Danno

"Obviously not everyone does this, as Danno has gone from 23.5mm to 25.5mm...

I think the previous owner must have done some upgrades on my car. It came with forged Fuchs, 26.8/18mm swaybars, and those 25.5mm torsion bars. I guess the guy wanted neutral handling using Porsche parts that were available in '88-89.

Also don't forget that the dampers makes a bigger difference in handling than the springs (IMHO). A car with stock springs and good dampers will easily out-handle a stiffly-sprung car with worn-out, mis-matched or bad dampers.



If you serious about racing the car, seriously look into racing Konis or Penske shocks.

*Rothmans*

Thanks Danno.... I don't want a wallowing or porpoising car so I found a new home for the turbo cup springs.
I am going to use the money to invest in better dampers, as suggested.
I also found out that I have a spare set of strut housings that have the factory Porsche threaded collars removed, so I can buy an aftermarket set up with more modern coil over type springs. (and racing Konis are to go inside them)
I will keep the set up soft as far as spring rates go, and play with the sways for now.
<img src="graemlins/burnout.gif" border="0" alt="[burnout]" />

*Rothmans*

Bump, for Michael in Germany

luckett

Thanks for the bump *Rothmans*, I almost missed this thread.

Two questions:

1) Without removing the torsion bar and measuring with a caliper, is there a way to determine what size torsion bar is installed, ie stampings on the ends, etc. ? Any clever ways to guesstimate? For example, sand bags in the trunk and measuring compression.

I suspect the previous owner of my 968 had the car set up for the track and installed thicker torsion bars. The rear ride height is lower than the front, and the fender lips have been rolled. I think when the car was sold he may have removed the front springs/shocks, rear shocks, but left the torsion bar in for obvious reasons.


2) I have Koni yellows and 250# front springs on the way to my garage. Assuming I have stock torsion bars (25.5 correct?), what would you guys recommend as a starting point for the shock settings? Bear in mind that I prefer oversteer to understeer, but I don't want to overdamp the rear to comphensate for the balance.

thanks

luckett

Danno,

Can you clarify what you mean by this?

[quote] Just to be clear, spring-rate and wheel-rate are not equivalent (on is an integral of the other). Wheel-rates is a function of the square of the wheel travel (^2). <hr></blockquote>

I was under the impression that the relationship between spring rate and wheel rate is a linear function. Looking at the rate data on the Tech Session site, it indicates a strict linear relationship between spring and wheel rates defined approximately by:

y = .56x

where y = wheel rate
x = spring rate

If you introduce and integration here, you are off by one order, thus creating a non-linear relationship between the rates.

Isn't wheel rate a function of wheel travel AND spring rate?

Although I'm no mechanical engineer, if you show me the equations to which you are referring, it should be sufficient to clarify what your saying.

Manning

Hey Skip, thanks for replying. I am really in a quandary about what direction to take with this car. I need to stay kind of mild for now as this is still my daily ride, hence the desire to stay with a lighter spring.

May just hold off on the rears for now, and then if I get a little more radical down the road I start to consider bigger bars or helpers.

Danno

Chris, those 250# springs & Konis should be fine for your car regardless if you have 23.5mm or 25.5mm torsion-bars.

"I was under the impression that the relationship between spring rate and wheel rate is a linear function."

We've just got a nomenclature mix-up that's all. Common-sense conclusions would tend to arrive at wheel-rate as the resultant spring-rate at the wheels given the leverage of the suspension arm-to-pivot-point lengths. However, this is more correctly referred to as: spring-rate at the wheels. Which can be determined by multiplying/dividing the actual spring-rate by the motion-ratio (the distance the wheels move versus the distance the springs compress/extends).

The actual equation for wheel rate is:

This is then used to compute the natural undamped resonant frequency of the suspension:

However, this is still an idealized highly-linear 2nd order system, whereas we know that spring rates change on a curve as they compress. Also the shocks affect the resonance frequency, so we have to use this equation:

where Z is the damping ratio, which is equal to:

Most production cars are tuned around 1.0-1.25hz. A high-performance car works best in the 1.5-1.7hz range with the rear being 10-20% higher than the front.

Danno

...bumped for Chris...