Setups
Vehicle
Weigh the car level, then raise the front axle a measured amount and weigh the rear again. More lift = better accuracy. For best results block the suspension solid so the springs don't move.
Links — axle point + bar length/angle
Axle point: X fwd · Y right · Z from axle c/l (+up). Incl +: front end higher. Chassis Y: front heim's distance from car centerline (+ = right). Fore-aft run is solved for you.
3D — drag to rotate · scroll to zoom
Side view — anti-squat
Rear view (from behind) — roll center
Top view — rear steer (axle yaw vs static, front up)
Frame. Origin at rear-axle centerline on the ground. +X forward, +Y to the right of the car, +Z up.
Input. For each link you give the axle-end point (X fwd, Y right, Z measured from the axle centerline), the bar's length, its inclination (angle off level on a gauge laid on the bar, + = front/chassis end higher), and the chassis-Y — how far the front heim sits from the car centerline. The bar's horizontal reach is fixed by length and inclination, so the chassis-Y sets the fore-aft run by Pythagoras; the tool derives the chassis point, fore-aft run, and plan angle and shows them under each link. Every input is a tape pull or a gauge reading — no plan angle to measure.
Static solve. Each link is a two-force member, so its axle pivot can only move perpendicular to the link. The four constraints leave two DOF — pitch (ride) and roll — solved in full 3D, so triangulation and left/right asymmetry are handled exactly.
Articulation. Two sliders set RR compression (bump — wheel rises toward chassis) and LR droop (wheel drops away from chassis) independently. The solver integrates both pitch and roll modes simultaneously using a Jacobian-based stepper — at each increment it computes the Z-velocity of each wheel under both twist modes, forms a 2×2 system, and solves for the pitch/roll rates that drive both wheels toward their targets. Link lengths stay constant to about 0.002" across the full range. Setting both sliders to zero gives the static geometry; moving only RR reproduces the old "roll onto RR" behavior; moving LR independently shows how droop-side geometry migrates.
Numbers. Anti-squat from the pitch-mode instant center projected to the front axle ÷ CG height. Roll center where the roll axis crosses the rear-axle plane. Both update live as you roll, so you can watch the geometry migrate off the corner.
Tire / stagger. Each rear axle end sits one loaded tire-radius above the ground, so the LR and RR diameters set both ride height and stagger. The average radius sets how high the linkage rides (raising it lifts anti-squat and roll-center height — roughly +13% anti-squat and +1" RC per 2" of average diameter). The difference tilts the axle housing, which cants the axle-end pivots and walks the static roll center toward the inside (~0.4" of RC lateral per 1" of diameter stagger here) — a built-in attitude before the car even turns. Enter loaded diameters as they sit at ride height; equal L/R = no stagger. Stagger quoted as circumference ÷ π gives the diameter difference. Axle points are taken square to the housing — the tilt is applied for you. Front tire size doesn't enter the rear geometry.
Kinematics only — no spring/shock/compliance loads.
Dirt left-turn oval — what these changes do
Under power, driveshaft torque tries to rotate the axle and the link geometry turns part of that into download at the rear tires. More anti-squat plants the rear under throttle — more forward bite driving off the corner, which is what you chase as the track goes slick. It also lifts the nose and tends to unload the front, so too much makes the car push on throttle, drive off snappy or hop the rear on a slick or rough track, and can break the tires loose by loading then unloading them. Less anti-squat lets the rear squat — softer, more forgiving delivery, usually better when the track is tacky and grip is everywhere, or to settle a car that's too aggressive off the corner. Rough rule: tacky early in the night wants less, slick late wants more — up to the point it starts hopping or going loose.
Rear roll-center height sets how hard the rear resists body roll and how quickly load piles onto the right rear through the middle of the corner. A higher rear roll center means less rear roll, quicker transfer to the RR and more jacking — it tends to free the car (loosen), because more of the roll couple is carried at the rear; good for rotating, but it can overload the RR and lose side bite on a slick track. A lower rear roll center means more rear roll, slower and more progressive load transfer and more total rear grip — it tends to tighten, hook the rear up, and ride a rough track better. The lateral migration you see when you roll the car matters as much as the static height: as the body rolls onto the RR and the roll center walks toward the inside, it keeps the rear loaded and driving forward. A lot of migration means the car changes character as it takes a set (more dynamic, can get edgy); a little means it stays consistent lap to lap.
As the body rolls, each rear wheel swings on the arc of its links, so it moves fore/aft as well as up/down. When one side moves forward relative to the other, the whole axle yaws — that's rear steer. The RR lead and rear steer readouts (in the roll strip) show it as you roll the car: a positive RR lead means the right rear has moved ahead of the left rear; negative means the left rear leads. That yaw effectively steers the rear of the car, and a bit of it is how a dirt car rotates and points off the corner — too much makes it loose, darty, or quick to step out. The direction (which corner leads) is set mostly by the lower-bar angle: flatter or down-to-front bars steer one way, up-to-front bars the other. The amount grows with how much the car rolls and with shorter or steeper bars, since a tighter arc drags the wheel through more fore/aft per inch of travel. Change a bar's length or angle and watch the RR-lead number move — that's the lever.
Two zones: the roll center governs entry and the middle — how the car rolls over, sets on the right rear and rotates — while anti-squat governs exit, how it hooks up and drives off. They overlap in the transition back to throttle, which is where dirt races are won. A common slick-track direction is enough anti-squat to find forward bite, paired with a rear roll center and migration that let the car set on the RR and rotate without going loose.
These are directional tendencies for the geometry only. Springs, shocks, sway/lift bars, weight and bite, tire stagger and track condition set the actual balance — this tells you what the linkage wants to do, not the final tight/loose result. With stagger the static roll center also sits off-center, which shifts these effects side to side.