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Unread 08-06-2016, 02:00 PM   #1
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Default Handling a car off or at the limit

We had a couple of discussions about vehicle choice where power and steering response were presented as something to be mindful of, particularly in relation to the drivetrain layout.

As a coach, my focus is the driver rather than the vehicle. Within the skillset required of a driver, the cornering skillset (and the car's capabilities to boot) are set ahead of straightline skillset, and the car's power.

I was therefore bent on writing about how the driver can squeeze more performance out of even the most humble vehicle through corners.

It is under "Motorsport" because it's main use is in a competitive environment. However, some of it is perfectly applicable to the public road.

In motorsport, the name of the game is to use the car's performance envelope more effectively, so that when you are using all of it - you are as fast as possible.

On the road, using the car's performance more effectivelly means you can maintain the same pace while demaning less of the car and keep more in reserve.

This reserve can come in handy if another driver uses more than their fair share of the road in a corner or if there is a unseen but slippery piece of surface like black ice or an oil spil.

To do this, we need practical coaching, but also to appreciate the nature of the car's dynamic response in a bend and how to match out inputs to it.

Now, the car doesn't go from straight to steady-state cornering and back to straight in an instant. It has to transition between each state and every transient is followed by a period in which the car "settles" again.

The problem with all parts of the corner is one of compliance in the parts of the car, mainly the rubber tires. Their ability to bend away from where you try to point them with the steering means that they don't react instantly and that they don't react accurately.

The tire will always travel a wider arc than the one the steering wheel points to (through the rim) because it folds away slightly - hence the "slip" angle. It's still turning, though. This creates a couple of effects:
1. Cornering force - pushing the car sideways into the corner.
2. Yaw moment - rotating the nose of the car into the corner
3. Induced Drag - additional friction that slows down the car.
4. Aligning Torque - a force that tries to bring the wheel back to straight.
5. Body roll - the car rolling away from the corner.

Now, in the transient into the corner, when the wheel is first turned, the fronts have to react and produce those forces. When they do, the car begins to produce a yaw moment before it starts to produce cornering force.

As the front moves into the corner, the rear is now placed in angle away from straight and into the corner, effectivelly being "steered" by the chassis, so it reacts and than generates a "slip" angle.

Only when all tires have began to produce a "slip" angle, can the car start to actually move sideways and go around the corner, while the yaw moment is in fact reduced.

This "lag" between the front and rear "slip" angles being generated and indeed the yaw moment and cornering force, is excaberated the faster you turn the wheel - the rear just can't keep up and the "slip" angles become larger than they ought to be, the steering has to be turned more and some of the car's performance is wasted.

At first this feels great because you get lots of yaw - the nose points in the right direction. However, the car than takes longer to settle and it still reaches steady state too sideways which isn't stable.

Lesson learned: Don't just "steer". Start to steer slowly and before the road actually starts to curve, and continue adding steering deep into the corner, until the response is satisfactory. The "line" becomes more elliptical than rounded.

This steering style lends itself to a cornering line where the car rolls around the outer part of the lane and "diving" into the inside of the lane later down the road - a "late-apex" line.

This line, where you only "clip" the inside of the corner closer to the exit than the entry, is good for getting a better exit (more on that below) and on the road - for getting a better look around the corner.

Lesson learned: Use of the outer part of your lane until you can straightline the exit. On the road, this requires having a clear view of your path out of the corner.

Coming out of the corner, you want to put power down. For one reason, it's the single most important element of your progress. Straights are typically much longer than corners, so there is (at least) fivefold more gains in speed to be made at the beginning of the straight compared to the end.

Lesson learned: Slow-in, Fast-out. There is no "fast-in, fast-out". For the public road, there is no such thing as "too slow" coming into the corner, only too fast. This is by far the most important lesson to be learned here.

Another reason you want to put the power down is that you want to start "pushing" the car out of the corner with torque and than start to undo the steering. The power shifts weight and effects all tires rather than the steering only having a direct effect on two of them. Lesson learned: Lead the exit with throttle and than start to undo the steering.

However, the tire can produce surprising amounts of drag that some of the power has to counteract, so it pays not to take too long with unding the steering either. Lesson learned: Add a bit more steering mid-bend to undo it faster and get rid of tire drag.

But what can be done with throttle, or brakes, coming into the corner? Well, since both produce a force that works in the direction of the wheel, they can be used to aid cornering.

Since the sharpest and slowest point of the corner is going to be roughly in the middle, there is a sense to be using the brakes gently so that as you continue to add steering and make the car travel a sharper arc, you reduce speed to boot, rather than going at the speed of the slowest point before you even reach it.

This is "trail braking". It's used in racing mostly to prevent overtakes, and it involves tapering off of the brakes in the same rate at which you add steering into the corner, to the point that when you finish steering, you finish slowing down, too.

To be capable of tapering off of the brakes rather than hanging at a constant pressure, you must enter the bend slower than what appears to be "right", below your personal limit of confidence.

The same thinking can be applied to using power on entry. If you reach the corner at a speed slightly slower than that of the slowest point of the corner, than you can increase speed by 2-3mph as you begin to turn the wheel.

As a technique for the public road, this has several advantages:
1. It forces you to go into the corner more slowly.
2. It makes you prepare for the exit in advance.
3. It generates force in the direction of the wheel, hence into the corner.
4. It forces you to sort out the gears (in a manual) before the corner.
5. It neutralises tire drag, which is "wasted" cornering force, hence turning it back to (more) cornering force.

Lesson learned: Enter the corner at a speed below your limit of confidence, so as to be confident of tapering off of the brakes or accelerate gently.

Since my focus was on the car's dynamic response, subjects I did not touch upon (although they are also important to discuss) are, namely, how to react when it goes wrong and the car slides, and how to work the hands about the wheel.

These subjects, as well as subjects of feedback, the dynamic response as we approach the actual limit and other interesting subjects, can be expanded upon through discussion below.

Last edited by Drivemaster; 08-06-2016 at 02:17 PM.

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