A common term thrown around in the suspension and chassis world is instant center. What the heck is it and what does it mean?
Instant center is located by drawing an imaginary line through the upper suspension links to where it intersects with a similar imaginary line drawn through the lower links. Think of instant center as the pivot or hinge point of the suspension at a specific given ride height. If you were to replace all the arms (say a front suspension), or all the links (in say a 4 link rear) with 1 big arm, where would that 1 arm be hinged.
So how does it relate to what we are doing? This pivot/hinge (instant center) is where all forces from the tires and/or engine are applied. This is key for both acceleration and braking. We want to control where this force is applied.
Think about the last time your buddy asked you to come help him move a his fridge. While your pushing the fridge into the kitchen do you decide to push on the very top? The bottom? Somewhere in the middle? Or do you just open the door grab a beer and watch him do all the work?
Obviously if you push on the top, you could topple the thing on top of him. Opposite would happen if you got a running start and showed off your old high school football moves and slammed into the bottom, it very well could end up on top of you. Chances are if you didn’t drink all his beer, you would think to push somewhere in the middle. We can call this the neutral line.
Same thing with a vehicle suspension. We want to apply the forces somewhere near the middle. Too low and the front of the car raises up and we spend a lot of time and power to raise the front end up before any forward motion is made. Too high and the front will go down (and the rear up losing traction) and again we just wasted a bunch of power going nowhere. So in particular on the rear suspension it is a balancing act of planting the rear tires just enough for traction, but not wasting anymore than necessary.
So the last topic to understand is where is the middle of the car? This neutral line is called the anti-squat line. Remember geometry class with the cute girls in it, that you didn’t pay attention in? This line extends from the rear tire contact patch up to the intersection of the front spindle at the center of gravity height.
If our instant center is in front of/below this line, that is equal to pushing the fridge lower than the neutral point. If the IC is after/above that line, your pushing above neutral point.
It is my belief, and those of experts I have talked with that for our AWD cars we want to be near the Anti-Squat line. We don’t need nor want to waste a lot of motion of the suspension as we don’t need extra traction for the rear, nor do we want to unload the front tires. We want it to take off with just slight rear weight transfer and go.
This can be clearly seen with the JSM Typhoon before and after video. The first “launch” is stock suspension, while the 2nd is with JSM typhoon 4 link rear.
Regardless if you are a past customer of JSM, future customer, or not a customer at all. Our goal with these tech topics is to educate and help you select the suspension that is right for you.