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Motorsport Wheel Alignment: Spring Rates

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Spring Rates

02.55

00:00 - Since in most cases, we'll be dealing with a coil over suspension design in a performance car, it's worth spending a little time discussing the elements that make up the coil over.
00:10 This comprises the shock absorber or damper and of course the coil spring.
00:15 These two components need to be selected to work in harmony if we want to achieve good control of the suspension and the ability to provide sharp response to driver input.
00:25 Coil springs are available in a variety of different shapes and sizes, particularly when we consider those used in OE applications.
00:33 In the motorsport industry however, things have been standardised somewhat and in most instances a 2.5 inch or 63 millimetre inside diameter spring is the norm.
00:43 This is handy since it makes it easier to source different spring rates for a given damper.
00:48 While the diameter may be standardised, the length and spring rates certainly aren't and the correct option will depend on your car as well as the suspension design.
00:57 In simple terms, the job of the coil spring is to absorb the suspension movement and store this movement as energy in the compressed coils.
01:05 Once the suspension is passed over a bump, this energy is then released from the spring which makes the spring extend again.
01:12 As we will see in the next module, the spring works in conjunction with a damper or shock absorber, otherwise we would have uncontrolled oscillation of the spring, which would make our car impossible to control.
01:24 One of the key aspects we need to understand with a coil spring is the spring rate.
01:29 This can be rated in either pounds per inch or kilograms per millimetre if you prefer metric.
01:35 Simply put, for a spring with a 250 pound per inch rating, this means that if we place a mass of 250 pounds on the spring, it will compress one inch.
01:46 A subtle aspect here that's often misunderstood is that assuming we have a linear spring rate, if we add another 250 pounds to the mass already on the spring, it will be compressed by another inch or two inches in total.
02:00 Of course this works exactly the same way if your spring is rated in kilograms per millimetre.
02:06 In order to convert between pounds per inch and kilograms per millimetre, we can simply divide by 55.9 and of course to convert from kilograms per millimetre to pounds per inch, we multiply by 55.9 Selecting the correct spring for your car is a complex topic that we will touch on later in this course.
02:27 However in broad terms, it's always a compromise between choosing a spring that's stiff enough to offer good response to driver input without adversely affecting the grip over bumpy surfaces.
02:38 In this respect, the correct spring rate will change from one track to another and will also depend to a degree on driver preference.