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Rear Steering, not really new

 
Old 03-07-2013, 09:18 PM
  #16  
C.J. Ichiban
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Originally Posted by axhoaxho View Post
I have a deposit on the RS; and if it also has the 4WS, yes, I will be an owner and I have my concern; and I am serious.

Have you bought a GT3/RS before? I have. The GT3/RS is not like Carrera's that dealers have plenty for test driving. Most of these cars were pre-ordered by owners and when they arrived at the dealer it would be picked up within hours. Very unlikely that any dealers (at least here) will have a car and will allow for test drive thoroughly (not just a couple miles around the block.)
+1000 there haven't been dealer demos in the past...

but who knows...this is the new porsche!!!
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Old 03-07-2013, 10:48 PM
  #17  
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When was the last time a GT3 needed to take turns at 31mph or less on a racetrack?

Was any previous GT3 unstable while changing lanes on straight line at speeds of 50mph and up?

If the answer to these two questions is NEVER, the Rear Axle Steering is just a gigantic Gizmo, with no performance gains at all, unless parking is a performance measurement.

I just checked my minimum Parking Lot Racing (Autocross) speeds, and they are above 31mph, so this Gizmo does nothing, not even at autocross.

Because of this gizmo, the car carries a weight penalty, and it cannot be offered with a Li-Ion battery (another penalty).
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Old 03-07-2013, 11:10 PM
  #18  
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I had the late 80's 4WS Prelude. It was fun and drove remarkably well. I didn't find the bum-steer to be disconcerting -- nowhere near as disconcerting as PDCC, PTV and electric steering on the 991S.

My only gripe is that they've designed the rear-steer to ride along as unsprung weight. At least, that's what it looks like in the diagram. That seems absolutely absurd. Why isn't it inboard and using a push rod control arm?

Presumably this kind of nonsense (like making the nose-lift unsprung weight) comes from production line engineering rather than focusing on driving and handling qualities.

As for the time line of gripes, that's not the history as it unfolded. The 993 was greeted as an advance from the 964, though some considered it too good, and in any case, the 964 was an unloved "bubble bumper" car. The 996 Carrera was rightly scorned as the "wet one" with fried egg headlights and slab-sided body with Ford family sedan ovals taking over the design of the cabin, which was promptly reversed to pay respects to the 993 when the 997 came along. But the 996 GT3 was well regarded from day one. When nav came to the RS, it was an option, not mandatory. When the RS had "nannies" they could be switched off completely. So these "gripes" simply never existed. Things started to curdle when PASM came along. I think it's fair to say the "step" from 997 to 991 is Porsche jumping the shark. I wonder if this GT3, like the 991 altogether, is one stop past the end of the line for a generation of 911 track drivers.

Porsche risks losing credibility with this GT3. But I don't believe that rear-steer alone can turn a car into a flop. Still there's always the risk that once drivers take it to the track, if it's not as enthralling as promised, by the time the RS comes along, if the GT3 is a flop due to rear-steer, the customer response might be "so what?" But I think it's going to be a tiny minority of dissenting opinions drowned out by a larger than ever population of customers.
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Old 03-07-2013, 11:21 PM
  #19  
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Originally Posted by NJ-GT View Post
When was the last time a GT3 needed to take turns at 31mph or less on a racetrack?

Was any previous GT3 unstable while changing lanes on straight line at speeds of 50mph and up?

If the answer to these two questions is NEVER, the Rear Axle Steering is just a gigantic Gizmo, with no performance gains at all, unless parking is a performance measurement.

I just checked my minimum Parking Lot Racing (Autocross) speeds, and they are above 31mph, so this Gizmo does nothing, not even at autocross.

Because of this gizmo, the car carries a weight penalty, and it cannot be offered with a Li-Ion battery (another penalty).
werd
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Old 03-08-2013, 12:39 AM
  #20  
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Carrera GT beat me to it. Honda had this about 20 years ago. It is gimmicky and is it really necessary?
I like the new GT3 overall and may end up buying one.

But this is a strong indicator that 991 is not your father's 911. It is a definite **** away from the traditional idea of what a Porsche is and is not.
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Old 03-08-2013, 01:03 AM
  #21  
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Originally Posted by SSST View Post
But this is a strong indicator that 991 is not your father's 911. It is a definite ****...
You got that right
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Old 03-08-2013, 01:11 AM
  #22  
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Originally Posted by NJ-GT View Post
When was the last time a GT3 needed to take turns at 31mph or less on a racetrack?

Was any previous GT3 unstable while changing lanes on straight line at speeds of 50mph and up?

If the answer to these two questions is NEVER, the Rear Axle Steering is just a gigantic Gizmo, with no performance gains at all, unless parking is a performance measurement.

I just checked my minimum Parking Lot Racing (Autocross) speeds, and they are above 31mph, so this Gizmo does nothing, not even at autocross.

Because of this gizmo, the car carries a weight penalty, and it cannot be offered with a Li-Ion battery (another penalty).
How else are they going to catch the GTR with a car that carries a significant fundamental design compromise in weight distribution and polar moment of inertia...and a car that must appeal as a daily driver. It's called gizmos.
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Old 03-08-2013, 01:31 AM
  #23  
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Originally Posted by axhoaxho View Post
I have a deposit on the RS; and if it also has the 4WS, yes, I will be an owner and I have my concern; and I am serious.

Have you bought a GT3/RS before? I have. The GT3/RS is not like Carrera's that dealers have plenty for test driving. Most of these cars were pre-ordered by owners and when they arrived at the dealer it would be picked up within hours. Very unlikely that any dealers (at least here) will have a car and will allow for test drive thoroughly (not just a couple miles around the block.)
Porsche Sport Driving School? You can rent a GT3 for the course and test it on the track. For a day or two. Not just a couple of miles around the block
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Old 03-08-2013, 02:22 AM
  #24  
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Originally Posted by StirlingMoss View Post
Porsche Sport Driving School? You can rent a GT3 for the course and test it on the track. For a day or two. Not just a couple of miles around the block
There are no GT3's at PSDS: https://www.porschedriving.com/PSDS-Cars.aspx
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Old 03-08-2013, 02:30 AM
  #25  
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Originally Posted by StirlingMoss View Post
Porsche Sport Driving School? You can rent a GT3 for the course and test it on the track. For a day or two. Not just a couple of miles around the block
The PSDS here is in Alabama and I am in California, and I don't beleive they have a GT3 (not the Cup) for rent.
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Old 03-08-2013, 02:44 AM
  #26  
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Originally Posted by NJ-GT View Post
Was any previous GT3 unstable while changing lanes on straight line at speeds of 50mph and up?
I don't know how to change lanes on a straight line, but take a GT3 through a 50+ mph box or offset slalom as fast as a Cayman can do it and you'll realize the rear of the GT3 is seriously trying to step out on the second move if you've got it set up neutrally. It's still one of the main drawbacks of the rear-engine layout.

Originally Posted by NJ-GT View Post
When was the last time a GT3 needed to take turns at 31mph or less on a racetrack?
Never. And despite that, the rear steer will still help.

As the O.O.P I'm going to try and clarify why you should consider driving the rear-steer car before making a judgement you many not yet be qualified to make.

First, I'll tell you to drive a 70s or 80s 911 at the limit and then drive a GT3. Notice that feeling when you lift off and turn in with the older 911 and the rear tries to come around? The feeling that's almost gone from the GT3? That's rear steering helping the GT3, plain and simple. It's not subtle, it's a huge effect, and the only reason the rear-engine platform is still viable today.

To get more technical, I'm going to pull some excerpts from an SAE paper, 2011-01-0987 "Investigating and Improving Vehicle Transient Handling Performance"- please look it up for a fuller explanation than I can give.

Basically rear engine cars are inherently going to try to oversteer during transitions. They have two big factors stacked against them: rear weight bias, and high polar moment.

The comparison graph below shows this, once you're able to decipher it. Below is the effect of weight distribution alone on rear tire slip angle, ie oversteer, during a "step steer" maneuver starting at one second, basically turn-in on a higher speed corner:



What you see: the larger the slip angle at the rear (6 degrees vs 3.5 peak here) the more the rear end of the car is beginning to drift. Notice how the angle overshoots as the car turns in before settling down, and also notice the size of the overshoot is greater the higher the weight on the rear axle? This is old school 911 behavior, where the rear steps out on turn-in and the driver needs to catch the rotation- any classic 911 driver who competes is familiar with it. The behavior gets magnified the higher the weight on the rear axle and the higher the polar moment, two things the 911 has in great abundance. Where you get into particular trouble is if the car turns one way, the rear adopts a slip angle, and then you need to quickly turn back the other direction- this is a recipe for a spin, and it's one of the things that defines how "neutral" one can tune the handling.

Now look at the second graph. This is what happens when you include a very rudimentary "rear steer" on the rear axle, basically bump-steer where the rear wheels turn as the body rolls:



This is turning the rear wheels slightly into the direction of the corner, and notice how much less slip angle (drift) the rear of the car has. Porsche already uses this in the GT3, but it also adds compliance steer (turns the wheels based on sideload, not position) on both the front and rear axles to neutralize the inherent oversteer of the rear engine layout much more than shown above. In a modern GT3, without you knowing it or doing anything both the front and rear are simultaneously countersteering as you enter a corner to prevent the rear end from starting to come around.

Now we know this works pretty well, so why bother adding an active system?

Two probable reasons as I see it:

First, you see significant overshoot in slip angle on the graphs, even with a little rear-steering as on the second graph. With a passive system this is probably unavoidable, as it by necessity reacts to body roll and the forces placed upon it. With an active system, however, it's possible to predict the overshoot and neutralize it entirely.

Second, the amount of this turn-in oversteer is very speed dependent. You might never notice oversteer in your car at high speed, but I guarantee you that if you've ever autocrossed you've noticed low-speed understeer in tight corners. This is easily fixable by stiffening the rear bar, but as you probably know if you do you'll take out every cone down the high speed slalom due to the oversteer effect above. Put simply you have few options in a standard car: because the transient oversteer is speed dependent you can choose which speed you want the car to be neutral at, then understeer or oversteer above and below that. Active rear steer simply corrects this issue: you just dial in as much rear steer as is needed to be neutral at any speed.

Because of the rear weight bias, rear steer is much more important in the 911 than in any Prelude or 300Z. This is not rocket science or marketing- any decent vehicle dynamics guy knows the advantages of steering the rear wheels these days (the paper noted above came out of GM), and all of the high end sports cars are using passive systems.

Just because you don't know you want active rear steer doesn't mean it's bad. My advice is to understand the system, then try it before you pass judgement. I expect it allows the car to be much sharper and faster in medium speed corners while maintaining or improving the high speed balance.

Last edited by Petevb; 03-08-2013 at 03:21 AM.
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Old 03-08-2013, 02:46 AM
  #27  
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https://www.porschedriving.com/Porsc...--GT3-Cup.aspx

Check the above....."The emphasis is on track time with the GT3, 911 Turbo and GT3 Cup Car."

Last edited by Bill_C4S; 03-08-2013 at 03:44 AM. Reason: Link corrected.
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Old 03-08-2013, 02:56 AM
  #28  
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Originally Posted by Petevb View Post
Never. And despite that, the rear steer will still help...............
Great post Pete. Really interesting and informative. Your autocross comments, among others, were dead on. Thanks.
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Old 03-08-2013, 03:12 AM
  #29  
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Originally Posted by Bill_C4S View Post
https://www.porschedriving.com/Porsc...l--GT3-Cup.asp

Check the above....."The emphasis is on track time with the GT3, 911 Turbo and GT3 Cup Car."
Link no workie.

The United States of America is a little bit bigger country than UK or Switzerland. Seriously, the PSDS is in AL and I am in CA, that is more than two thousand miles away.

The US PSDS don't 'just' have a GT3 for rent. Even for the 997 GT3 Cup Experience ($10K 2-day, $3.5K 1-day), who will 'realistically' spend days of time plus so much money, travel two thousand some miles just for a 'test drive.'
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Old 03-08-2013, 03:20 AM
  #30  
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Porsche had rear steer axle some 35 years ago on 928: http://en.wikipedia.org/wiki/Weissach_axle... so the japanese came after
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