Motion ratios for E36's?

**NegativeCambre** · 05-25-2005, 08:25 PM

Looking for the motion ratio for front and rear suspension for the E36.

Thanks!

Steve

**SG_M3** · 05-25-2005, 08:29 PM

.88 front, i think
.46 rear

**Deadphan** · 05-25-2005, 11:00 PM

Originally Posted by SG_M3

.88 front, i think
.46 rear

Wow, it's that low in the front I figured it would be a lot closer to 1.

**NegativeCambre** · 05-25-2005, 11:03 PM

Originally Posted by Deadphan

Wow, it's that low in the front I figured it would be a lot closer to 1.

I imagine its because of the wheels is kicked out a little from the strut mounting point by the hub and brake assembly.

Steve

**Tourenwagen** · 05-26-2005, 10:25 PM

The front is approx .94 for spring and damper.
The rear is approx 1.05 for the damper, approx .65 for the spring.

Those are approximate because this all changes as you change camber, ride height and other things.

The front is not 1 to 1 because of KPI (kingpin inclination).It has nothing to do with the fact that the wheel is kicked out a little from the strut. If KPI is zero (the line from the outer control arm pivot to the upper strut mount pivot when viewed from the front) then you will get 1 to 1. If it has an angle, it will be less than 1.

Hope this helps,
Barry

**SG_M3** · 05-26-2005, 11:26 PM

Originally Posted by Hellabad

The math shows that the rear of an e36 or e46 is 0.46:1 ratio, also called motion ratio.

Jay

Where did Jay pull that from then?

**BlueMaxx9** · 05-26-2005, 11:59 PM

Probably was thinking of the multiplyer to find effective wheel rate. Unless I am confusing things, the effective wheel rate percentage is the square of the motion ratio. Assuming Barry has his motion ratio numbers right (and that is probably a safe assumption), square them and you end up with numbers close to the Jay quote. I'm sure I will be corrected if I'm wrong about this.

-Bret

**B.Watts** · 05-27-2005, 12:24 AM

Yes...assuming I haven't gone crazy:

WHEEL RATE = SPRING RATE x (MOTION RATIO)^2

Thus, Jay's number of .46 is pretty close to Barry's motion ration of .65^2 = .42. The number Jay listed is somtimes also referred to as the motion ratio, but is probably more accurately defined as the wheel rate conversion ratio...it just depends on who you are talking to.

You need to square the motion ration in order to calculate wheel rate because you must first consider the compression of the spring relative to the movement of the wheel, and then consider the amount of force that the spring is able to transfer back out to the wheel along the lever.

This is all working from memory, but I'm pretty sure I have it right. Can anyone confirm?

EDIT: To use ^2 instead of writing out SQUARED.

**SG_M3** · 05-27-2005, 12:35 AM

yup, i got confused. Bryan is right.

wheel rate = spring rate x (displacement ratio)^2, wheel rate = spring rate x (.65)^2
or
wheel rate = spring rate x .46

edit: same for my front number .94^2 = ~.88

**Tourenwagen** · 05-27-2005, 12:50 AM

Bingo to all. The terminology gets confusing sometimes.

Barry

**Kos-motate139** · 05-27-2005, 02:46 AM

Is that the same motion ratio for the rear of the e30 style cars too?

**BlueMaxx9** · 05-27-2005, 08:55 AM

I'd imagine not, seeing as how the suspension design is so radically different. Motion ratio should be all that hard to calculate for an E30 rear suspension though. You just need a tape measure and some freetime

-Bret

**Steve J.** · 05-27-2005, 09:58 AM

Bret, the problem is not physically measuring it, its measure the exact geometry that you are trying to design for, or better yet, designing the geometry and making sure what you build is maintaining those dimensions (i.e. using hardcore jigs). Just measuring the suspension with a tape measure and running them through some formula's won't do much, since you are not getting anything out of it, the advantage comes from plugging in different numbers to see which results can benefit you.

**B.Watts** · 05-27-2005, 10:06 AM

Steve - It's beneficial if all you want to know is what your wheel rates are so that you can have a better understanding of what changing your actual spring rates does to your car's balance. That seems to be all Kos wants.

Kos-motate - According to Gustave's E30 site, the rear motion ratio for the spring is .67. I don't know the info for the damper.

**Steve J.** · 05-27-2005, 10:08 AM

I guess, it depends on how much you really understand of the dynamics of the suspension, otherwise its just a bunch of numbers

**B.Watts** · 05-27-2005, 10:13 AM

There's nothing all that confusing about the basic equation to give approx. wheel rates so that you can understand why a 100 pound spring rate change in the rear doesn't have the same effect as a 100 pound spring rate change on the front. That's all most folks are looking for. It's only as complicated and confusing as you want to make it.

**Steve J.** · 05-27-2005, 10:15 AM

Ah you are just talking about realizing what spring rate changes do, gotcha.

You can also try to get a hold of Excel spread sheets to play with to see what happens when you change the roll centers slightly, and sway bar/spring rates, you can get the same corner weight with a specified load in several weighs, but with differnt compromises. Very easy and simple way to see what the potential of the car is

**NegativeCambre** · 05-27-2005, 03:20 PM

Originally Posted by BMWRacerITS

Steve - It's beneficial if all you want to know is what your wheel rates are so that you can have a better understanding of what changing your actual spring rates does to your car's balance. That seems to be all Kos wants.

Kos-motate - According to Gustave's E30 site, the rear motion ratio for the spring is .67. I don't know the info for the damper.

Bingo--looking to see actual spring rates. I've had the equation wheel rate = (motion rate)^2 x (spring rate) for awhile now--and what are the consensus for the motion ratios then?

.96/.65?

Steve

**B.Watts** · 05-27-2005, 03:29 PM

Steven - Those numbers will get you close enough for figuring out how your wheel rates compare to each other If you really want exact numbers, you could always go measure your car.

**NegativeCambre** · 05-27-2005, 03:51 PM

Originally Posted by BMWRacerITS

Steven - Those numbers will get you close enough for figuring out how your wheel rates compare to each other If you really want exact numbers, you could always go measure your car.

Ok, good--so... to figure out the wheel rate, I should use this (rough) equation:

Front:

wheel rate= (.96)^2 X (spring rate)

??

Steve

**B.Watts** · 05-27-2005, 03:53 PM

Yup.

**NegativeCambre** · 05-27-2005, 03:54 PM

Originally Posted by BMWRacerITS

Yup.

Thanks alot guys--appreciate the help.

Steve

**NegativeCambre** · 05-27-2005, 04:09 PM

Wow, using these new values, I achieved ideal spring rates that are *much* more reasonable--I had a funky rear motion rate value before, and the equation was telling me I needed to run 1400ft/lbs/in springs in the rear--yikes.

Steve