汽车制动能量回收 中外文翻译

THE BRAKES

Don't expect miracles from tuning on the brakes-improvement, yes-but no miracles. There are two reasons for this. First, the racing disc brake system has been developed to a very high state indeed so that there just isn't a lot left in the line of practical improvement and, second, we just don't spend very much time under the brakes. On the average road racing circuit, something less than ten percent of the time required to complete a lap is spent braking. Therefore, a five percent improvement in braking performance (not brake efficiency) would net a theoretical improvement in lap time of one half of one percent or about one half second in a 90 second lap. In actuality, the improvement would be somewhat less because human and practical limitations always prevent us from realizing the full potential benefit from any performance improvement.

The big payoff of a well sorted out braking system comes, not from any increase in braking power itself, but in the confidence, consistency and controlability that it provides to the driver. This is particularly true when it comes to corner entry-entry speed, placement, precision and repeatability are all directly dependent upon braking performance and consistency.

I would be astonished to learn of a modern road racing car which was delivered with inadequate brakes. Badly arranged or badly set up I'm willing to believe, but inadequate-NO. This statement is valid only so long as we do not change tire size, power output and/or gross weight all out of proportion to the original design. It is definitely not true in those classes of production based touring car and G.T. Car racing where the sanctioning body, through sheer ignorance and/or bloody mindedness prohibits changes to the braking system.

BRAKING POWER: WHERE DOES IT COME FROM?

It takes an astonishing amount of energy to decelerate a moving vehicle-in fact it takes the same amount of energy to decelerate from one speed to another as it would to accelerate between the two speeds-except that we can decelerate faster because most of the inertial forces are working for us rather than against us. The actual energy required to decelerate our racer is given by the equation:Energy (Ib/ft) = .0335 x [(mph max)2 (mph min)2] x gross

weight (lb).For a 1760 Ib car braking from 150 mph to 60 mph we are talking about .0335 x [( 150)2_(60)2] x 1760 = I, I 14,344 Ib/ft.No matter what terminology we use, this is a hell of a lot of energy absorbed in a very short period of time. Somebody once converted the braking energy put out by a GT 40 over the twelve hours of Sebring and came to the conclusion that the same amount of energy could supply the electrical requirements of a fair sized city for a goodly period of time. So where does the energy come from-what actually stops the car?

Some comes from the rolling resistance of the tires-not much, but some. A notable amount, at least at high road speeds, comes from the vehicle's aerodynamic drag. A little bit comes from the friction generated between the moving parts of the entire mechanism. Most of it, however, must come from the vehicle's braking system which converts the kinetic energy of vehicle inertia into thermal energy which must then be dissipated into the airstream-because we have yet to figure out a practical method to collect it, store it, and use it for propulsive thrust. We really aren't very efficient. This chapter is devoted to investigating the braking system itself. We shall conveniently ignore the other factors which slow the car because what we really want to do with them is minimize them to increase the acceleration of the vehicle.

WHAT WE CAN EXPECT FROM THE BRAKES

What exactly are we looking for in braking system performance? First of all we need a braking system which is capable of developing enough braking force to exceed the deceleration capacity of the tires-at any speed that the vehicle can reach-time after time, for the duration of the race. All racing cars, and many modified production cars, have such a system-provided that it is properly installed, adjusted and maintained. The braking effort produced must be directly and linearly proportional to the pedal pressure exerted by the driver. Further, the driver effort required must be reasonable, pedal pressures must be neither so great that Godzilla is required to stop the car nor so light that it will be easy to lock the tires. The pedal position must be correctly matched to the geometry of the driver's foot and ankle, must remain at a constant height and should be really firm and have minimum travel. The system must deliver optimum balance of braking force between the front and rear tires so that the driver can maintain steering control under very heavy braking and yet use all of the

decelerative capacity of all four tires. Lastly, the system must offer complete reliability. If the driver is braking as deep and as hard as he should be, any brake system failure will inevitably result in the car leaving the circuit. What happens after that is up to the man upstairs. Brake failure in a racing car at the limit has to be experienced to be understood. This is why even the most heroic drivers are liable to give the brake pedal a reassuring tap before they arrive at their braking marker.

We need a vehicle suspension system capable of dealing with the loads and forces generated by heavy braking without wheel hop, suspension bottoming, compliance, adverse camber effects, pull or darting. Most of all we need a driver sensitive and skillful enough to balance the car on the edge of the traction circle under braking and under the combination of braking and cornering. If we do not provide the driver with all of the system parameters listed above, he can not provide us with the skill and daring necessary to ride the edge of the traction circle.

EVALUATION AND DRIVER TECHNIQUE

We'll start out with what is probably the most difficult part of the whole braking scene-evaluation of what you have. Measuring the braking performance of your particular projectile against that of the competition is no easier than comparing any other aspect of vehicle performance-and for the same reasons-too many variables and too much ego involved. This is where instrumentation is invaluable. The biggest variable is, of course, the driver. The very last thing that a really good racing driver learns to do truly well is to use the brakes. Most people take too long to get them on hard, leave them hard on too long and brake too heavily too deep into the corner. Almost invariably the lap times generated by the King of The Late Brakers are slow. His adrenalin level is liable to be abnormally high and he has a tendency to fall off the road. There are several reasons for these characteristics. When you leave your braking too late you are very liable to arrive at the point on the race track where you really want to start your turn only to find that the car will not turn. It will not do so because, in your efforts to save your life, you have the binders on so hard that all of the front tires' available traction is being used in deceleration and there is none left to allow the generation of the side force necessary to turn the car-or, should you somehow succeed in initiating a turn, to keep it

in a balanced cornering state. In addition, the front tires are liable to be very nearly on fire and dangerously close to the compound temperature limit. Thirdly, if you are still hard on the brakes when the turn is initiated, forward load transfer has unbalanced the car, the front suspension travel is about used up, the front tires are steeply cambered and, if the thing turns at all, things are going to happen a bit quickly.

If you persist in braking too late, the spectators will \no end and the announcer will mention your name frequently-noting that you are really trying out there. You will complain pitifully about corner entry understeer followed by an incredibly rapid transition to power on oversteer. The other drivers will pour by you-either while you are exploring the grey areas of the track in your frantic scrabble for traction or on the way out of the corner when they have both higher exit speed and a better bite. You will do a lot of exploring as your self induced understeer forces you into unintentional late corner entries. Your team manager will eventually catch on, wander out on the course and observe your antics. If the ensuing frank discussion of technique does not inspire you to mend your ways, he will seriously consider either another driver (if you don't own the car) or another job (if you do).

So any time that you are going in noticeably deeper than the competent opposition (assuming similar cars) but your lap times are not reflecting the degree of heroism that you feel they should-and the car is entering corners badly-have a good think about the wisdom of your braking points. Slow in and fast out will beat fast in and slow out every time. Of course fast in and fast out beats either of the above-and that's what we are trying to achieve-but you won't come out fast if you go in with the car unbalanced and the front tires on fire. This is not to say that a super late brake application followed by a deliberate early corner entry and a bit of slithering around which uses up a portion of the track that might otherwise be useful to someone else is not a valid desperation maneuver. It is, and it will continue to be-but it isn't very often fast and it is even less often repeatedly fast.

THE BRAKE PEDAL

Having disposed of the system actuator, it is time to discuss hardware. We will begin with the brake pedal since it is the closest part to the driver. The brake pedal should be very

strong-and so should its attachment to the chassis. This may sound very basic and a bit ridiculous-and so it should. Any fool should be able to figure out that if the brake pedal, or any of the associated bits, fractures, bends or tears out of its mountings, big trouble is about to happen. And yet it happens-not very frequently-but it does happen. It has even happened to very good operations. Don't let it happen to you. Take a long hard look at the pedal/master cylinder setup and, if anything even looks like being questionable, redesign and/or reinforce as seems necessary. Remember that the typical brake pedal has a mechanical advantage of at least 3: I and it may be as much as 8: I. The pedal arm must be plenty stout and it must be generously gusseted at the intersection of the bias bearing tube. If the pedal bracket is a chunk of 20 gauge aluminum pop riveted to the floor, it won't be good enough. If it doesn't eventually tear out, it will distort and it is difficult enough to modulate brake pressure without the pedal waving about. The pedal pivot support should be at least 18 gauge steel, it should be either boxed or flanged and it should tie into a corner of major structure. You are going to lean on the pedal very frequently and plenty hard. If the master cylinders are mounted to either sheet metal or to slightly stiffened sheet metal, you will end up with a soft or vague pedal. This particular

design sin is nowhere near so rare as it should be-especially in those vehicles which do not employ a front bulkhead (a major Sin In Itself). I'll be damned if I know why this is ever allowed, but it is easily detected and remedied. Also look at the master cylinder push rods. It is very desirable that they should not bend. In normal lengths the stock Girting bits will do just fine. Trouble starts at about six inches. We can also get into trouble with really thin wall tubular extensions and with butt welds.

PEDAL GEOMETRY AND ADJUSTMENT

Take some time and adjust the fore and aft position of the brake (and clutch) pedal to suit the driver's geometry and preference. To do this right may not be as simple as it sounds. The easiest method is that practiced by Lolas, who screw the foot pad into the pedal shaft with a long bolt which is welded to the pad. This gives lots of adjustment without deranging the pedal geometry and offers the added advantage of allowing you to install the pad at an angle should your driver prefer.

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