Spring Rates
Spring rates seem to get a lot of emphasis from car owners. This article shows how optimal spring rates can be chosen and even compared between two different cars, and why your dampers should be taken into consideration while choosing spring rates.
Manufacturers rate their springs in kg/mm or lb/in, but neither is how you should be making your spring rate decision. (For reference, though, one kg/mm is equal to 56lbs/in.)
No, instead you should be looking at the natural frequency of the spring. This is also called a spring frequency. This magical number takes into account the suspension geometry, weight, and weight distribution of the car- which just so happens to mean that natural frequencies can be fairly compared between cars. Want your car to ride like a Cadillac? Copy its spring frequencies, not its spring rates.
For comparison, here are some spring frequencies for various cars (information gathered online on various forums, so its accuracy has not been verified):
- 1990 Mazda Miata ~ 1.15hz front, 1.01hz rear
- Mitsubishi Evo VIII ~ 1.3hz front, 1.2hz rear
- Honda S2000 ~ 1.3hz front, 1.4hz rear
- GD Subaru WRX STI ~ 1.6hz front, 1.8hz rear
- Lotus Elise ~ 1.8hz front and rear
- C4 Corvette ~ 2.0hz front and rear
- Mazda RX-7 ~ 1.22hz front, 1.16hz rear
- Acura NSX ~ 1.6hz front, 1.5hz rear
Most experts and textbooks would suggest use around 1.6hz front, 1.8hz rear for a sports car and 2.0hz front, 2.25hz rear for a race car without downforce. Of course, there's some wiggle room in those numbers, and autocrossers frequently go into the 2.5hz region. The important thing here is that you do not go too stiff for the road you are driving on or you will lose grip. Remember that on a perfectly flat surface, you wouldn't need a suspension- the suspension is there for the bumps, so optimize for them accordingly.
The reason we frequently see a higher spring frequency in the rear is 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 usually happens at highway speeds or even higher. While the concept was created for passenger comfort, many race cars use reduced-pitching spring frequencies to aid chassis stability.
Other vehicles use equal spring frequencies front and rear. This should give bumps an equal magnitude front and rear- that is, the front and rear of the chassis should respond very similarly. This may feel more predictable than varying spring frequencies. At low speeds, where chassis pitching is inevitable with any rate, equal spring frequencies generally feel the most predictable and normal.
Some of the cars listed above use softer spring rates in the rear, contrary to conventional teachings. This may indicate that jounce bumpers have been deployed in the rear for use as secondary springs, adding to the main spring rate. It may also indicate that the manufacturer was trying to compensate for something, perhaps much softer rear dampers, which brings up two new points:
- Do not assume that increasing your front and rear spring rates by the same percentage over stock is the correct thing to do! Very frequently the vehicle's manufacturer had different goals than you do now.
- There may be other forces in your suspension, such as strong anti-squat geometry, jounce bumpers, strange damper settings, or a progressive springs that may necessitate deviations from these recommended rates.
Dampers have the effect of lowering a spring frequency. Soft dampers will have a minimal impact, while stiff dampers completely eliminate the spring frequency because the suspension will no longer oscillate. (If there's no oscillation, then there is no period between oscillations!) The natural frequency of a spring with a damper is known as the damped frequency and is a little more complicated due to velocity-sensitive dampers. That will be covered in more depth in another article.
For now, this article is here to illustrate the point that spring rates are only useful for determining spring frequencies, but spring rates are somewhat irrelevant by themselves. Spring frequencies are comparable between vehicles and have a large influence over how a car behaves.