Slip angle and power

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Jersey Tom
166
Joined: 29 May 2006, 20:49
Location: Huntersville, NC

Re: Slip angle and power

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Erunanethiel wrote:So lets stay on the subject.
Is there a correlation between slip angle (lateral grip) and slip ratio (longitudional grip)?
Are there relationships between lateral and longitudinal forces? Of course. Though you should be careful of what you're calling lateral or longitudinal "grip."
Erunanethiel wrote:Oh yeah I drift everywhere :mrgreen: :mrgreen:

Am I not brilliant? =D> =D>
No, you aren't.

Honestly I've had a hard time here following what exactly it is you want to do or learn. I think what would be helpful would be if you can be very specific of what end goal you have that you're trying to achieve, and we can work back through technical details (if necessary) from there. Much easier to answer a question like that than poking and prodding around some technical terms without knowing where you're going with it.

At some point it sounded like you asked.. if you have a low power RWD car and bolt on some UHP tires (Pilot Sport Cup, whatever) - will you be able to drift it? Gut feeling - no. Especially in a production car that's set up out of the box to have some steady state understeer.. put on high traction tires and you'll just have power-on understeer (yes, such things happen).

Here's what a lot boils down to, in simple terms: Some tires have a very easy traction transition and are easy to modulate with steer and throttle on the limit, by a professional driver on a closed course. Other tires have a much more violent and abrupt transition where you push them a bit too hard and just be out of control = wrecked car with amateur driver and with obstacles around.

So taking that and adding to what I said early - is trying to drift around everywhere brilliant? No, pretty dumb and unimpressive.
Grip is a four letter word. All opinions are my own and not those of current or previous employers.

mrluke
33
Joined: 22 Nov 2013, 20:31

Re: Slip angle and power

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Tommy Cookers wrote:nowadays drifting has become a meaningless term relating to operation beyond this range of torques
where only the driver knows whether there is significant front slip hidden by armloads of countersteer/opposite lock
99% of the time there is not, little skill is involved, and it is only what was in earlier times called a powerslide
ie it looks good but the (apex) speed is low

the way to get into the drifting range is to go faster into the corner
maybe you are doing this already ?
Going to take issue with this.

There is a reason a lot of the top USA drift guys are now doing hollywoods precision driving.

It may not be your "thing" but that isnt a good reason to say its crap and skill less.

http://vimeo.com/40196006

Im not going to derail the thread any further :)

Erunanethiel
1
Joined: 26 Oct 2013, 10:17

Re: Slip angle and power

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Jersey Tom wrote:
Erunanethiel wrote:So lets stay on the subject.
Is there a correlation between slip angle (lateral grip) and slip ratio (longitudional grip)?
Are there relationships between lateral and longitudinal forces? Of course. Though you should be careful of what you're calling lateral or longitudinal "grip."
Erunanethiel wrote:Oh yeah I drift everywhere :mrgreen: :mrgreen:

Am I not brilliant? =D> =D>
No, you aren't.

Honestly I've had a hard time here following what exactly it is you want to do or learn. I think what would be helpful would be if you can be very specific of what end goal you have that you're trying to achieve, and we can work back through technical details (if necessary) from there. Much easier to answer a question like that than poking and prodding around some technical terms without knowing where you're going with it.

At some point it sounded like you asked.. if you have a low power RWD car and bolt on some UHP tires (Pilot Sport Cup, whatever) - will you be able to drift it? Gut feeling - no. Especially in a production car that's set up out of the box to have some steady state understeer.. put on high traction tires and you'll just have power-on understeer (yes, such things happen).

Here's what a lot boils down to, in simple terms: Some tires have a very easy traction transition and are easy to modulate with steer and throttle on the limit, by a professional driver on a closed course. Other tires have a much more violent and abrupt transition where you push them a bit too hard and just be out of control = wrecked car with amateur driver and with obstacles around.

So taking that and adding to what I said early - is trying to drift around everywhere brilliant? No, pretty dumb and unimpressive.
If you look at my question again, you are going to see that I only used drifting as an example to explain my question. And ofcourse I am not drifting everywhere like you think. I actually don't drift at all. But understanding what makes you drift, can keep you out of slow drifts that you don't want to do.
Aren't you brilliant? :wtf: :wtf:

Jersey Tom
166
Joined: 29 May 2006, 20:49
Location: Huntersville, NC

Re: Slip angle and power

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Erunanethiel wrote:Aren't you brilliant? :wtf: :wtf:
Yes.

Now then. Let's circle back. Is there anything you're still trying to figure out? Or better yet - how would you summarize your thoughts so far?

I'd put things concisely as such: When you get on throttle coming out of a corner in a RWD car there are (in very simple terms) two things happening. One, by adding torque to the rear tires.. all things being equal you're moving closer to the limit of traction, losing some cornering stiffness, and your rear tires' slip angles will increase (as will the body yaw angle) promoting oversteer. Extreme case would be exceeding the traction limit and you spin out.

However, all things are not equal. At the same time you have rearward longitudinal load transfer which is adding "dynamic grip" to the rear tires, reducing slip angles, and promoting understeer.

How those two weigh against each other and which one wins out is a function of (a) how much torque you're delivering to the wheels, (b) the tires, (c) load distribution including aero, (d) the inertial state accels.

As a result, a RWD car can both exhibit power-on oversteeer and power-on understeer depending on the circumstance, and even which corner you're taking on a given racetrack and which gear you're in.

Class dismissed.
Grip is a four letter word. All opinions are my own and not those of current or previous employers.

Erunanethiel
1
Joined: 26 Oct 2013, 10:17

Re: Slip angle and power

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Jersey Tom wrote:
Erunanethiel wrote:Aren't you brilliant? :wtf: :wtf:
Yes.

Now then. Let's circle back. Is there anything you're still trying to figure out? Or better yet - how would you summarize your thoughts so far?

I'd put things concisely as such: When you get on throttle coming out of a corner in a RWD car there are (in very simple terms) two things happening. One, by adding torque to the rear tires.. all things being equal you're moving closer to the limit of traction, losing some cornering stiffness, and your rear tires' slip angles will increase (as will the body yaw angle) promoting oversteer. Extreme case would be exceeding the traction limit and you spin out.

However, all things are not equal. At the same time you have rearward longitudinal load transfer which is adding "dynamic grip" to the rear tires, reducing slip angles, and promoting understeer.

How those two weigh against each other and which one wins out is a function of (a) how much torque you're delivering to the wheels, (b) the tires, (c) load distribution including aero, (d) the inertial state accels.

As a result, a RWD car can both exhibit power-on oversteeer and power-on understeer depending on the circumstance, and even which corner you're taking on a given racetrack and which gear you're in.

Class dismissed.
Well done brilliant boy.
Is the increase of slip angle on rear tires due to the lateral force or the torque itself?

Jersey Tom
166
Joined: 29 May 2006, 20:49
Location: Huntersville, NC

Re: Slip angle and power

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Erunanethiel wrote:Well done brilliant boy.
Is the increase of slip angle on rear tires due to the lateral force or the torque itself?
The combination of both. Can't know one without the other two.

Dug this up in a quick 60 second Google search. Believe it or not, the relationship between the three is actually pretty clear here. (By the way, what they mean by "longitudinal slip angle" would be more conventionally called longitudinal slip ratio).

Image
Grip is a four letter word. All opinions are my own and not those of current or previous employers.

Erunanethiel
1
Joined: 26 Oct 2013, 10:17

Re: Slip angle and power

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Jersey Tom wrote:
Erunanethiel wrote:Well done brilliant boy.
Is the increase of slip angle on rear tires due to the lateral force or the torque itself?
The combination of both. Can't know one without the other two.

Dug this up in a quick 60 second Google search. Believe it or not, the relationship between the three is actually pretty clear here. (By the way, what they mean by "longitudinal slip angle" would be more conventionally called longitudinal slip ratio).

http://www.dinamoto.it/dinamoto/8_on-li ... _1_eng.jpg
Can you comment on that graph too please?

And have you seen the other thread I opened?

Tommy Cookers
617
Joined: 17 Feb 2012, 16:55

Re: Slip angle and power

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Erunanethiel wrote: ........ Extreme case would be exceeding the traction limit and you spin out.
...... Is the increase of slip angle on rear tires due to the lateral force or the torque itself?
the increase of slip angle can be either due to the lateral force or due to the torque itself
according to the driving technique (which depends on the driver's purpose)
that's the crucial point !!

Case A
today's 'drifter' chooses to corner at a relatively low speed and produces his desired very large rear slip angle by applying high torque
and so develops a rather low rear lateral force
which he necessarily balances by reducing the front slip angle by countersteering, so also lowering the front lateral force

Case B
he is a racer, so he chooses the highest corner speed
at this speed most of the slip angle is due to the higher lateral force developed (needed to corner at this higher speed)
so a smaller torque increment will further increase the slip angle (beyond the point of highest rear lateral force capability)
immediately reduction of the steering angle, even countersteer, will be needed to give a matching reduction in front lateral force
and some reduction in torque to restore the initial optimal slip angles, and the associated higher cornering capability

Case A has a safety margin because the corner speed is lower
Case B has in principle no margin because that's what a race is
Case A and Case B are totally different, but they look the same !!!
that's the big point
the Maserati eg Fangio 1957 WDC looked Case A when being driven Case B
his previous 4 WDCs (Mercedes,Alfa Romeo, and Ferrari) looked Case B when being driven Case B

and 'my' British TV journos have led the world in driving Case A and presenting it by implication as Case B
(this is what I had in mind, mr luke)
when for 60+ years 'drifting' meant '4 wheel drifting' ie Case B


to put this another way .....
countersteering is throwing away loads of front cornering capability (lateral force)
the best cornering speed will always come from having similar slip angles front and rear (ie both relatively large)
this means only slight oversteer or understeer or neutral steer
so the car will bodily be pointing 'inside' the corner tangent by a conspicuous angle, the slip angle
look eg at footage of the old Milwaukee races
they corner 'tail-out' but they are 95% of the time using neutral steering
they are 4 wheel drifting (Case B), not powerslide 'drifting' (Case A)

today's F1 of course drives Case B but modern race tyres (working with high DF) operate at small slip angles, so people don't see it
and often sacrifices apex speed for exit acceleration

track days allow any rwd driver to get both ends of the car into the non-linear slip angle region and so try Case B
stiffer rear springs are the easiest way to make the necessary reduction of the otherwise built-in understeer
if you do this you'll learn how 'handling' is the behaviour in the non-linear region, not the journo bs that we are given

Erunanethiel
1
Joined: 26 Oct 2013, 10:17

Re: Slip angle and power

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Tommy Cookers wrote:
Erunanethiel wrote: ........ Extreme case would be exceeding the traction limit and you spin out.
...... Is the increase of slip angle on rear tires due to the lateral force or the torque itself?
the increase of slip angle can be either due to the lateral force or due to the torque itself
according to the driving technique (which depends on the driver's purpose)
that's the crucial point !!

Case A
today's 'drifter' chooses to corner at a relatively low speed and produces his desired very large rear slip angle by applying high torque
and so develops a rather low rear lateral force
which he necessarily balances by reducing the front slip angle by countersteering, so also lowering the front lateral force

Case B
he is a racer, so he chooses the highest corner speed
at this speed most of the slip angle is due to the higher lateral force developed (needed to corner at this higher speed)
so a smaller torque increment will further increase the slip angle (beyond the point of highest rear lateral force capability)
immediately reduction of the steering angle, even countersteer, will be needed to give a matching reduction in front lateral force
and some reduction in torque to restore the initial optimal slip angles, and the associated higher cornering capability

Case A has a safety margin because the corner speed is lower
Case B has in principle no margin because that's what a race is
Case A and Case B are totally different, but they look the same !!!
that's the big point
the Maserati eg Fangio 1957 WDC looked Case A when being driven Case B
his previous 4 WDCs (Mercedes,Alfa Romeo, and Ferrari) looked Case B when being driven Case B

and 'my' British TV journos have led the world in driving Case A and presenting it by implication as Case B
(this is what I had in mind, mr luke)
when for 60+ years 'drifting' meant '4 wheel drifting' ie Case B


to put this another way .....
countersteering is throwing away loads of front cornering capability (lateral force)
the best cornering speed will always come from having similar slip angles front and rear (ie both relatively large)
this means only slight oversteer or understeer or neutral steer
so the car will bodily be pointing 'inside' the corner tangent by a conspicuous angle, the slip angle
look eg at footage of the old Milwaukee races
they corner 'tail-out' but they are 95% of the time using neutral steering
they are 4 wheel drifting (Case B), not powerslide 'drifting' (Case A)

today's F1 of course drives Case B but modern race tyres (working with high DF) operate at small slip angles, so people don't see it
and often sacrifices apex speed for exit acceleration

track days allow any rwd driver to get both ends of the car into the non-linear slip angle region and so try Case B
stiffer rear springs are the easiest way to make the necessary reduction of the otherwise built-in understeer
if you do this you'll learn how 'handling' is the behaviour in the non-linear region, not the journo bs that we are given
Yeah, but in the case A, wouldn't he also be able to drift by creating a load lateral load and use little throttle? Would he be able to get high slip angles by that too?

olefud
79
Joined: 13 Mar 2011, 00:10
Location: Boulder, Colorado USA

Re: Slip angle and power

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Maybe it will help to think of this way; a rear tire has a traction oval, usually with the lateral axis a bit larger. Thus if tire is at steady state maximum lateral traction with no power (forward thrust) and power is applied and torque is added to the tire, part of the traction budget will be taken by the torque and, available lateral traction will decrease. Since lateral traction was originally was at the max, all things being equal, the slip angle will increase big time. Of course all things won’t be equal since the torque will accelerate the vehicle thus adding wheel weight and changing the traction circle which, in turn, could either increase or decrease the slip angle.

Erunanethiel
1
Joined: 26 Oct 2013, 10:17

Re: Slip angle and power

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olefud wrote:Maybe it will help to think of this way; a rear tire has a traction oval, usually with the lateral axis a bit larger. Thus if tire is at steady state maximum lateral traction with no power (forward thrust) and power is applied and torque is added to the tire, part of the traction budget will be taken by the torque and, available lateral traction will decrease. Since lateral traction was originally was at the max, all things being equal, the slip angle will increase big time. Of course all things won’t be equal since the torque will accelerate the vehicle thus adding wheel weight and changing the traction circle which, in turn, could either increase or decrease the slip angle.
Yeah, once you break the traction of the rear by loading the tires with too much lateral force, will you be able to keep the tires spinning with the throotle even if you straighten up the car?

Greg Locock
233
Joined: 30 Jun 2012, 00:48

Re: Slip angle and power

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Yes, but it isn't easy. the easiest way to try it is in the rain, in an auto RWD, on flat tarmac. In a straight line at say 60 kph floor the throttle and shift into reverse. The rear wheels will break traction and yet the car will continue forward. Now turn the steering wheel and try and catch the resulting spin. It is a bit hard if you let it go too far because the tendency is for the rear to spin round. If the road is cambered you will find the back end of the car slides down the camber. If the car were in a forward gear my guess is that it would be slightly more unstable.

of course you may have a RWD car that is powerful enough to continuously break traction at a reasonable speed in a forward gear, somehow I guess from the general trend of this thread that that is not the case.

Erunanethiel
1
Joined: 26 Oct 2013, 10:17

Re: Slip angle and power

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olefud wrote:Maybe it will help to think of this way; a rear tire has a traction oval, usually with the lateral axis a bit larger. Thus if tire is at steady state maximum lateral traction with no power (forward thrust) and power is applied and torque is added to the tire, part of the traction budget will be taken by the torque and, available lateral traction will decrease. Since lateral traction was originally was at the max, all things being equal, the slip angle will increase big time. Of course all things won’t be equal since the torque will accelerate the vehicle thus adding wheel weight and changing the traction circle which, in turn, could either increase or decrease the slip angle.
I also found out that, even after the tire has past its best (most lateral force) slip angle, the tire doesnt lose much ability to take lateral force. Like, for examle, if the tire can produce its top g (3) at 8 degrees of slip angle, the tire also still makes 3 g at 12 degrees of slip angle. Then, why do we care about slip angle that much?

Greg Locock
233
Joined: 30 Jun 2012, 00:48

Re: Slip angle and power

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You are thinking in a steady state fashion. Suppose the rear wheels start to slide sideways, generating X sideforce, and then slip a little more. After the force peak, say at ten degrees, the gradient is gently negative. So the axle slips more, and generates , if anything, less sideforce, so it'll slip some and so on and so forth.

So there isn't a whole lot we can do with that axle to get it back under control.

The driver still has options, but he has to exercise them, and they will REDUCE the lateral acceleration of the car compared with the case where he'd been able to hold both axles at their optimum.

Erunanethiel
1
Joined: 26 Oct 2013, 10:17

Re: Slip angle and power

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Greg Locock wrote:You are thinking in a steady state fashion. Suppose the rear wheels start to slide sideways, generating X sideforce, and then slip a little more. After the force peak, say at ten degrees, the gradient is gently negative. So the axle slips more, and generates , if anything, less sideforce, so it'll slip some and so on and so forth.

So there isn't a whole lot we can do with that axle to get it back under control.

The driver still has options, but he has to exercise them, and they will REDUCE the lateral acceleration of the car compared with the case where he'd been able to hold both axles at their optimum.
Oh I see now! Even tho it almost generetas the same sideforce, that doesn't change the fact it slips more, right?
But, how does the thottle application affect sliding of the tires? I mean it doesnt power itself doesnt change the slip angle of the tire right? Then how can it make it slide more (laterally)?

Thank you for your time and answers btw!