Ran across some software on the Internet that, for a hundred bucks, will tell you the location of your instant center when using an adjustable 4link. Well, I don't know about you, but I hate to spend money on software that I'll use once, for 5 minutes, and then it just sits on my computer for the next 5 years. Especially when, in this case, it's so easy to figure this out with pencil and paper. And, this software didn't even tell you if the car was going to squat or rise!

Here's a way to save that money, though you might want to spend a portion of it on something that you'll have opportunity to use more than a few times over the next five years. I'm referring to one of these fancy new tools for measuring the slope. I've seen them advertised complete with digital readout. Sounds plenty handy for general fabrication. And, if you're going to calculate the location of the instant center, you'll be needing to know the slopes of the links.

With a 4link, you'll need to know the slopes of the upper and lower links. If you've done any construction work, you probably refer to the slope as "the rise over the run." If a surface rises 2 inches in every horizontal 10 inches, the slope is two-tenths or 0.2!!! (Didn't want to put a period immediately after that number.) The slope will be positive if the front of the link is higher than the rear and negative if the opposite is true. IT IS IMPORTANT, WHEN MAKING THE CALCULATIONS, THAT YOU DO NOT FORGET THAT NEGATIVE SIGN WHEN THE SLOPE IS NEGATIVE!!

You'll also need to know the distance forward, from the rear tire patch, to the rear mounting point of the upper link and the forward mouting point of the lower link. The vertical locations, from the track surface, of these two points must also be measured.

If you want to know if the car will squat or rise, you must also measure the wheelbase and make a reasonable estimate of the vertical location of the car's center of gravity.

The distance forward, from the rear tire patch, to the instant center, is:

( X(d)S(L) - X(a)S(U) +Y(a) - Y(d) ) divided by ( S(L) - S(U) )

where X(d) is the distance forward, from the rear tire patch, to the forward mounting point of the lower link, S(L) is the lower link slope, X(a) is the distance forward, from the rear tire patch, to the rear mounting point of the upper link, S(U) is the upper link slope (don't forget the negative sign if the slope is indeed negative), Y(a) is the vertical distance, from the track surface, to the rear mounting point of the upper link, and Y(d) is the vertical distance, from the track surface, to the forward mounting point of the lower link. Your biggest problem will probably be with keeping the signs straight, so be careful!

The vertical distance, from the track surface, to the instant center is:

S(L)(X - X(d)) + Y(d)

where X is the value you just calculated.

One more thing to calculate:

Z = hX/L - Y

where Y is the vertical distance you just calculated, "h" is the CG height you've estimated, and "L" is the wheelbase. If "Z" is positive, the car will squat; if negative, it will rise.

There! I've saved you some money. You can buy me a Pepsi when we meet.

good post............

thanks for the math . wonder if the cars rake and weight , front vs. rear , would enter into the figures ?

Only as it affects the value of "h."

Check out this link:
http://www.pirate4x4.com/forum/show...&highlight=link

I started thinking the same thing so I started screwing around with Excel. I did it for triangulated 4-links on 4x4's...but for just finding the IC and antisquat it should work fine with any link suspension (as long as there in an intersection to find).

Anyway it will draw your suspension, links, IC; give you some #'s and %'s...but best of all it is 100% free!

I am in the process of working on getting out some bugs (errors when the links are parallel) and am also almost done with a 3-link/Panhard version.

This is just some other junk I've messed with
http://home.earthlink.net/~triaged/Files/