Cross Weight Calculator checks corner scale readings from LF, RF, LR, and RR tires to show the car’s diagonal balance. Formula: Cross Weight % = (RF + LR) ÷ Total Vehicle Weight × 100.
The Diagonal That Controls Your Lap Time
Cross weight has nothing to do with left-right or front-rear balance on its own. It measures a diagonal relationship — the combined weight on the Right Front and Left Rear tires expressed as a percentage of the car’s total weight. That diagonal pair is what loads up under cornering, and even a 1-2% deviation from your target figure changes how the car rotates through the center of a turn.
Most scale pads give you four numbers. The question is what to do with them. This calculator takes those four corner weights, computes your current cross weight percentage, maps it against your target, and tells you exactly how much weight needs to shift across the diagonal — and which direction.
Formulas Used
Core Weight Totals
- Total Weight = LF + RF + LR + RR
- Cross Weight (diagonal sum) = RF + LR
- Front Axle Weight = LF + RF
- Rear Axle Weight = LR + RR
- Left Side Weight = LF + LR
- Right Side Weight = RF + RR
Percentage Distributions
- Cross Weight % = (RF + LR) ÷ Total Weight × 100
- Front % = (LF + RF) ÷ Total Weight × 100
- Left % = (LF + LR) ÷ Total Weight × 100
Wedge Reference Point
- Ideal 50% Cross = Total Weight ÷ 2
- Wedge vs 50% = (RF + LR) − (Total Weight ÷ 2)
Target Adjustment
- Target Cross Weight = Total Weight × Target% ÷ 100
- Cross Weight Gap = | (RF + LR) − Target Cross Weight |
- Shift Per Corner = Cross Weight Gap ÷ 2
Direction Logic
- If (RF + LR) < Target Cross Weight → Add Wedge to RF/LR
- If (RF + LR) > Target Cross Weight → Remove Wedge from RF/LR
- If gap ≤ 0.001 → Balanced Target
How It Works
Enter the weight reading from each corner scale pad — Left Front, Right Front, Left Rear, Right Rear. The measurement system toggle switches between pounds and kilograms and reloads default example values when you change it, but it does not convert your existing entries. Units are cosmetic labels here; all arithmetic runs on the raw numbers you type.
The cross weight percentage is calculated first: add RF and LR, divide by total vehicle weight, multiply by 100. That single figure is the primary output. Everything else branches from it.
Front-rear and left-right splits are computed independently from their respective axle and side totals. These tell you the longitudinal and lateral bias of your setup, but neither of those figures is cross weight. A car with perfect 50/50 front-rear and 50/50 side-to-side balance can still carry asymmetric cross weight if the corners are mismatched within each axis.
The wedge reference compares your actual RF+LR sum against the absolute 50% mark — half of total vehicle weight. This is a fixed reference point regardless of your chosen target percentage.
The target adjustment section is what makes the tool actionable. Set your desired cross weight percentage in the Target field. The calculator finds the gap between your current diagonal sum and the weight that would produce that target percentage, then divides that gap by two. Changing one spring perch shifts load diagonally across both corners of that pair simultaneously — raising one corner by X affects the diagonal total by roughly 2X — which is why the correction is split per corner.
Why Shift Per Corner Is Half the Gap
When you raise a spring perch, you pre-load that corner of the chassis. That load doesn’t stay local — it travels diagonally through the chassis structure and unloads the opposite corner. Raise the RF perch by a meaningful amount and the LR gets heavier; the LF and RR get lighter in roughly equal measure.
This means a 10 lb cross weight gap doesn’t require 10 lb of perch change at a single corner. Two corners are working together. Adjust RF and LR perches by 5 lb each, and the cross weight shifts by 10 lb total. The calculator expresses this directly: Cross Weight Gap ÷ 2 = the adjustment needed at each corner of the RF/LR pair.
This only holds under the assumption that the chassis transfers weight symmetrically, which is not guaranteed on asymmetric or heavily modified frames. On stiffly-sprung oval cars this relationship is consistent enough to use directly. On road course setups with significant suspension compliance, treat it as a starting point and verify back on the scales.
Worked Example
A late model stock car comes off the scales with the following readings: LF = 925 lb, RF = 850 lb, LR = 875 lb, RR = 850 lb. The team is targeting 52% cross weight for a banked oval. Measurement system is set to Imperial.
Total Weight: 925 + 850 + 875 + 850 = 3,500 lb
Cross Weight (RF + LR): 850 + 875 = 1,725 lb
Current Cross Weight %: 1,725 ÷ 3,500 × 100 = 49.29%
Wedge vs 50%: 1,725 − (3,500 ÷ 2) = 1,725 − 1,750 = −25 lb (below the 50% mark)
Target Cross Weight: 3,500 × 52 ÷ 100 = 1,820 lb
Cross Weight Gap: |1,725 − 1,820| = 95 lb
Shift Per Corner: 95 ÷ 2 = 47.50 lb
Direction: current cross (1,725 lb) is below target (1,820 lb) → Add Wedge to RF/LR. The front-rear split comes out at 50.43% front; left-right at 51.43% left. The team needs to raise the RF and LR spring perches to shift 47.5 lb per corner across the diagonal, then re-scale to confirm.
Frequently Asked Questions
Can I enter a corner weight of zero?
Yes, individual corners accept zero without triggering an error. The calculator only rejects negative values. However, total vehicle weight must sum to more than zero — entering all four corners as zero produces a validation error because dividing by zero is undefined.
What range is valid for the target cross weight percentage?
The code enforces a strict range of 1% to 99%. Values of exactly 0% or 100% are rejected. Those extremes represent physically impossible setups where the entire vehicle weight rests on one diagonal pair.
Does switching between Imperial and Metric convert my corner weight entries?
No. Switching the measurement system only changes the unit label displayed and reloads the default example values (Imperial: 850/780/810/760 lb; Metric: 380/350/370/340 kg). Any corner weights you entered before switching are overwritten by those defaults. Enter your numbers after selecting the unit system.
The Wedge vs 50% figure is negative — does that mean something is wrong?
Not at all. Wedge vs 50% is simply how far your RF+LR diagonal sum sits above or below the absolute 50% mark of total vehicle weight. Negative means RF+LR is currently carrying less than half the total weight. This is a reference value, not a performance judgment. Most race setups run somewhere between 48% and 54% cross weight depending on track type.
Why does Shift Per Corner stay the same even if I change which corners I’m adjusting?
The calculator computes the required weight shift on the RF/LR diagonal only. It does not offer a separate calculation for adjusting the LF/RR pair, because those corners are the inverse diagonal — removing wedge from one pair is mechanically equivalent to adding wedge to the other. The shift-per-corner figure applies regardless of which physical perch you choose to turn.