Setup Lab
Fore-aft center of gravity
Use axle loads and reaction-plane spacing to locate the car-driver system's fore-aft center of gravity without implying that level scales also measure its height.
ILLUSTRATED FREE-BODY MODEL
Axle reactions locate fore-aft CG.
moment about front axle: R·L = W·x
x = R·L / W
gravitational change: ΔE = m·g·ΔzCG
- F, R, W
- front, rear, and total static loads in one force unit
- L, x
- wheelbase and fore-aft distance in one length unit
- m, g, ΔzCG
- mass, gravitational acceleration, and actual vertical CG drop
Illustrative generic example, not a target
With R = 102.5 lb, generic L = 84 in, and W = 200 lb, x = 102.5 × 84 / 200 = 43.05 in behind the front axle. The force unit cancels in the ratio.
A higher CG does not automatically add usable energy. ΔzCG depends on start and finish orientation, pitch, path, and driver pose. Height can also change stability, frontal area, and control.
ASSUMPTIONS AND CHECKS
Static math is only as good as the load state.
- Measure wheelbase between the reaction planes used by the model.
- Level, zero, and repeat axle loads with full race posture.
- Verify W equals F + R within measurement resolution.
- Keep raw readings and assumptions beside x.
- Predict the direction before a permitted mass or posture change, then remeasure.
GARAGE WORKSHEET
The minimum record needed to act on this guide.
QUESTION THIS STATION ANSWERS
Where is the combined car-driver fore-aft center of gravity between the two axle reaction planes?
BRING
- Coplanar front/rear scales or four pads
- Tape or trammel for reaction-plane spacing L
- Driver-position landmarks, wheel chocks or safe restraint, and a calculation sheet
LOCK BEFORE READING
- Level car, full race state, steering neutral, and no outside contact
- Front and rear reaction planes named consistently with the wheelbase measurement
- Driver pose and secured ballast unchanged through repeated weighs
WRITE THE RAW RECORD
- Front F, rear R, total W, reaction spacing L, and x = R L / W
- Raw repeated loads, driver landmarks, wheel IDs, and ballast map
- Predicted and observed x change after one permitted known mass move
DECISION GATE
Reject a result when F + R does not agree with W within the demonstrated scale process. This level-scale method locates fore-aft CG only; it does not measure CG height.
VERIFY NEXT
Check final legal weight distribution and stability, then test one controlled mass-position hypothesis.
THE SYSTEM POINT
Center of gravity belongs to the car and driver together.
The combined center of gravity is the balance point of every mass in the moving system: floorboard, shell, wheels, hardware, ballast, helmet, and driver. Moving the driver a small distance can matter more than moving a small weight plate.
With the car level and the axle loads known, the fore-aft location measured from the front axle can be estimated from static equilibrium:
This gives a horizontal position. Measuring CG height requires a controlled tilt method and careful geometry; for most teams, consistent axle loads and driver position are the more practical setup tools.
SPEED AND STABILITY
Farther rearward or higher is not automatically faster.
Rearward CG increases rear axle load and changes the tail differential. A higher CG does not automatically add usable energy: the potential-energy change depends on the actual vertical drop of the combined CG between the start and finish, including vehicle orientation and path. It can also increase sensitivity to pitch, cross-slope, steering, and disturbances. A low, compact driver position usually helps frontal area and stability, while a legal rearward distribution may help a particular ramp or car.
The correct target is the fastest repeatable point inside the organization's distribution limits, with positive steering and a driver who can hold the required position. Do not trade safety or inspection margin for a theoretical gain.
TRACK-TEST METHOD
Move one known mass by one known distance.
- Record total, front, rear, and four-corner loads.
- Mark the driver's repeatable race position.
- Move only a legal adjustable weight, keeping all fasteners and positions compliant.
- Reweigh and calculate the new CG position.
- Run multiple paired tests with lane and wheel effects controlled.
- Stop if steering, braking, or driver confidence worsens.
Loose ballast is never a tuning method. Official rules require weight to be secured in the prescribed manner and prohibit movable weight.
SOURCES & FURTHER READING
Check the primary material.
- ISBD Rule Book - linked for the 2026 World Championship Official PDF currently linked from Race Week resources; the file itself says revised June 2025.
- ISBD: Adding weights to your car Official basic examples for Stock and Super Stock combined weight.
- National Derby Rallies rules hub Official NDR landing page, 2025 tail-weight update, rule books, and inspection links.
- OpenStax University Physics: Conditions for static equilibrium University-level reference for force and moment balance used in axle-load and center-of-gravity calculations.
- NIST Engineering Statistics Handbook: Uncertainties of calibrated values NIST guidance on uncertainty in future results corrected by a calibration curve, including calibration-curve and future-measurement contributions. It does not characterize the whole Derby measurement process by itself.