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Calculating motion ratio

Motorsport Wheel Alignment Fundamentals

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Discussion and questions related to the course Motorsport Wheel Alignment Fundamentals

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In order to calculate the motion ratio, the course instructs to:

- measure the distance between the damper and lower control arm

- divide that by the total length of the control arm and get the ratio

But all examples are for coilovers ( damper and spring packaged together ). Is this the way to calculate if the spring is not located on the damper body, as it is common in BMW rear multilink suspension ? Or I should measure from where the spring is located on the lower control arm and use this to calculate the ratio ?

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If I understand the question correctly, a qualified yes.

Qualified because some suspension designs, including many multi-link ones, don't have a consistent motion ratio and can vary through their travel, even without the problem of components flexing.

It's a lot of messing around, but to check you need to measure the distance between the spring perches for every 1"/25mm of hub travel (more/less travel between would be more accurate). You may wish to double check with 1/4"/5mm changes in the spring distance. You'll need to remove the spring, the damper and disconnect the ARB bar so you have free movement.

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