IcemanG17

I am having my bilstein shocks rebuilt at Performance Shock inc (Infineon)....I need to determine my unsprung weight and motion ratio

I know my wheel-tires are about 45lbs.....I have Big Red front brakes with stock hubs...I have standard S4 rear brakes

My shock are custom heim jointed, but the mounting points are stock in front and nearly stock in rear, just slightly inboard....

Here is my "before" shock dyno test....of course I really have no idea what it means

Vilhuer

Motion ratio or something very much like it has been discussed years ago when 928 Cup car specs were talked about. Basically if wheel goes up 1" shock goes up x fraction of it. These numbers for both front and rear suspension should be in archives.

Cortina

The figs I used are out in the garage - will get them for you tomorrow.
Not critical to get them dead right - since the whole exercise is a wee bit hit and miss (for example - you have no idea what you want your graphs to look like ... am I right ?)
Your before graphs look similar to mine - they measure damping force against velocity for compression and rebound stroke .. the tricky part is saying what you want.
From memory the rear motion ratio is heading towards 1:1 (since mounting point is near to wheel) I think I used 4:5. Front is much closer to 2:1 (from memory I used 1.75:1) .. will check in the morning.
They should also be asking for your spring rates !! , and the corner weights of the car.

Have fun , it worked for me !!

GlenL

Lemme see if this works...

______To Tire____Shock Pivot___Angle___Spring Multiplier
Front _____13 ______6.5__________10_______0.492
Rear______17______11___________15_______0.625

I measured from the pivot axis to the center of the wheel. The hub doesn't count! And added in something for the angle of the shock at the resting height. I was surprised the way the rear turned out. Mostly people say "1:1" but it was a lot less.

Do with this want you will!

Hilton

Which suspension setup was this with Glen? Early or late control arms/hubs?

I had this other thread bookmarked with the S4 motion ratio values being 0.5 and 0.82 for front/rear:

https://rennlist.com/forums/928-foru...heel-rate.html

wheel rate = spring rate * (motion ratio)^2

Which puts spring multipliers being x4 for front, and x1.5 for rear

e.g. if you want 500/300 wheel rates, you need springs at 2000/450.

This explains why Mike Simard is running 2000lb springs on the front of his racer

Having said which, I haven't measured the distances myself to calculate the motion ratio (i.e. distance from shock mounting point to pivot point/distance from pivot point to wheel centre). Your point Glen about wheel center is important - it means that offset also affects spring performance

GlenL

That's a stock early suspension.

The rear evaluation is suspect as I'm unclear on exactly where the axis of rotation is.

Why square the motion ratio? The force is on a simple lever.

mark kibort

my shock goes up 1" and i get 5" of wheel travel. (about). seems like a lot, but thats the reality.

Mike Simard

Glen's numbers are good. The front is about .5 and the rear is slightly higher but not nearly as much as .82. I did take measurements of my car but have a modified rear shock mount. I suspect the stock rear wouldn't be much over .6 if at all.

Hilton's post has a real gem of spring setup secrets. If you get the wheel rate correct, you could have all the sway bars fall right off and still keep driving!


It's also possible to move spring points up front and gain some benefit. Mark A knows something about that.

brutus

Right , "It's also possible to move spring points up front and gain some benefit. ..." With lowered suspension the stock front shock is less vertical and as the wheel moves higher under compression the increasing angle as the shock moves more toward horizontal results in a regressive spring rate. With smaller diameter springs you can move the upper mount out for a more vertical starting point allowing for a more constant spring rate in the range of suspension travel you will be using.

IcemanG17

Interesting about the lowered stance causing a regressive spring rate........maybe a progressive spring with proper valving to match is the answer using stock mounting points?

GlenL

Lowering the car makes the angle between the arm and the shock closer to 90 degrees. That makes the spring effectively stiffer.

Why do you think lowering the car makes it regressive?

brutus

as the shock get more horizontal an inch of movement along the arc of the lower mount is less than an inch of compression of the spring shock. At the hypothetical extreme ,horizontal, there would be nearly no compression as the lower mount scribed an arc. Mounting the shock at an angle is a compromise needed to fit a spring and shock with out hitting the inner fender or wheel.

GlenL

Okay, I see what you're saying. The actual geometry isn't like that, though. Over the range of motion with running deflection and ride height variations the angle of the spring isn't changing much. I'm not sure it's getting more leverage or less as the hub moves up. I think less but it's marginal. It's not like an F1 car or other open-wheel racer where the springs are at a large angle to start with.

Mike Simard

One of the advantage to the pushrod/rocker of an open wheel car is to place the pivot points in a way to have increasing spring travel per wheel travel as bump increases. With a rocker it's easy to do that.

It may be tough to visualise the 928's front but there is something going on there that can be improved. If the shock is 'layed down" then it becomes less so with bump and is therefore more 'progressive'.
Mark A is the guy to talk to about that. I would call, order some parts and then ask about this