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After years of trying maybe we can finally get the point home as to what Porsche were actually trying to achieve when they [very cleverly] designed the suspension system.
For those playing along at home who are interested the diagram Errka attaches [from the WSM] shows the S4+ suspension layout with the NSR of -10.2mm clearly depicted and to achieve this you need ET65mm. So by the time you get down to ET55 the scrub radius is zero and by ET50 it is +5mm [British Rail suspension territory].
As to how this translates in practice probably depends on how far one pushes the envelope as it were- going to the shops etc no problem.
What caught my attention was what the chap called "squirming". No idea what that is unless it is what I understand to be "tramlining" - this condition [tendency?] would logically exist at ET55 surprise surprise.
A question perhaps Errka can respond to- is it logical to understand that the wider the rubber the more NSR one might need? My front wheels for instance are ET68 and seem to work very well as I can tell with my limited abilities.
My first road-going experience with NSR was with a Saab 900 FWD. With the car came a book of interesting "why we did it" information. Along with equal-length half-shafts (eliminates torque steer they said), the negative scrub radius killed almost all tramlining. Perhaps more important, Saab suggested that you could experience a front tire failure at speed with your hands off the steering wheel, and not have the car do any violent direction changes. I never tested that feature, but the car was pretty rock-steady at speed; I did test that regularly!
My first road-going experience with NSR was with a Saab 900 FWD. With the car came a book of interesting "why we did it" information. Along with equal-length half-shafts (eliminates torque steer they said), the negative scrub radius killed almost all tramlining. Perhaps more important, Saab suggested that you could experience a front tire failure at speed with your hands off the steering wheel, and not have the car do any violent direction changes. I never tested that feature, but the car was pretty rock-steady at speed; I did test that regularly!
Bob,
The thing is most folks "know" the 928 handles very well by reputation but what they do not necessarily know is why. Pratt around with parameters that impact such design and for the most part I suspect owners would not know what they have done until such time as they really need that characteristic and then of course it is not there when needed.
Those of us who have researched the design of the steering system and appreciate its finesse perhaps understand this concept. If for some folks such concept is irrelevant then I just wonder what they are doing in this car when inferior machines like a Camaro or a Mustang are much cheaper to buy and run [possibly?].
Thats why the 928 handles as well as it does and actually gets better with larger rims and tires.
maybe not as great under brake failure problems (one side) but great for all other situations , unless you want much more stability and forgiveness under braking, like a BMW or other strut based vehicles.
so my 9 inch wheels with a 64mm offset are nearly stock as far as scrub radius goes. I wonder what my 12 inch rears with 37mm offset do to the equation...
Rears will apply more torque with greater mechanical advantage to the self steer, what impact that has is a good question. My SWAG, which is a scientific WAG, would be that it would initiate the self steer sooner and make the car feel more stable, and less exciting. Applying throttle would allo even greater adjustment of line.
Scrub radius, I drove a several years with +ve fitment on the 90S4, very little ill effect. So I think it's a bit overrated? I mean it's nice if it's correctly lined up as it keeps the steering feel during road bumps. Just so long the forces on the rack are equal each side, you're golden.
e.g. I drove around the block tonite, happen to have one stock 17" wheel and one 18" wheel. Car was pulling pretty hard to the 17" side. Probably due to height difference more than anything. It was horrible!
The differential in front track vs rear track. There is something.
This to me seems to define the turn-in character of the car, how it transitions to the one edge into and through a corner.
so my 9 inch wheels with a 64mm offset are nearly stock as far as scrub radius goes. I wonder what my 12 inch rears with 37mm offset do to the equation...
You are probably in a different territory altogether given you are [were?] in the 600 rwhp club and if I remember correctly [many years since I read your website] you probably pinned the Weissach axle.
With such rims I suspect you are going to be in the 335 region rubber wise and with such a rear offset you probably have custom rear flares along the lines Louie runs with more clearance than the stock GTS fender.
I know Mark feels that pinning the Weissach is not necessary, I have no opinion on such albeit I understand the logic behind such [I think?].
The early 928's had diagonal brake split - the later cars have F/R brake split. It would be pretty much impossible to have a hydraulic failure on one side of a later car that lasted longer than a couple of seconds.
You could have a mechanical failure (friction material popping off the backing plate, for example) on one side.
No one has mentioned the height of the wheel tire combo may also affect this.
A low offset wheel with a taller tire would seem to self correct this issue.
Scrub radius, I drove a several years with +ve fitment on the 90S4, very little ill effect. So I think it's a bit overrated? I mean it's nice if it's correctly lined up as it keeps the steering feel during road bumps. Just so long the forces on the rack are equal each side, you're golden.
e.g. I drove around the block tonite, happen to have one stock 17" wheel and one 18" wheel. Car was pulling pretty hard to the 17" side. Probably due to height difference more than anything. It was horrible!
The differential in front track vs rear track. There is something.
This to me seems to define the turn-in character of the car, how it transitions to the one edge into and through a corner.
Deciding what is "ill effect" is a crapshoot based solely on normal US road driving. Most changes in wheel offset are part of a tire-and-wheel change, with wider modern tires adding to the total available friction. So the car may in fact handle much better with the new tires and wheel offset. Unless there is a difference in offset from side to side, same-diameter same-width tires should not cause a "pull" regardless of the rim diameter. But the 18" wheel with 55mm offset will "pull" less than the 17" wheel with 50mm offset with tire diameter and width being the same. NSR helps manage the "Pull", so that a flat or failing tire causes the tire/wheel to toe in rather than out, as will changes in normal road friction/drag on one wheel/tire. Hit a road irregularity with NSR, and te car won't pull towards that irregularity; the forces applied to the slowing wheel actually turn that wheel (and the other front wheel via the steering linkage) away from the problem just enough to offset the effect of the slowing wheel. It essentially neutralizes the effect and allows the car to continue relatively straight.
Moving the tire outward (less offset) alters the realtionship of the front tires to the center of the car's turning circle differently. To correct steering geometry, the steering arms need to be shortened slightly and bent outward slightly, so there is more Ackerman included. Racers will note that stuffing seriously wider front tires under the fenders means effectivelt widening the front stance, otherwise the tires will rub on the inner pieces in the wheelwells. Without adjusting the geomery of the steering arms, they will set front toe to zero or even slightly toe'd out to smooth th etransition from balanced front weighting to weight on the outside tire entering a corner. The toe adjustment reduces steering upset on the transition, commonly referred to as "initial turn-in". The toe adjustment helps mask the basic problem with incorrect geometry, particularly when there is additional bump-steer with an already-lowered front suspension.
Lots to think about. There are software packages for racers to help understand and correct anamolies in steering and handling caused by different geometries. To make them work, you need a slew of data on height changes and other suspension movement. The output is often surprising and always revealing.
so my 9 inch wheels with a 64mm offset are nearly stock as far as scrub radius goes. I wonder what my 12 inch rears with 37mm offset do to the equation...
My one experience watching you leave a monthly breakfast suggests that you were doing more steering with throttle, and only adding small corrections with the steering wheel.
The early 928's had diagonal brake split - the later cars have F/R brake split. It would be pretty much impossible to have a hydraulic failure on one side of a later car that lasted longer than a couple of seconds.
You could have a mechanical failure (friction material popping off the backing plate, for example) on one side.
I blew a front brake line in a race, approaching the hairpin at laguna, the entire system went dead. i think i had a little rear brakes, but that might have been my imagination.when the pedal goes to the floor, there is no brakes
01-17-2016, 02:32 PM
Scrub radius
