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A Primer on Performance Driving. By a Novice for a Novice.
Itís far more complex then I had any idea. Stating the obvious, first you have to figure out what you are supposed to be doing, then you have to figure out how to do what you are supposed to be doing, and finally you have to practice it enough such that you can do it really smoothly, really fast, and (occasionally) while youíre really "anxious".
Driving fast is all about tire adhesion. As the oft quoted remark goes, "your tires can only do a 100% of something". It can be 100% braking, a 100% turning or 80/20. But if you try 85% braking + 25% turning you might be off into the dirt. You donít get 110%.
Braking. "Threshold braking" is braking at the very limit. That is to say, all the braking you can do without skidding. Surprisingly, at least to me, when you are braking at threshold your tires are turning ~15% slower then you are moving. So a fair amount of skidding is actually already occurring. ABS and Traction Control systems vary significantly in sophistication so different cars will react differently when approaching threshold conditions.
A novice starts out by doing all his braking in a straight line. That is to say, get the braking done before "turn-in". That way you can brake at 100%, get off the brake, execute the turn-in, and have 100% available tire adhesion to resist the lateral forces of the turn. Ok, thatís a bit of a simplification, but itís by far the safest technique.
As you get better, youíll start staying on the brake thru turn-in (itís called "trailbraking"), but you have to be careful. On a rear engine car, the rear end will REALLY want to come around on you when you do this, so it takes a gentle touch. But by extending the braking zone into the turn, you can start braking a little later, and that might be good for a car length or two.
Turning. In general terms, the key to driving fast is getting thru turns quickly, and getting out of them with a high exit speed. Those two issues are not the same. If a straight-away follows the turn, then getting out of the turn with the higher exit speed is more important then getting thru the turn quickly. If you start a 1/2mi straight-away with a 10mph speed advantage, it will take a lot of horsepower for someone else to catch you.
There are lots of ways to take a corner. The "line" that you choose is dependent on whatís before and after the corner, and the cornerís road surface. But to make things easy, weíll stick with an isolated theoretical corner.
The diagram to the left is intended to show why the theoretical standard racing line is more efficient than an inside line or an outside line. Note that by turning in early, hitting at "apex" (note thick arrow) at the center of the turn, and then tracking out, you can create a turn "radius" far larger then the actual geometry of the turning track. And, stating the obvious, a turn with a larger radius can be taken faster.
Note that although the theoretical standard racing line is 4.8% longer, it allows for 37% higher speed.
The "apex" of a turn is a critical issue. Note the "rumble strips" on the inside edge of the turn. The apex of the turn at left is depicted by a small circle in the center of the rumble strip. The idea is that the apex is the halfway point of the car's change of direction.
Like everything else, itís far more complicated then this simplification describes. The apex that a driver chooses doesn't have to be at the geometrical halfway point of the pavement's inside edge. A driver will often choose to "late apex". This means that he chooses a line that hits an apex late in the turn, and therefore doesn't hit the inboard edge of the track until after the center of the turn. This line allows a higher turn exit speed because it gets more of the turning done early. And if you don't have to do as much turning to do late in the turn, you can go faster. Stating the obvious, when there's a lot of turning to do, you have to slow down. When there's not so much turning that needs to be done, you can go faster.
Often there is another turn immediately after. In that case you might not want to "track-out" all the way to the outside of the track. Because if you do, you might not be in position to take that next turn. And that will completely change how you take the first turn.
If you go back to the Track Tales page, there is a link that takes you to a page that discusses analysis of a "run" using a data logger and analysis software. The analysis discusses Roebling Roadís turn 1 and 2. The following diagram shows a similar turn, and also provides a good example of how different kinds of turns can create complicated lines.
At right is a "constant radius" "hairpin". Most hairpins are not constant radiuses, because track designers are evil. Note how the chosen line breaks the turn into 2 turns, each with itís own turn-in, apex, and track-out. This gets especially fun if you have to shift gears between turns 1 & 2 because the car really doesnít ever get "settled" in the millisecond of straight travel (between 1 & 2) that you might create with this line. Unless you are very skilled, the act of shifting is going to change some car balance variable a little, and when youíre on the edge of tire adhesion, a little change is all that is needed to take you off of the track. But more on the carís balance next.
Because this diagram is similar to Roeblingís turnís 1 & 2, if youíre interested in really studying the data loggerís graphs, this will help you visualize what was happening.
The carís balance. A tireís ability to hang on to the road is a function of how much weight is on that tire. Obviously when you brake, the weight of the car shifts forward. And that means that your rear tires are "light", and will have less traction. When you accelerate the opposite occurs. Consider how a dragster "rearís back" when it leaps off of the line. Accelerating adds weight to the rear tires, but lightens the front. This issue is very significant. The whole process of getting thru a turn is a product of choosing the desired line, and then manipulating the carís balance such that you can stay on the track.
One of the characteristics of a rear engine car is that you generally need to be on the gas while turning. All else being equal, if youíre going around a corner hard in a rear engine car, the rear of the car will start sliding out before the front does. The engine is heavy and it wants to keep going straightÖ..itís that whole "momentum" thing. So the rear tires have to grip hard to hold the rear of the car "on the line". As mentioned before, when you accelerate, you push down the rear tires and help them grip. But itís a delicate dance, because the front tires need to grip too. And you can also make minute steering adjustments to make small changes in the "slip angle" of the front tires.
Itís important to note that itís not a matter of tires slipping or not slipping. Tires are most efficient in a turn at a slip angle of around 7deg. So the trick is to keep managing front and rear tire slippage such that youíre staying on the line and also the car is rotating as it should. Note that turning (staying on the line) and the carís rotation, are two different things. You can choose to keep the car pointed neutrally on "the line", or you could choose to rotate the car a little early. Well, you could if you were good. Unlike, say, me.
Quick definition of critical terms: Understeer and Oversteer. Understeer is when your front end is washing out so the car does not want to turn as much as you want it to. The general solution is to get some more weight on the front tires so theyíll grip better. You do that by backing off of the accelerator. Oversteer is the opposite (there's some other ways too). Oversteer is occurring when the carís rear end is starting to slide out from under you, you end up getting more rotation then you want. In other words, the car wants to go into a spin. The general solution to oversteer is to get on the gas so you can get some more weight transferred to the rear tires that are slipping.
Itís so important in a rear engine car to be on the throttle thru the turn that if you donít have the available torque to do it, you might not be able to "plant" the rear end down hard enough to make the turn. Lightly loaded rear tires might not be able to hold on to the road and youíll spin. When you are dancing at the limit, things need to go right. Something as seemingly unrelated as missing a shift can be the difference between making the turn and not. Youíve got a heck of a lot more torque available in 3rd gear, then in 4th. So if you hose up the shift and you end up in 4th when you meant to get into 3rd, you might not have enough available torque to provide the acceleration necessary to keep the rear planted. So being in 4th gear might be all that it takes to send you off the track. Backwards.
An oversteer condition becomes challenging when you have come into a turn too fast. If you come in too hot, you canít get on the gas much thru the turn. If you canít get on the gas, then your rear end starts to slide out. If you do get on the gas, you end up going even faster. So if you are in trouble because too much speed, and the general solution is more speed, youíre in a fairly unhappy place.
When you are going thru a turn at the limit of tire adhesion, you have to balance understeer and oversteer, second by second, with throttle and steering inputs. If you heard the phrase "steering with your accelerator", thatís what is happening. And it does work. With your gas pedal you can change the amount of weight on the front tires vs. the rear tires second by second. The changing inputs changes how much adhesion youíve got front and rear (both of which are slipping a bit), and therefore you can exercise some control of how much turning and rotation is occurring.
Itís important to note that understeer and oversteer can be significantly altered by tire size and suspension set up. American cars are all set up to understeer. This is to help shield car manufacturers from law suits. Apparently juries are more apt to blame the driver if his front end washes out (understeer) and he goes off of the road, but more apt to blame the manufacturer if he spins off (oversteer) of the road. You can "dial-out" some understeer by increasing the negative camber of the front wheels. Negative camber looks like your wheels are leaning a bit inboard.
Heel-and-Toe. Most red-blooded lads with any interest in cars have heard of this. But I, for one, didnít know what it really was until I got involved in all of this. Heel-and-Toe means "blipping" the gas pedal while you are still on the brake. Anyone that has driven a manual understands the occasional need to downshift. Most folks understand that you should make an effort to match RPMs when you downshift and release the clutch. Your clutch will thank you, even in city driving. The issue is far more significant on the track tho.
RPMs are high on the track. Youíll rarely be below 4k RPMs, because thatís not where the power is. So when you downshift youíll want the motor spooled up pretty high when you release the clutch. What makes it tricky is that at the same time you are braking. Braking really hard. Braking harder then you ever would in normal driving. So your right foot has to be two places at once, hard on the brake, and also blipping the gas. The way to do it is to brake with the ball of your foot such that if you twist or rotate your foot a bit, you can hit the gas pedal too.
Performance cars have their pedals close together for this reason. Many folks, however, put on after-market pedal covers of some sort that have the net effect of getting the brake and gas pedals even closer.
If you blow the heel-and-toe heading into a turn, a number of things could happen. All bad. You could position your foot not quite right such that you couldnít apply enough pressure to the brake. I did this the other month. With the ball of my foot not quite centered on the brake pedal, if felt like if I pushed as hard as I needed to, my foot was going to slip off of the brake pedal. And, going into Roeblingís turn 4, there was absolutely not even a millisecond to correct it. With not enough braking pressure applied, the track turned right and I didnít.
Blowing the heel-and-toe could also hose up your braking by locking up your rear tires.
Your clutch matching RPMís will put an additional braking load on the rear tires. That could break them freeÖremember that your rear tires are already "unweighted" because during braking all the weight is on your front tires. So it doesnít take much to break your rear tires free. And that means youíll get significantly less braking force out of them for a second. If youíre doing any turning, any at all, the lateral force is going to want to whip your rear around on you. An all too realistic scenario.
Finally, if you are doing poorly at matching RPMs, your clutch could decide that itís tired of this kind of mistreatment. The recovery technique from that scenario involves your checkbook.
Itís a hard thing to practice heel-and-toe, since you canít really practice on the way to work. When you brake gently your brake pedal doesnít go down very far, which prevents you from sneaking your foot over to the gas. This is something you do at >100mph, not 65.
The only place that I found to practice was the garage. Not quite the same, but very forgiving of mistakes.
Putting it all together. Ok, youíre entering a turn before a straight-away. You get the car pointed at the apex and you floor it. You exit the turn at 105mph. The tach is almost redlined so you shift to 4th. You know you canít make it down the straight-away in 4th so at ~125mph you shift to 5th.
You glance ahead at the "corner worker" to see if anyone might be jumping up and down frantically waving flags.
The braking markers prior to turn 1 approach at 133mph. You are on the far outboard side of the track. You havenít gotten around to identifying a "brake point", so you look hard at the rapidly approaching corner and make your best guess. You hit the brakes pretty hard for a heartbeat, letting the car settle with itís weight forward, and then you press down on the brakes just as hard as you think you can without engaging the ABS. Which is to say REALLY DAMN HARD. It looks to you like youíre going to run out of track really really damn soon. A cold nugget of wide-eyed worry appears in your belly.
As you pass about 80mph and while still pressing the brake pedal hard, you hit the clutch, make the awkward shift from 5th to 3rd (Folks that are good shift from 5th to 4th, and then to 3rd). Then, while keeping the ball of the foot firmly on the brake, you sneak the outboard side of your braking foot over to the gas to give it a "blip" as you release the clutch. If it goes well, you wonít chirp the rear tires (reducing their braking efficiency) and annoy your motor and clutch. If it goes poorly youíre in 6th, not 3rd, and this turn is going to be an adventure because you're not going to have enough torque to keep the rear end planted.
Your eyes have already shifted to the turnís apex and track-out.
With the weight still on the front tires (because youíre braking) you turn in, quickly but smoothly reducing braking pressure significantly. Remember, your tires can only do 100% of something. Braking and turning canít add up to 110%. Youíre doing about 75mph.
You feel the rear getting a little "loose" and with no conscious thought you are already easing up on the brake just a hair, and turning into the slide just a fraction.
With turn-in complete, you smoothly press down on the throttle. At about ĺ throttle your front starts washing and doesnít want to head for the apex. You back off the throttle just a bit and the front end starts cooperating better. Once you think that the front end is going to cooperate for the rest of the turn, you smoothly press the throttle to the floor.
Your eyes are now on the straight-away after the turn.
As you pass the apex your speed is building. More speed means youíll need a larger turning radius than the first half of the turn.
Your eyes are now on the next turn up ahead. You are rapidly planning how you are going to take that next turn. But your automatic reflexes are still in this turn. Youíre automatically making estimates on track out. Based on your carís rotation, whatís happening with under and oversteer, and what speed youíre going to have when you get to track-out, is there enough track width or do I need to ease up?
Of course "easing up" in a rear engine car has itís own perils. Itís looking like you might end up going to wide, so you ease up on the throttle ever so slightly to transfer more weight to the front. The front tires get a bit more grip and now it looks like youíll make it.
Everything I just wrote is rift with simplifications. Thereís a number of exceptions and alternatives to every point I attempted to make. If it was easy it wouldnít be a challenge.
There are all sorts of books on performance driving. A particularly good one is based on the Skip Barber Driving Schoolís. Itís called Going Faster. If you get the bug, get more then one book. Memorize the books and then take the apparent contradictions and ambiguities to instructors or licensed racers and get their input. Then get on the track and try to make sense of it all.
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