So I'm trying to calculate suspension frequency for the RX8, so hopefully it'll help me make a decision on suspension.

So from what I gather:
Reading up in the High-Performance Handling Handbook, it says 3.2 cps for "ultrasmooth asphalt track" and 1.2cps for the "very bumpy dirt road found on rally stages"

Then 1.9-2.2 cps for autox or road racing on slightly bumpy course.

So that's all fine and dandy, the problem I'm having now is with the formula, which is:
Frequency = 3.13 * (wheel rate/sprung weight)^0.5
Wheel Rate = Sprung rate * motion ratio^2

Now sprung weight at say 2900/4 (as the car as 50/50 balance, and the 2900 lb to be generous, as the lighter it is, the higher the freq will be for the spring rate) that's 725...that's pretty easy.

But the wheel rate...is what I can't calculate, because I do not know the motion ratio of the RX8's front nor rear suspension. Anyone have a good way to calculate this?

 

interesting topic.

google? http://www.stockcarproducts.com/pstech9.htm

 

Well better question is does anyone already know what it is :p I'll do it my self, but I'm lazy ^^

 

the numbers i'm more familiar with for steet type cars is ~ 1 for the street and ~2 for the track.

... at least do some searching on here for corner weights as they do exist

 

1 and 2? what units are you using?

corner weights in a discussion about suspension frequency... yeah..... ok.....

 

The units would be hertz of course. Using corner weights is alot more accurate then saying the car has 50/50 weight distribution (which it does not) and deviding the curb weight by 4 to do the calculation.

 

Corner weights will give you more accurate numbers - I suspect some have been posted somewhere on this forum (if not here then perhaps the autocross forum).

Also, as long as you are running some basic numbers, be sure and remove the unsprung weight (wheel, tire, brakes, upright, etc) from the sprung weight. It's probably in the range of 65-75 lbs per corner (WAG on my part; could be way off).

As for the motion ratio, you can get an approximation by some careful measuring.

 

Corner weights are dynamic, it's not like the weight that you measure with scales, stay static over that wheel. The weight transfer will not occur proportional to the static corner weight. You can't damp each wheel for the difference in corner weight and actually expect it to be like that. Set it up evenly on both sides.

 

Using a corner weight is better then saying total weight / 4. Your not going to get a different spring rate for each corner of the car so a little common sense should prevail when doing the calcs.

 

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Corner weights are static and you need accurate numbers or your calculations are garbage. The RX-8 is not 50-50 in most cases because you must use the corner weights *including* driver. The dynamic aspect is swaybars, which is not inclusive to the calculations.

 

At this level, it's rediculous to even think about adjusting for each individual corner. And it's not that simple. The "accurate" static numbers mean nothing in a dynamic situation. Especially if you're talking about damping.

Anti roll bars are NOT the only dyamic aspect in the equation and it IS inclusive of the calculations. If affects your spring rate in single wheel bump. Go find a way to decouple bump and roll on our cars easily and you'd be rich.

Take the accurate on scale front and rear weights and divide them by two for this purpose. I suggest using some type of CMM to measure the suspesion mounting points to find your motion ratio. You might also want to check if the car has a rising rate geometry and if so how much?

This is not a task for the "lazy." So may be we should just drop the whole discussion.

 

Calculating and setting the natural frequency of the spring rate has nothing to do with damping. The frequency effects the period at which the car will oscillate and the general theory of 1-3 hz puts the motions in a range that the human body can deal with. The age old wisdom on the subject is the human body likes 1hz (you walk at about 1 hz) and can stand up to a little over 3hz at which point your vision is blurred. Street cars should aim for 1-2hz which is considered between comfortable and harsh.

A simple spring-mass system oscillates with a period determined by the said spring rate and the mass its attached to. The mass does not change dynamically (it can not). Forces change dynamically which change the amplitude of the oscillation.

Dampers (aka shocks) effect the rate of decay of the oscillations - otherwise known as damping... which of course need to be matched to the suspension after you figure out what springs your using.


hence the suggestion to take 30 sec and search for some corner weights on this forum....

 

he must have a PhD in Anality ...

 

Oh, crap I skipped a step huh? Why on earth are you talking about an undamped car. Even a car without dampers will be damped to some minute degree... I made the assumption that if you're figuring out the suspension freq, you're going to use it to figure out how much damping you need.

Thanks for the physics lesson r0tor. I think I'm enlightened.

Yes, I do have a PhD in anality. I've been told that before, and I'm damn proud of it. Deal with it.

 

haha, i actually had to look and make sure you didnt make that post