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Z bar

Z bar

your comments on this would be appreciated,... 2 litre vw polo run in clubmens class suffering severe wheel spin on inside wheel to the turn. Cannot afford lsd if a Z bar could be packaged into the front would this help with the wheel spin or is there another "demon tweak" that might help? If the Z bar could be made to work were or how would I start with the calcs for diam of bar lever arms lenghts etc. bty know about poor mans lsd .thanks Golfpin

RE: Z bar

In fitting a 'Z' bar you are trading roll stiffness against heave stiffness.

Have you already disconnected the front anti-roll bar ('U' bar)?

Have you already / can you afford to soften the front side springs?

Do you need more front heave stiffness?

Regards, Ian

RE: Z bar

Hi Ian, thanks for the quick reply but could please ask you to define "heave" that will clarify a point for me and enable me to answer more accurately. 4 lines down on the main page of the suspension posting ,under the heading "nomenclature in engineering" I asked the the same question, the term heave in suspension geometry parlance is foreign to me. It took awhile to get to grips with the American term jounce as an example.I still have to pause and think if jounce is bump or rebound equally plough=under steer, loose= oversteer.

Front roll bar is still connected. My thinking on the Z bar is that it would push the unloaded wheel down and help with the wheel spin but I am not sure how it impact on the various forces that come into play when in a cornering state. would it aggravate under steer, would it conflict with the anti roll bar properties?
I would have thought that softening the springs would have been the wrong thing to do this would lead to further body roll and further unloading of the inside wheel, or am I wrong?
As I said, it is at this stage hypothetical because the packaging might be insurmountable any thought on this would be appreciated.
In the spirit of free thinking and enterprise cheers

RE: Z bar

Heave means "vertical motion"

Not having an LSD will make your life in racing very hard, but if that is a matter of fact I would also in a first step disconnect the U-Bar in order to see what that does. Typically an OEM rollbar rate equals more or less the same rate as a coils spring (for info). When removing the front ARB you will most likely be suffering from more oversteer and more rollangle. This oversteer can be reduced by mounting stiffer coil springs on the front (which will to some degree again make the gain in traction less) or by working at the back end of the car using a smaller rear rollbar/springs (which will increase your roll angle even more, a thing you do not want) or simply mount a wider tire. If that does not help consider shifting weight around to the front end (this will also help your traction but hurt you braking).
As you can see, not an easy answer to your question. A z-bar can transfer load in roll from one wheel to the other but will work in heave as pure vertical spring which would mean that you would have to reduce the rate on coils springs too, a chicken and egg thing.

As a rule of thumb you can assume that your load transfer is caused by 3 mechanisms
a) load transfer due to rollrate (total value of springs and rollbar rate in roll motion) and rollangle (sprung mass)
b) load transfer due rollcenter height (sprung mass)
c) load transfer due to unsprung mass

Since an LSD does not interfere with neither of those 3 it is the perfect solution to your problem.


RE: Z bar

two other possible options (if rules permit) perhaps worthwhile to consider are:

a.) - a connection of the front ARB (sway bar) to the uprights/knuckles/hubs , if done at the "right place" this can be used to transfer load via the steering system to the inside wheel. The steering systems "rolls/ anti - rolls" the car with respect to steering angle used.
Such systems were/are quite commonly used on FWD race cars, in a variety of series.
Please keep in mind that this puts additional loadings onto your steering and if present "power assist" system, if not accounted/modified/designed for, this can lead to some problems/issues and premature failures.
It also has an effect on "steering feel/feedback", so will take some "getting used to" from the driver - but both "problems" can be overcome with a bit clever engineering.

b.) - you could consider a "poor mens LSD", as designing a system, which let's you brake the front wheels independently - a.k.a. "fiddle brakes".
Such a system, along the lines of an "hydraulic handbrake" as used in most rally cars, could be implemented via two paddles behind the steering wheel.
It would allow you to brake the inner wheel interdependently, and thereby preventing it from spinning up under acceleration.
The pressures needed are not too high, so hand operation is normally not an issue.
Such a system, can also be used to aid turn in and to help with fighting understeer on exit.
Similar systems, a bit more "advanced" were used on some of the 2ltr. Super Touring cars in the mid 90's, before they got banned by the rules.

RE: Z bar

Thanks for the input chaps the idea of the fiddle brake got me on a thinking curve and I realised that the hydraulic hand brake set up from one of the modern bikes could be adapted for this. Much excitement put together a proposal ......I may as well have been talking to a wall the party concerned did not know what I was talking about!!! Thanks again All who contributed.

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