Build A Faster Car - helping drivers engineer

Higher spring frequency are frequently used in the rear of a car to produce what is known as a flat ride. With a rear spring frequency slightly than the front, the rear of the car will oscillate in a sightly shorter amount of time. Done correctly, the front and rear of the chassis will complete one oscillation at exactly the same time, reducing pitching in the chassis. This article will elaborate a bit on the subject, and show how damper tuning can make this feel even better.

Let's say our front spring frequency is 1.8hz. If we make our rear frequency 1.8hz too, our chassis motion will look somewhat like what you see in the following graph:

The two lines represent chassis movement at the front and rear axle. Any time they are different, the chassis is pitching fore and aft, creating an unpleasant rocking sensation. (And the speed at which this happens determines ride harshness.)

You can see that equal spring frequencies and (with dampers matched identically to the springs) will produce two bumps that feel of roughly equal harshness, but the car will see substantial pitching due the front and rear of the car not settling into a rhythm with one another. We can fix this by running a higher spring frequency in the rear, which will cause the rear of the car to oscillate faster and catch up to the front oscillation:

Notice how the front and rear of the spring settle together, very nice! However, now the front and rear bumps are of different magnitudes due to the firmer rear springs. While there is less chassis motion in this simulator, the change in direction is more abrupt and that's what determines ride harshness: rate of acceleration. We can soften the dampers to allow more of the bump to be absorbed by the spring, and in doing so we get this:

In this graph I've reduced the rear damping to 36% critically damped, down from 50% critically damped from the previous graph. Notice the bump is now of equal magnitude front and rear, except we clearly need more damping for the rear! This is what being severely underdamped looks like- lots of body motion. We don't want this. So, what we want is that same low bump resistance, but higher rebound resistance. This graph was made in Photoshop (this particular simulator was too simple to allow dynamic damper rates), but the end result is intended to look somewhat like this (the particular program I was using did not support variable damping rates):

What you have there is an ideal spring and damper combination for stability. Stability leads to increased grip from the tires, too. You can achieve this reduced pitching regardless of how soft or firm your springs are (to an extent). Pretty neat, eh? This is why many engineers recommend and use firmer springs in the rear with less bump resistance relative to the front.