Getting on the gas while cornering
Discussion
Martin A said:
Hi All
Has anyone else noticed that sometimes getting on the gas while cornering seems to increase understeer and on other occasions it lessens it. Or is it just me? And if so any theories as to why?
Article I wrote fot Clubdriving may help you ...Has anyone else noticed that sometimes getting on the gas while cornering seems to increase understeer and on other occasions it lessens it. Or is it just me? And if so any theories as to why?
Following on from the Cornering masterclass I thought I would put together a short article covering this topic. I hope that this acts as an "aid memoir" for those of you who were able to make the class, and an insight for those of you who were unable to attend. Thanks to Mark for teaching this to me in the first place.
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.
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 ...
Mark explains the following in the Advanced Roadcraft DVD, I cannot put it any better, so I've “borrowed it” ...
“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, say £10. So if you spend £5 on steering, you can only spend £5 on 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 of front wheel drive vehicles 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:
One interesting feature of front wheel drive vehicles 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.
Is that what happens in your experience? It isn't in mine. Once you have gone beyond a neutral throttle, more power results in more slip angle results in the car running wider.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.
You're mixing two effects there: slip angle is a function of speed (amongst other things), so if the speed goes up because you've gone beyond 'neutral' throttle, the the slip angle actually goes down.
But rather than disappearing off on a mathematical oddessy, let's keep on topic. Adding more power while than is necessary while cornering creates more cornering force - by a small amount. The disadvantage is that adding more power than necessary accelerates the car and you need more cornering force to keep the car on the same radius (F=mv^2/R for those who want a momentary horrendous flashback to O-level physics). In the end adding more power than is necessary is self defeating as sooner or later you'll breach that condition. On the other hand, many corners on the road are completed by then so it's a valid technique for road driving where you aren't normally limited by grip but by vision.
The other thing to consider is that with this effect there are a (small) range of neutral throttle settings, so how you reach a neutral throttle can have a measurable (at the steering wheel) effect on the amount of steering required to maintain a path at a given speed. Most people I take out don't even get to neutral throttle - often irrespective of expertise and experience.
But rather than disappearing off on a mathematical oddessy, let's keep on topic. Adding more power while than is necessary while cornering creates more cornering force - by a small amount. The disadvantage is that adding more power than necessary accelerates the car and you need more cornering force to keep the car on the same radius (F=mv^2/R for those who want a momentary horrendous flashback to O-level physics). In the end adding more power than is necessary is self defeating as sooner or later you'll breach that condition. On the other hand, many corners on the road are completed by then so it's a valid technique for road driving where you aren't normally limited by grip but by vision.
The other thing to consider is that with this effect there are a (small) range of neutral throttle settings, so how you reach a neutral throttle can have a measurable (at the steering wheel) effect on the amount of steering required to maintain a path at a given speed. Most people I take out don't even get to neutral throttle - often irrespective of expertise and experience.
mph999 said:
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'.
I realise that what I am going to refer to now is part of a quote of a quote from mph999 and the part I am looking at in this post is not quite on topic, however. What I would like to point out is that unless the passengers' heads are well supported and prevented from twisting which is unlikely, their comfort is, due to the extra sideways force on their necks, not likely to be improved.StressedDave said:
You're mixing two effects there: slip angle is a function of speed (amongst other things), so if the speed goes up because you've gone beyond 'neutral' throttle, the the slip angle actually goes down.
I'm still not following you here.Taking slip angle to be the angular difference between the direction the wheel is pointing and the direction it is moving, this is not a function of speed. Increasing the wheel speed changes the direction that the contact patch is scrubbing, changes the thrust vector (which may subsequently change the direction of travel and hence the slip angle) but does not directly change the slip angle. Increasing engine torque beyond the 'neutral' position moves the thrust vector forward so that the lateral component is reduced, less lateral thrust, the car follows a wider line, the slip angle increases.
Perhaps when you refer to slip angle you are thinking about the direction of scrub on the contact patch, or something else?
GreenV8S said:
StressedDave said:
You're mixing two effects there: slip angle is a function of speed (amongst other things), so if the speed goes up because you've gone beyond 'neutral' throttle, the the slip angle actually goes down.
I'm still not following you here.Taking slip angle to be the angular difference between the direction the wheel is pointing and the direction it is moving, this is not a function of speed. Increasing the wheel speed changes the direction that the contact patch is scrubbing, changes the thrust vector (which may subsequently change the direction of travel and hence the slip angle) but does not directly change the slip angle. Increasing engine torque beyond the 'neutral' position moves the thrust vector forward so that the lateral component is reduced, less lateral thrust, the car follows a wider line, the slip angle increases.
If you analyse the contact patch kinematics, you get a pair of equations for slip angle:Rear: slip angle = (sideslip speed - b x yaw rate)/forward speed
Front: slip angle = (sideslip speed + a x yaw rate)/forward speed - steering angle
(a and b are the distances from the centre of gravity to front and rear axle centrelines respectively)
Both are inversely proportional to speed I'm afraid. I appreciate that most people on here aren't either applied mathematicians or vehicle dynamics engineers, which is why I didn't descend into algebra in the first place. While your scrubbing analogy makes sense to you, I'm afraid it's not a realistic representation of what is really happening in the contact patch under most conditions.
opps, looks like I'll have to ammend my article ...
To be fair, I got the details for a univeristy phd article if I recall correctly. However, certainly in my mini, the application of power mid-bend does, to a degree tighten the line quite noticably.
The effect was first demonstrated to me by a police driver in an Audi ...
Martin
To be fair, I got the details for a univeristy phd article if I recall correctly. However, certainly in my mini, the application of power mid-bend does, to a degree tighten the line quite noticably.
The effect was first demonstrated to me by a police driver in an Audi ...
Martin
Martin A said:
mph999 said:
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'.
I realise that what I am going to refer to now is part of a quote of a quote from mph999 and the part I am looking at in this post is not quite on topic, however. What I would like to point out is that unless the passengers' heads are well supported and prevented from twisting which is unlikely, their comfort is, due to the extra sideways force on their necks, not likely to be improved.mph999 said:
opps, looks like I'll have to ammend my article ...
To be fair, I got the details for a univeristy phd article if I recall correctly.
Just goes to prove the usefulness of a post-grad education rather than getting out and doing it for real... To be fair, I got the details for a univeristy phd article if I recall correctly.
mph999 said:
However, certainly in my mini, the application of power mid-bend does, to a degree tighten the line quite noticably.
No doubt, but is the reason for the tightening not getting enough gas on in the first place - IME this is the cause, and from my previous career the cause of a lot of the fatal RTAs I investigated.mph999 said:
The effect was first demonstrated to me by a police driver in an Audi ...
Quite possibly... I've been banging on about it for quite some time, although I spend more time getting people not to do it as it's just a waste of tyre grip up until the point where you actually get on the gas.The other factor, which doesn't appear to have been mentioned yet, is that acceleration causes the weight to shift towards the rear. You would be surprised how much the F/R balance can shift under acceleration and braking. With less weight upon the front tyres, other things being equal, the less grip they will have.
flemke said:
The other factor, which doesn't appear to have been mentioned yet, is that acceleration causes the weight to shift towards the rear. You would be surprised how much the F/R balance can shift under acceleration and braking. With less weight upon the front tyres, other things being equal, the less grip they will have.
Oh, don't I know it! I have to be so careful with all that spine crushing power at my disposal. 
Best wishes all,
Dave.
7db said:
Weight distribution really happens when accelerating (longitudinally) rather than with a little power to settle speed.
Of course less load at the front means more at the back. Power understeer anyone?
To see the effect of weight distribution on contact patch size and consequent grip, all you need to do is to try trail-braking.Of course less load at the front means more at the back. Power understeer anyone?
Obviously, the effect is proportional. The OP referred to "getting on the gas". To me, that implies a noticeable increase in throttle application, certainly enough to increase understeer.
Not entirely sure that's true that you can see the contact patch change when trailbraking...
For me, the principle advantage of trail-braking is facilitating later braking and brake-steering overlap (and so a faster entry) -- although it does promote o/s and help cure u/s to help turn-in.
Don teaches the technique of lifting / braking to promote front-end grip in limit situations, and it does work when the u/s is approaching terminal u/s. It is well worth understanding that the grip doesn't come from nowhere and if you take some grip away from the back, it doesn't all appear at the front -- the tyre becomes less efficient with increased load. Maximum grip is available to the vehicle when it is fully balanced.
However, if you are in a front wheel slide then getting weight forward is a good idea, and then worry about the back if the nose makes it round the corner.
For me, the principle advantage of trail-braking is facilitating later braking and brake-steering overlap (and so a faster entry) -- although it does promote o/s and help cure u/s to help turn-in.
Don teaches the technique of lifting / braking to promote front-end grip in limit situations, and it does work when the u/s is approaching terminal u/s. It is well worth understanding that the grip doesn't come from nowhere and if you take some grip away from the back, it doesn't all appear at the front -- the tyre becomes less efficient with increased load. Maximum grip is available to the vehicle when it is fully balanced.
However, if you are in a front wheel slide then getting weight forward is a good idea, and then worry about the back if the nose makes it round the corner.
flemke said:
7db said:
Not entirely sure that's true that you can see the contact patch change when trailbraking...
By definition, if more weight is put on a tyre, its contact patch will expand.
Not necessarily. In a world where power understeer can cause the car to take a tighter line, you may not be safe making assumptions like that.
GreenV8S said:
flemke said:
7db said:
Not entirely sure that's true that you can see the contact patch change when trailbraking...
By definition, if more weight is put on a tyre, its contact patch will expand.Not necessarily. In a world where power understeer can cause the car to take a tighter line, you may not be safe making assumptions like that.
Cheers.
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