About Wheel Frequency

by Racing Aspirations on November 24, 2017


Wheel frequency is about keeping your tires in contact with the road at (almost) all times and the springs effective for all foreseen situations.

 

Static Deflection

Springs are measured in kg/mm or lb/in. So for each kg placed on top of the spring it will compress by the mm value.
Spring Rate +Corner Weight =Spring Deflection
 3 kgs/mm  90 kg  30 mm
 400 lbs/in  220 lbs  0.55in
In the first example if the spring is 200 mm long out of the box, it will compress to 170 mm long when it is fitted to the car and the car is placed on its wheels. But wait! Springs are sometimes fitted at an angle or are mounted inboard behind rockers and pushrods. These factors will change the relationship between the spring compressing and the ride height changing. Thankfully, the calculators work out this relationship (called the suspension leverage).

 
Given this model, we can build a table and return the static deflection. You can find the suspension leverage value by clicking .

The suspension leverage value is printed on right.
 
Spring Rate +Corner Weight +Suspension Leverage =Static Deflection
 3 kgs/mm  90 kg 1.59:1 75.63 mm
 400 lbs/in  220 lbs 1.59:1 1.39 in
Note: You can also view the dynamic suspension leverage as well as the spring travel by turning on reporting
You don't have to do the math - the Wheel Frequency Calculator will work it out for you.
 

 

Suspension Travel

What the static deflection gives us is the available suspension travel if you like. So if you were to find yourself on uneven ground and one of the wheels was to fall down a rut or pot hole, using the first example, the spring would still be effective if the pot hole was up to 75.63 mm deep. Equally, if your car was to roll during cornering the inside wheel spring would still remain effective as long as the wheel didn't drop more than 75.63 mm relative to the chassis.

You can use the Wheel Chassis Offset metric within reporting to return the suspension travel measured at the wheel.
 

 

Downforce

If your car has downforce and you can calculate or approximate the amount of weight added at a given speed then add this value (for a given axle divided by 2) to the total corner weight. This will change the static deflection and give you the car's true ride height at any given speed.
 

 

CPM

CPM or cycles per minute is the frequency at which the spring oscillates. That is to say, if you were to compress a spring and release it, it would bounce at the same rate (bounces per minute if you like) even if the distance it travelled during the bounce reduced over time (don't confuse frequency and amplitude). The CPM gives a good approximation of what to aim for when selecting a spring.
Use From To
Comfortable road car 60 cpm 80 cpm
Sports road car  80 cpm 100 cpm
Racing cars without wings or ground effects  100 cpm 125 cpm
Racing cars with downforce  200 cpm 350 cpm
Ground effect era cars  500 cpm
 

 

Conclusion

Selecting the right springs is a balance of achieving the correct CPM and achieving the right static deflection that gives your suspension enough travel to handle its environment.

 

 

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to the Suspension Geometry Calculator


to the MacPherson Geometry Calculator


to the Wheel Frequency Calculator
 

 

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