Natural Spring Frequency
 

how does Natural Spring Frequency come into play in relation to specing out spring rates?

I know that a NF of 1 is about where normal street cars are. A NF of 2 is about where race cars are.

NF is:

nf = 3.13 x sqrt (wheel rate / sprung weight per wheel)
or
nf = 3.133 / sqrt(static deflection)


What kind of split should there be between the front NF and the rear NF? I would appreciate if anyone had any technical info on this. I realize there are plenty of proven rx7 spring rates, but I'd like to know how to spec out myself, if anything, just to know the process.

Thanks to anyone that has answers. :)

The answers your looking for are not quite as easy to come by as one might think. Your ideal spring rate is really dependent upon a number of factors such as the type of driving you do, the road surface you are running on, the comfort level you wish to withstand, roll rates, roll bar to be used, etc..... If you REALLY want to know how to do it, pick up a copy of "Racecar Vehicle Dynamics" by Milliken and Milliken. It's about 1.5" thick and is considered by many to be the technical bible of racecar suspension design and development. A second book that might help and is much cheaper (but MUCH less technical and in depth) is "Competiton Car Suspension" by Allan Staniforth. From his book, the simplistic calculations of

Wheel Rate = Coil Rate / [(Susp Leverage)^2]
= [(wheel Frequency / 187.8)^2] x Sprung Weight

Wheel Rate is in units of (lbs/in)
Coil Rate is in units of (lbs/in) and is what the spring is measured as (i.e. 500 lbs/in, etc...)
WHEEL Frequency is in units of CPM (Cycles per Minute) ...thic can be converted from Hz by recognizing that Hz is cycles per second
Sprung Weight is in units of lbs
Suspension Leverage is in units of inches.

A good rule of thumb for sports cars is 80-100 CPM and for racecars without ground effects it's 100-125 CPM. Bear in mind that you can arrive at the same wheel rate by changing any of those factors and therefore just because you have caluclated a wheel rate that is within that range, it may not equate to a great handling setup.

Note that the leverage comes into play as Leverage = front view length of lower A-Arm / front view distance from chassis a-arm pivot to bottom shock mount.

Enjoy!!

PS: The front and rear suspension strut inclination angle is so small that it causes very little change in the calculations.

If you are considering these changes for a street car that you don't want to hate driving... There's a point at which subjective ride comfort starts to drop quickly, like 1.3Hz or something like that. I also believe that you want to make to make the rear frequency about 10% higher than the front for ride comfort. If you are interested, I could find out a little more precisely.

-Max

If everything is working good on a suspension(unlike the stock rear on an FB) you would use the speed you plan on driving as part of your calculations for your spring combo.

The easy answer is to +1 the first answer you got. There are some cheaper books out there that cover this question but if you are playing at the level that you don't want to just buy what somebody else sells(good for you) then invest in the Milliken book because it will help with any of your next questions.

Yes. This helps prevent porpoising.

Interesting. I appreciate the response. (from everyone for that matter) A shame I didnt ask this question BEFORE my trip to amazon.com. I ordered "How to Make Your Car Handle by Fred Puhn" the other night on advice from a friend. I suppose if its not good enough I'll go with the book reccomendations above.

Again, thanks everyone. This is interesting stuff to me now that I know it exist :)

Dedicated autox cars can go up to 2.2-2.5 Hz

Thats actually what I am trying to avoid. lol.

Don't forget that all the above discussion only pertains to LINEAR springs. If you're dealing with PROGESSIVE rate springs, you've got a different (more complicated) animal, because the spring stiffness is nonlinear (depends on compression distance), so you'd have to incorporate that into your frequency calculation to be more accurate. :)