Motion Ratios

[Error404]

Does anyone have a formula for calculating the motion ratio of the McStrut or is it really a case of measuring it? If so does anyone have a rough idea what the motion ratio is for a coupe?

[quattro84]

Which one are you after? I guess you want to calculate coil spring rates from wheel rates? Then you need
MR = (B/A)²

I did this once some time ago, not that difficult to do at home with a jack, gaffer tape, piece of string and a tape measure, depends a great deal of what ET you are running.
Make sure you get it to the same ride height & horizontal level.
Visualize a line between inner pivot point (bolt) of your wishbone and middle of your tire contact patch. Measure the lenght "A"
Then remove the wheel and visually extend centerline of you struth until you reach previous angled line "A" (easier if you use a string fixed with gaffer tape)
Point where both meet is you endpoint for line "B"

And no, the ratio isn't 1 as a lot of sources suggest for McP struts.

[Error404]

Great thanks, yes I wanted to measure the motion ratio to calculate the wheel rate, so I can look at spring rates a bit more scientifically
As you say lots of "web" information says it is 1:1 but it can't be as the spring must be inboard of the tyres so the arc of the contact patch is longer

[quattro84]

You are welcome.

If you are going to do DIY coilovers i would suggest to try different sets from Faulkner springs and try your butt feeling compared to the maths.. they may differ by some margin i noticed over time...

Try to find an online version of Milliken's books, he uses guidelines and his professional knowledge which is very handy for us, like natural frequencies for both axles compared to weight distribution, saves you on finding the frequency range whereby you start fouling yourself whilst driving on harsh roads

[Acidburn]

He's got my kw's to play around with.