4-Link Measurement Sheet

Everything here feeds the triangulated 4-link calculator. Field names in orange mono match the tool's input labels.

Read first — ground rules

• Ride height, race trim. Correct ballast/driver weight, fluids, tires at race pressure, on a flat level floor. Roll it and let the suspension settle. Don't jack it until the CG step.
• Measure to heim/bolt centers — the pivot is the center of the rod end, not the end of the bar.
• Frame: +X forward · +Y toward the car's right side · +Z up. All "Y" values are measured straight out from the centerline string: left side = negative, right side = positive.
• Tools: tape, digital angle gauge (magnetic), centerline string + plumb bob, 4 corner scales, jack + stands, marker.

Step 1 — Set your datums

1. Mark the center of the rear axle housing (pinion centerline); drop a plumb line and mark the floor.
2. Mark the front center (center of the crossmember / equal off both frame rails); plumb to the floor.
3. Stretch a string between the two marks = car centerline (this is Y = 0). Measure every Y perpendicular to it.
4. Zero the angle gauge on a known-level surface (or the frame, if it's level).

Step 2 — Vehicle numbers

1. Wheelbase — front axle CL to rear axle CL (measure both sides, average):   in  → Wheelbase
2. Rear track c/c — center of LR tire tread to center of RR tire tread:   in  → Rear track (c/c)
3. LR loaded radius — floor straight up to center of LR hub:    ×2 =   in  → LR tire diameter
4. RR loaded radius — floor straight up to center of RR hub:    ×2 =   in  → RR tire diameter

Measuring floor-to-hub-center and doubling gives the loaded diameter the tool wants (it accounts for tire squat). The LR/RR difference is your stagger. Front tire size isn't needed.

Step 3 — CG height (with the scales)

1. Weigh level: Total   lb → Total weight  ·  Rear (LR+RR)   lb → Rear wt, level
2. Block the rear suspension solid (clamp the shocks or fit solid links) so the springs can't move when you tilt the car. This is the #1 accuracy item.
3. Raise the front axle a measured amount (10–15" if safe), rear stays on the scales. Vertical rise =   in → Front raised by
4. Re-weigh the rear (LR+RR) while raised:   lb → Rear wt, raised
5. In the tool, open "Find CG height with scales," enter the four numbers, hit compute — it fills CG height. Then lower the car, unblock the suspension, re-settle.

Step 4 — Each bar (do all four: LL, LR, UL, UR)

Measure to the heim centers. Per bar you need six numbers:

• Axle X — fore/aft from the axle centerline plane to the axle-end heim (+ = forward).
• Axle Y — perpendicular from the centerline string to the axle-end heim (left −, right +).
• Axle Z — vertical from the axle tube center to the axle-end heim (+ = above center, − = below).
• Length — axle-end heim center to chassis-end heim center.
• Incl — angle gauge laid on the bar, degrees off level (+ = front/chassis end higher than the axle end).
• Chassis Y — perpendicular from the centerline string to the chassis-end (front) heim (left −, right +).

Take both sides even if it "looks symmetric" — dirt cars rarely are. The fore/aft run is solved for you, so there's no plan angle to measure.

Step 5 — Enter & check

1. Type the vehicle numbers, then each bar's six numbers (use Mirror L→R if a side truly is symmetric).
2. Under each link the tool prints a derived "chassis pt." Eyeball it against the car — does that point land where the front heim actually is? If not, recheck that bar's length / incl / chassis-Y.
3. Name and Save the setup, and Export a file so it's backed up.