motion ratio
- Thread starter sb_fd3s
- Start date
sb_fd3s said:
It would be most ideal to measure & calculate motion ratios for yourself on your own vehicle.
Which specific motion ratio are you interested in? General usage/semantic for motion ratio is the amount something moves for a given amount of tire contact patch movement.
Spring motion ratio is the amount the spring compresses and extends for a given amount of movement of the tire contact patch. Shock motion ratio is the amount the shock compresses and extends for a given amount of movement of the tire contact patch. On a coil-over shock like on the NSX, the spring motion ratio and shock motion ratio are the same. The bar or motion ratio is the amount the end of the bar moves for a given amount of movement of the tire contact patch, etc..
Before taking measurements, establish the suspension position (elevation) with reference to the chassis while the car is on the ground. Use a flat surface obviously when you take this measurement.
Here's a step by step procedure-
1. Place the NSX on stands. Disconnect and remove the wheel and tire, 17mm nut/washers for the spring/shock, and end link for the sway bar (if used) as necessary to allow free movement of the suspension.
2. Put a jack under a point that will represent tire contact patch movement. The tranny jack from Harbor freight works excellent for this sort of thing. This point can be anything outboard of the lower ball joint. The brake rotor, hub or spindle will work as these items, ignoring camber change, move vertically the same as the tire contact patch.
3. Move the suspension up to ride height with reference to the chassis per your reference measurement in the very first step.
4. On the side being measured, move it up another inch with reference to the ground.
5. Measure what would be the installed length of the spring, strut or shock if installed.
6. Measure a reference distance, ideally from the ground to the position of the end of the sway bar.
7. Lower the suspension on the side being measured by 2 inches with reference to the ground.
8. Re-measure what would be the installed length of spring, strut or shock if installed.
9. Re-measure the reference distance to the position of the end of the sway bar.
10. Divide the difference in spring, strut, shock or bar movement by 2”. The resulting motion ratio is usually a fraction between 0.50 and 1.00.
11. Obviously, record results for each measurement on the data sheet.
The general formula for motion ratio is as below for an A-arm suspension design-
MR - Motion Ratio
d1 - Distance from spring centreline to control arm inner pivot centre (in)
d2 - Distance from outer ball joint centre to control arm inner pivot centre (in)
sb_fd3s said:
You'll need the spring rate or wheel rate to calculate this. Again, use your specific vehicle as accuracy is relevant for calculations.
Angle Correction Factor:
AFC - Angle Correction Factor
A - Spring Angle from Verticle (see diagram)
C - Spring Rate (lbs/in)
sb_fd3s said:
You were given in great detail best if you take the time to explain fully your intention or situation.
its mainly for my own curiosity. i noticed that the nsx-r suspension has a higher front spring rate and SOS reccomends a higher front spring rate. from my limited experience with mr, the rear always had stiffer spring rates. so i figured it probably had something to do with the motion ratio, i wanted to do my own calculations. also i plan on getting an nsx in a year or two. so that information would be valuable when picking out spring rates for coilovers. finally because im bored im making a list of vehicles that intrest me and their motion ratio's just for fun. so far i have the miata, rx7, and s2000.
so if anyone knows it off hand and could tell me id appreciate it.
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