CHASSIS SET UP EQUIPMENT
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Motion Ratio Gauge®
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The motion ratio describes the ratio of motion between the wheel and the damper. For example 5mms of damper compression and 10mms of wheel movement would result in a motion ratio of 0.5.
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This ratio is a crucial factor in determining which spring rate to use, best thought of by considering a simple rocker arm type suspension as shown below:
There is a distance A between the outer bearing and the pivot point at the chassis and a distance B between the damper mount and the pivot point at the chassis. The lengths of A and B determine the motion ratio, but considering it as a lever and a force being applied via the wheel it can be seen that the greater the distance A, the greater the leverage affect this force would have over the spring and therefore the stiffer the spring rate that would be required to balance the system.
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Using this ratio and the weight supported by that damper it is possible to calculate the spring rate required to reach a particular chosen spring frequency.
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This gauge is designed to provide a simple solution for measuring the motion ratio. It’s easy to use, adjustable to suit a range of damper lengths and comes supplied in a hand held flight case.
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Each kit is supplied with bearings to suit ¼”, 5/16”/M8 or 3/8” damper mounts and a range of studs to make the gauge adjustable between damper lengths of approximately 200mms to 320mms depending upon the application.
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Five folded plates increasing in increments of 5mms are included and can be positioned under the tyre. With the lowest plate positioned, the dial gauge should be set to zero, and the gauge reading then noted after each of the taller wheel plates have been positioned under the tyre.
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Inputting the readings into the excel program on the memory stick provided will output the motion ratio, wheel rate and spring frequency. A table provided gives typical spring frequencies for a range of vehicles and hence this helps for determining a baseline setting for a vehicles spring rates.
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There is also the option to include progressive bump rubbers into the spreadsheet if the rate is known and to compare the front/rear spring stiffness relative to the weight distribution.
There is a list of other factors involved to determine the optimum set up for any given race car and circuit, this gauge is designed to aid with a starting point for the selection of spring rates, therefore aiding with one of the variables from that list.