rear suspension design help needed

[wedged]

please explain a bit more. I know enough about this to get myself in trouble.

[wedged]

I found this:

"The instant center is found by extending the lines through the pivot points until they intersect. From there, anti squat is found by drawing a line from the IC to the center of the tire at the ground. Where that line crosses the center of gravity line it creates our measuring point ‘A’. The percentage of the line from the ground to our intersect point is the percentage of anti squat."


Right now I have no idea where my center of gravity will be. I'm guessing it will be further back than a stock pick up truck and maybe slightly lower.


From a Car Craft article:

vehicles have a specific point around which the entire car will balance called the center of gravity (CG). For most domestic front-engine, rear-drive cars, the CG is generally located forward of the mid-point of the car at around camshaft height off the ground. While all rear suspensions pivot around a given point, this is not necessarily the point at which the rear suspension applies power or lift. Suspension engineers call this lift point the instant center (IC). Different suspensions place this IC at different positions in the car. Because suspension components tend to shift as the body lifts or squats, this position is dynamic, meaning that it moves as the car pitches or rolls. One definition of IC is the unseen center of an arc created by the moving suspension links. The simplest instant center is a drag race ladder bar. The forward mounting point for the ladder bar where it hooks to the chassis also happens to be its instant center. With other rear-suspension designs, the instant center is an imaginary point in space.

Kevin Gertgen's Performance Trends has created a drag race four-link computer simulation program called 4 Link that offers pictures that tell the story much easier. If you look at the illustration, you'll notice a pair of dotted lines that extend from the lines drawn by the two upper control arms and the two lower control arms. The intersection point of those two lines is called the instant center. The 4 Link program allows you to reposition the IC by moving the mounting points of the upper and lower control arms. Also notice the dotted line that extends from the rear-tire contact point forward at an angle. This line intersects a point created by the intersection of the horizontal CG line with a vertical line drawn through the front spindle. This angled line is called the 100 percent antisquat line, or sometimes called the neutral line.

By changing the location of the upper and lower four-link bars, you can move the IC location either above, directly on, or below that 100 percent antisquat line. When the IC is positioned below that 100 percent antisquat line, the rear of the car will squat on acceleration and "hit" the tires relatively softly. When the IC is positioned above the 100 percent antisquat line, the rear of the car will tend to rise on acceleration and "hit" the tires harder. Obviously, if the IC is placed directly on the 100 percent line, the rear will remain neutral.

This explanation holds true for all rear-drive cars, but there is plenty of confusion around the location of the instant center with different suspension systems. For example, with leaf-spring cars, the IC is the front spring eyes, but with ladder bars, the IC is the front pivot point. Factory four-link cars are determined exactly the same way as drag race four-link systems. If you extend imaginary lines forward on a factory four-link rear suspension, the IC generally falls in front of the car, well below the 100 percent antisquat line. This is why all factory four-link cars squat on acceleration. By installing the Lakewood anti-hop bars (for example), this kit raises the rear locating point of the upper control arms roughly 2 inches. This shortens the IC length and also places it above the 100 percent antisquat line, which now helps plant the rear tires. "


I guess if I make the bottom bar to angle upward from the rear to to front mount point and reduce the angle of the upper bar the same amount, that will increase the anti squat ?

[Roadbuster]

For calculations and simple explanation:
Schope's site has good instructions, illustrations and lets you plug in the numbers.

Jon

[Bryce]

To calculate the A/S you need to know the height of the CG.

There are probably pictures online on how to calculate it but I will try to explain it.

Draw a vertical line at the front tire contact patch.
Draw a horizontal line at the CG height and intersect the vertical line you just drew.
Now draw a line from this intersection point to the contact patch of the rear tire. You should have an upward slopping line from the back to the front.
This line is your 100% anitsquat line. If your IC is on the line you have 100%, below the line is less and about is more than 100%.

If you want to have a street/autocross car I would keep the LCA parrallel to the ground. This keeps you roll steer neautral.

[wedged]

If i keep the lower parallel to the ground, then i would need a steeper angle on the upper bar. i think.

[Bryce]

Originally Posted by wedged

If i keep the lower parallel to the ground, then i would need a steeper angle on the upper bar. i think.

Yes, if you wanted a higher A/S. For a street car build it to 50 to 100% and make it adjustable. For a drag car I would build it from 90 to 150%. Leaf spring cars have well over 100% but have wheel hop issues, where they cant put the power down. My 65 drag mustang has leafs and slapper bars. I lauch with the front wheels off the ground and I only have 300 hp.

[wedged]

I just spent some time messing with the CAD drawing. I dropped the upper forward mount point a bit and ended up with the instant center at 50". I guesstimated where the center of gravity would be, then drew the anti squat line. What resulted was interesting. The instant center was directly on the 100% line.

Then i went over to the mocked up truck and fired up the FARO ... I wish. I grabbed a tape and held in place to see about where the front pivots points would be. The upper would be outside of the inner edge of the rear tires, making the frame kind of interesting. It would either need to stay low and pass under the upper bar or go really high over the bar. If I shortened the upper bar to about 18" OAL, it could land on the frame rail with the frame a width that would fit between the tires. Of course the lower bar would then be about 20" long. I'm going to think this over a bit before I make a decision on what way to go.

[64pontiac]

Not that I know as much as some on suspension design, but one thing I have found is that IF you have to go with a shorter upper link, you can overcome crazy pinion angle changes and binding by making the attachment point on the housing farther back from the axle centerline than the lowers. It seems when I am playing around with suspension worksheets that a similar ratio as the arm length helps. My 3 link on my 64 has a super short upper link, but is mounted farther back from axle centerline and I have NO pinion angle change over the whole sweep, and Im on bags. I also punched in everythign into the suspension calculator, and I still have acceptable antisquat and roll centers, which are all adjustable.

If you are going for Air ride later, and have a narrow frame, huge rollers, and tight fender clearance stay away from a panhard if at all possible. you will probably touch the tire to the fender on full air-out otherwise.

A watts linkage or sliding wishbone are ideal, as a triangulated 4 link works great for coilovers but tends to bind on air ride systems.

I know you probably have figured everything out, but just wanted to chip in my half cent.