The Z-Bar Illustrated

[redhot]

So, for me:

I have a 1970 1200 without the Z-bar. Guess it has the 22mm torsion bars of the short type, not through the retainer. 21mm is for the z-bar equipeed I think to remember.

It seems that the best option for me is to add a pivor-type camber-compensator limiting the droop and not adding any more roll stiffness and oversteer. The car is not used with a lot of load in the rear, not needing any more stiffness I think.

Usage of the car is Autobahn-driving, some gentleman circuit driving and narrow windy roads; no drag-racing or real racing.

Is that reasonable?

[FJCamper]

Hi RedHot,

Yes, a simple compensator will work for you. Porsche originally intended the camber compensator to work as an overload spring as well as to try and keep camber reasonable, but we VW guys just want some camber control.

FJC

[hoghead]

Still not clear if one also should install a rear sway bar in combination with t the stock Z-bar, or space the end of the downlink to limit travel

If the spacer is the preferred route, how long a spacer or how much "space" to leave?

[FJCamper]

Hi Hoghead,

If I were you, I'd mount a standard camber compensator (not an anti-roll or sway bar), and put spacers on the Z-bar down-links.

This is the belt and suspenders route.

Use tube steel stock and washers, leave the rubber bushings but no slack whatsoever. The factory mount has nothing but slack. And the bar itself is a small diameter. That's why you may want the compensator.

I'd try the street handling with the Z-bar shimmed, and if you like it, you may not need the compensator ... but for racing, I'd have both.

FJC

[Sneaks]

I know this is 180 off from the spirit of the topic, but from how you have explained the Z-bar (very clearly and easy to understand, thanks!!), I cannot see any benefit to removing it from my baja I'm building, correct? I'm going to be using it on the street as well as in the woods.

[hoghead]

I will give it a go with both CC and Z however the application is in a Speedster replica and there will be a bit of work on the inner fender well to carry the Z bracket.

The car is used for sometimes spirited driving on winding mountain roads, with not so good paving at times.


I had intended to fit a 19mm front sway bar as well.

[hoghead]

If I understand correctly neither the CC or Z performs an anti roll function.

Bentley tells me that the Z bar cars have a 1mm smaller OD torsion bar, hence the softer ride until the Z kicks in.

Does one not need a rear roll bar in addition to the shimmed Z bar?

[kdf]

Yes, the Z-bar or CC doesn't add roll resistance.

The geometry of the swing axle resists roll very highly. When there are cornering forces, the swing axle suspension stiffens up in roll. The anti-roll effect is good, but the bad part are the jacking forces involved.

The stiffer the rear torsion bars, the stiffer the already too stiff rear is in roll.

The extreme of Z-bars can be seen on many formula vee race cars. They have a suspension that has effectively no torsion bars, only a monoshock that performs like a very stiff Z-bar.

Some people put anti-roll bars at the back and have good success with it, but I don't think it's necessary. An anti-roll bar connects the rear wheels, making the rear act more like a solid axle. The Z-bar is a bump spring, so it works in the very opposite way compared to an anti-roll bar.

Quoting Colin Chapman: any suspension will work, if you dont let it. And that's exactly what we're trying to do to the Swing axle rear by creating more restraints with z-bars and droop limiters.

[Piledriver]

Would a Z-bar help an IRS car keep both back tires planted better?
(I'm thinking AX)

[FJCamper]

Hi Piledriver,

Interesting thought (seriously).

I can see where, with a Z-bar on an IRS, that if one diagonal arm goes up it pushes down on the other, but the arm being pushed down on would keep it from going light and causing wheelspin.

FJC

[Piledriver]

Cool, it seemed logical based on how it works, just needed a sanity check... I usually fail

Most IRS cars are (IMHO) too stiff in the back anyway, making the rear "softer" in roll will reduce the dreaded oversteer, and reduce the roll stiffness requirements up front. (and a Z bar can be made very easy to adjust)

Maximum traction is probably best achieved with all 4 tires on the ground.

At least it would be easy to try...

[kdf]

Piledriver:

Is your back end aerodynamically sensitive? It's the only reason to install a Z-bar at the rear in a IRS vw.

Many formula cars have 3rd springs that only work in bump. They work the same as the stock VW Z-bar, they have free play and come in effect only after there is enough bump. It's very important in these cars to have especially the front ride height just right.

On a IRS (or mcpherson) car you don't want to soften roll too much. The suspension geometry is sensitive to roll so it loses camber when the chassis rolls.

If you want to change spring rates quickly you could make spring perches that "grab the coil spring in different places". It's a bit difficult to describe, but they work so that they can leave a part of the spring inactive. The less coil spring that is in use, the more stiff the suspension is. We had spring perches like these this summer, it had 4 stiffnesses for each spring, changing the spring rate one adjustment stiffer or looser meant that you had to rotate the spring perch 3 revolutions up or down. Corner weights and ride heights were not affected by the adjustment and the factory claimed you could adjust springs easily in less than 15 minutes. For us it took more time, every time the dampers got hot the adjustment jammed up, so you had to wait for it to cool down.

[Piledriver]

Thanks for the input!

The idea was primarily for low speed/throttle steered conditions in order to keep the inside wheel solidly planted.

Limited slip diffs have their place, but they are expensive, and have some downsides, often provide funky/inconsistent action.

It will probably be tried on a 914... it will be traction limited.

[kdf]

The idea was primarily for low speed/throttle steered conditions in order to keep the inside wheel solidly planted.

Limited slip diffs have their place, but they are expensive, and have some downsides, often provide funky/inconsistent action.

Try droop limiters at the front. Open diff cars (without limited slip) usually have limited droop travel at the front. They are set so that they are not effective when cornering, but when you press the throttle it lifts (or lightens) the inside front wheel. When a front wheel is lifted it means that the front axle is at maximum stiffness in roll, which transfers grip to the inside rear.

The droop limiters are very well engineered and built into the push rod suspension on for example formula fords, and they can be adjusted very accurately so you can get best possible traction out of corners with an open diff.

On a bigger scale car like a beetle I would have a rubber or another spring element built into the droop limiter to have softer application. The suspension can make a very loud noise banging the "heavy" unsprung mass against the droop limiter, this can be very unnerving to the driver...

Running droop limit at the rear will work the opposite way, helping the car to turn into corners by lifting (or lightening) the inside rear wheel.

[Piledriver]

Hmmm.. interesting ideas all.

I'm visualizing off-road style strap travel limiters up front (w/fine adjustment)

I may still try the Z-bar out back, if only because it will provide some anti-squat and lift effect.