In true RU style .... an article on cornering.
Discussion
Wrote this for a separate forum, but though you guys might be interested.
Martin
One of the reasons often given for road positioning on bends is to increase the radius and therefore reduce the cornering forces on the car. This method is acceptable and typical for the IAM test, but is it the only, or best way?
At some point in a bend, you have to start to straighten the car, so that you are in a straight line as you exit. However, the point at which I do this is perhaps different to the "typical" IAM style.
My approach into a bend starts off the same, and for safety I go for “slow in, fast out”. To do this, I need to get the car pointing straight out of the bend, at the earliest opportunity. Let us take the example of a left hand bend.
Providing there is no approaching traffic I will be positioned towards the center of the road, if I have the visibility, I may well be over the center line, providing the road markings allow this.
Before I get to the bend I will access the speed required using a number of factors, not only the limit point but other visual clues such as the cross views, the angle of the center line, tree line or lampposts. Once completed I will balance the car on the throttle, maintaining my speed but not accelerating. I will also “hint” at the steering, that is put on a tiny amount of lock, a split second before I actually enter the bend, why, I hear you ask.
For a tyre to corner, something called a “slip angle” must be set up. In a nutshell, when turning the steering wheel, maximum cornering from the tyres, for the given amount of steering is not immediate, it can actually take up to half a second, dependent on many factors. By turning the wheel very slightly, I 'wake the tyres up' and set up the slip angle. Once set up, any increase in steering will have a more immediate maximum effect, and produce smoother turn in.
When the front wheels turn (or rear, for that matter) the tyre footprint twists, due to the friction between it and the road surface, and it is the angular distortion of the direction of the footprint relative to the direction of the wheel that is the slip angle. This develops a sideways force which is what turns the vehicle, and can take up to ¼ second to fully develop, which means, that for ¼ second after you turn the wheel, you don't get the full benefit of however much steering lock you applied. The slip angle causes a “self aligning torque” which is felt through the steering wheel. If the tyre loses grip it cannot sustain the slip angle and the slip angle is reduced. This in turn reduces the torque and the steering is felt to go “light”.
As the front of the car moves, the body rotates and sets up the slip angle in the rear tyres, this takes and additional ¼ second. Add these together and we can see that it can take upto ½ second after you turn into a bend for the vehicle to develop full “cornering grip”, hence the requirement to “hint” at the steering.
Once in the bend, I'll keep an eye on the limit point to ensure it doesn't start to move towards me (ie. a double apex bend). Providing this doesn't happen, at some point it will start to move away from me, signaling the start of the exit of the bend. At this point my vision is increasing and I'll start to accelerate and begin to “shave the bend”, that is actually tighten my line (and therefore increase the cornering forces on the car). As the limit point “sprints” away, I'll bring the car back towards the nearside. I can now begin to straighten the car, and as the steering comes off I can increase my acceleration even harder (dependent of course on the speed limit and if it is safe to do so).
So what have I achieved. By tightening my line I have got the car pointing straight at the earliest possibility and have not driven “all the way around the bend”. This allows my simply to get maximum acceleration sooner, and therefore increase my progress. By using acceleration in the bend (a big gasp I hear ...) I gently press myself, and any passengers back into the seats, giving them more support and preventing them from moving sideways, so I have additionally improved the quality of the ride and passenger comfort'.
Accelerating mid-bend, I better justify this ...
“For the given road conditions, you have an amount of grip available which you can 'spend' on either acceleration (+ve or –ve (braking)) or steering, or a combination of both. However, you have only a certain amount, so if you spend half on steering, you only have half left for accelerating”. Personally I will drive so that I always keep some grip in reserve, in case the unexpected happens. However, especially in the dry, I have plenty of grip for my driving style and can safely use some of this reserve to accelerate mid bend, and then as the steering is straightened (I am now spending less on steering, so have more available for acceleration) I can accelerate harder.
One interesting feature, that can be quite fun, is the ability to steer the vehicle on the throttle. If I have a nice flowing bend (eg. 40 - 50-mph) that tightens slightly, by an amount that would require say an1/8ish increase in turn of the wheel, I can actually leave the wheel completely still and just accelerate. Contrary to what you may think, the car will actually tighten its line, as if you had turned the wheel.
Why is this, back to the slip angle. Within limitations, an increase in the slip angle causes more cornering force. If you apply power to a cornering wheel, the slip angle increases, this provides more cornering force and so the car turns in. Of course there are limits, if the slip angle increases too much, the tyre breaks traction and understeer develops.
Martin
One of the reasons often given for road positioning on bends is to increase the radius and therefore reduce the cornering forces on the car. This method is acceptable and typical for the IAM test, but is it the only, or best way?
At some point in a bend, you have to start to straighten the car, so that you are in a straight line as you exit. However, the point at which I do this is perhaps different to the "typical" IAM style.
My approach into a bend starts off the same, and for safety I go for “slow in, fast out”. To do this, I need to get the car pointing straight out of the bend, at the earliest opportunity. Let us take the example of a left hand bend.
Providing there is no approaching traffic I will be positioned towards the center of the road, if I have the visibility, I may well be over the center line, providing the road markings allow this.
Before I get to the bend I will access the speed required using a number of factors, not only the limit point but other visual clues such as the cross views, the angle of the center line, tree line or lampposts. Once completed I will balance the car on the throttle, maintaining my speed but not accelerating. I will also “hint” at the steering, that is put on a tiny amount of lock, a split second before I actually enter the bend, why, I hear you ask.
For a tyre to corner, something called a “slip angle” must be set up. In a nutshell, when turning the steering wheel, maximum cornering from the tyres, for the given amount of steering is not immediate, it can actually take up to half a second, dependent on many factors. By turning the wheel very slightly, I 'wake the tyres up' and set up the slip angle. Once set up, any increase in steering will have a more immediate maximum effect, and produce smoother turn in.
When the front wheels turn (or rear, for that matter) the tyre footprint twists, due to the friction between it and the road surface, and it is the angular distortion of the direction of the footprint relative to the direction of the wheel that is the slip angle. This develops a sideways force which is what turns the vehicle, and can take up to ¼ second to fully develop, which means, that for ¼ second after you turn the wheel, you don't get the full benefit of however much steering lock you applied. The slip angle causes a “self aligning torque” which is felt through the steering wheel. If the tyre loses grip it cannot sustain the slip angle and the slip angle is reduced. This in turn reduces the torque and the steering is felt to go “light”.
As the front of the car moves, the body rotates and sets up the slip angle in the rear tyres, this takes and additional ¼ second. Add these together and we can see that it can take upto ½ second after you turn into a bend for the vehicle to develop full “cornering grip”, hence the requirement to “hint” at the steering.
Once in the bend, I'll keep an eye on the limit point to ensure it doesn't start to move towards me (ie. a double apex bend). Providing this doesn't happen, at some point it will start to move away from me, signaling the start of the exit of the bend. At this point my vision is increasing and I'll start to accelerate and begin to “shave the bend”, that is actually tighten my line (and therefore increase the cornering forces on the car). As the limit point “sprints” away, I'll bring the car back towards the nearside. I can now begin to straighten the car, and as the steering comes off I can increase my acceleration even harder (dependent of course on the speed limit and if it is safe to do so).
So what have I achieved. By tightening my line I have got the car pointing straight at the earliest possibility and have not driven “all the way around the bend”. This allows my simply to get maximum acceleration sooner, and therefore increase my progress. By using acceleration in the bend (a big gasp I hear ...) I gently press myself, and any passengers back into the seats, giving them more support and preventing them from moving sideways, so I have additionally improved the quality of the ride and passenger comfort'.
Accelerating mid-bend, I better justify this ...
“For the given road conditions, you have an amount of grip available which you can 'spend' on either acceleration (+ve or –ve (braking)) or steering, or a combination of both. However, you have only a certain amount, so if you spend half on steering, you only have half left for accelerating”. Personally I will drive so that I always keep some grip in reserve, in case the unexpected happens. However, especially in the dry, I have plenty of grip for my driving style and can safely use some of this reserve to accelerate mid bend, and then as the steering is straightened (I am now spending less on steering, so have more available for acceleration) I can accelerate harder.
One interesting feature, that can be quite fun, is the ability to steer the vehicle on the throttle. If I have a nice flowing bend (eg. 40 - 50-mph) that tightens slightly, by an amount that would require say an1/8ish increase in turn of the wheel, I can actually leave the wheel completely still and just accelerate. Contrary to what you may think, the car will actually tighten its line, as if you had turned the wheel.
Why is this, back to the slip angle. Within limitations, an increase in the slip angle causes more cornering force. If you apply power to a cornering wheel, the slip angle increases, this provides more cornering force and so the car turns in. Of course there are limits, if the slip angle increases too much, the tyre breaks traction and understeer develops.
mph999 said:
However, you have only a certain amount, so if you spend half on steering, you only have half left for accelerating
Strangely enough, if you £10 of grip and you spend a fiver on steering, you still have £8.50 left to spend on accelerating (or more likely braking). That's the Tesco Value rule of tyre grip.Incidentally, I'm not sure I buy your explanation of power tightening the corner - I don't believe the slip angle changes - just that the drag component of the tyre's lateral force is killed off, making the available grip more usefully directed. Some people don't even believe that.
7db said:
mph999 said:
However, you have only a certain amount, so if you spend half on steering, you only have half left for accelerating
Strangely enough, if you £10 of grip and you spend a fiver on steering, you still have £8.50 left to spend on accelerating (or more likely braking). That's the Tesco Value rule of tyre grip.Incidentally, I'm not sure I buy your explanation of power tightening the corner - I don't believe the slip angle changes - just that the drag component of the tyre's lateral force is killed off, making the available grip more usefully directed. Some people don't even believe that.
mph999 said:
One interesting feature, that can be quite fun, is the ability to steer the vehicle on the throttle. If I have a nice flowing bend (eg. 40 - 50-mph) that tightens slightly, by an amount that would require say an1/8ish increase in turn of the wheel, I can actually leave the wheel completely still and just accelerate. Contrary to what you may think, the car will actually tighten its line, as if you had turned the wheel.
Why is this, back to the slip angle. Within limitations, an increase in the slip angle causes more cornering force. If you apply power to a cornering wheel, the slip angle increases, this provides more cornering force and so the car turns in. Of course there are limits, if the slip angle increases too much, the tyre breaks traction and understeer develops.
Dear Mr Mph - is this anything to do with understeer and oversteer.Why is this, back to the slip angle. Within limitations, an increase in the slip angle causes more cornering force. If you apply power to a cornering wheel, the slip angle increases, this provides more cornering force and so the car turns in. Of course there are limits, if the slip angle increases too much, the tyre breaks traction and understeer develops.
I think it was John Miles who suggested to sit at the side of the road (in your car, obviously) in a road that is just wide enough for you to do a full u-turn. Do a u turn and see what space you have. He then suggestedt to do it faster (not fast, necessarily)and said that a rear wheel drive will oversteer, such that the turning circle will be noticeably smaller, giving room to spare. He said that a front wheel drive will understeer (even at the low speeds talked about) and will not be able to complete the manouvre (it will run out of space).
Is this the same thing you are talking about, or something different. Wouldn't a front wheel drive car tend to run out, not turn in, if you increased the acceleration. Or is it simply that I dont know what I'm talking about, which is a real possibility.
Regards
Sally
[quote=Lady GodivaDear Mr Mph - is this anything to do with understeer and oversteer.
... wouldn't a front wheel drive car tend to run out, not turn in, if you increased the acceleration. Or is it simply that I dont know what I'm talking about, which is a real possibility.
Regards
Sally
[/quote]
Hi Sally,
No, not under/oversteer.
You would imagine that the car would understeer, you are quite correct, and indeed if you overcook it, indeed that will happen.
What I'm taking about is an effect that should be noticeable before understeer develops.
The effect depends on a number of factors, amount of steering applied, speed, and even the car.
If you imagine you're traveling round a large roundabout, at say 40mph (big roundabout), the radius being such that there is plenty of grip available, and the steering is set at a "fixed position". Now simply accelerating should have the same effect as turning the wheel more, you would actually start to head towards the kerb, but without turning the wheel - bit odd the first time you experience it.
M
... wouldn't a front wheel drive car tend to run out, not turn in, if you increased the acceleration. Or is it simply that I dont know what I'm talking about, which is a real possibility.
Regards
Sally
[/quote]
Hi Sally,
No, not under/oversteer.
You would imagine that the car would understeer, you are quite correct, and indeed if you overcook it, indeed that will happen.
What I'm taking about is an effect that should be noticeable before understeer develops.
The effect depends on a number of factors, amount of steering applied, speed, and even the car.
If you imagine you're traveling round a large roundabout, at say 40mph (big roundabout), the radius being such that there is plenty of grip available, and the steering is set at a "fixed position". Now simply accelerating should have the same effect as turning the wheel more, you would actually start to head towards the kerb, but without turning the wheel - bit odd the first time you experience it.
M
Lady Godiva said:
a rear wheel drive will oversteer, such that the turning circle will be noticeably smaller, giving room to spare.
I don't think that's right, on two counts.Being rear wheel drive doesn't necessarily mean that the car will oversteer or understeer. It depends on the setup and also how it is being driven.
A car that is oversteering will not necessarily take a tighter line as the oversteer increases.
GreenV8S said:
Lady Godiva said:
a rear wheel drive will oversteer, such that the turning circle will be noticeably smaller, giving room to spare.
I don't think that's right, on two counts.Being rear wheel drive doesn't necessarily mean that the car will oversteer or understeer. It depends on the setup and also how it is being driven.
A car that is oversteering will not necessarily take a tighter line as the oversteer increases.
I will return the favour by baking you some of my cherry scones.
Regards
Sally.
Lady Godiva said:
GreenV8S said:
Lady Godiva said:
a rear wheel drive will oversteer, such that the turning circle will be noticeably smaller, giving room to spare.
I don't think that's right, on two counts.Being rear wheel drive doesn't necessarily mean that the car will oversteer or understeer. It depends on the setup and also how it is being driven.
A car that is oversteering will not necessarily take a tighter line as the oversteer increases.
I will return the favour by baking you some of my cherry scones.
Regards
Sally.
RWD will under or oversteer it depends what you do. If you steer sharply in the wet, it may understeer (go in a straightish line), if you apply to much throttle in a corner, it may oversteer (back steps out).
Certainly, a RWD car could be made to understeer so that the turning circle is reduced (think of a car stationary but doing donuts ....)
It is often written that if the slip angle at the front is greater than the rear, you have understeer - kinda true.
If you are driving in a constant radius, and have been for a short time, front and rear slip angles should be the same. If you increase the steering, tighten your turn, then momentarily the front slip angle increases (therefore you turn more), and a split second later (about .25) the back slip agle catches up. SO for 1/ sec, the front slip angle is greater than the back, but we do not have understeer.
If you now increase your speed, the front slip increases, keep accelerating till the front cannot grip, the slip angle is too large to maintain and we break into understeer. Now we have the case where the front slip angle in greater than the back, and we do have understeer.
I need to check a few points, I think the above is correct - I will check and edit later if need be.
Martin
mph999 said:
Wrote this for a separate forum, but though you guys might be interested.
Martin
Martin,Martin
While not dismissing the possibility that your article be an original work, it does read like a cut and paste of concepts expounded by Don Palmer and Stressed Dave, right down to the language and terminology that they use to explain stuff.
If they are your inspiration, it would be polite to credit or mention them.
WilliBetz
Edited by WilliBetz on Thursday 30th August 22:02
Lady Godiva said:
GreenV8S said:
Lady Godiva said:
a rear wheel drive will oversteer, such that the turning circle will be noticeably smaller, giving room to spare.
I don't think that's right, on two counts.Being rear wheel drive doesn't necessarily mean that the car will oversteer or understeer. It depends on the setup and also how it is being driven.
A car that is oversteering will not necessarily take a tighter line as the oversteer increases.
I will return the favour by baking you some of my cherry scones.
Regards
Sally.
Although, by now I expect you're ABS standard - Advanced Baker of Scones - so maybe it's no longer a problem.
PS. I'm not sure that I understand slip angles particularly well either - among numerous other things.
Best wishes all,
Dave.
The clearest simple explanation of slip angles I've seen is a short video called "Don Palmer Talks Tyres" on his web site: http://www.donpalmer.co.uk/videos/tyretalk.htm
James
James
I think the usual definition of slip angle is the angle between the direction the wheel is pointing, and the direction it is travelling. Tyres have to run at a slip angle in order to generate any side force, but the angle is usually very small until the side load on the tyre approaches the limit of grip.
Oversteer and understeer can be defined in different ways, but the most obvious description is that oversteer is when the rear slip angle is more than the front, understeer is the opposite. Neutral steer is when the two slip angles are the same.
It can be shown that a car which understeers or has neutral steering will follow a wider line (for a given steering input) as the speed increases and hence the slip angle increases.
A car which oversteers may follow a tighter line, or a wider line, depending on the absolute slip angles and the relationship between them. As the speed rises the line may tighten over some speed ranges and widen over others.
Whether a car understeers or oversteers depends on a lot of factors such as tyre size/compound/pressure/condition/temperature, suspension geometry, weight distribution, spring and damper rates, drive and braking loads, road surface conditions etc etc. It can't be predicted just based on the transmission layout. Rear wheel drive cars are particularly liable to oversteer in certain circumstances, but it is absolutely wrong to say that they will *always* oversteer.
Oversteer and understeer can be defined in different ways, but the most obvious description is that oversteer is when the rear slip angle is more than the front, understeer is the opposite. Neutral steer is when the two slip angles are the same.
It can be shown that a car which understeers or has neutral steering will follow a wider line (for a given steering input) as the speed increases and hence the slip angle increases.
A car which oversteers may follow a tighter line, or a wider line, depending on the absolute slip angles and the relationship between them. As the speed rises the line may tighten over some speed ranges and widen over others.
Whether a car understeers or oversteers depends on a lot of factors such as tyre size/compound/pressure/condition/temperature, suspension geometry, weight distribution, spring and damper rates, drive and braking loads, road surface conditions etc etc. It can't be predicted just based on the transmission layout. Rear wheel drive cars are particularly liable to oversteer in certain circumstances, but it is absolutely wrong to say that they will *always* oversteer.
WilliBetz said:
mph999 said:
Wrote this for a separate forum, but though you guys might be interested.
Martin,
While not dismissing the possibility that your article be an original work, it does read like a cut and paste of concepts expounded by Don Palmer and Stressed Dave, right down to the language and terminology that they use to explain stuff.
If they are your inspiration, it would be polite to credit or mention them.
WilliBetz
Martin,
While not dismissing the possibility that your article be an original work, it does read like a cut and paste of concepts expounded by Don Palmer and Stressed Dave, right down to the language and terminology that they use to explain stuff.
If they are your inspiration, it would be polite to credit or mention them.
WilliBetz
Edited by WilliBetz on Thursday 30th August 22:02
Have indeed read Don Palmers, and many similar ...
"... and it is the angular distortion of the direction of the footprint relative to the direction of the wheel that is the slip angle"
Only so many ways you can say that, so yes, it is indeed similar to Don's and also many others ... I could list them (if I could remember all the pages I went to, but it'll be a long list).
If thanks should go to anyone, it would actually be Mark Kendrick of Bespoke / ClubDriving, as it was he who taught me to corner in that fashion in the first place.
Took me quite a while and a few versions, but is indeed my own work ...
Martin
Edited by WilliBetz on Thursday 30th August 22:02[/footnote]
[footnote]Edited by mph999 on Saturday 1st September 00:44mph999 said:
Lady Godiva said:
Dear Mr Mph - is this anything to do with understeer and oversteer.
... wouldn't a front wheel drive car tend to run out, not turn in, if you increased the acceleration. Or is it simply that I dont know what I'm talking about, which is a real possibility.
Regards
Sally
Hi Sally,... wouldn't a front wheel drive car tend to run out, not turn in, if you increased the acceleration. Or is it simply that I dont know what I'm talking about, which is a real possibility.
Regards
Sally
No, not under/oversteer.
You would imagine that the car would understeer, you are quite correct, and indeed if you overcook it, indeed that will happen.
What I'm taking about is an effect that should be noticeable before understeer develops.
The effect depends on a number of factors, amount of steering applied, speed, and even the car.
If you imagine you're traveling round a large roundabout, at say 40mph (big roundabout), the radius being such that there is plenty of grip available, and the steering is set at a "fixed position". Now simply accelerating should have the same effect as turning the wheel more, you would actually start to head towards the kerb, but without turning the wheel - bit odd the first time you experience it.
M
I used to have a an old Ford with wide tall profile tyres. In reality they were probably too big for the car, it meant that you could hoon about at reasonable pace, well within the limit of grip.
I seem to remember that if you chose to do it wrong, and go around a sharp bend at good speed, removing the steering input leaving the car pointing suddenly in a straight line, you could feel the wheelrims bouncing around in the tyres.
The sharp removal of lock, and a big hoof into the throttle would set up a declining oscillation.
Knowing the state of the car, it was more than likely the suspension as well!
Edited by dilbert on Saturday 1st September 03:15
Dear All - finally, I found the bit I was looking for. Expert driving the Police Way, chapter 4 - Balance.
He talks about slip angles, understeer, over steer, centrifugal force, etc. He then describes an experiment, briefly as follows:
Understeer car - do a U turn, full lock, as slow as possible. See what space is needed. Do it again slightly faster. A few extra feet of space is needed.
Oversteer car - do the u turn at the slightly faster speed first. See what space is reuwired. Then repeat but much slower, and see the extra space required.
I don't think this has anything to do with the original post on here, but I thought that some other bookworms would find it interesting.
Regards
Sally
P.S. no Dave, the scones are no better than the original ones! From now on, I'm going to do sherry, not cherry, scones.
x
He talks about slip angles, understeer, over steer, centrifugal force, etc. He then describes an experiment, briefly as follows:
Understeer car - do a U turn, full lock, as slow as possible. See what space is needed. Do it again slightly faster. A few extra feet of space is needed.
Oversteer car - do the u turn at the slightly faster speed first. See what space is reuwired. Then repeat but much slower, and see the extra space required.
I don't think this has anything to do with the original post on here, but I thought that some other bookworms would find it interesting.
Regards
Sally
P.S. no Dave, the scones are no better than the original ones! From now on, I'm going to do sherry, not cherry, scones.
x
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