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 accuracy 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.
I would ask you to please abide by this request. Thank you.
In the beginning stages of your driving career, it is unnecessary to spend too much time fussing over your tires. If they are in decent shape, round, hold air, and have the right amount of it in them, then forget about them for now and just drive. Whether they are great tires or not is not overly important, as you won’t be needing max performance yet. However, the better you get as a driver, the more you need to think about them. Let me give you different method of utilizing them properly than you are likely thinking of. This article will be combining many of the previous ones, dovetailing them together to bring about awareness of a more esoteric aspect of driving well. We will also be adding a new term to the distilled list of commands. First off, the technical term for the topic is Slip Angle. Simply stated, the Slip Angle is the difference in direction between where your tires are pointing, and where your car’s momentum is taking it. If you are approaching a corner in a straight line, your tires are pointing in the same direction as your car. If you were to graph this on paper, you would draw a line for the direction of both the car and the tires. At this point, the difference in the angles of the two lines, the Slip Angle, would be zero. If you are not going very fast, and turn in very gently, the difference in the angle of the tires –vs- the angle of the car’s momentum is fairly slight. How your car corners will also affect the slip angle. Understeer is represented by large slip angles, oversteer by small or even negative slip angles (counter-steer). If your car is well balanced and you gradually increase your speed, still driving smoothly, your slip angle will increase somewhat linearly… to a point! At some point, you will either find that the rear tires will start to step out into a four-wheel drift (perfect!), or the front tires will have given all the mechanical (stiffness), and chemical (rubber compound) grip that they have, and they will simply give way. When this happens, you will have the tires pointed sharply along the line you want to travel, but the loss of grip will have the car’s momentum heading somewhere you really don’t want to go! This would represent a very large slip angle. You can also induce the same scenario at lower speeds if you are abrupt with your steering inputs, which is precisely why you never SHOULD be. Incidentally, if you are driving fast enough to access these limits, it is relevant at this juncture to point out again that this is the time when you will be learning the most about car control; our main objective. Street tires allow you to access this limit much easier than R-compound tires. You will have some ways to go yet before the Slip Angle of an R-compound tire begins to grow to an unacceptable level. That point also comes VERY abruptly relative to a street tire, and if you do not have your Car Feel and Car Control chops, you won’t stand a chance! It should be clear now what Slip Angles are, and that we want to manage them effectively. But to think that this is only important at the turn-in point is to miss the point of this article. If you recall the “Big Mo” article you will remember that what we are trying to attain is a consistent high-speed arc through the turns. If you have mastered the basics of cornering theory, you no longer need the slow in, fast out, straight exit approach to stay off the fence. You will carry more speed into the corner, and the car will be in a smooth and balanced four-wheel drift. Could we reasonably expect at this point that you will be right on target for a perfect apex every time you take this or any turn? Of course not. Life – and driving – just don’t work that way. However, we need to mitigate a circumstance that is common to many drivers, particularly those that gain a fair amount of skill in dissecting cornering theory before they have enough experience to deal with the extra speed this creates. If the Business & Acceleration Cone lessons have been learned and applied, the car will set in its drift, and you will be traveling at an increasing rate as you approach your apex. Remembering that speed is the ultimate hallucinogen, it is all too common for novice drivers to assume they are going to be early for their apex. The notion of “Trust is a Must” now looms large, and is usually not given consideration. If one is accelerating through the turn, this means speed and momentum will build progressively throughout. The driver must factor in the distance they are from the apex, and how much more momentum will have been gained by the time they actually get TO the apex to decide what, if any, corrective steering input is needed. If you’ve done it right, it is virtually none.
Now we come back to slip angles. The example of yanking the steering wheel at turn-in is an extreme one. We are now dealing with a more subtle and sensitive form of slip angle. When we have done it all right, the car is in a smooth
four-wheel drift, the slip angle is actually very close to zero because we have used proper cornering technique to set the car on the desired path. All the ingredients have been measured and applied properly to gain cornering success. Now is a critical time, and tendencies need to be mitigated.
That tendency is for the novice driver to not trust, not factor in the momentum building, get wired on the speed drug, and over react. This is perfectly understandable of course. The “need” of the student to steer AWAY from the apex is an absolutely devilish adversary, as discussed previously. Of course, we will never be perfect every lap, and we will indeed need to make steering corrections from time to time, mid turn. However, it is the amount of those steering corrections that is key. When one is in the special high speed slip angle situation, the counter steering input necessary to make the minor corrections in driving line needed are EXTREMELY SMALL… much smaller than the nervous mind realizes! What invariably happens is that the student does not factor in the momentum nor the trepidation factor, and ends up steering 2-3 times more than necessary. When you have well exceeded the optimum slip angle, it takes fairly large steering inputs to correct this. In a high-speed drift, the input required is only a fraction of that, and the result of too much correction is a complete loss of your proper cornering arc. Thinking back now to the cornering diagrams in Turning article #21, by steering too much away from that apex, you will have drastically altered your geometric cornering line, and invariably caused yourself to run out of room at the track out point just as if you had turned in way too early. We need only very minute steering inputs to correct any little early or wide apex situations. We will now add that new command to the Distilled List: RELEASE. This means release pressure on the steering wheel to achieve a minor correction in the cornering arc. In reality, this is all that is needed in most cases. The car is pulling so many “Gs” in the corner that large inputs will effect an exponential change in the slip angle, heading it much too far toward zero again, if not into negative territory. Movements as slight as half an inch will make the small but necessary changes we need. In fact, it can be as subtle as simply easing pressure on the wheel, barely moving it at all. Any more than just a tad and we will lose completely the fine line we worked so hard to attain. The Slip Angle will also be talking to you all the while. It can be envisioned by drawing lines as described previously. It can also be heard. It is important to note that there is a quantifiable difference between a tire screeching, and howling. Granted, tires are different, and sing different songs, but the distinction between the two states is still to be made. A screeching tire is one that is happily doing its utmost to take you through the turn. A howling tire is one that needs more air, is driven by someone who has made a serious blunder, or commanded by a ham-fisted lout without the sensitivity to mitigate this torturous scenario. If you are cruising through a turn, you should be listening to your tires for a change in song. Unless doing so will foul up your turn and run you into the tires, you should always be seeking to reduce the slip angle at any opportunity throughout the corner. This is what is generally referred to as “freeing the car up.”