DWS964

Can someone provide a good technical reference on the impact of adding spacers to the front wheels (or decreased ET offset) and the subsequent impact on geometry and handling. I understand that in principle that the center of the tire contact patch should match with the axis of rotation of the steering system. I would assume that small increments - 5mm +/- would not be noticeable in handling. I recall seeing references to impacts of under/oversteer by incremental changes in front or rear track width. In the extreme case of using 25mm+ spacers, what will the effect be? Increased steering effort? Stability/Instability/Darting? Braking (any uneveness will be exaggerated in pulling one way or other)? Thus, any safety concerns?
My situation - 90 C4 with a widebody conversion. The front suspension has not been moved to the outboard mounting points (approx 25mm each side) for increased track width. Instead, wheels with decreased offset (ET23 vs ET55) have been used. I am picking up the parts to do the suspension conversion eventually. Street driving seems fine, but I will be taking it to the track later this spring.
A good technical explanation would be helpful, along with some practical experience. Thanks for the help, as always.

wanna911

I was told by a guy in the paddock at Barber that adding spacers in effect reduces the spring rate by moving the wheel outboard from the spring. How much he wasn't clear. If it's true, I have no idea.

I run 19 mm spacers occasionally and honestly the car feels more planted because I have so much more track width. It's really not much different than having the wheels custom built to maximize the width and track areas. However big spacers carry big weight, mine weigh almost 10 lbs for the set. But I can feel the track width benefit more than any unsprung weight penalty (however I have lots of power).

jdistefa

5mm I wouldn't worry about, but trying to increase the track width by 20+mm will significantly change your scrub radius and make the car a real handful on turn-in at the track.

For street driving not a major problem, but I wouldn't drive the car approaching the limit of adhesion at the track. Having said that, it depends on your skill level, risk, braking point/style, and corner entry speed.

IMHO, do the job right and move the front suspension to the outboard mounting points

bobt993

As Matt explains you need to do this properly. I think the increased spacing should be 30mm which is what is standard on the 964/993 chassis. To do this there is a laundry list of parts: tie rods, uprights, sway bar drop links extended........ watch the solid brake lines also. I am not sure if the pedal cluster is affecting on the earlier car, but it sure is on the 993.

DWS964

Agreed - it needs to be done right. I am sure that a bunch of Germans smarter than me knew what they were doing when they moved the suspension out, rather than adding spacers. (although.. I have heard that the earlier 930's used spacers on the front?) - I don't want to hijack my own thread - I was interested in a technical discussion of the geometry, forces, actions/reactions - all explaining why I should expedite the change.
"We don't have time to do it right, but we always have time to do it over."

Bill Verburg

To understand scrub radius you irst have to understand SAI -steering axis inclination. The steering axis(SA) is the line between the top pivot point of your hub and the lower ball joint of your hub. On a MacPherson strut, the top pivot point is the strut bearing, and the bottom point is the lower ball joint. The angle between the SA and vertical is SAI, add static camber to that and you have the included angle.

The scrub radius(SR) is the distance on the ground between the centerline of the tire contact patch and the point at which the SAI intersects the ground. If these two lines intersect at ground level, then you are said to have zero scrub. If the SAI intersects the ground at a point inside or outside of the centerline of the contact patch, you are said to have positive or negative scrub respectively.

older 911s have a small (+) SR, from 964 up they have a small(-) SR

the point at which the steering axis line contacts the ground is the fulcrum pivot point on which the tire turns. The location of this point within the contact patch has a great effect on steering effort, feel, and stability.

The bigger the scrub radius the more kickback there is at the steering wheel. If you change to wheels w/ larger o/s you will feel the increased kickback at the steering wheel


If the scrub is zero, the scrubbing action of the contact patch is equal on either side of the pivot point causing the tire to act like a car with a welded differential, inducing a condition called 'squirm'. In a straight line the tire tends to be stable and tracks well. As you turn though, the portion of the contact patch on the outside of the pivot point moves faster than the portion on the inside of the contact patch. Since the scrubbing area is equal on each side of the pivot point, yet the forces are different, the tire tends to fight itself and it becomes 'grabby' causing tire wear to increase and the steering to become unstable.

Positive and negative scrub radii have benefits in different types of suspension. A MacPherson strut assembly(as used on 911 front) typically performs well with a lot of SAI and caster(sound familiar?), a system negative scrub works well in. Because both SAI and caster increase the amount of camber on the outside wheel when steering, the fulcrum pivot point is at a point that has more leverage, requiring less steering effort. Negative scrub also helps reduce torque steer in front wheel drive cars. Positive scrub radius works well with suspensions that use dual control arms that use less caster and SAI to optimize geometry.

As with anything else, a little of a good thing is great, but lot of a good thing is not necessarily better. When you have excessive scrub, whether it be positive or negative, steering effort increases and road 'feel' increases, as the steering is more susceptible to road shock. Additionally, if you plan on modifying your scrub radius, you must take into account the amount of sidewall flex your tire will encounter under hard cornering. When the sidewall flexes, the contact patch moves in relation to the SAI and can make a slightly negative scrub radius become zero.

abs equipped cars want a little (-) SR to allow for uneven braking forces between the left and right front wheels

jdistefa

True dat

Because positive scrub radius (i.e. your proposed cheating on the offset) creates more fore-aft rolling of the wheel in conjunction with steering, you'll get more bump steer and 'pulling' with asymmetrical road surfaces. This can be a problem if you're threshold braking on uneven pavement (texture, ripples, whatever can effect grip), or dealing with mid-corner bumps or elevation changes that can potentially upset the car.

jdistefa

Oops, looks like Bill beat me to it!

DWS964

Bill, outstanding information, as always. Matt, hope you are satisfied with a 2nd place finish.
I have already made an immediate "improvement" in the car after getting it home. It came with ET23 wheels AND 17mm spacers on the front. Looked kool with the wheels pushed all the way out to the fender lips, but I knew that was really wrong (for a total of about 45-50mm positive SR !), so I removed the spacers completely. The information I found shows the front track width on C2/C4 is 1380mm and Turbo3.3 is 1434mm, giving 27mm extra on each side.
Bill (or Matt, giving you a chance to lead here) - just curious about how much negative SR is built into the 964 geometry? And thus, how much variation do we get with the range of ET wheels out there on a stock set up? (ET52, ET55, ...)
THANKS to everyone.

thingo

What would be a good way of measuring the scrub radius? I have just changed from 18x10 et 60 to 18x9 et 46, and on the street the car feels a lot different, but I also have camber plates which are set pretty well on 'max' which would have increased my negative scrub significantly correct? I am wondering what sort of range I should be aiming for.

Larry Herman

I can't answer that exactly, but keep in mind that the scrub radius is partially a factor of the width and offset of the wheel too, it is not an inherent "setting" on the suspension.

No, changing the angle of the strut will not change the scrub radius significantly. It is fixed through the spindle. On 944s, 964s, and later cars you can change the angle of the spindle carrier in relation to the strut, and that will absolutely change the scrub radius. In fact, I do this on purpose to bring the scrub radius back to where I think that it should be, but I too am not certain on how to measure it. The next time I have a car on my turn plates, I am going to carefully watch how they move when I turn the front wheels.

thingo

The angle at my spindle carrier was dictated by clearance issues to fit the larger wheels, then we used the camber plates to set the negative camber, but since the camber plates adjustment at the top (though at the end of a long lever) I would still expect the change in SAI will have an effect on the scrub radius? I am intending to use the spindle adjustment to keep the scrub radius in check once I have spent a bit of time at the track, but the noticeable change on the street has got me trying to understand the actual geometry.

Sterling Doc

It's interesting that Porsche varied the offset on the 944/924S series cars by 30mm each side, with no other changes. The 924S has the same front suspension as an early 944, but with late offset wheels (30mm less offset). I've often wondered how the got away with this, and what it all means for the relative handling.

Bill Verburg

here's how the factory measured it on a 964, scrub radius is 'r'

thingo

So judging by that diagram the 'r' is not a large number at all, say under 10mm.