Most motorsports enthusiasts are familiar with “The Save.” This is when a driver somehow manages to avert a catastrophe when control threatens to slip away momentarily, or, when it seems to have completely vanished. To the extent that he was an actual participant in this save, the most famous example might be Danny Sullivan’s 360° twirl during his winning performance at Indy in 1985. I don’t necessarily want to ruin anyone’s illusions here, but the reality is that when things get this far gone, most of these “great saves” are pure luck. True, there are instances where the skilled pilot can add inputs to the car that are helpful to its ultimate survival, and there can be some skill applied to lessening the effects of a loss of control, but it is also equally true that many more times than not, it is these inputs by variously skilled pilots that perpetuate or worsen the situation, and in many cases have caused it. Here’s a classic example. Ever see a car going “wigwagwigwagwigwag” coming out of a corner? It is a fairly common situation, and some of us have likely been behind a car that is locked in this rhythmic pattern. I myself had a front row seat to our own Brian Daley performing just such a ballet exiting South Bend at Virginia International Raceway some years ago at a not inconsiderable rate of speed. Your first reaction on seeing something like this right in front of you might be something like, “SH%@!” The second reaction is almost certainly “NICE SAVE.” Really, truly? Not to pick on Brian, because everyone that finds themselves in that position has gotten there the same way. It’s what we who stew & babble a lot about this driving thing refer to as “Slow Hands.” Slow hands are responsible for most losses of control in all drivers of modest talent, which includes all of us at the hobby level. Let’s discuss what is happening, and how we get ourselves into and out of these situations. When a car is traveling straight down the road, the rear wheels are following the fronts in a straight line. In this mode, the only force in the chassis is that of the car’s mass pressing vertically downward on the tires. When the front wheels are turned, this creates a horizontal component of movement – a side load - that is know as a Yaw, which can be defined as; The rotation of an object in motion about its vertical axis so as to cause the longitudinal axis of the object to deviate from the line or heading in its horizontal plane. More simply put, the addition of a steering input to the left initiates an equal and opposite reaction of the mass of the car to want to resist this change in direction, shifts weight that was previously equally distributed on all four tires to the right side pair, and creates a delayed reaction of this mass relative to the steering tires. In all cars, this delayed reaction creates a sway in the rear of the car, or a yaw. The following diagrams will illustrate this more clearly for you visual types. When you are traveling straight ahead (A), both the direction of the car and its inertia are represented by the same line (1). When you initiate a left turn (B), the front wheels (2) and the car itself (4) will move to the left, but the cars inertia will continue to want to pull it straight ahead (1). This creates what is (arguably) known as centrifugal force. If the grip of the front tires is sufficient to effect the desired change in direction, this sets up not only centrifugal forces in a corner, but a yaw moment will also occur as the rear tires and the mass they support (with its inertia) attempt to react to this change in direction. This will set the car to rotating about its polar moment of inertia (The center of its mass, arbitrarily illustrated here by the black dot in the center. The direction of the yaw is represented by #3. This is the basic recipe for most every cornering motion you initiate. To the extent that you turn in smoothly, and grip is maintained in the rear tires as they react to this change in direction, the diagram on the right (B) will remain in a fairly static state if nothing upsets it. However, we all know that there are any number of variables in play here. We are concerned in this discussion of The Save with what happens when the grip of the rear tires is compromised for some reason, which can include a clumsy turn in, a bump, crack, patch, or other change in the pavement, some slippery substance (dirt, fluid), or just simply exceeding the grip of the tires. The inertia of the car is still pulling straight (1) while the car travels left (4), and the yaw sees the rear of the car pulling right (3). If grip at the rear is lost for some reason, creating an excessive yaw and a slide, everyone should know that a counter-steer (Diagram C) of the front wheels (2, now steering right) is in order. We do know that this counter-steer is the way to cancel the forces of the yaw and reestablish chassis balance again if rear tire grip is lost. You are countering the rear of the car wanting to step out to the right by briefly heading the nose of the car over in the same direction. However, there are a couple of variables that will govern what happens next. How much you counter-steer is governed by how far out the tail has hung. If you catch it quick, the steer can be short and sharp Left-to-Right because you are countering it before it gets too far. If you are a bit slow to react or the car really jumps out on you significantly, you’ll have to give it a substantial yank, and maybe keep it there for a moment. All
Driver's Ed. Education - A Series of Specifics for Success
by John Hajny
All Text and Graphics herein are Copyrighted (C) 1995-2015 by John L. Hajny
I have striven to make this an extremely well written and accurate series on a subject that is not to be taken lightly and can obviously be dangerous. To maintain the accuracey and proper presentation of that message, I would ask that absolutely no use whatsoever of any text herein be made without my express written consent.
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of this happens in the blink of an eye, and success in making The Save is down to skill, experience, and how deeply these reactions are ingrained in your subconscious driving arsenal. If you have to think about it, you are too dang late!
The key to avoiding all of these excessive yaw moments is awareness and reflex. A talented driver has developed an almost clairvoyant sense of when a yaw motion will occur. Ride with one, and you will see the steering wheel moving very slightly and rapidly in seeming anticipation of nearly imperceptible changes in the car’s attitude caused by changes in the side load that produces yaw. Quite apart from merely sawing on the wheel because it looks cool or satisfies nervous impulses, this driver is countering very minute yaw motions or changes in the car’s dynamic balance before they require anything more drastic than a quick twitch. However, even the most skilled drivers get surprised occasionally and cannot account for every motion before it registers as a noticeable yaw moment. If the steering counter input is applied swiftly, the moment can be corrected. This steering motion is nothing more than a very rapid jerk of the wheel in counter-steer, followed immediately by a correspondingly quick return to the previous steering angle. This latter point is
key. If you catch it quickly enough, you are merely trying to counter a momentary step out with an equally brief counter steer. You will be approximating the angle to which the tail steps out with a roughly equal steering angle to cancel the
yaw and ease the excess side load that produces the yaw. An equal and opposite reaction, as Newton offered with his Third Law of Motion. If you have not been quite quick enough to rely solely on the rapid Out & Back technique, you may have to hold the counter steer for an appropriate length of time, and to a greater degree, to allow the rear tires to regain some grip. Beyond the pure mechanics of the counter steer comes a highly interpretive but fleeting transition period. In the brief moments when the car is sliding sideways and you are holding a counter-steer (between C & D) comes the second true test of awareness, feel, and timing.
I don’t know when I first heard it, but it is quite true that it’s usually not the wig that gets you, it’s the wag. If you do not return the steering just as quickly as you made the counter input, you will cause a wag and you will likely spin in the opposite direction from whence the tail first stepped. In the very short moments between wig and wag, there is a window of opportunity to keep the wag from even happening at all, and yes, this is completely within the driver’s control… if he’s good enough. Even if the initial steering correction was swift and effective, the driver may be surprised or stunned by the initial yaw, freeze, and negle4ct the return portion of the steering equation. This is where a wag or reverse yaw motion is created. When corrective counter steer is initiated because the rear tires have momentarily lost grip and started to slide, do know that you will only counter for a short time until the side load is eased and those tires regain grip. A split second before this resumption of grip happens is the ultimate moment of truth as to whether the driver will make The Save, or join the ranks of the many that almost caught it. What you have in the case of the continuing wigwagwigwag scenario is a driver that started out behind the car, and never caught up. Either this driver did not react quickly enough to the initial yaw to cancel it, or did not return the wheel quickly enough to prevent the reverse yaw from happening. The car yaws and the driver counter steers, but either not soon enough or for too long. Grip returns and snaps the rear end into a yaw in the opposite direction, and he repeats. It yaws back again, and he repeats. This can go on as long as he stays behind the car and perpetuates it. There are two ways of beating this wigwag scenario, short of being good enough to not let it happen in the first place. The first is to have the razor sharp anticipation and timing required to return your steering just as the rear tires are regaining grip. In the split second it takes to go from the third to the fourth drawing above when the rear end regains grip and threatens to snap around the other way, the skilled driver will kill that impulse by steering back straight again. This is where skill and a big dose of luck really come in handy. There’s another method of preventing a wigwag from happening, or stopping it once it has started. It is a method that no one would ever think of as effective if they didn’t see it work, and might consider quite insane in the abstract. Arguably the best way to eliminate the wag is to let go of the steering wheel and let the forces of nature fix it for you. Yes, you read that right. There are many times when letting go of the wheel will correct a situation that the driver has caused, or is unwittingly about to, and has no real prayer of avoiding or correcting. There is no better example of this technique being used with absolute brilliance than Danica Patrick in her ARCA stock car debut. Take this link, watch, and learn. What she accomplished by letting go of the wheel was to allow the front wheels to seek their own path free of any encumbrance. When she sensed that the slide was ending, she allowed the car to remain free for just a moment so as not to add the steering input that might trigger the wag. As the rear tires gained grip, she let the car flow free until such time as it began to roll straight on its tires (down into the grass, toward the end of the pit lane), at which point she resumed steering control. Anybody still think she can’t drive? If you are surprised, stunned, or are otherwise slow to react to a slide, you are going to have to counter hard and long to have a prayer of saving it, and you better be lucky and/or good enough to return the wheel at the exact right moment. If you are on top of your game and experience a yaw motion, yank that wheel out & back, take no prisoners, and cancel it right now. The sooner you sense it and correct for it, the shorter your counter needs to be. If you have to hold the counter, you are too late and you’d better pray a little. If you’re really good, you don’t let it happen. The best save is one you never have to make!