Budget FWD suspension upgrade, Part 1

This issue, measuring the standard spring rates

by Julian Edgar

Click on pics to view larger images

At a glance...

  • What spring rate is
  • Directly measuring spring rate
  • Measuring spring rates on the car
  • Calculating spring rates
  • Measuring natural frequencies

In this series we’re going to spend minimal money to upgrade a light-weight, front-wheel drive car’s suspension. The car uses MacPherson front struts and a torsion beam rear axle. Making it harder, it’s also a car for which there’s no readily available aftermarket parts.

This issue: measuring the standard spring rates

Spring rates

The stiffness of a spring is described by its ‘rate’. The rate of a spring is most often measured in kg per millimetre (or pounds per inch).

The rate indicates how much weight is required to compress the spring a given distance. For example, a 2 kg/mm spring requires 2kg sitting on top of it to compress it by 1mm. If it needed 4kg to compress it the same distance, it would be a stiffer spring.

Making this a bit harder than it first appears is that most original equipment springs are variable rate, at least to a degree. That is, the first 50mm of deflection is likely to require less weight than the next 50mm of deflection. This variable rate is achieved in springs by using:

  • varying coil spacing (the tightly-spaced coils close right up, so stiffening the spring as it deflects)

  • varying diameter (the smaller the diameter of the coils, the stiffer they are)

  • varying wire thickness (thin wire is not as stiff as thicker wire).

Therefore, when measuring rate, always do so at a certain relatively large spring deflection.

So why are spring rates important? Well, the first step in specifying new springs is to measure the rate of the old springs. Without knowing the old spring rate, you’re just blundering around in the dark. There are four ways of measuring spring rates.

  • Direct measurement

The first approach is to remove the springs from the car and directly measure their rates. (It is sometimes easier to source a spare set of factory springs for your car, rather than remove the ones you have for measurement. People often get rid of standard springs at little or no cost.)

To directly measure the spring rate, place the spring on a pair of bathroom scales and then position a weight (for example, you!) on top of the spring. Very carefully measure how far the spring deflects.

If you have a vertical mill or large drill press, you can do the same with the head of the machine tool bearing down on the spring. Note that bathroom scales are available that measure up to 150kg.

This measurement is best done at two different loads. For example:

Measured mass

Spring length

45

250mm

64

240mm

So for a weight change of 19kg, the spring length changed by 10mm. 19 divided by 10 = 1.9 kg/mm.

(To convert to pounds/inch, multiply by 56 – therefore, this spring rate is 106 pounds/inch.)

  • In-car measurement

The second approach is to measure the spring length in the car with the car’s weight acting on it. If you know how long the spring is in its free state, the weight of the car and its front/rear weight balance, you can get a good idea of the spring rate. (If the weight and front/rear distribution are unknown, you can use a weighbridge to find out this data.) Note that in this case the motion ratio of the suspension is assumed to be 1:1, as it is for example in a MacPherson strut.

For example, a small and light weight front-wheel drive car might have a mass of 850kg, with about 60 per cent on the front axle. That is, on each of the front wheels, a weight of 255kg is being supported.

The free length of the front spring (that is, with the spring off the MacPherson strut and lying on the ground) is 280mm. The length of the spring when mounted on the strut and with the car’s weight on it is 160mm.

Mass

Spring length

0kg

280mm

255kg

160mm

So for a weight change of 255kg, the spring length changed by 120mm. 255 divided by 120 = 2.1kg/mm (118 pounds/inch).

Note: measuring the spring rate off the car gave a rate of 1.9 kg/mm (see above). Measuring on the car gave a rate of 2.1 kg/mm. In the new spring design calculations, an average of these figures (2 kg/mm) was used to represent the car’s standard front spring rate.

  • Spring design

If you can measure the spring diameter, wire thickness and number of free coils, you can use a software application to calculate the spring rate. For example, at the time of writing, the calculator at http://www.thespringstore.com/spring-calculator/spring-rate-calculator.html is available.

Using such a calculator looks really easy and accurate but it does not take into account the variability in rate that is common to many springs. It also requires accurate measurement of the spring.

  • Natural frequency measurement

This approach is almost never performed in modified car circles but it is potentially very helpful. You will need a smart phone and a specific vibration measuring app. You place the smart phone on the car above the front or rear axle line and bounce the car manually.

The smart phone uses its internal accelerometer to measure accelerations, and from this the frequency (the number of times per second) that the car is bouncing up and down can be displayed. This measurement reflects both the stiffness of the spring and the weight of the car acting through that spring.

The higher the natural frequency, the stiffer is the spring for the mass (or the lighter is the car for the same spring stiffness).

Go to Measuring wheel suspension rates to see more detail on how to do this measurement.

In the case of the example car we’ve been talking about, the front spring natural frequency was 1.6Hz. From this figure we can work out the static deflection – here is a graph that allows you to do that. Note that the lower the frequency, the greater is the static deflection (i.e. the softer is the suspension).

However, a frequency measurement is much more useful in making comparisons between different systems – for example, how does the front natural frequency compare with the back?

In our car, this was the standard suspension:

Natural frequency

Front

1.6Hz

Back

1.7Hz

So the rear suspension stiffness is higher than the front. And here’s why this is so important: when the springs were measured individually, the following applied:

Spring rate

Front

2.0 kg/mm

Back

1.4 kg/mm

This shows that the rear springs are much softer than the front. But despite the rear spring rate being much softer than the front, its natural frequency is higher – this car has a stiffer suspension at the back than the front. Even with a softer spring! How does that work? Because there’s much less weight acting through the rear end.

This shows the importance of measuring frequency as well as spring rates if you want a good feel for what is actually going on. Note: measuring actual frequency also takes into account the motion ratio of the suspension, something that measuring the spring rate alone does not.

Conclusion

The manufacturer of your car spent a huge amount in developing the suspension. The result might not be as you would prefer, but the factory specs make an excellent starting point from which you can then go your own way. Always measure the factory springs before devising the specs for new springs.

Next issue: specifying the new springs.

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