333pg333

No but at the wheels those t/b - coilover combos will come out very differently no? Based on what you have said that the t/bs are basically what you see is what you get, but the coils are way different. Or am I missing something. At work and busy so hard to concentrate but it's not merely a process of adding the two rates together to get the wheel rate.

Van

Those numbers I used were the wheel rates - the rate at the wheel.

I'll walk you through the math - first, coilovers:

The spring vendor will tell us what a spring is rated at - this is straight compression at the spring in a test machine. For this example let's use a 600 lb/inch spring. This means, if you put a 600 pound weight on the spring, it'll compress 1 inch. Put 1,200 lbs on it, it'll compress 2 inches. Put 300 lbs on it, it'll compress 1/2 inch.

The most accurate way to measure motion ratio is to set up a controlled environment with a shock absorber with no spring installed. At ride height, move the wheel up exactly 1 inch (when I did this, I actually started 1/2" below ride height and raised the wheel up to 1/2" above ride height). When the wheel moves 1 inch, the shock compresses 0.63 inches. The motion ratio is 63% - for every inch the wheel moves, the shock moves 63% of that amount.

To calculate wheel rate, we take the motion ratio and square it, then multiply it by the spring rate. In this example, it would be (0.63*0.63)*600 = 238 -- this means a 600 lbs/in spring in the rear of a 944 will have a 238 lbs/in wheel rate.

With me so far? Let's move onto the torsion bar:

A torsion bar is essentially a metal rod with one end fixed and one end that can rotate, thus putting a "twist" on the bar.

First, we need to know the length of the bar - the force required to rotate the end of a long bar 1 degree will be less than the force required to rotate the end of a short bar 1 degree. A 944 torsion bar is about 24.75 inches long.

Second, we need to know the diameter of the torsion bar - a fat bar will resist the twisting action more than a skinny on will. Let's use a 30mm bar for the example. A hollow bar will resist twisting a little less than a solid one - because it has less material that needs to be "twisted" - but it will also weigh less. If you like, we can make this example a hollow torsion bar - the ID (hole down the center) will be 10mm.

In inches, this torsion bar will have an OD of 1.181" and an ID of 0.394".

Third, we need to know the "Polar Moment of Inertia" - essentially the area/volume of the torsion bar that will be resisting the twist. This is a formula that is: (3.14*(OD^4 - ID^4)) / 32 -- which, in this case, is 0.188691154.

Fourth, we need to know the "strength" of the material - rubber torsion bars will be far less strong than steel ones. My best guess on the "Modulus of Rigidity" of the spring steel torsion bars used by Porsche is about 11,600,000 PSI.

Fifth, we can now calculate the amount of force it takes to rotate our torsion bar 1 degree - remember, one end is held still, the other end can be rotated. The formula is: ((2 * 3.14) / 360) / (Length / (Polar Moment of Inertia * Modulus of Rigidity)) -- which equals, in our example, 1,543,5 inch/lbs of torque (divide that by 12 if you want ft/lbs of torque).

Now, finally, we can finally figure out what the wheel rate will be. Notice we have the torque needed to twist that torsion bar 1 degree... All we need to know is our moment arm (distance from the hub center line to the torsion bar center line) and how many degrees the torsion bar will have to twist to move the wheel 1 inch. The moment arm is about 16.5 inches, and 1 inch of linear movement of the hub is 3.475 degrees. We divide the torsion bar torque by the moment arm, then multiply by the # of degrees the torsion bar will twist. In this case, it's: (1543.5 / 16.5) * 3.475 = 325.1 -- the rear wheel would have a wheel rate of about 325 lbs/inch.

If your car has both the coilover and the torsion bar in this example, you just add the 2 wheel rates together: 325 + 238 = 563 lbs/inch total wheel rate.

It's not rocket science... just race car science.

Van

Boy, I'm tired after all of that!

Want to know what I work on when I'm not calculating race car math?

DDP

Great post, Van! You took more time in that one post then I think I ever have in all my posts combined. I'm sure someone will be much appreciated.

Here is another link to check out:

http://web.archive.org/web/200412111...x_v2&id=22&c=4

spoolin51

Get back to your vacation dude....
And take some pictures damit

superloaf

so where do you get the proper values? because that is one question i've been dying to know since i have a vw scirocco which i'm running 400# front springs in and i can't understand how a 951 sport spring is 250 or so. i realize that there may be a difference between the 2 cars but i would think that most mac struts are very similar when it comes to spring rate versus wheel rate--am i wrong?

BTW physics ain't my forte! ha ha. but i would appreciate your knowledge on this, van. thanks.

uh, ok, just read you're last post, van. do you mean to say that you figured out the 944 wheel rate equation on your own? wow, you may have too much time on your hands! impressive, though.

333pg333

Ok I think I'm mixing up wheel rate with equivalent coilover rate. Thanks for taking the time Van. Great post and I love the YouTube clip. Ha, great stuff.

Van

I did not figure out the wheel rate equation on my own - I found it both on-line and in books about race car suspensions.

What I did do was calculate the motion ratio myself - that was actually quite easy. I took the spring off the strut, with my floor jack, raised the hub to 1/2" above ride height, put a zip tie on the strut shaft and a dial indicator under the centerline of the hub, and then lowered the floor jack exactly 1 inch:




I then carefully measured the space between the zip tie and the top of the strut with a caliper. For exactly 1 inch of movement at the wheel, the strut moved 0.91 inches. So, the motion ratio is 91%:




To answer the question about your sirocco, the answer is simple: due to the suspension geometry - the length of the control arm, where the strut connects, and the angle of the strut are different, so the motion ratio is different. You could use the same method I've done to figure it out. I have a hunch that even pre- and post-'87 cars have different motion ratios due to the different control arm lengths.

For all of those that care, this wheel rate stuff is really only part of the picture - it's also very important to understand how much "sprung weight" the springs are holding up. You need to strike a balance here (called suspension frequency). A 5,300 lb Mercedes S class will have a different "ride" with a 500 lb/in wheel rate than a stripped out 944 SP1 car weighing 2,400 lbs running a 500 lb/in wheel rate.

Hope that's informative!

lee101315

Thanks for the info guys

2Bridges: You mustve read my mind because thats exactly why I was asking. Thank you

vt951

Van,

That is the definitive 944 wheel spring rate post.

One more wrinkle to throw in... effects of weight distribution (hold your chebby motor jokes for a moment).

For cars with 50/50 weight distribution, is it always best to shoot for equal wheel spring rate front and rear, or do you want a little more rate in front to give natural understeer, or a little more in the rear for natural oversteer? Also, for cars that don't have 50/50 weight distribution... let's say a car with 60% over the front wheels, would you want a correspondingly higher spring rate in the front in order to achieve neutral handling (eg. 600 lb front and 400 lb rear)?

333pg333

My understanding is that the LS motor weighs a very similar amount to our i4...or so I've read on the internet Further to your question, I wonder if the power characteristics of a V8 vs a turbo i4 call for different setup too? Having said that, DVC runs KW v3s and loves them.

vt951

Yeah, I believe the LS motors are comparable to our turbo 4 cyl weight, all said and done. My question was not directly related to my upcoming swap... just a general question.

2bridges

generally most want a little more front to get a very slight understeer.

Van

I've heard of a few theories on front vs rear wheel rates (suspension frequencies) - the "primary" theory is that you want a slightly higher suspension frequency in the rear because, as you drive over a bump, the front wheels hit the bump first, so the rear frequency needs to be a little higher (more cycles per minute) so the oscillation of the rear "catches up" with the front.

This may be true for true purpose built race cars that have double-wishbone suspensions at all 4 corners, but I haven't found it to be true for 944s.

As a general rule of thumb, the "stiffer" a suspension, the "less" grip it'll make. (When the car is in mid corner, the more supple the suspension, the more the tires' contact patch will stay in contact with the road.) This is why, if you're feeling a little over steer, you soften your rear sway bar, or stiffen your front sway bar, to compensate and balance the car.

Making the rear of the car too stiff will make the rear "loose" and promote oversteer. Going a little stiffer in the front will promote understeer - which most people like more.

Does that answer your question Alan? The best thing to do is to corner balance your car once the engine is in, then select a "target" suspension frequency. Get springs to match that, then evaluate how it feels while you drive it and see if you want to fine-tune by adjusting one of the rates a little bit.

333pg333

I thought it was always a compromise between body roll and grip. Also we have found that what 'feels' faster on the track doesn't always translate in times. In the past we had the sway bars set to full hard, but this was to negate hitting the bump stops. It felt fast because there was very little body roll, but you don't take into account where you have to back off mid corner because you can't put the power down.