This walks you through a rear 4-link without the algebra. Pick a rig close to yours, tell it how you wheel, plug in what you can measure, and it'll tell you in plain English how the thing will behave — and what to move if you don't like the answer.
Every number here is a starting point. Measure your own rig, mock it up, and cycle it before you burn anything in.
Pick whatever’s closest to your rig — you’ll fine-tune the numbers next. These are starting points, not gospel.
Loaded now: Jeep XJ — trail 4-link, 35s
This sets what “good” looks like when we grade your geometry.
Tape measure, level, notepad. Everything below is asked the way you’d actually measure it under the truck.
Measure from the center of the rear axle. Forward is positive, up is from the ground, and 'out from center' is toward the driver or passenger side — measure one side, we mirror it.
Cycle the suspension by hand if you can. Guessing here changes how the pinion and squat numbers get graded.
Not sure about a number? Every field has a starting value from the preset — change what you know and leave the rest.
This is your rig from the side, rear axle on the left. Drag the slider (or hit play) to cycle the suspension through full stuff and full droop, and watch what the axle actually does.
The dashed orange line is the squat line — where it lands at the front axle compared to your center of gravity is what sets whether the rear squats or lifts under power. The purple crosshair is where your upper and lower links cross: the invisible pivot the axle swings around.
Every check we could run came back inside the numbers for how you said you wheel it. Tack it up, hang the axle, and cycle it by hand through full stuff, full droop and full flex before you burn in the final welds. Make the upper frame mount multi-hole anyway so you can tune it after a few trips.
Right around neutral, so getting on the throttle neither jacks the rear up nor squats it down. The suspension stays free to work over rough ground and you keep your bite.
For comparison: Most experienced builders aim for 70% to 110% for trail running, with an adjustable upper frame mount so they can move it after a few trips.
That is more than a plain single-joint shaft wants to see, but it is normal territory for a built rig. Run a CV (double-Cardan) shaft, point the pinion straight at the transfer case output, and the joint soaks up the swing without buzzing.
For comparison: Forty-inch lowers with thirty-inch uppers works out to about half a degree of pinion roll per inch of travel. That is the classic 75% arm ratio, and it is why most long-arm rigs end up on a CV shaft.
There is enough angle in the links, looking down from above, that they resist side load by themselves. No panhard bar, no track bar, nothing to shove the axle sideways as the body moves. The axle stays put under the rig.
For comparison: Forty degrees total is the widely used cutoff for deleting the bar. Most builders land somewhere between twenty-five and forty-five degrees per side on the uppers.
There is a good vertical spread between the upper and lower link mounts at the axle. That spread is the lever arm that stops the housing twisting when you put power to it, so the pinion stays where you set it instead of rolling up under load.
The rear body pivots around a point that sits close up under your centre of gravity, so there is not much lever arm for cornering load to work with. Less lean means the rig feels planted on an off-camber traverse and it does not load one spring up and unload the other.
The lowers sit near level and they are long enough that the axle travels on a flat arc. That keeps the axle under the rig where you put it, and it keeps the squat behaviour steady from full stuff to full droop instead of changing on you halfway through the stroke.
Opens the advanced view: instant center, roll axis, target bands, and the formulas behind all of this.
░▒▓ Learn the Basics ▓▒░
What each number means on the trail, in shop language. The Advanced view carries the math.
Four bars and a way to keep the axle from sliding sideways. That is the whole machine. It looks simple on the bench, and then it decides almost everything about how the rig behaves on the trail.
THE FOUR JOBS
- - Hold the axle fore and aft, so your wheelbase stays put and the axle does not walk under the truck.
- - Hold the axle side to side, so it stays centered under the rig instead of wandering across the frame.
- - Control how the housing rotates, which is axle wrap under power and pinion angle through travel.
- - Decide what the rear does when you get on the throttle: squat down, stay level, or lift.
All four of those come out of one thing: where you put eight bolt holes. Four link ends at the axle, four at the frame. Move a frame mount an inch and every one of those behaviors changes. That is why it is worth laying out on a screen before you strike an arc.
The single most useful thing to take away up front: squat under power and nose-dive on the brakes are geometry problems. Springs and shock valving will not fix them. Neither will a stiffer sway bar. Get the link layout right and the springs go back to doing the job they are actually good at.