Natural Frequency [CPM - cycles per minute] =
187.8 times {square root of (wheel rate [lb/in] divided by sprung weight [lb])}
Wheel Rate = Coil Rate [lb/in] divided by {suspension leverage raised to the second power}
(Low leverages - MacPherson struts - mean that wheel rates are approximately the same as coil rates.)
Since the tire is also a spring acting in series with the coil, you can take it into consideration by calculating:
Reciprocal value of combined spring rate [lb/in] =
reciprocal value of coil rate [lb/in] + reciprocal value of tire rate [lb/in]
Tire Rate = say 1,500 lb/in
Suspension Frequency values:
60-80 for comfortable road cars
80-100 for firmer and sport suspension
100-175 for non-ground effect racers
Sky is the limit for Formula 1 with minimum suspension movement
In order to avoid uncontrollable pitching, the front frequency is 10 to 20 CPM lower than the rear frequency.
Progressive springs are highly desirable for traction and road holding. Progressive springs will result in a softer suspension at full droop (e.g. on landing after a jump or a bump) and much stiffer suspension at full compression in order the delay (not to completely eliminate) grounding to higher G levels.
When designing the suspension, care has to be taken to divide the useful stroke of the shock absorber correctly between the bump and rebound. Adjustable spring perches do not automatically solve this. For instance, a higher preload on the spring will result in more bump and less rebound.
Special notes:
1. All of the above will be more difficult to do for progressive springs since you do not have a single value of rate like for linear springs and it will prove to be impossible to do it correctly unless you obtain the deflection diagram for each progressive coil from the manufacturer.
2. Typically, in order to get a progressive rate coil with a sufficient crush length, you stack a linear coil on the top a progressive coil. Resulting combined coil rate is calculated as follows:
Reciprocal value of the combined coil rate = the sum of the reciprocal values of the two coil rates
The correctly designed progressive coil will work through the whole useful stroke of your shock absorber.
3. Beware of progressive "helper" coils which are fully crushed at the ride height or sooner! As you can see, they result in a linear rate suspension for the most part of the suspension movement!
Regarding the setup of the shocks:
For rally suspension, the ratio between the rebound and bump resistance is usually 2.5 to 1. For racecars operating on smooth surfaces perhaps 1 to 1. For really bad rally surfaces the ratio can be as high as 5 to 1.
Some rally suspensions, in order to compensate for insufficient stiffness of the spring at full bump (typical deficiency of linear springs) use higher bump values (i.e. rebound to bump 1.5 to 1), since the resistance of the shock in bump acts as an additional spring.
The shocks are typically rated in Newtons at the shock shaft speed of 0.52 meter/second. [Pounds of force x 4.448 = Newtons]
Of course, there is a range of values for rebound and bump for corresponding shaft speeds. Unfortunately, this is rarely volunteered by manufacturers but can be obtained easily on a shock dyno.
Regarding a 2,300 lb car:
You have not given the weight distribution.
I am not familiar with two wheel drive cars. It seems that 250 lb/in spring rate in front will work. However, the 110 lb/in in the back may be way too low (unless your weight distribution is 70/30 or worse) and will give you a much lower frequency in the back than in the front (highly undesirable) and severe bottoming out problems in the back.
(Yes, I know that these rates were published this year by a certain magazine in connection with a front wheel drive car that will remain nameless.)
Of-the-shelf racing shock absorbers, such as Bilstein 300/120 and 270/100 [deka Newtons stamped on the body of the shock] should work well for you and if you find in the future that you need different valving, Bilstein will revalve them for you at $55 a piece. This would require extensive testing - look for the guidelines in the literature listed below.
Literature:
Race and Rally Car Source Book, Allan Staniforth, Haynes, 1993
Car Suspension and Handling, Donald Bastow and Geoffrey Howard, SAE, 1993
Performance Handling – How To Make Your Car Handle, Techniques for the 1990s, Don Alexander, Motorbooks International, 1991
How To Make Your Car Handle, Fred Puhn, HPBooks, 1981
Topic: rear coil over (Read 6377 times)