HotCars explains: A new way to measure suspension stiffness with your phone | Taza Khabre

Key findings

  • Comparing springiness between different cars is ineffective due to differences in suspension systems and gear ratios.
  • Only the dimensions of the spring do not take into account the weight acting through the spring, which affects its stiffness.
  • Measuring the natural frequency of a suspension system provides a universal measure of its stiffness, taking into account spring force, movement coefficient and weight.

Let’s say you’re interested in a suspension modification and you’re comparing notes with someone else. You say to the owner of another car: “What kind of springs do you have?” The person says, “340 psi in the front and 910 psi in the rear.” Jeez, you think those are pretty stiff rear springs – is my car’s rear suspension supposed to be similar?

This information is not suitable for comparing different cars

But unfortunately, the information you just received told us next to nothing about how stiff their car’s suspension is compared to yours! Ha? Isn’t that how the characteristics of spring are always expressed? It is, but it is not good for comparing different cars, especially if they have different types of suspension. Let’s see why and then explore this technique does allows you to compare cars.


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Spring prices are not what they seem

Universal automotive spring
via: Flickr

The stiffness of the spring, as in the conversation above, is expressed in pounds per inch. That is, if you have a coil spring that is standing vertically on the ground and the spring is rated for 250 psi, it will compress one inch if you put a 250 lb weight on it. (And then 2.0 inches of compression at 500 pounds, etc.) However effective The stiffness of the springs when working in the car suspension system depends on two more factors.

The suspension is independent, McPherson
Via: Carthrottle

The first additional factor is called the motion coefficient. You can think of it as the lever that the suspension has on the spring. IN McPherson, where the spring is mounted near the wheel, one inch of wheel travel will be close to one inch of spring travel. So the spring rate (in this example) of 250 psi will be similar to the wheel rate. (So ​​the wheel speed will also be close to 250 psi.)

The suspension is independent, double levers
Via: Cartretments

But what if the spring is mounted differently, for example, as with double transverse suspension? The spring is now located inside the wheel and the spring is actuated by a lever (usually one of the levers). This leverage means that to achieve the same 250 psi wheel speed we saw with the MacPherson strut, the spring rate should be much higher. Since spring and wheel rates can vary greatly, and wheel rates matter, we can see that comparing the spring rates of different suspension systems is not a good approach!

The second reason that comparing spring rates alone is a poor method is that it does not take into account how much weight acts through the spring. A given spring, if it can handle more load, will behave as if it is softer than the same spring that can handle less load. So that makes things pretty complicated, doesn’t it? This means that to compare the stiffness of the suspension of different cars, we need:

  • Spring norm
  • Coefficient of movement of the suspension (lever).
  • Weight acting through a spring

Or, instead of all this complexity, we can simply measure the natural frequency of the system. All you need is a car and an iPhone with a $5.00 app. So how does it work? And what really? is natural frequencies of the suspension?

Natural frequency is the rate of bounces up and down

Let’s say we take the coil spring out of the car again, stand it upright, and put a 250 pound weight on it again. But this time we are pushing the weight hard. When released, the weight bounces up and down at a rate of (say) three times per second—that is, at a frequency of 3.0 Hz (Hz).

The natural frequency is how fast the weight on the spring likes to bounce up and down.

No matter how far we push the weight before releasing it, this spring and weight combination “likes” to bounce around at 3 Hz. This is called a system natural frequency.

If we kept the spring the same but changed the weight, the natural frequency of the system would change. This would also change if we kept the weight the same but changed the spring rate. In other words, there is a direct relationship between the effective weight acting downward through the spring, the stiffness of the spring, and the final natural frequency.

Therefore, if we can directly measure the natural frequency of the suspension, we get a number that takes into account the stiffness of the spring, the coefficient of movement of the suspension and the weight working through the spring. It takes everything into account with just one measurement! (And that’s why suspension design textbooks always use natural frequencies, not springs.) Sound difficult? The measurement takes less than a few minutes.

Using an iPhone to measure the pendant’s own frequency

This approach uses the iPhone as a measurement tool—you just need to install a cheap app. The program was developed by Diffraction Limited Design and is called vibration. At the time of writing, it costs just $5. programs like MyFrequency and Physics Toolbox available for Android.)

Download and familiarize yourself with the program

The first step is to download and then play around with the Vibration app to see how it works. It’s pretty simple, but like a lot of things, it’s a lot quicker to learn by learning the software features on your phone than when I write about it here.

Compilation of Vibration app setup screens
Julian Edgar / HotCars / Valnet

Sample settings of the Vibration program

  • Data source: internal accelerometer
  • Data collection: 50 Hz
  • Sampling delay: 4.0 seconds
  • Sample duration: 10 seconds

Leave the other settings on this screen disabled. Enable only Z-axis to display time series and frequency. This means that when the phone is on its back, it will only record vertical movements. You may have to play around with the vertical settings for a moment, but start with a vertical scale of 0.1 and a multiplier of 1x.


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Place the phone at one end of the car and bounce it off

After sorting the program, do the following:

  • Place the phone on top of the car across one axle, for example on the hood. It is best if the car is in neutral gear with the electronic brake off. (Use multiple stops that allow a little forward movement but prevent the vehicle from rolling.)
  • Bounce that end of the car up and down. The suspension will be very resistant to any bounce outside of its natural frequency, so you’ll feel when you need to push very quickly. (It’s like a baby swing – it’s obvious when to push.) If the car has stiff suspension and/or springs, you may need a helper.

Collection of applications Vibration 981 front
Julian Edgar / HotCars / Valnet

Press the sample button in the software and the phone will start registering after a short delay. (The delay gives you a chance to make the car bounce smoothly.) You should end up with an up/down track that looks something like the image on the left above. This is a Porsche 981 Cayman front suspension measurement.

Now switch the program to “Frequency” and move the cursor to the peak. If there are many peaks, look for one in the range of 1.0-2.5 Hz – this will be the suspension frequency. (Others would be buses around 10Hz.) Here the cursor is placed on the peak that reads 2.15Hz (note the ellipse I placed around that number).

Front view of the 2014 Porsche 981 Cayman.

Make notes of what you read. You can then do the same at the other end of the car.

The higher the frequency, the stiffer the suspension.

In the case of the Porsche, the frequency of the front suspension was 2.15 Hz, and the rear – 2.64 Hz. The higher the natural frequency, the stiffer the suspension. To reduce the angle, most, but not all, cars have a higher rear frequency than the front.

via Wikipedia

Here are the test results of a mid-Eighties Mercedes 230 W123 (pictured above) with the optional hydraulic self-leveling rear suspension.

Front: 1.3 Hz

These numbers show that the Mercedes suspension is much softer than the Porsche!

To be as accurate as possible, during a static test the car should be loaded as usual, for example by one or two people. Note that Porsche and Mercedes underwent static tests in an unloaded state.

Accurate comparison of different cars

Measurement of front and rear natural frequencies allows comparison of any vehicle suspension, regardless of spring stiffness, movement coefficients or weight acting through the springs. This is a universal suspension stiffness meter. On road cars, you’ll find a range that extends from about 1.2Hz to about 2.5Hz (the Porsche is pretty stiff for a road car). Some air suspension cars from the 1950s had less than 1 Hz – see my book from the history of automobile suspension to learn more about these exciting systems. Racing cars can develop a frequency of up to 3.5 Hz.

This approach can also be used to measure suspension systems with non-linear springs, including variable rate coil and leaf springs, as well as gas (air and nitrogen) springs. This measurement will give you the natural frequency with small deviations.

You can also check the stiffness in Pitch and Roll

In addition to testing bounce rates (up/down), this type of testing can easily be done for roll frequency (how hard the car rolls) and pitch frequencies (how hard the car rolls – where when the rear is up the front is down and vice versa). These tests are easiest to do statically by pushing the car (to do the height test you need two people working in unison, one on each side of the car).


Throw rates will usually be higher than bounce rates. This is due to the action of additional springs formed by stabilizers of lateral stability. Except for very special suspension systems, the pitch frequency will be numerically between the front and rear rebound frequencies. (Special cases? When the front and rear suspension systems are connected in such a way that if the front wheel lifts over a bump, the rear wheel goes down to keep the car more level. These cars have a lower roll rate than any other front or rear bounce frequency.)


If you want to do a detailed comparison between the suspension systems of different cars, or just for fun you want to know what the natural frequencies of your car are during rebound, pitch and roll, the cheapness of the program and the ease with which you can do the testing opens up a whole new world. It also makes the suspension design tutorials a lot easier to understand when you can visualize what “1.6Hz bounce rate” actually means!

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