The 'Maverick' Classic Porsche Forum and Classic Porsche Club for Porsche 356, Aircooled Porsche 911, Porsche 912, Porsche 914 & Porsche Replicas. 'We join for the cars but stay for the people'
I think the key is to use the compensator with the softer rear torsion bars and stiffer front ARB to get the overall chassis balance improvement
If you just fit a compensator with the standard torsion bar, you're effectively increasing the rear stiffness which is kind of the opposite effect of what they were trying to achieve..
Compensator + Stock torsion bars = similar stiffness to stock in roll, more stiffness in bump
Compensator + Soft torsion bars = softer in roll, same in bump
..the combination of softer roll on the rear plus stiffer roll in the front (larger arb) seems to be where its at - presumably better turn-in and more rear-end grip
The compensator is a second spring that acts in both directions from its neutral state. This combines with the torsion bar to provide a complex net spring rate at the wheels.
If I may I be so presumptuous, I would offer another view.
Strictly speaking, Mr.Skirmants' device is more correctly described as a "virtual Z bar", and as such, its operation in cornering should not add spring rate (stiffen) the rear roll couple. It is true that when the body is level, movement of one wheel will experience an increased resistance to movement in either direction, but that is but a side effect; its primary function is to control jacking by transmitting the force of the spring-induced drop of the inside wheel to force the rise of the outside wheel, without increasing the roll couple.
The center connection is a key element of its operation, in that it should be free to pivot without being influenced by the transverse angular movement of the chassis, just like a true Z bar. However, it is the case that the pinned connection on Mr.Skirmants' device is not the ideal configuration, as it does transmit some force in angular transitions as the transverse leaf is not truly free to rotate.
I recognized this issue when I conceived my own version of a "virtual Z bar" in the mid-eighties, independent of any knowledge of any other approaches. The realized design of this concept uses a proprietary true hinged connection at the center, so that the leaf can pivot freely regardless of the transverse angle of the chassis.
The other issue of note is that the pre-load should put the end links in slight compression, that is, one would set the suspension for zero camber, so that when attached and adjusted, one would induce approx. one-half degree neg. camber. This eases transitions and makes sure that the neutral force position is never positive camber.
In any case (of design), the actual/virtual Z bar is the best way to get the most out of a swing-axle rear suspension; the a-r bar can only make things worse.
As always, the employ of a fat a-r bar at the front (I run a 19mm) that increases the front roll couple induces a more predictable balance with smoother transitions that make the desirable oversteer more manageable.
Awesome info
I've always been really intrigued with suspension design ever since I started a module on analysis at university - I love the puzzle and figuring out the interactions
I also love that 19mm is considered a fat anti roll bar on a 356
My BMW is running 39mm front / 28mm rear, but working on some 75mm hollow front and rear with adjustable blades
Horses for courses - my bimmer prob weighs 2x 356's
Last edited by smallspeed on Sat Jul 30, 2016 4:29 pm, edited 1 time in total.
Many thanks for all this information and comments. It really does help
when trying to evaluate a change or upgrade. As this does not seem like
a simple bolt on fix I think it will add it to the wish list. Mainly because
there are probably higher priority fixes to complete first.
I would like to make sure the body work is tidy and rust free if possible
and also mechanically sound which it nearly is.
Also that it looks the way I want it.
Finally it also needs to perform the way I would like too, which I think
other than a gear box rebuild and maybe a remote oil cooler I would say
the suspension set up would fit here.
I think a lot of the above is cosmetic which means that once done I could
have this sort of change completed before the next Le Mans trip ???
Fingers crossed and thank you so much for the information. It really is
appreciated.
Hopefully see you guys at an event soon
Probably going to Goodwood Breakfast club next weekend but not bothering with registering
there was a noticeable (3 seconds a lap) improvement with the Vic Skirmants camber compensator versus without it when we fitted it to my 356A. I'd wholeheartedly recommend them. they are a bit agricultural in that they weigh 9 kg's and it's all at the back of the car which isn't so good for a racer but on a road car you won't notice the weight penalty. on the front i'd go for a 17mm or even 19mm front bar because it will reduce the roll significantly without making the ride much harsher, and a decent rebuild of the koni's will make it like a new car. in fact it will be beyond any 356 you've experienced!
Cheers
Steve
'53 356 Pre-A (a box of bits!)
'54 Okrasa/Porsche/VW special
'58 356A Super (GS-spec)
'65 Razoredge Ghia
'66 Westfalia Camper
'70 911S (2.3 ST Le-Mans spec engine)
i've always scratched my head about the rear suspension....i've assumed that it works better than it should because, when cornering, the outside half-shaft joint, just outboard of the transmission case, dips and in so doing tries to keep neutral or negative camber to the outside wheel, although it is being opposed by the couple, due to the tyre friction, tending to want to lift that joint, and the whole car, upwards and flip the thing over.
Also, perhaps, (i have to say i' ve not driven a 356, as my T5 is taking a long time to restore), the more body roll that there is at the rear, the more force that the spring plate will exert, and it is the spring plate, perhaps, that is making the rear suspension progressive.
The C, if it was fitted with a factory compensating spring, was recommended to have its torsion bars reduced to 22, from the standard 23, in the same way that the B should have its t-bars reduced from 24 to 23.
The factory c.s. was not held to the transmission case, there was simply a rubber buffer between the spring and the case, so it is not surprising that it did not solve the tucking under problem (perhaps the spring even went past its flat shape and flipped the other way, under the extra forces of the momentum of the suspension falling, when leaving the brow of a hill at speed, i'm just guessing).
I've wondered why the C could get away with so much softer rear t-bars than the B, and i've thought that, maybe, because the C was supposed to have had quite different rubber buffers (they were said to be progressive), compared to the B, and the C had a separate bump stop, whereas the B had none, that these large buffers came much more into play, than in the B....perhaps the designers had seen the success of the Mini's suspension.
I must admit, i still don't really understand why the Skirmants bar helps in cornering, except that i suppose the central fixture to the transmission case, decouples the forces, side-to-side, so the outer wheel springing is stiffened more than the inner wheel is being tended to be pulled down.
Have there been any transmission case fractures on vars with the Skirmants bar?
So, on my T5, i will fit 23 t-bars, 17.5 or 19 front anti roll bar and maybe see if the C buffers will fit, and possibly the Skirmants bar.
The dynamics of a swing axle are indeed complex, and it is little wonder that it can be a tricky thing to get a handle on.
Sadly, on the 356, it does not work better "than it should"; when pressed, the outside wheel shows the same propensity to tuck under as all other versions of the concept.
The mechanics of a "solution" were not unknown, thus Porsche reduced the rear roll couple by reducing the diameter of the rear bars, thus relying more in the higher (stiffer) front roll couple to keep the chassis shiny side up. However, this also reduced the rear load-bearing capacity, so the euphemistically named "camber compensator" (which is no more than a helper spring) was introduced. Its sole purpose was to provide the static load capacity lost with the softer bars.
To make sure it did no more, the ends (at the wheels) had one-way (push down, not up) connections, while the high center of the bowed spring was loosely pinned thru a rubber cushion to the chassis (via a bracket attached at the transaxle side plates), so that it would pivot fairly freely. The one-way connectors at the ends made sure that the compensator added no effect to non-static chassis dynamics, that is, it does not add anything to the rear roll couple. In turns, what happened to one wheel had no effect on the other, exactly as intended.
I attempted to explain the dynamics of the Skirmants bar in my previous post; perhaps I was not clear. I would suggest that to best understand it, one should first get a grip on how a "Z bar" works. The Skirmants device provides approximately the same effect, but with a different geometry of operation. Hence, its more accurate description is a "virtual Z bar".
From a practical perspective, my 356C corners rather faster and rides better than my similarly aged Healey 3000, which is a bit of a muddle at speed. I haven't had much experience of the Porsche yet, but it is a revelation on bumpy country roads, corners quickly and seems surprisingly well balanced when the tyres start squealing. It rides better than my 987 and is easier to manage on bumpy roads, although ultimately the Boxster has quite a lot more grip.
As usual, that's a very nice working description you have written. I guess they must have spent much time deliberating on the handling options for road use. You change one thing and it upsets something else.
By the time I was driving my 356C which in my opinion handled really well I found little difference than when driving my 356A which I already had been driving for five years. If anything I found the ' C' a little softer.
Trouble is one worn element can give you wrong idea's unless you are specialist in geometry. I think guys like Steve who race them can quickly establish if the handling is incorrect but in his case his engine is pushing out so much more power than the normal 60 or 75hp versions he needs suspension to be perfect.
Its a very interesting subject though and thanks again for that input.
isn't this what happens with a swing axle when cornering? the outside wheel (left on drawing) tries to go into negative camber....which is a good thing isn't it?...that was what i was trying to get at
and why is not the increased stiffness given by the twisting of the spring plate (as the suspension becomes loaded) not mentioned more....is it because it wasn't that significant?
this, and the fact that C's were fitted with a rubber buffer (with a separate bump stop, not used previously) which was supposed to be much more progressive, (and, due to the softer suspension of the C, this buffer was presumably called into action much more in normal driving than the B set-up) all makes for the C having a much more progressive , yet compliant suspension than before.
as far as I've understood just from my old vw knowledge .. but from your diagram , the loaded outside wheel ( on the left of your picture ) can have a habit of tucking under and going into positve camber ( bear in if you come off the throttle with the tyre still loaded and the rear of the car lifts , ) .. thats where the camber compensators are trying to reduce / stop this effect
yes, and i suppose that, because the throttle unloading is always in a specific rotational direction, the effect of this unloading on the car in right and left hand corners will be slightly different
![[URL=http://s255.photobucket.com/user/a ... .jpg[/img]](%5BURL=http%3A//s255.photobucket.com/user/ade55/media/912/various%20022%20Small_zpsy3sv9bmc.jpg.html%5D%5Bimg%5Dhttp://i255.photobucket.com/albums/hh129/ade55/912/various%20022%20Small_zpsy3sv9bmc.jpg%5B/img%5D){kind=link}