wheels is the last concern when trying to brake and steer away from an obstacle. Like it's not a really natural reaction to stamp the brake pedal, it's not so natural to brake and steer. First off, don't just steer away. There can be other cars around you, and your original speed may be too high to enable an evasive manouver. While you are thinking on where to swerve to, the car is traveling and is bearly slowing down. Always brake, wipe off speed before making an evasive action or before hitting the object. The problem is, that once braking, it's your natural response to stare at the obstacle, and pray you will stop in time. While it is important to see the obstacle and establish that there's a danger, it's not a good thing to fix your eyes on it, because this will make you unable to steer away from it. What you need to do, is to look away towards a possible route of escape. Practically, this is done by swinging the whole head sideways, forcing the eyes to follow, and than by focusing on a visual target. If you don't have ABS, looking away will also help you get over the urge to stay on the brakes, you release a nice amount of pressure, steer, bring it straight, re-apply brakes.
Actually, braking is also a good method of regaining traction which was lost regardless of braking. Take for an example, a front-wheel skid. Such a skid happens when the steering does not seem to work and the car does not turn or turns very wide. Some driving instructors will tell you to steer into the skid, that is, point the wheel towards the outside of the bend. The problem is that this solution counters every human sense, and not fot nothing: a skidding car is slowing down, untill eventually regaining grip by itself. The last thing you want to do is to stop that slow-down and have the car rolling at where you really don't want to go. Such a manouver must be fullfiled in great accuracy. A better solution is to ease-off of the acclerator, gently apply a bit of braking, and let the car pull itself back to it's original lane of travel.
A not so different scenario has to do with a rear-tire skid (oversteer), in this situation, the rear is breaking traction due to progressive braking while cornering sharply. People are usually told to "steer into the skid" (generally, away from the direction of the corner) and usually to apply the throttle, or even de-clutch. While this can work, it might as well not, or it can work "too good": the accleration can pitch the car forward before it pulls out of the skid, so it's now understeering towards the inside of the corner, or (more likely) it would create a much more voilent oversteer skid. By countersteering and applying power, you are turning the inertia of the car, it's accleration and slip, towards the outside of the corner. This makes for an unrecoverable skid. Moreover, in the original skid, the grip of the front wheels has a chance of keeping the car inside the general line of the corner, but when you skid in the other way you will immedialy be facing oncoming traffic or might even fly off of the road.
A much better solution is to retract some or all of the steering, and brake to the maximum, just as you feel the back slipping away. Usually, it's told not to brake during such a skid, but that's because people brake progressively. A quick slam on the brakes might very well pull the car out of the slide. If not, it can wipe off speed very quickly, to enable the driver to pull out of the slide later, or to stop the car quickly.
The only situation where it gets a bit trickey is when a driver needs to brake while cornering. This can very well cause understeer because the wheels lock-up, or oversteer due to a forward weight shift. Even though, in normal conditions, even if the road is wet, just braking in a corner will usually not cause serious oversteer, and even understeer due to locked wheels isn't so bad because the car is wipping off speed quickly. Therefore, it's wrong to say "don't brake while cornering". If you need to brake while cornering, undo a bit of steering while doing so. This will make sure that the centrifateral force will not throw the car away. Also, make sure the wheel don't lock-up so you can keep up turning. With ABS, you can keep the pedal stamped. Without it, you need to brake hard, but not full-force, about 70%, and release a bit more if nessecary.
About the Author
The author has vast experience is race and rally driving, as well as in instructing defensive driving techniques and racing techniques to other drivers.
TIB769Z on 980 (Problematic Bus Stop Brake/Air System) PART 1
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