911 Carrera T
10-28-2017, 07:32 PM
#227
Three Wheelin'
Can I ask a dumb question. I get why RWS helps in slow tight corners - helps rotate the car. And why it helps with fast lane changes. But it’s not obvious how it helps with fast corners. Can anyone explain?
10-29-2017, 05:45 AM
#228
^^ Slow corners the rear wheels steer in the opposite direction to the front wheels whereas in high speed corners the rear wheels and the front wheels turn in the same direction.
mmm I didn't say RAS gave you more grip - I said it allows you to exploit the grip you have at higher speeds relative to a vehicle that doesn't have RAS.
Draw the vectors, a 997 GT3 rear wheels will track away from the apex whereas with RAS equipped cars the rear wheels will track towards the apex (in a high speed corner). The difference is significant - if both cars are at the same speed, have the same tires and with the same widths. In the RAS equipped car, the contact patch is under less torsion and the lateral shear stress is lower at the contact patch surface interface. Thus the RAS equipped car can go round the corner at a higher speed before the limit of the tire is reached.
randr,
This reminds me of your views on turbo-lag: that it's immaterial on modern cars such as your own. And for the way you drive that may well be true, but you'd prefer not to admit that for the way others drive there is an impact that can be significant.
Rear wheel steering is similar- if you're driving in such a way that you're not losing the rear end it's quite possible you won't notice a difference. If you are driving on and over that line, however, the difference is substantial. I'll try to explain why:
First, rear wheel steering does not inherently increase the grip of the car or the rear tires mid-corner. The rear tires don't know or care if rear wheel steering is angling them by a degree and a half relative to the car or if the whole car is rotating by a degree and a half. From their point of view the effect is identical: they are operating at 1.5 more degrees of slip angle to the road.
Also 1.5 degrees is a lot try taking 1.5 degrees of camber from your rear wheels or add 1.5 degrees of toe to the front wheels.
I see plenty of 997s losing it, far fewer RAS 991s
. I think you'll very much enjoy RAS on your GT3. You'll find it makes it more drivable without losing its inherent character.
This reminds me of your views on turbo-lag: that it's immaterial on modern cars such as your own. And for the way you drive that may well be true, but you'd prefer not to admit that for the way others drive there is an impact that can be significant.
Rear wheel steering is similar- if you're driving in such a way that you're not losing the rear end it's quite possible you won't notice a difference. If you are driving on and over that line, however, the difference is substantial. I'll try to explain why:
First, rear wheel steering does not inherently increase the grip of the car or the rear tires mid-corner. The rear tires don't know or care if rear wheel steering is angling them by a degree and a half relative to the car or if the whole car is rotating by a degree and a half. From their point of view the effect is identical: they are operating at 1.5 more degrees of slip angle to the road.
Also 1.5 degrees is a lot try taking 1.5 degrees of camber from your rear wheels or add 1.5 degrees of toe to the front wheels.
I see plenty of 997s losing it, far fewer RAS 991s
. I think you'll very much enjoy RAS on your GT3. You'll find it makes it more drivable without losing its inherent character. Draw the vectors, a 997 GT3 rear wheels will track away from the apex whereas with RAS equipped cars the rear wheels will track towards the apex (in a high speed corner). The difference is significant - if both cars are at the same speed, have the same tires and with the same widths. In the RAS equipped car, the contact patch is under less torsion and the lateral shear stress is lower at the contact patch surface interface. Thus the RAS equipped car can go round the corner at a higher speed before the limit of the tire is reached.
Last edited by randr; 10-29-2017 at 08:20 AM.
10-29-2017, 07:09 AM
#229
I'm curious why Porsche made it an option on this car. They didn't have to. I can only come up with they thought that the shortened wheel base feeling on tight twisty roads added to the overall feel of the car (or they just want more money). I would love to hear Jethro B's opinion of both after watching his 996 video.
- Lower base price.
- Lower base weight.
If I'm correct, the RWS system weights something like 20kg.
I personally would order a Carrera T without, just because I'm not a good enough driver to need the additional performance on track, and I think that the simplier a car is, the better it is to improve yourself.
10-29-2017, 03:18 PM
#230
mmm I didn't say RAS gave you more grip - I said it allows you to exploit the grip you have at higher speeds relative to a vehicle that doesn't have RAS.
Draw the vectors, a 997 GT3 rear wheels will track away from the apex whereas with RAS equipped cars the rear wheels will track towards the apex (in a high speed corner). The difference is significant - if both cars are at the same speed, have the same tires and with the same widths. In the RAS equipped car, the contact patch is under less torsion and the lateral shear stress is lower at the contact patch surface interface. Thus the RAS equipped car can go round the corner at a higher speed before the limit of the tire is reached.
Draw the vectors, a 997 GT3 rear wheels will track away from the apex whereas with RAS equipped cars the rear wheels will track towards the apex (in a high speed corner). The difference is significant - if both cars are at the same speed, have the same tires and with the same widths. In the RAS equipped car, the contact patch is under less torsion and the lateral shear stress is lower at the contact patch surface interface. Thus the RAS equipped car can go round the corner at a higher speed before the limit of the tire is reached.
Let's talk about slip angle first. Every tire must slip (both laterally and longitudinally) relative to the road in order to generate grip. Wider width, shorter sidewalls and fewer groves all reduce the slip angle before maximum grip is achieved, so slip is going to be far more obvious in a 50s racer with bias ply tires than a modern LMP1 car, but both are slipping substantially. At a professional level every tire's slip angle is quantified with charts similar to this:
As you can see, no slip means no ability to provide lateral force.
Now let's look at what this means for the car. We know the tires need to slip by ~4% relative to the road to provide corning force, but how is that achieved if they're pointing straight? The answer is simple. They're not:
As you can see above the entire body of the car adopts a slip angle. In a dirt circle track racer that slip angle can be ridiculous, and in a hot-rod 912 with tall skinny 185 R compounds only marginally less so: I've competed in a car that made peak grip at 12 degrees of slip, and it's a blast! In both cases it's obvious that the rear tires are tracking towards the apex because the whole car's body is rotating in order to point them there. With modern low profile radials on a 997 GT3 it's far less obvious but equally true- the entire car adopts a slip angle, so rear wheels are pointing in exactly the same direction as on a 991 GT3 with rear wheel steering. The 991's body is actually rotating as well, just 1.5 degrees less than the 997's because that's the extra amount the RWS is kicking in (out of around 4 degrees needed).
This is why RWS helps so much in transition- you don't need to wait for the car's body to rotate that 4 degrees before it really starts gripping, and that saves a lot of time. However it's not going to show any advantage to steady-state speed mid-corner- at that point in both cases all wheels are pointing the same direction and undergoing the same loads.
Follow?
Last edited by Petevb; 10-29-2017 at 06:52 PM.
10-29-2017, 03:46 PM
#231
First and perhaps most obvious is an increase is perceived stability. Low speed you want very sensitive steering, but at high speed this can make the car feel nervous. With rear wheel steering you can dial that out. At low speed 8 degrees of angle on the front wheels is added to 2 degrees of angle on the rear, giving you a total of 10 degrees of wheel angle. Put the steering wheel in the same position at higher speed, however, and you can get a very different answer: 8 minus 1.5 equals 6.5 degrees of total angle and a more stable feel.
The other big advantage is keeping the rear-end in check. A car with "neutral" balance will tend to understeer at low speeds then transition towards oversteer as speeds increase. Weight transfer due to trail braking adds to this, and can make the rear end very challenging to manage on corner entry. On an early 911 this results in a particularly engaging and challenging dance as you get the car rotated into a corner and the tail steps out, but while fun it's not terribly fast. From the 964 onwards Porsche has been using increasingly sophisticated passive rear wheel steering to limit the amount the rear steps out, and with active rear wheel steering they've been able to take that one step further making high speed corner entry far easier and faster- essentially the rear steps out 1.5 degrees less than it otherwise would, and you don't need to wait for the car to rotate that extra degree and a half before you get full grip from the back.
10-29-2017, 04:35 PM
#232
Advanced
RAS Stable or Nervous
As you can see from the above there is no benefit in outright grip in the middle of a fast corner, but it still has a couple advantages.
First and perhaps most obvious is an increase is perceived stability. Low speed you want very sensitive steering, but at high speed this can make the car feel nervous. With rear wheel steering you can dial that out.
First and perhaps most obvious is an increase is perceived stability. Low speed you want very sensitive steering, but at high speed this can make the car feel nervous. With rear wheel steering you can dial that out.
What I think I'm hearing is that at higher speed, the RAS will make the 911 T "feel" like it has a longer wheelbase and thus be more stable. (?)
10-29-2017, 05:23 PM
#233
I have two very different Porsches. One is a 2008 Cayman which can feel nervous at high speed or if the tire pressure is off. The other is a 2016 Panamera 4 which feels like it is on rails... which I attribute to the long wheelbase.
What I think I'm hearing is that at higher speed, the RAS will make the 911 T "feel" like it has a longer wheelbase and thus be more stable. (?)
What I think I'm hearing is that at higher speed, the RAS will make the 911 T "feel" like it has a longer wheelbase and thus be more stable. (?)
Your Panamera has a couple other things going for it that make it feel more stable, in particular much higher "polar moment". Often described as moving the weights out to the end of a barbell, a high polar moment makes a car reluctant to change direction quickly. The long Panamera with its heavy engine in the nose has a very high polar moment when compared to your Cayman, and this increases stability at high speed but reduces agility at lower speeds.
10-29-2017, 05:36 PM
#234
Advanced
Yep, Porsche themselves use the "long wheelbase" explanation to describe how RWS behaves at speed. With RWS in phase the car responds to the steering wheel much like the wheelbase has been extended, while out-of phase is the opposite.
Your Panamera has a couple other things going for it that make it feel more stable, in particular much higher "polar moment". Often described as moving the weights out to the end of a barbell, a high polar moment makes a car reluctant to change direction quickly. The long Panamera with its heavy engine in the nose has a very high polar moment when compared to your Cayman, and this increases stability at high speed but reduces agility at lower speeds.
Your Panamera has a couple other things going for it that make it feel more stable, in particular much higher "polar moment". Often described as moving the weights out to the end of a barbell, a high polar moment makes a car reluctant to change direction quickly. The long Panamera with its heavy engine in the nose has a very high polar moment when compared to your Cayman, and this increases stability at high speed but reduces agility at lower speeds.
r
10-29-2017, 08:09 PM
#235
Rennlist Member
I see two reasons to keep it optional:
- Lower base price.
- Lower base weight.
If I'm correct, the RWS system weights something like 20kg.
I personally would order a Carrera T without, just because I'm not a good enough driver to need the additional performance on track, and I think that the simplier a car is, the better it is to improve yourself.
- Lower base price.
- Lower base weight.
If I'm correct, the RWS system weights something like 20kg.
I personally would order a Carrera T without, just because I'm not a good enough driver to need the additional performance on track, and I think that the simplier a car is, the better it is to improve yourself.
However, there is an opposite truth: RWS actually helps less skilled drivers by stabilizing the car under heavy braking and/or during sudden steering inputs at speed. I've tried 991.2 Carreras with and without RWS back to back and it is noticeable—and even Walter Röhrl says he'd prefer have RWS in an emergency situation (though it sounded like he preferred the 991.2 Carrera without RWS for fun).
10-30-2017, 05:17 AM
#239
Yes indeed.
However, I was looking at it from a fundamentals point of view e.g. the shear stress at the contact patch.
The point being that the rear tyres on a RAS car will be under less shear stress at the contact patch relative to a non-RAS car going through the same corner, at the same speed with the same tires e.g. lower slip angle - to me whats interesting is what controls the deformation at the contact patch which in turn defines the slip angle.
Fundamentally its the shape, area, contact pressure distribution and coefficient of friction of the contact patch - road interface that governs how "well" or how quickly a car can take a corner.
Thus from a technical point of view, a RAS equipped car should always be able to go through corners more quickly than a non-RAS car all else being equal. Which is what is observed
Couple of articles that may interest you.
Circuit racing, track texture, temperature and rubber friction
R. S. Sharp, P. Gruber & E. Fina
Vehicle System Dynamics Vol. 54 , Iss. 4,2016
Normal and shear forces in the contact patch of a braked racing tyre. Part 1: results from a finite-element model
Patrick Gruber, Robin S. Sharp & Andrew D. Crocombe
Vehicle System Dynamics Vol. 50 , Iss. 2,2012
Investigation of Shear Stresses in the Tire-Road Contact Patch
Anghelache, G., Negrus, E., and Ciubotaru, O.,
SAE Technical Paper 2003-01-1273, 2003
In essence you're looking at the conventional slip angle where as I was looking at what actually occurs at the contact patch itself (which ultimately defines the slip angle). I guess the simplest way to look at is a car will exhibit understeer if the slip angle of the front tires is greater than that of the rear tires, conversely a car will exhibit oversteer if the slip angle of the front tires is less than the slip angle of the rear tires.
The effect of RAS (crudely) is to reduce the slip angle of the rear tires e.g. therefore if the front slip angle and rear slip angles are broadly similar the car will behave in a more neutral manner. This in turn feeds into the ability to generate the optimal maximum frictional capacity of the tires.
What does it translate to in the real world? Ten days ago I had to good fortune to be invited to a 488GTB day at a race circuit I know well. There are two corners that favour RAS equipped cars - in this case my humble X51 .2 S was taking those corners ~7kmh faster than the GTB. I attempted the same speeds in the GTB and had the back end out everytime, the pro had several cracks at it with the same outcome. This despite the GTB being shod with Trofeo Rs relative to my S which was on MPSC2 N0 (it now has N1).
Whilst the technical explanations are very interesting, there is no doubt the practical outcomes are significant from a control/time perspective
However, I was looking at it from a fundamentals point of view e.g. the shear stress at the contact patch.
The point being that the rear tyres on a RAS car will be under less shear stress at the contact patch relative to a non-RAS car going through the same corner, at the same speed with the same tires e.g. lower slip angle - to me whats interesting is what controls the deformation at the contact patch which in turn defines the slip angle.
Fundamentally its the shape, area, contact pressure distribution and coefficient of friction of the contact patch - road interface that governs how "well" or how quickly a car can take a corner.
Thus from a technical point of view, a RAS equipped car should always be able to go through corners more quickly than a non-RAS car all else being equal. Which is what is observed
Couple of articles that may interest you.
Circuit racing, track texture, temperature and rubber friction
R. S. Sharp, P. Gruber & E. Fina
Vehicle System Dynamics Vol. 54 , Iss. 4,2016
Normal and shear forces in the contact patch of a braked racing tyre. Part 1: results from a finite-element model
Patrick Gruber, Robin S. Sharp & Andrew D. Crocombe
Vehicle System Dynamics Vol. 50 , Iss. 2,2012
Investigation of Shear Stresses in the Tire-Road Contact Patch
Anghelache, G., Negrus, E., and Ciubotaru, O.,
SAE Technical Paper 2003-01-1273, 2003
In essence you're looking at the conventional slip angle where as I was looking at what actually occurs at the contact patch itself (which ultimately defines the slip angle). I guess the simplest way to look at is a car will exhibit understeer if the slip angle of the front tires is greater than that of the rear tires, conversely a car will exhibit oversteer if the slip angle of the front tires is less than the slip angle of the rear tires.
The effect of RAS (crudely) is to reduce the slip angle of the rear tires e.g. therefore if the front slip angle and rear slip angles are broadly similar the car will behave in a more neutral manner. This in turn feeds into the ability to generate the optimal maximum frictional capacity of the tires.
What does it translate to in the real world? Ten days ago I had to good fortune to be invited to a 488GTB day at a race circuit I know well. There are two corners that favour RAS equipped cars - in this case my humble X51 .2 S was taking those corners ~7kmh faster than the GTB. I attempted the same speeds in the GTB and had the back end out everytime, the pro had several cracks at it with the same outcome. This despite the GTB being shod with Trofeo Rs relative to my S which was on MPSC2 N0 (it now has N1).
Whilst the technical explanations are very interesting, there is no doubt the practical outcomes are significant from a control/time perspective
Last edited by randr; 10-30-2017 at 06:45 AM.
10-30-2017, 08:18 AM
#240
Three Wheelin'
Practicing slip angle control with my .1 GT3 has nearly eliminated my initial concerns. Feels very natural now, and reaction occurs as desired for me. I think a better approach for Porsche would be to allow the RWS cars (at start up) to chose it to be On/Off. Plenty of blank button spots, and it would be as simple as cutting power to the RWS motor at start up.