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GerardWon
Master Racer



Joined: May 10, 2011
Posts: 46
Location: NYC Area

PostPosted: Sun Oct 23, 2011 2:13 pm Reply with quote Back to top

myownworld wrote:
My eyes must be playing tricks on me! I swear I read a reply by Gerard here - some great advice - and now it's disappeared! Shocked


It was incomplete and then I lost the completed post.... Then I got sick for 3 weeks and now at last I have almost caught up at work and on my websites.

But I still have to rewrite the post at least I have a copy of the incomplete one.

I hate having to rewrite things... such is life I guess.
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GerardWon
Master Racer



Joined: May 10, 2011
Posts: 46
Location: NYC Area

PostPosted: Sat Nov 05, 2011 11:46 pm Reply with quote Back to top

myownworld wrote:
My eyes must be playing tricks on me! I swear I read a reply by Gerard here - some great advice - and now it's disappeared! Shocked


As I mentioned in my last post, my answer was not as complete as it should have been so I deleted it. Then I lost my completed post -- sigh. Anyway here is a good response...

How well or poorly a car responds to steering inputs (how well it changes direction) is a result of how much loading (or weight) the front tires have on them. There are other factors I mention a key one later in this post.

Myownworld; have you noticed in your driving, for example, that when you slam on the brakes your head and body go forward? You have just increased the amount of loading on the front tires (the amount of weight they are carrying) while at the same time decreased the amount of weight on the rear tires. The car is now poised to change directions very aggressively if this is followed by turning the steering wheel and releasing the brakes at the right moment in time. Timing is crucial here, as is how much you have turned the steering wheel. This is a flat out roadracing technique. The car will rotate Very Hard into the corner And Frequently Must Be Stopped From Spinning Out With A Well Timed Application Of the Gas Pedal (throttle) to Stop It From Spinning OUT, and maybe with some opposite lock on the steering wheel too . The car can easily spin out of control because as the weight has gone onto the front tires so it has also come off of the rear tires. The car is very unbalanced front to rear at this time. This Is NOT something to fool around with on the street.

To a lesser extent: a lift off of the gas pedal (throttle) as you are about to turn the steering wheel -- or even after you have turned it will also increase the load on the front tires ( while decreasing the load or weight on the rear tires) and make the car turn at a sharper angle(turn more).

There are of course other factors involved -- like are we using the brakes the same time we are trying to change direction. Remember a tire has only so much overall grip or bite and if you are using some of that grip for slowing down then it has less it can give you to turn the car. Look at it this way-- if a pizza has 8 slices and you have eaten 4 of them (using that for slowing) then you only have 4 slices left for turning.

This is an oversimplification but I feel its a good explanation of what is happening with the car.

'Effective driving lines': He is talking about making a turn less sharp by using a wider path -- therefore you can turn the car in a more gentle way and still execute the corner. Personally I have mixed feelings about this in the real world. I live in area where they are lots of trees and bushes. In the summer time they can obscure your vision into a corner so personally I frequently do not apex (come to the inside of the turn) where I normally would. This allows me to see further down the road much sooner than if i had turned into the corner in a normal racing type of a fashion. To put it a slightly different way: apexing the corner would cut off my vision down the road.

Like wise, on a divided 4 lane highway -- for a right hand curve I'll generally stay in the left lane (on the far left - hand side) as I approach and execute (turn into) the right the curve. Doing this lets me see much further down the blind corner sooner.

On the street ( where vision is obscured by bushes, etc.) I always sacrifice lane position (a less sharp turn-in angle) for the view that will let me see furthest down the road. Doing this is a huge reason why I'm able to type this response today and am not a fatality statistic.

http://www.GerardWon.com | High Performance Driving School

We Own The Road.
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Astraist
Master Driver



Joined: Mar 27, 2010
Posts: 209

PostPosted: Sun Nov 06, 2011 7:04 am Reply with quote Back to top

Well put, Gerard. I have a few comments, though, beginning with the subject of car dynamics that you mentioned. You touched two key concepts of automotive dynamics: Weight transfers and the friction circle. However there is another, often overlooked concept which is the critical wheels. At this point I will also back it up with the physics, if you may.

When the car is moving, it is acted upon by two forces: Inertia and Friction. The force of inertia (K) originates from the movement of the car and equalls to the square of the car's mass, multiplied by it's velocity and divided in two. The force of friction occurs between the tires and road and is classically defined as the coefficient of friction by the "normal force" (which increases, albeit none-linearly, by the weight of the car).

When we turn the wheel we generate a slip angle unto the front tires. This slip angle in turn, generates lateral grip. It basically uses the force of friction described above to make the front accelerate laterally (i.e. turn) and this rotates the body of the car, making the rear wheels experience a slip angle that in turn further rotates the car into the corner. At this point the suspension (tire sidewalls, springs, dampers, roll bars) and chassis also come into play.

This is what happens to the force of friction in a corner, but what about the force of inertia? Just like the tire's friction is turned into lateral grip or ressistance, the force of inertia turns into a side force which tries to pull the car towards the center of the corner (the centripetal force), but since the vector of that force is longer than the radius desired for the driver, it is experienced as a force that tries to push the car away from the corner, called (inaccurately) a centrifugal force.

When the relevant values (velocity, friction, normal force) cause the centripetal force to be greater than the tire's lateral ressistance, they will get pushed away from the desired radius, either at the front or rear respectivelly. This causes the phenomena you and I know as "understeer" and "oversteer."

But this model isn't perfectly applicable unto a turning vehicle. A car that its being handled through a bendy road section or track is also expected to exhibit dynamic capabilites that are formally described as the car's cornering stiffness. When you first turn the wheel, up to the point where the car "takes a set" after you finish turning the wheel into the corner, the car is said to experience a transient during which the "critical wheels" are the two front wheels that turn the car into the corner.

This also means that during this transient or "change of direction" the car will not and cannot experience oversteer, only understeer. So no matter how far you bias the weight forward under braking, the tail won't actually break away untill you finish turning the wheel. It also means that you can as if "discover" more reserve grip by use this technique, instead of braking in straight lines. As I've said in the past, both styles are acceptable.

So, we brake (gently) coming into the corner. By the time that the required steering amplitude is achieved (i.e. when we finish turning the wheel into the corner) we should already be off of the brakes and on constant throttle and at the apex we need to increase the power to facilitate some acceleration. Steering goes in - brakes are on. Steering is held in place - constant throttle is on. Steering winds out - acceleration is on.

My other note regards the subject of cornering lines. When I am talking about a road-driving line, I am talking about the same late-apex line from the race-track, but with the actual apex placed much further down the turn (I like to call this the "last apex") so that you can get a better view around the corner in advance. You can sometimes see this line practiced in rally racing. Also, this should only be done within the limitations of the driver's own lane, without invading other lanes or the shoulder lane. If a normal late apex is around 50-60 degrees around the corner, the last apex line is at least 60 degrees around the corner.
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Misha
Site Owner



Joined: Aug 02, 2006
Posts: 705
Location: McLean, VA, USA

PostPosted: Mon Nov 07, 2011 12:55 am Reply with quote Back to top

Astraist wrote:
This is what happens to the force of friction in a corner, but what about the force of inertia? Just like the tire's friction is turned into lateral grip or ressistance, the force of inertia turns into a side force which tries to pull the car towards the center of the corner (the centripetal force), but since the vector of that force is longer than the radius desired for the driver, it is experienced as a force that tries to push the car away from the corner, called (inaccurately) a centrifugal force.
Sorry Astraist, I have to make a correction here. Inertia, by definition, tries to force our car to keep its speed and direction. In our case the vector of this force is strictly perpendicular to the current effective radius of the turn. No way inertia can pull the car towards center, grip between tires and road is responsible for that part. If/when car loses traction completely, it goes straight until it runs off the road, this is exactly the work of inertia in the absence of grip.

Not sure about the rest of your observation, yet it does not exactly correspond to my experience. I certainly did experience many times the slippage of the rear axle while I still was turning the steering wheel. I don't really see why we have to stop turning the steering wheel for rear axle to break loose. In fact, we can easily send an rwd car spinning without turning steering wheel at all, just by giving it more gas than it can handle. Smile
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Astraist
Master Driver



Joined: Mar 27, 2010
Posts: 209

PostPosted: Mon Nov 07, 2011 5:43 am Reply with quote Back to top

We are generally saying the same thing. The tires form a force of friction that acts as lateral ressistance. When the car is turned, it is acted upon by a Centripetal force which pulls towards the center of the circle. The classic example is of spinning a weight tied to a string. However, the physics does not "know" exactly what turning radius the driver wishes to achieve, and as a result the vector of the centripetal force is wider than the cornering radius that you try to achieve as a driver, as a result the actual direction in which the force is pushing you, is outwards.

Let's take the classic example and reconstruct it to fit this situation: This time, put a ring halfway across the string and start swinging it around. The force will act towards the center of the circle. If you swing lightly enough, the friction between the ring and the rope will keep it in place. Once you swing too hard, it will slip towards the outer edge of the rope, even though the force is pulling inside. Due to the ressistance, some of the force turns into torque that tries to roll the car, which is why we see cars "leaning" down on their outside wheels and why big cars tend to roll over.

The subject of direction changes is related to the car's steering configuration. A car that turns by tilting it's FRONT wheels into a corner will always experience understeer during the transient, as part of a steering configuration known as the "Ackerman steering." Turn the front wheels into a corner, and now draw lines perpendicular to the direction of each of the four wheels of the car. Where will these four lines meet? On a certain point which is parallel to the location of the rear axle.

However, when we also calculate the elastic nature of tire rubber, we can see that the tires will deform laterally into a "slip angle" that will change the direction of those perpendicular lines so that the point where they meet (which marks the actual center of the corner) will be placed "closer" to the car and further forward. This generates understeer. When you further turn the wheel, you generate greater slip angles and a great weight transfer and the center of the corner changes agains and "tightens up." Under these conditions the car cannot oversteer.

What you might feel as oversteer is what Gerard described as "rotation" or "yaw" or basically as very little understeer. Furthermore, many modern road cars have certain "passive" steering attributes to their rear axles that can also lead to a sensation that the car is kicking it's tail out - where in fact the opposite happens. There is also an option that you perform a double weight transfer which changes the whole equation, or that you were not perfectly aware of the exact timing of the skid or the steering.

When I train people in car control and in planning and executing skids, I show them that if they induce a provocative action (such as lifting off of the throttle, pulling the handbrake or what not) while the steering is applied, the reaction will be delayed (untill the steering input is stopped) and less accurate than desired. However, if they time their input percisely with the moment after they finish steering, they will achieve the greatest possible angle of sliding. You can also see this performed in rally racing or in drifting leagues.

My point is that you might be feeling the tail about to come out, but it will not actually come out untill the steering has been turned. Alternativelly, one might, upon feeling this, stop steering and this will initiate the slide.
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myownworld
Site Admin
Site Admin



Joined: Jan 06, 2010
Posts: 485

PostPosted: Thu Nov 10, 2011 1:21 pm Reply with quote Back to top

GerardWon wrote:
myownworld wrote:
My eyes must be playing tricks on me! I swear I read a reply by Gerard here - some great advice - and now it's disappeared! Shocked


As I mentioned in my last post, my answer was not as complete as it should have been so I deleted it. Then I lost my completed post -- sigh. Anyway here is a good response...

How well or poorly a car responds to steering inputs (how well it changes direction) is a result of how much loading (or weight) the front tires have on them. There are other factors I mention a key one later in this post.

Myownworld; have you noticed in your driving, for example, that when you slam on the brakes your head and body go forward? You have just increased the amount of loading on the front tires (the amount of weight they are carrying) while at the same time decreased the amount of weight on the rear tires. The car is now poised to change directions very aggressively if this is followed by turning the steering wheel and releasing the brakes at the right moment in time. Timing is crucial here, as is how much you have turned the steering wheel. This is a flat out roadracing technique. The car will rotate Very Hard into the corner And Frequently Must Be Stopped From Spinning Out With A Well Timed Application Of the Gas Pedal (throttle) to Stop It From Spinning OUT, and maybe with some opposite lock on the steering wheel too . The car can easily spin out of control because as the weight has gone onto the front tires so it has also come off of the rear tires. The car is very unbalanced front to rear at this time. This Is NOT something to fool around with on the street.

To a lesser extent: a lift off of the gas pedal (throttle) as you are about to turn the steering wheel -- or even after you have turned it will also increase the load on the front tires ( while decreasing the load or weight on the rear tires) and make the car turn at a sharper angle(turn more).

There are of course other factors involved -- like are we using the brakes the same time we are trying to change direction. Remember a tire has only so much overall grip or bite and if you are using some of that grip for slowing down then it has less it can give you to turn the car. Look at it this way-- if a pizza has 8 slices and you have eaten 4 of them (using that for slowing) then you only have 4 slices left for turning.

This is an oversimplification but I feel its a good explanation of what is happening with the car.

'Effective driving lines': He is talking about making a turn less sharp by using a wider path -- therefore you can turn the car in a more gentle way and still execute the corner. Personally I have mixed feelings about this in the real world. I live in area where they are lots of trees and bushes. In the summer time they can obscure your vision into a corner so personally I frequently do not apex (come to the inside of the turn) where I normally would. This allows me to see further down the road much sooner than if i had turned into the corner in a normal racing type of a fashion. To put it a slightly different way: apexing the corner would cut off my vision down the road.

Like wise, on a divided 4 lane highway -- for a right hand curve I'll generally stay in the left lane (on the far left - hand side) as I approach and execute (turn into) the right the curve. Doing this lets me see much further down the blind corner sooner.

On the street ( where vision is obscured by bushes, etc.) I always sacrifice lane position (a less sharp turn-in angle) for the view that will let me see furthest down the road. Doing this is a huge reason why I'm able to type this response today and am not a fatality statistic.

http://www.GerardWon.com | High Performance Driving School

We Own The Road.


Thank you for this. I have learnt so much from you and Astraist - as must the rest of our members.

Yes, I see that one should basically avoid sharp turns, esp. slamming on the brakes while sharply turning. In lay man's language, I've been practicing moving into the appropriate lane earlier on, anticipating the turn and slowing down and gently turning. So far so good! Smile
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Astraist
Master Driver



Joined: Mar 27, 2010
Posts: 209

PostPosted: Fri Nov 11, 2011 8:32 am Reply with quote Back to top

GerardWon wrote:
The car will rotate very hard into the corner and frequently must be stopped from spinning out with A well timed application of the gas pedal (throttle) to stop it From spinning OUT, and maybe with some opposite lock on the steering wheel too .


I almost forgot this. In a modern front-wheel driven car, oversteer is not recovered from via opposite lock. Instead, the driver should accelerate quickly and positivelly enough so that the rear grips again with the wheel pointing straight or near straight (slight counter-steering). If there is enough torque or little grip, you can also spin the front tires slightly to push them back onto the right line. Proper opposite lock is very dangerous in a front-wheel driven car and, in general, once you had to steer against the corner - you lost a lot of momentum.

N.B. Most road drivers cannot acquire the skill of car control all that easily - so I, like many other trainers, teach them to go hard for the brakes in skidding situations, much like a track driver would do "both feet in" upon losing control of the car.
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