Anybody know what the rear motion ratio is for the FC?

Thanks

 

It'll be something like 0.95 from the look of it (I've never measured it).

 

how do you measure it? i have a whole subframe sitting out and can do it quick, I tried measuring distance from the pivot to the shock and shock to the brake rotor and i got a .92 motion ratio in the back

 

It's the distance from the pivot to the shock divided by the distance from the pivot to the hub all squared.

A = distance from the pivot to the shock
B= distance from the pivot to the hub
Motion ratio = (A/B)^2

The squared comes from the fact that they move in an arc.

If it's on the car then you can take the springs out and set it up at ride height then progrssively measure the shock travel and the hub travel and get the ratio from there.

 

You guys are right.

Today I took some measurements with the wheel, spring and bumpstop off. I measured every inch. The wheel moved up at almost the same rate as the shock. So both the front and rear of the FC have a spring/shock motion ratio of ~1.


At the lowest point:
Center of axle to fender brim: 17 3/4", top of shock to bottom of GC mount: 8 3/4.

At the highest point:
Center of axle to fender brim: 9 3/4", top of shock to bottom of GC mount: 1".


Note: "Center of axle to fender brim" measured with a measuring tape up against the lowest part of the fender brim right above the wheel to the center of the axle. The method of measuring in the FSM, using the outer part of the underside of the fender brim would probably put this 1/16 or 1/8 higher.

Note: "bottom of GC mount" is the flat part of the mount. There is a 1" tube that is below that that the bumpstop will be in. This tube or cylinder is what hits first. The top of the shock hit's this at 9 3/4"

Thanks for your help!

 

what does knowing this help?

 

Knowing a car's motion ratio will help you consider what spring rates to use. You can imagine your suspension as a level, and the longer your lever, the softer your spring gets.

So if your spring is way inboard, and you have a motion ratio of like .5, your 400lb/in spring is really more like a 200lb/in at the wheel. If it's closer to 1:1, your wheel rate is closer to your spring rate.

 

Well, I'm no expert and haven't tried to use the formulas, but I'm trying to figure out spring lengths and rates to use.

A Miata or FD will have the shock/spring travel say 3 or 4" for every 5" of wheel travel. The FC is around 1 to 1, so it's easy to figure things.

Like today I measured that my lower spring perch could screw down another 1 3/4". So I know that I can get a spring that is 1" longer but no more. If it was a Miata or FD, I could probably fit a 2" longer spring, assuming the same spring rate and ride height.

On the front, my ride height is 12.5". I measured 1" between the shock top and bump stop. Since the front has about the same ratio (1:1), I know I will start hitting it at 11.5" ride height. My bumpstop setup (2 hard rubber pieces and a 1.5" GC microcellular) should crush down to no less than 1" I figure, which will keep me from bottoming at 10" ride height. Actually I want to change this so I have more travel before hitting bumpstops and use harder bumpstops.

Also, if you know the corner of the car weighs about 800 lbs., you know a 400 lb./in. spring should compress 2". On a Miata or FD the spring travel is less than the wheel, so there would be leverage on the spring, so you'd have to know the motion ratio to calculate...

 

From those measurements I get 0.945, incredibly close to my wild *** guess. Now, were the measurements parallel for the hub to fender measurements, or did the angle keep changing? That'll have an effect on the accuracy.

 

Well, here's what I got, but I didn't measure all that accurately, and would want to measure more carefully to get anything else out of this.
Probably the main reason for inaccuracy is that as the hub/axle gets higher, it goes inside (neg camber of course) and goes away from the tape measure. I'd need a level and to get that right. Anyway, here's what I recorded.
The number after the dot is in 16ths of an inch. (17.12 = 17 12/16).

Wheel Shock diff
17.12 8.12 1
16.12 7.13 1.1
15.12 6.14 1.2
14.12 6 1.4
13.12 5 1.4
12.12 3.14 1.2
11.12 2.15 1.3
10.12 1.14 1.2 1" from cylinder on bottom of RB mount to shock top
9.12 1 1.4 0" Contact is shock top to cylinder on bottom of GC mount.