The Super 90 Camber Compensator: Yea or Nay?
- Category: Research & Identification
Camber compensators were installed on Super 90's and offered as an option on 356C's. Porsche's objective with the camber compensator was to improve handling. Their formula was to use smaller rear torsion bars (23mm) combined with a type of spring called a camber compensator.
Whether or not they accomplished their objective is controversial. There are some who believe it improves handling as it was intended to do -- and there are others who believe it actually detracts from handling and should be removed. Presented here is information compiled from the 356 Registry Talk List and other sources.
The handling of the Super 90 was improved by the installation of a compensator spring. This was not a stabilizer bar, but a leaf spring fixed to the two ends of the rear axle that was supported under the case of the gearbox without being mounted to it. Because this leaf was under tension, it supported part of the weight of the car and it was therefore possible to use less rigid torsion bars at the rear. This increased the body roll and in a corner transferred a greater part of the weight to the front outside wheel, which decreased oversteer. The rear spring was eliminated on the 356C (but remained optional), where a similar effect was obtained by increasing the diameter of the front swaybar, while decreasing the rigidity of the rear torsion bars. For competition use it is recommended to use a Z-Bar, a similar device to the Porsche compensator spring, but which pushes the outer rear wheel up when cornering.
There has been a myth built up over the S-90 camber compensator since 1960. By itself, it does nothing to improve the handling! The factory put it on when they installed softer rear torsion bars, so the back of the car could still have the same load-carrying capabilities. The softer bar made the rear of the car roll more, so the front end would take more of the cornering loads. This increased understeer, only making it seem like the car had decreased oversteer. The factory compensator is incapable of preventing the rear wheels from "jacking up" during hard cornering, which is the biggest detriment to a swing axles cornering abilities. The S-90 compensator, when pre-loaded, actually pulls the rear wheels toward positive camber.
356 Registry Talk List Posts
From: Dan Metz
The name "camber compensator" says it all. The 356 (all models) has a swing-axle rear suspension, as did the VW bug. This is a suspension designed as a cost-savings measure, because only one u-joint/halfshaft is required. It is called "swing axle" because the half shafts or axles can and do pivot about a u-joint near the transaxle. This means that during large body roll angles (hard cornering, hitting a bump with one side of the car and not the other), large camber changes can occur. Large means > 5 degrees, sometimes much greater than that even. By contrast, a modern car such as a 996 with a multi-link rear suspension will only experience camber changes of a few degrees, possibly even less than that, from full jounce to full rebound.
When a tire runs at a significant camber, the slip angle vs. sideforce curve shifts along the slip angle axis, thus changing the tire sideforce generation characteristics. On the rear axle, tire sideforces work against the turn, thus reducing oversteer or inducing understeer. The tire performance changes are bad enough, but what's even worse is that under very severe cornering, the camber change can be large enough to actually allow a wheel to tuck underneath the car, becoming parallel to the ground. Of course, this results in a rollover accident and broken suspension. I emphasize that this would require an extremely severe maneuver, one which most 356-ers would not encounter in the entire lifetime of the car. The problem is accentuated even more if the throttle is dropped in a corner, thus transferring weight from the rear wheels to the front wheels because of deceleration. This where the terms "rear wheel tuckunder" and "dropped throttle oversteer" come from.
All the camber compensator does is try to limit the amount of camber change by connecting the two rear wheels together through a flat spring. There are plenty of them available for sale at swap meets, but the bushings aren't so plentiful and they cost an arm and a leg. One of the Registry List members at one time was talking about actually manufacturing some for sale, but I don't know if that ever occurred.
Consider a camber compensator as a band-aid applied to an antiquated suspension design. It has some effect, for sure, but it simply isn't possible to eliminate all of the deficiencies of the swing axle that easily. Useful, but not a panacea.
From: Stan Hanks
It's critically important to recall that the camber compensator as originally fitted actually compensated for using SMALLER rear torsion bars. That is, it was a decoupled spring (as described) that didn't change the rear roll axis, but did add stiffening in the vertical plane for things like bumps, etc.
The real gain in using the camber compensator was USING SMALLER REAR TORSION BARS.
This radically changed the roll geometry and let the front anti-sway bar carry a larger portion of controlling chassis roll.
And while the design of the suspension on the 356 is antiquated, it is also stunningly effective if it's set up right. The trick is finding someone that actually knows how to do it...
From: Geoff Fleming
There is really no performance benefit to installing an ORIGINAL camber compensator. The handling advantage was due to the softer rear torsion bars and increased diameter of the front sway bar. The compensator merely "compensated" for the reduced load carrying capacity of the car with the softer torsion bars. It can be set in two positions and will elevate the rear end of the 356 to provide more ground clearance.
If you are looking for a bar that will certainly increase the handling, then contact Vic Skirmants, ( co-founder of the 356 Registry). Vic produces a fantastic bar that makes a real difference in the handling of the 356 without reducing the nice feel of the car.
Articles from the time and later said that in 1963, the inflation of the German mark was hurting sales in the US. Therefore, the factory did some (heresy) cost cutting. Among the obvious to the eye things to go on the T6 was the compensator spring. The factory was spending all its engineering effort on the intro and tooling of the new 901 and 902. Another change was the parking lights from white to amber lens required by CA by 4/1/64 and the rest of the states by 1/1/65 (I think). Any other changes were for cost or to repair a problem. There were others that are not remembered that well.
From: Steve Gurney
No, it's fine. Mostly it just compensates by helping carry the load for weaker rear torsion bars, if installed to shift more cornering power to the front. It's not intended to improve the handling, but the weaker torsion bars are. Then, combine it with a stiffer front roll bar and handling improves. It least, mine sure has. Steve
From: Ron LaDow
Not sure Vic is on the list, and I wouldn't presume to speak for him.
A compensator 'compensates' for softer torsion bars at the rear. Without going into a lot of suspension theory, the end of the car that rolls the most, sticks the most. At the limit.
Does a compensator help? In racing, certainly. On the road? Depends on how hard you push the car. The way the C coupe is driven, it would just make the rear feel a bit strange. But then it might end up saving me from some bone-head mistake...
The compensator spring was highly written about in 1960 when the S90 came out and that they had cut off XX seconds on laps around Nurnburgring, etc. w/ the spring vs. w/o the spring. That seemed the basis for the addition of it. My new S90 coupe in '61 had it and it was one of the reasons I bought the S90 over the Super. I put one on my '64 SC and Vasek Polak made me adjust the suspension settings. It seemed better, but then I wasn't racing it, except on the street.
From: Al Zim
I think selling your camber regulator is a mistake. Mr. Skirmants device works entirely differently than the factory one. You are preloading camber into the rear suspension with his device. With the factory one you are removing camber. The camber regulator allows transfer of weight to the inner wheel in cornering. It took me a while talking with Vick to understand what he has done. My personal belief is that this is a better option than a Z bar. Take some time to learn how it works and you will keep it.
From: Pat Tobin
I think they worked fine, but the factory re-tuned the suspension on the C models so that they got much the same benefit without the added spring leaf ("camber compensator").
I never owned a Super 90, but I drove some when they were new and the rear end really did stick better than on my '60 Super Coupe.
Others have correctly stated that the improvement in cornering came not so much from the bar itself as from the softer rear torsion bars which softened the roll stiffness in the rear so that more of the cornering load was transferred to the outside front wheel. Since the car is tail heavy, that tended to even up the dynamic weight distribution between the outside tires.
But just softening the rear torsion bars would then mean that the tail would be too soft, hitting bottom all the time especially if there was a load in the rear seat. The spring bar added spring stiffness in the vertical plane without adding to the roll stiffness.
From: Al Zim
I agree with your analysis and that was what I was trying to say for the C cars. They found a better and cheaper solution and that is what they used. I believe all the Carrera 2 had the compensator (both B and C).
From: Jim at EASY
If deleting the Factory installed camber compensator, a person should also change the torsion bars in the read of the car. In the case of 356B's the "standard" torsion bar is 24mm diameter....cars equipped with a camber compensator have 23MM bars.
356C's (they have a shorter torsion bar) have 23MM bars as standard and 22MM bars if equipped with a camber compensator.