Vilhuer

This was possibly ABS related thing.

FredR

We keep seeing wheel threads at the rate of one a week [or so it seems]. The discussion is invariably the same "will they fit?" dialogue whereas the question that perhaps should be asked is "will they work?". I have written about the concept of NSR heavens knows how many times given my interest in the subject matter and figured it might be an appropriate time to refloat this excellent thread that did not really get the traction I feel it perhaps should have.

The S4 suspension by design has a negative scrub radius [NSR] of 10.2mm. To achieve this NSR the suspension has to be set with a specific camber and have wheels with ET65. The worst case scenario is when wheels are fitted that have zero scrub radius which is what happens with wheels that are ET55 and such examples will likely exhibit quite noticeable squirm. Thus not difficult to understand that wheels that have ET52 [i.e. most other Porsche sports cars] have a positive scrub radius of 3mm.

As with most things suspension, altering specific parameters invariably has an eclectic mix of positives and negatives but Porsche knew exactly what they were doing and departing from the design configuration has some downsides with somewhat drastic potential consequences. For those who race having a positive scrub radius can have quantitative benefits. However for street driven cars the NSR concept is solid.

Now we often see comments about how some wheels have "good NSR" but then I often wonder what constitutes good. If the design requires 10mm NSR and the wheels proposed to be fitted have a 2mm NSR is that good? If the recommended tyre pressure is 35 and we used 20% of that [i.e. 7 psig] would that be considered "good"? To be clear, the two concepts are not directly comparable but the principal remains to some extent

In short the NSR drives a number of failsafe concepts- in the case of a rear wheel drive vehicle like ours and more so with high output motors, during acceleration and braking more front toe-in is induced. The NSR also stabilises the car under braking in the event that one side is weakened for whatever reason and perhaps more importantly if you hit standing water on the curb side of the road in a down pour under braking the car will tend to drive itself out of the water [rather than pull you into it under yaw].
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To help those interested in the concept I attach an interesting extract that explains in a bit more detail the effects of scrub radius. For those who drive gently in all conditions this subject matter is largely irrelevant but if one wants to drive as Porsche intended the car to be driven it boils down to a question of whether one feels lucky [well do you?]. Ironically, despite living in one of the driest climates in the world I have driven into unexpected ponded water at some speed and despite one helll of a water splash the car pulled through effortlessly [my rear end didn't half pucker up though and I do not mean the car].

jpitman2

My experience in another make, going from ET52 to ET47, same rim and tyre diameter, was a small loss in directional stability. This was noted immediately after the change, so there were no other variables involved. After getting used to the new wheels, its not noticeable or remarkable any more.
Can somebody explain how a change in 'height' , or rolling radius could affect the scrub radius, as postulated above? If caused by KPI and camber, it would have to be gross to be significant IMHO.
jp 83 Euro S AT 57k

FredR

JP,

Simple trigonometry is the answer to your query. The scrub radius is defined by the difference between the centreline of the wheel and a virtual projection of a line formed by projection of the upper and lower ball joint centres onto the ground. Increasing the rolling diameter means that the same virtual axis touches the deck a little further outboard but the difference is negligible. Thus the point of the demo is that when reducing wheel offset by 10mm or whatever the impact is non recoverable and folks should understand what they are doing when they dick around with wheels as the impact on steering geometry is potentially quite profound. Most folks who ask queries on the list do not even know about the concept of NSR yet alone what it does and why it does it- quite some basis for modification-huh? Do this in Germany and your insurance is invalid because the mod would not have TUV approval. Are the Germans ***** or wise folks?

When things go wrong at high speed most punters are simply not capable of responding with appropriate inputs. Porsche know this and went to quite some trouble to build stability into the 928 and the chorus we typically hear is "it works OK for me"- and indeed it does- until such time as it does not that is! Just about every Porsche sports car uses a front suspension design that requires an ET52 offset but there were some quite significant changes to suspension geometry between early and late model 911's with the scrub radius going from positive to negative or so I understand- I wonder why?

By the way the term Kingpin dates back to the horse and carriage era when the steering wheels had a physical kingpin as did the Ford Model T and many other automobiles. The advent of modern suspension eliminated the need but the term was retained to define the virtual concept we refer to as the kingpin angle.

Suspension design is a fascinating subject for those so interested and not really an area to dick around with unless one knows what one is doing and what you want to achieve.

jpitman2

Fred,
I think you have confirmed what I was getting at - changes to camber or rolling diameter will have negligible effect on scrub radius - much less than changes to offset anyway. Re king pins, or their current manifestation, ball joints, I have observed some models of MBs display visibly large camber (?)angles of wheels when on full lock - any idea why this was ? What behaviour was it designed to achieve?
jp 83 Euro S AT 57k

Geza

Caster angle

dr bob

A reminder that changing camber on our cars, be it via the normal lower ball joint adjustment or by swapping different upper control arms, does not directly affect the scrub radius. As Fred reminds us, the scrub radius is figured from a line that traces the axis through the two ball joints, and where that line intersects the tire contact patch. That number only changes if you change the offset of the wheel, or otherwise move one or both of the ball joints relative to the contact patch. Adjusting camber moves the axis through the ball joints relative to the chassis, while the relationship to contact patch does not change. Wider tires on the same wheel offsets, no change. So long as the rolling diameter of the tire stays consistent, changing the wheel diameter will have no effect on actual scrub radius.

JP's observation of the MB camber changes at high steering angles: MB uses lots of caster to maintain on-center steering stability, with less dependence on scrub radius. Good idea, as it allows a large range of wheel and offset combinations with smaller effect on steering "feel". They make up for a lot of that too with their power steering assist configuration. It also compensates for body roll and helps maintain correct contact shape at speed. Reality is that we tend to make much smaller steering inputs as the speed goes up, with less than -maybe- +/- 90º steering wheel angle. "High speed" steering and on-center feel happen in a pretty narrow range of steering angle changes. Those bigger angle changes at low speed are masked with high steering assist.

FredR

Bob,

Thanks for pointing that out- in trying to make the case for the impact of ET on the system I even managed to get myself confused due to lack of sufficient coffee I suspect. You are quite correct to say that the camber does not impact the NSR but unless I am missing something the diameter does and my calcs suggest that if the diameter changes from 25 inches to say 26 inches [the biggest diameter that could be fitted?] the NSR would increase from 10.2mm to 12.4mm- not huge but nonetheless discernable and the point I was trying to make was that even this is small compared to the impact that an incorrect offset has. This assumes a kingpin angle of about 10 degrees. The maximum diameter I have used was 25.4 inches running a 265x35x18 up front.

Trust I have that correct.

Rgds

Fred

Richard Armstrong

Does anyone know what the standard scrub radius of the rear is. I know the front is -10.2, I can't find any info on the rear. I have unique wheels on my road car and I removed the (17" 7.5J) fronts for a refurb and checked the Offset - only to find a number on the inside of the wheel - ET 52. I have not removed the rears yet. The car handles well but does have a slight tendency to tramline, however the tyres are now well worn - especially on the very inside edge. This, I think, is probably more likely to be incorrect toe out. The suspension is slightly lower than standard as well. I have tracked the car on many occasions and it handles well - with some understeer. My racecar, on the other hand, with 16"8J D90s and Toyo 888s has incredible turn in, but the suspension is heavily modified.
I think I will live with them - it will be interesting to see what the rear width and offset is.

jpitman2

Scrub radius is only relevant for a wheel that does steering, so it cant apply to rears - its the distance from the point that the effective kingpin line hits the road, and the tyre centre, where negative means the tyre centre is inside the kingpin line. This usually improves directional stability, tendency to straighten rather than wander. Normal front ET is 65, so some tramlining at 52 makes sense. ET52 is not uncommon on rear wheels, and doesnt seem to have a bad effect. On the front, ET52 seems to have more effect on S4s onwards than earlier models.
jp 83 Euro S AT 57k

FredR

Richard,

As I am aware the concept of scrub radius only applies to the front suspension steering geometry.

Alignment settings will impact the tyre wear characteristics and too much or too little toe will cause differential wear. Too much rear camber will cause differential wear of the inner heel. The obvious question is what alignment setting are you running? You should know this and have the car setup to your specific needs. I have a custom wheel set and experimented with rear wheel alignment and found that setting the rear toe at the lower end of the acceptable range killed drive out of slow bends and thus set rear toe bang in the middle of the acceptable range. I have run rear camber up to 1.5 degrees and never seen accelerated differential tread wear.

If you really do have have a "toe out" condition on the rear that would not be a good thing. Reducing rear toe theoretically helps the car steer out of a bend but too little toe can also kill rear grip when powering out of a bend as I found out - a question of balance as it were.

Turn in is impacted by front toe setting. As with all things suspension wise, adjustments that improve one thing usually have a negative impact on something else and thus why a custom alignment suited to one's specific needs is critical to performance and especially so on a car that is to be tracked. Try driving a NASCAR tuned machine in a straight line at low speeds! Thus a front toe setting at the lower end of the positive toe-in range helps turn in but too much reduction makes the steering more twitchy with a loss of straight line stability- a question of balance. Remember steering is a process designed to create instability so that the vehicle can change direction. The quicker it changes direction the more unstable it has to be- quite a paradox!

Regarding your front wheels- nothing unusual there- most Porsche wheels sets for the 911 and Boxster crowd have ET52 fronts- the 928 has a somewhat different geometry. Not necessarily a bad thing for a track car but for a street vehicle it is [theoretically speaking] potentially "lethal". To be fair, chances of getting into such a situation are probably low to non existent but that is hardly the point. Offset is critical to the NSR. Single carriageway roads crown in the centre, dual carriageway roads drain to the side. When the heavens open up rain water collects on the side of the road. At some point the drainage is overwhelmed and one gets standing water on the side of the road- now drive into that with the passenger side wheels and that creates yaw- the NSR negates this effect as it naturally keeps the car tracking in a straight line- with your front wheels this will not happen. Needless to say a sensible person would ease up in such conditions but drive enough in such conditions and sooner or later one will find oneself in such a scenario- it has happened to me over here where it hardly ever rains [but when it does all hell is let loose]. Tramlining is most prevalent when the scrub radius is zero- i.e. ET55mm. I have run fronts with ET52 - hated them. Sold them to a friend for use on his 996 C4 and used the funds to purchase my custom wheels.

Finally, by design, the 928 under steers- a comparatively safer option than over steer. It is generally accepted that the fastest lap times are derived with a car set up to oversteer- fine for Lewis Hamilton and such talents. With 7.5 inch front wheels you are probably running a 225 section up front and doubtless something wider [255?] on the rears. The wider the rear section relative to the front the more it under steers. Nothing much you can do about this without fitting wider rims and rubber. You could fit grippier rubber up front. Stiffer rear links also favour reduction of under steer as would less "stiffness" up front. For dual purpose vehicle 255 or 265 rubber up front and 285 or 295 rubber on the rear is about as good as it gets- fit that with a Devek [or equivalent] adjustable front anti roll bar wherein one can tune the oversteer/understeer characteristics, fit slightly stiffer springs and that is about as good as it gets.

Vilhuer

I knew I had seen old style suspension value somewhere. It's not in any old style suspension pics I know but is in '86 MY yearly changes document:

Negative Scrub Radius - 10.2 mm
The negative scrub radius is reduced from -16 mm to -10.2 mm due to the new front axle geometry, which was necessary because of the new braking system.