The Z-Bar Illustrated

[FJCamper]

The Z-bar is named after it's "Z" shape. Downlinks from the ends connect to axle brackets. The drawing above depicts the basic version.

Swing axles catch a lot of criticism, but stop and think for a moment. They've run and won on many a race car. The mighty Gran Prix Auto Unions, the world-class Mercedes 300SL, 356 Porsches including the Carreras, LeMans-class British Triumphs and Alpine-Renault Monte-Carlo rally winners all had swing axles.

If a swinging axle is what you have or like, no apologies are necessary.

Porsche had developed the leaf-spring camber compensator for the 1960 Super 90, which with softer rear torsion bars helped get that car around the Nürburgring almost 20 seconds (!) a lap faster.

The Porsche 356 series ended in 1965, so the "Z-bar," an in-house project for a better swing axle modification, was passed on to the VW. In 1967 (Pre-Nader), all VW Type-1 models got this simple but effective device. It stayed in production for all VW swing axle models until the last one was built in 2003.

Savvy Porsche 356 racers were soon fabricating their own Z-bars, which worked better than the leaf-spring camber compensator!

The Z-bar does everything right, pushing the outer rear wheel up when cornering, as it tries to drop, yet adds no rear roll resistance to cause oversteer.

Inexpensive bushing kits are available from go-fast parts vendors that let you shim your factory stock VW Z-bar to make it act full time rather than just in extremes.

[redhot]

Hi,

I enjoy these threads you have started on the handling of our beloved car.

Short question: does the Z-bar in any way function differently than the camber compensator, since you , qoute "fabricating their own Z-bars, which worked better than the leaf-spring camber compensator! "


[u][b]Found this on the VZi forums:[/b][/u]

"The Z-bar is what VW calls an "equalizer spring" and is not obvious in its operation. Cars fitted with these have reduced over-steer tendencies and therefore more neutral handling characteristics - Which was what VW was trying to achieve. IRS vehicles, on the other hand have, (IMO) too much under-steer but, in the minds of manufacturers, this is inherently more safe (I don't necessarily agree with that policy).

Anyway, how the swing axle Z-bar works is like this: VW softened the rear torsion bars, so as to make the car roll more, during cornering. By doing this, the cornering forces are transferred to the front axle which would then 'break away' sooner. This increased under-steer, prevents the conditions for 'wheel tuck' to occur, except in (possibly) the most extreme of circumstances.

The beneficial by-product of the softer rear bars is that of increased ride comfort. The Z-bar, which is actually a long torsion bar, replaces the force lost by the softer main suspension and only comes into effect when the vehicle is heavily laden or when the suspension is working harder. This actually creates another beneficial by-product of 'progressive suspension'. However, when the car is cornering, the Z-bar has no effect whatsoever, allowing the softer rear torsion bars to perform as described above. That is why the left & right links, on the Z-bar, operate in opposite directions.

The camber compensator was originally designed for non Z-bar VW's & Porsche's. What it does is cradle the axle tubes and physically prevents the wheels from 'jacking up'; a condition liable to produce wheel tuck, which normally results in roll-over. However, unlike the Z-bar system, you could have the situation whereby only three wheels are on the ground (during cornering) plus the under-steer effect is not inherently there! The CC also prevents the same 'jacking up' condition occurring at high speed, when the Bug has a tendency to raise up. This is a major factor affecting the instability often felt by drivers on Motorways, especially when passing other vehicles or in high winds. This is also the area which the Z-bar system does not appear to address.

However, some people have said that they can create the CC effect with the Z-bar by removing the nut and washer at the end of the link rods, on either side, and adding spacers, washers or rubber blocks so as to reduce the in-built free-play in the system. This means that the progressive nature of the Z-bar will come into effect sooner than VW intended while still allowing the car to produce the under-steer which gives it that more neutral handling. However, it is likely that in reducing the free-play, the Z-bar may exert some force on the suspension, at some point, during roll, but I am not certain about that.

I don't know if I have been particularly clear in this explanation, but I hope I have helped somehow. "

http://www.volkszone.com/VZi/showthread.php?t=180244[u][/u]

[FJCamper]

Hi Redhot,

Aerodynamic lift on a swing axle VW feels scary because the rear axles are going into positive camber and you're beginning to run on the outside edges of the tires.

The stock Bug starts to lift at about 75mph, but most calculations are done with 100mph as the baseline.

My 1973 IMSA Super Beetle driven by Dr. Jim Roberts set and still holds the closed course land speed record for a stock-bodied VW at Talladega Superspeedway.

We were able to get 125mph on the straights, and with IRS rear suspension and cutaway of all air traps inside the hollow body (it was essentially a tube frame car with a full bellypan) and all other tweaks it was still a handfull at top end.

The tail end was riding higher up the track than the front end.

If we'd had better aero we'd gotten more top end -- but we had to have a rule book ride height and no aero dams, wings, etc. The sheet metal flap looking things you can see under our car were legal and classed as brake cooling vanes, but we were using them for some air-dam effect.

FJC

[gimmesomeshelter]

Hello-

Aerodynamic lift on a swing axle VW feels scary because the rear axles are going into positive camber and you're beginning to run on the outside edges of the tires.

Does this mean that anytime a car experiences lift (driving on the freeway, etc.) the rear end pulls up and the wheels move towards positive camber?

Thanks,

Paul

[FJCamper]

Hi Paul,

Rear lift on a swing axle car does cause positive camber. The Ghia has better aerodynamics, but tends to have more rear lift than front.

Bugs lift more up front, contributing to the scary "instability in a high wind" feeling at speed. The Bug is trying to rotate around its rear wheels.

The fix is to decamber, and kill as much lift as possible with aerodynamic aids such as front airdams or rear wings.

The IRS Ghia stays planted better, and actually goes into nose lift once the rear suspension has risen up as far as it's going to go.

FJC

[volkdent]

Hi Paul,

Rear lift on a swing axle car does cause positive camber. The Ghia has better aerodynamics, but tends to have more rear lift than front.

Bugs lift more up front, contributing to the scary "instability in a high wind" feeling at speed. The Bug is trying to rotate around its rear wheels.

The fix is to decamber, and kill as much lift as possible with aerodynamic aids such as front airdams or rear wings.

The IRS Ghia stays planted better, and actually goes into nose lift once the rear suspension has risen up as far as it's going to go.

FJC

You may have answered this elsewhere, but do you have any concrete info on how the roof lip affects rear lift? It was made by Remmele and set about 18" forward of the top edge of the rear window. I think they are attached with adhesive.

Jason

[FJCamper]

Hi Jason,

By "roof lip" do you mean a spoiler? And on a Bug?

Air dam = blocks air
Spoiler = disrupts air flow
Wing = creates downforce

FJC

[volkdent]

Jason

[FJCamper]

Hi Volkdent,

The roof spoiler (nice picture) shown is very effective on the Bug. Note that the New Beetle turbos have a motorized version of this in the same location. At this location, it does not have to be big to work well.

The spoiler breaks the airflow and prevents a low-pressure area drag behind the car that normally would literally pull backwards on it. You get a higher top speed and more overall aerodynamic stability.

Some aero devices don't actually provide downforce, but do kill lift or drag. This is one of them.

Of course, this one is rear mounted. If you spent 51% or more of your time in reverse, you'd put it over the windshield.

That's a joke. It's OK to laugh. -- FJC

[volkdent]

Ha ha ha ha

Ok, that was a good one but stick to cars OK?

Thanks for the advice. I don't want to hijack, do you want me to continue this on a new thread? I've got more questions(always...).

Jason

[FJCamper]

http://www.shoptalkforums.com/viewtopic ... light=aero

Hi Jason,

Start a new topic. We shouldn't go to aerodynamics from Z-bars.

And take a look at the thread above. Lots of VW aero info.

FJC

[volkdent]

Start a new topic.

FJC

Just opened up an old can...

[kdf]

Short question: does the Z-bar in any way function differently than the camber compensator, since you , qoute "fabricating their own Z-bars, which worked better than the leaf-spring camber compensator!

The camber compensator and z-bar are two different devices that are meant to do the same thing. I consider the z-bar better engineered and it is loaded in a more natural way than a camber compensator. A camber compensator is also heavier in construction. Camber compensators I've seen don't work in the same way as a z-bar either because of the way they are mounted (works in bump only) or they are designed to have too much friction or they interfer with ride stiffness (bump stiffness). You can manufacture a camber compensator that works just like a z-bar, but for the effort it's usually more wise to use the lighter z-bar.

The vw z-bar with slack works different than a solid mounted z-bar. But the z-bar with slack works pretty much in the same way as a camber compensator with slack.

Like mentioned in the above text. You want the car to roll more, but you want to have stiff suspension in ride. The camber compensator and z-bar both affect ride stiffness, but don't affect much how the car rolls.

[redhot]

I have some difficulty relating all this information - it seems to be a bit contradictory in some aspects.

Relating this to the camber-compensator thread (perhaps they should be merged? They are so interconnected that it could be easier to understand then perhaps?)

http://shoptalkforums.com/viewtopic.php ... c&start=15

The first quote from thesamba is what I`m thinking of.

What do we want from the rear suspension?

1) Soft torsion bars = increase rear roll to move more forces to the front but still keep sitff enough

2) Limit positive camber at rear in both cornering, braking and aerodynamic lift

So, the Z-bar is an overload spring, that depending on the shimming can be made to both stiffen and soften the suspension from the base-setting?

And the CC with center pivot does nothing else than limit the positive camber of both wheels, which occurs in both cornering because of jack-up and in straight line driving? And depending on the preloading, softens the supsension somewhat by pressing against the torsion bars until the CC has no more preload.

[kdf]

The swing axle resists roll as a design, and getting enough bump resistance is an issue. The Z-bar adds bump stiffness while it doesn't affect roll stiffness.

If you add a CC with center pivot, it makes the suspension act stiffer in droop (when the wheel moves down). Less suspension movement = stiffer. But depending on how you think of it, you can say that the spring rate gets softer with less movement (just like with stiffer springs). Any camber compensator or Z-bar stiffens the rear in bump in some way.

Some formula vee race cars have whats called a zero-roll rear suspension. They have a swing axle which has only a monoshock. On a beetle this would be compared to just having a very stiff Z-bar and no torsion bars at all.
This seems to work well for many of them.