21 replies to this topic

[red300zx99]

I entered a debate with my boss about the friction circle and filling in the gap, by slowly and smoothly releasing the break till the apex. I can't explain it to him in words that he understands, he knows that F=ma, but much more then that he hates numbers and science, oh and the guy knows a thing or too about seting up a car. I've show'd him the graphs, no love. I've showed him literature, "well don't believe everything that you read". I don't want him to live in this ignorance anymore and I want to hear if anyone knows of any better way of explaining the concept.

[Greg Locock]

Stirling Moss' short note is completely non-mathematical and authoritative. P362 in Milliken. The preceeding diagrams and discussion of gg are also very intuitive.

[red300zx99]

Been there, that quote was the cause of the "don't believe everything you read" comment

[Greg Locock]

He's beyond help, then. Am I to assume he doesn't accelerate /out/ of corners until the steering wheel is straight?

[red300zx99]

Thats what I was thinking, the application of braking should be very simular to the accelerating out of the turn, only backwards. With the front brake bias needed to keep such driving stable would it still be a faster way of filling the circle?

[Greg Locock]

re-read the Moss quote. To stay on the circumference of the circle every bit of grip that is not being used for latacc should be used for traction.

As a matter of interest, what is his counter-argument?

Well, I've just thought of one.

Typically there are two locally optimum solutions for a given corner, one in which the vehicle is basically understeering (OK, boo) using the brakes, and one in which it is basically oversteering, scrubbing off speed by slipping the tires sideways. The same more or less applies on the way out of the corner.

In certain circumstances either can be a faster technique, and I imagine everyone here knows which is more fun.

[RDV]

As everything in life and the universe, it is dependent on many factors, and has several answers.

For corners where driver has to practicaly turn around, I.E. more than 90 deg and most of all at non cambered crown flat hairpins (uphill and downhill hairpins are other subsets), and also in the case of front wheel drive, the technique of braking in straight line alows you to brake harder and later, cutting across track and when close to apex (and at lower speed )release brakes completely and just turn on a smaller radius (car will do this because of smaller speed requirement for cornering force, remember speed function in centripetal force is a square function..) , second gain is also on exit, car will be earlier on acceleration with wheels straight. If you have a surplus of power (big cars) or lack of traction (front wheel drive) section time will be faster.

This also seems counter intuitive, but carefull analysis of cornering speeds and section times , plus umpteen hours in simulation produced something that when applied to track was faster.

It is very difficult to train driver for this unless he has a grasp of the reason why he is doing it, he has to learn to brake much later (on average 6 to 10 meters later, which is huge ) and at a harder rate.

On faster corners or open ones this technique is actually slower, so driver must be selective on when to apply, or be steered by his engineer.

[Yelnats]

There is an excellent article in one to the technical mags (RaceTech?) on the stands now that explains this very well with graphs etc, but if your boss can't figure this out by himself (it's been accepted since the days of Moss and Clark) maybe he's in the wrong business.

[McGuire]

Hey, your boss may very well be right. LOL, what I mean by that: if he is old school and came up running undertired, badly dampered cars on back roads and rural race courses, his way may really be faster... for him. Before good tires, downforce, and real brakes, the benefits of the friction circle approach were far more subtle, and took considerable touch and talent to exploit.

Engineering is a pure science, while driving shares many characteristics of The Arts. Meanwhile, it's not like Classic Technique flat doesn't work. Warning, analogy alert... Mike the bike never did really learn to hang off. Blasting around bolt upright like a human gyroscope with his knees clenched to the tank, he was still faster than most of the pups with the clearly superior technical style.

...as I got older I gradually came to the realization that it is not mandatory for everyone to know everything like myself. You could look at it this way: If you know something the boss doesn't know and refuses to learn (not an uncommon trait in humans) maybe that means you will be boss pretty soon. That's how it tends to work.

[Ben]

RDV has mentioned certain types of vehicles that respond to driving techniques other than trying to follow the perimeter of the g-g diagram. We should also add that the vehicle's stability characteristics in a trail braking situation can affect how much of the g-g envelope the driver can use.

red300zx99: Would it be fair to say that this sort of subtlety isn't what your boss is referring to?

Ben

[Ross Stonefeld]

Well there's also the practicalities to consider of doing a 50lap GP than one perfect corner. Most drivers turn in much earlier than the 'ideal' line just because it gives them better odds of making it to the apex. You're allways told "the exit is the most important part of the corner" but they never tell you the entrance is the most important, because it affects the entire corner and the exit.

[Greg Locock]

McGuire wrote "Engineering is a pure science, while driving shares many characteristics of The Arts"

Hoho. No, sadly not. Engineering perhaps aspires to be a science, but we really know we are applying a veneer of maths over gut feeling, more than half the time!

Here's a nice quote "Scientists measure and explain what already exists. Engineers and artists create things that have never existed before"

and "A science with more than seven variables is an art"

[Fat Boy]

I'm going to agree with your boss.

If you have any racecar data of a good driver, take a look at a G-G plot as a conditional of brake pressure and throttle.

Let's say the friction circle is a clock face. A good driver will be on the brake pedal from about 4:30 to 7:30. From about 8:30 to 3:30 they will be on the gas. From 3:30 to 4:30 and from 7:30 to 8:30 they'll won't be on either pedal much. Remember, this is only for drivers that are going truely fast.

The throttle application starts while the tire drag is slowing the car. As the throttle is applied the tire drag and engine torque balance each other out and the car has 0 longitudinal acceleration. This is where you get maximum cornering force, 3 and 9 o'clock on our friction circle.

To corner heavily and to slow down doesn't take brake pressure, tire drag is enough, that's the range on the circle where they really aren't on either pedal.

If you are on the brake pedal all the way to the apex, you've stayed on the pedal _way_ too long. Too often, drivers think they constantly have to be doing something. The really fast guys understand that there is a point in the corner where you have to be off the pedals and just let the car roll. If you've entered the corner with enough speed, you'll be right on the edge of the traction circle through the whole thing. The trick is getting the car to accept the entry speed that it takes to make it happen correctly.

[red300zx99]

I'm going to let it go, if he want's to learn it fine, if not, oh well

[Yelnats]

Originally posted by RDV
As everything in life and the universe, it is dependent on many factors, and has several answers.

For corners where driver has to practicaly turn around, I.E. more than 90 deg and most of all at non cambered crown flat hairpins (uphill and downhill hairpins are other subsets), and also in the case of front wheel drive, the technique of braking in straight line alows you to brake harder and later, cutting across track and when close to apex (and at lower speed )release brakes completely and just turn on a smaller radius (car will do this because of smaller speed requirement for cornering force, remember speed function in centripetal force is a square function..) , second gain is also on exit, car will be earlier on acceleration with wheels straight. If you have a surplus of power (big cars) or lack of traction (front wheel drive) section time will be faster.

This also seems counter intuitive, but carefull analysis of cornering speeds and section times , plus umpteen hours in simulation produced something that when applied to track was faster.

It is very difficult to train driver for this unless he has a grasp of the reason why he is doing it, he has to learn to brake much later (on average 6 to 10 meters later, which is huge ) and at a harder rate.

On faster corners or open ones this technique is actually slower, so driver must be selective on when to apply, or be steered by his engineer.

An excellent explaination to which I can only add ; The more power the car has (both in braking and accelerating), the more this technique pays off. The old saw about carrying speed through the corner in low powered cars is another way of saying, maximize apex speed with low powered cars because speed is precious as you ain't gonna get it back in a hurry on the way back onto the straight.

[McGuire]

Originally posted by Greg Locock
McGuire wrote "Engineering is a pure science, while driving shares many characteristics of The Arts"

Hoho. No, sadly not. Engineering perhaps aspires to be a science, but we really know we are applying a veneer of maths over gut feeling, more than half the time!

Here's a nice quote "Scientists measure and explain what already exists. Engineers and artists create things that have never existed before"

and "A science with more than seven variables is an art"

Quite right, thanks for the correction. Engineering may only *aspire* to pure science. But if you guys are artistes, why do so many of you dress so badly?

...the Science tells us that maximum speed is found by exploiting the tires' maximum lateral and longitudinal grip all the way through the corner. Meanwhile, the stupid driver thinks he is "trail-braking."

[Greg Locock]

" But if you guys are artistes, why do so many of you dress so badly? "

I resemble that remark

[desmo]

Originally posted by Greg Locock
McGuire wrote "Engineering is a pure science, while driving shares many characteristics of The Arts"

Hoho. No, sadly not. Engineering perhaps aspires to be a science, but we really know we are applying a veneer of maths over gut feeling, more than half the time!

Here's a nice quote "Scientists measure and explain what already exists. Engineers and artists create things that have never existed before"

and "A science with more than seven variables is an art"

Just another, seemed apropos:

"...logic is man's most destructive illusion.
All thinking is done with the glands, and
the logic part gets stuck on afterward to
neaten things up."

-- John D. MacDonald

[merlyn6]

While driving the circumference of the friction circle, (actually an ellipse due to the brakes ability to generate more torque than the engine), may be the fastest way around a given corner it is often not the fastest way around the track, which is after all, the point of the exercise.

The straights preceding and following the corner are vastly more important in achieving fast lap times. Therefore it follows that whatever will allow optimization of the straights is the preferable cornering method.

Straight line braking to the turn in point, (maximizing the preceding straight) then accelerating through the apex as lock is released gives increased speed at the apex, which translates to a higher corner exit speed, and more distance traveled in a given time period on the following straight.

Sometimes a slight turn in while still at speed then braking in a straight line to the apex, or turn in point works well, as it tends to deny the corner to any passing attempt.

Another point is that race cars, especially with down force, can brake much more effectively in a straight line, carrying speed longer which maximizes the preceding straight.

[Greg Locock]

Using the friction circle tells you what to do on the straight as well. What it does not do is tell you what line to pick, but, as Moss said (more or less), once you have entered a corner on the limit for that car you are on a 'ballistic trajectory', you have not got much choice about your path.

Path selection is the most interesting part (to me) of the various lap time simulators. It is also not very important, since the simulators should be used for AvB testing, not estimating achievable lap times.

[Ben]

Exactly. The relative strengths of the driveline and the cornering potential due to suspension and aerodynamics surely alter the ideal line for the vehicle but don't invalidate the argument for trying to keep the car at the limit of the g-g envelope.

What hasn't been mentioned yet is that braking and turning into a corner is a 3-d manouevre over the g-g-V envelope. I suspect a front wheel drive super tourer generates much more front downforce when braking and turning in resulting in the centre of pressure migrating forward - an oversteer effect, makes sense to do more braking in a straight line and try to carry more speed into the apex under these conditions.

Ben

[merlyn6]

On reflection I see you are correct. We are both talking about using all of the cars friction potential all of the time. I confused “Friction Circle” with trail braking. They are not the same thing.