Rear steering
06-07-2011, 09:29 PM
#1
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Rear steering
I have been seeing things about the 928 assisted rear steering. Can someone explain that to me what models have it and how does it work?
THANKS IN ADVANCE
THANKS IN ADVANCE
06-07-2011, 09:36 PM
#2
the way the rear suspension was designed allows for passive rear wheel steering, so basically it helps prevent lift-off oversteer (to a limit)
que the wiki:
que the wiki:
The Weissach axle (pronounced 'Vise-' as in 'advise' and '-ach' with guttural 'ch') is a special rear suspension arrangement devised for the Porsche 928. The goal of the Weissach axle was to eliminate lift-throttle oversteer by allowing the rear suspension to adjust itself during cornering maneuvers.
The Weissach axle is a variant of the semi-trailing arm suspension. The tendency of a vehicle to oversteer when decelerating is compounded by the compliant bushings found in most trailing arm suspensions. When the vehicle is decelerating, the trailing arm pivots towards the rear as the wheel is "pulled" backwards relative to the chassis. This results in toe out, which makes the vehicle unstable.
For the Weissach axle, the front pivot bushing of the trailing arm is replaced by a short link. In this arrangement, when the vehicle decelerates and the wheel is "pulled" back, the result is toe in. This adds to stability and thus, reduces oversteer.
A similar, but simpler concept to the Weissach axle is seen in the rear suspension of the second generation Mazda RX-7. Instead of an extra link, a special bushing is used in the same position in the lower control arm to perform a similar function.
Suspensions with this or a similar function are sometimes called "passive rear steer". The concept of passive rear steering has fallen out of favor for new car designs, as semi-trailing arm suspensions have almost completely given way to multi-link designs with dedicated toe control arms.
The Weissach axle is a variant of the semi-trailing arm suspension. The tendency of a vehicle to oversteer when decelerating is compounded by the compliant bushings found in most trailing arm suspensions. When the vehicle is decelerating, the trailing arm pivots towards the rear as the wheel is "pulled" backwards relative to the chassis. This results in toe out, which makes the vehicle unstable.
For the Weissach axle, the front pivot bushing of the trailing arm is replaced by a short link. In this arrangement, when the vehicle decelerates and the wheel is "pulled" back, the result is toe in. This adds to stability and thus, reduces oversteer.
A similar, but simpler concept to the Weissach axle is seen in the rear suspension of the second generation Mazda RX-7. Instead of an extra link, a special bushing is used in the same position in the lower control arm to perform a similar function.
Suspensions with this or a similar function are sometimes called "passive rear steer". The concept of passive rear steering has fallen out of favor for new car designs, as semi-trailing arm suspensions have almost completely given way to multi-link designs with dedicated toe control arms.
06-08-2011, 01:59 AM
#4
Been selling Twinkies on Ebay,
have some extra cash right now.
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have some extra cash right now.
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My understanding is that the decelerating toe-in feature also works oppositely when accelerating, making the car easier to powerslide.
Me likey that part!
Me likey that part!
06-08-2011, 02:25 AM
#6
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Adam beat me to the above from http://www.autozine.org/technical_sc...spension21.htm
Here is the equivalent on the later style 911s:
My friend in Weissach insists that if there had not been a 928, the WEISSACH axle would have debuted in the 911 sooner or later. It kind of did, but it was quite a bit later... about 12 years.
Here is the equivalent on the later style 911s:
My friend in Weissach insists that if there had not been a 928, the WEISSACH axle would have debuted in the 911 sooner or later. It kind of did, but it was quite a bit later... about 12 years.
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06-08-2011, 02:47 AM
#8
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Nicole said:
Great quite Nicole. Thats the conundrum Porsche 911 engineers face. There is SO much history and loyalty that they are boxed in on radical new ideas. The engineers are pinned in the Evolutionary pig pen.
They can't venture out into the revolutionary pens for very long or if EVER.
The one time they did has been the 997.1 to 997.2 Base model 911s. That was a revolutionary move for Porsche if there ever was one. Completely new motor with 40% less parts and then the NEW Tranny the PDK all in one so called "Mid model refresh" Mid Model my donkey!!!!
The 928 engineers had clean pieces of paper in their designs. that never happened before in Porsche evolutionary steps foreard.
My friend in Weissach insists that if there had not been a 928, the WEISSACH axle would have debuted in the 911 sooner or later. It kind of did, but it was quite a bit later... about 12 years
They can't venture out into the revolutionary pens for very long or if EVER.
The one time they did has been the 997.1 to 997.2 Base model 911s. That was a revolutionary move for Porsche if there ever was one. Completely new motor with 40% less parts and then the NEW Tranny the PDK all in one so called "Mid model refresh" Mid Model my donkey!!!!
The 928 engineers had clean pieces of paper in their designs. that never happened before in Porsche evolutionary steps foreard.
11-21-2019, 08:18 PM
#9
Burning Brakes
I understand how the Weissach bushing acts as a (semi) articulated joint to induce toe-in under deceleration.
BUT... are there more geometry tricks hidden in the 928 rear suspension? I seem to recall reading a long time ago that body roll, alone (without the fore-aft forces which would induce movement of the Weissach joint), would change toe during pure cornering (toe-in on the outside and toe-out on the inside). But I can't confirm that and it could simply be a journalist's misinterpretation of how the Weissach joint operates or just bad memory on my part. Is that true?
Perhaps the same question but I'm wondering why the link pin is angled the way it is, as it would seem to cause some twisting motion relative to the upper control arm as the suspension moves up and down. It doesn't appear to be quite parallel to the joints the control arm pivots on (?). It's also interesting that the bushing centers for the Weissach-joint-to-body bushing and the camber bushing (lower and innermost bushing) are not colinear, so something has to give as the suspension moves. Finally, lateral forces might induce some deflections in the bushings that affect toe. Does any of this induce desirable toe changes, and if so is there a discussion of that anywhere? I've searched and come up blank so far.
BUT... are there more geometry tricks hidden in the 928 rear suspension? I seem to recall reading a long time ago that body roll, alone (without the fore-aft forces which would induce movement of the Weissach joint), would change toe during pure cornering (toe-in on the outside and toe-out on the inside). But I can't confirm that and it could simply be a journalist's misinterpretation of how the Weissach joint operates or just bad memory on my part. Is that true?
Perhaps the same question but I'm wondering why the link pin is angled the way it is, as it would seem to cause some twisting motion relative to the upper control arm as the suspension moves up and down. It doesn't appear to be quite parallel to the joints the control arm pivots on (?). It's also interesting that the bushing centers for the Weissach-joint-to-body bushing and the camber bushing (lower and innermost bushing) are not colinear, so something has to give as the suspension moves. Finally, lateral forces might induce some deflections in the bushings that affect toe. Does any of this induce desirable toe changes, and if so is there a discussion of that anywhere? I've searched and come up blank so far.
Last edited by rjtw; 11-21-2019 at 08:40 PM.
