Setup Lab
Triangulation
Use repeatable diagonal measurements to establish a square chassis reference before attempting spindle or steering alignment.
ILLUSTRATED MEASUREMENT
Diagonal equality depends on valid references.
comparison magnitude = |ΔD|
- Dᴸ, Dᴿ
- matched diagonal lengths in one unit
- ΔD
- signed difference; sign requires a documented side and hook direction
- uᵣ
- repeatability band from remove-reset-repeat trials, in the same unit
Illustrative generic example, not a target
Dᴸ = 61.125 in and Dᴿ = 61.000 in, so ΔD = +0.125 in. If complete fixture resets normally span 0.010 in, investigate the difference. It is not a universal tolerance or a Stock-car dimension.
What it proves: equal diagonals support square or centered geometry only under symmetric-reference and validated-centerline assumptions. Offset reference pairs or a false centerline can produce misleading equality.
PROCEDURE AND FAILURE MODES
Make the reading survive a complete reset.
- Verify the support plane and plan-defined reference points.
- Validate symmetry and centerline independently.
- Use one tool, contact height, hook direction, and controlled tension.
- Measure, remove and reset, then repeat each pair at least three times.
- Record raw results, unit, resolution, load state, operator, and steering or fastener state.
GARAGE WORKSHEET
The minimum record needed to act on this guide.
QUESTION THIS STATION ANSWERS
Do the two matched chassis-to-axle diagonals agree after the tool is completely removed and reset?
BRING
- Current division plan with the four permitted reference points marked
- Rigid level supports plus one trammel, tape, or string method used in one direction
- Fine reference marker, inspection mirror, and raw-reading sheet
LOCK BEFORE READING
- Named FL, FR, RL, and RR references
- Contact height, hook direction, pull or contact force
- Steering neutral, cable state, clamp state, and declared loaded or unloaded condition
WRITE THE RAW RECORD
- D left and D right for at least three complete remove-reset trials
- Unit, tool ID, resolution, operator, time, temperature, and load state
- Centerline validation method and every rejected reading with its reason
DECISION GATE
Compare |D left - D right| with the spread from full resets. If the signal does not clearly exceed that spread, improve the fixture or method. If it does, recheck reference symmetry and the support plane before any plan-permitted correction.
VERIFY NEXT
Check the axle reference plane, then loaded spindle direction. Repeat triangulation after final cable and kingpin state.
WHAT IT IS
Two diagonals test a defined reference rectangle.
Triangulation is the practice of measuring from matched front references to matched points on an axle. Equal diagonals support a square, centered axle only when the front and rear points are actually symmetric about a separately validated centerline. It is a chassis-reference operation, not a complete wheel alignment.
Start with reference points the official plan actually defines: kingpin centers, the manufactured floorboard centerline, axle square-stock ends, or legal centering marks. Pull every measurement with the same tool, tension, height, and contact point. A tape hooked differently on each side can create more error than the car has.
SEQUENCE
Build a trustworthy reference stack.
- Verify the floorboard and fixture. Warped wood or an uneven bench can move the target while you measure.
- Mark the centerline legally. Never drill, groove, reshape, or alter a controlled component just to create a reference.
- Square the rear axle first. It becomes the stable reference for the steering axle.
- Center the front axle in neutral steering. Set cable tension and bow-tie before final spindle readings.
- Repeat under race load. Axle flex and steering tension can move the spindles even when the unloaded square stock measured perfectly.
LIMITS
Triangulation does not prove the wheels are parallel.
Equal diagonals can coexist with bent or misaligned spindles, crossbind, unequal axle bow, a shifted steering cable, or corner-load imbalance. After triangulation, check crossbind on a level rail fixture, then measure spindle toe and camber under simulated wheel load. Finally, repeat the full stack after applying final kingpin torque.
NDR's published general car rules permit triangulation centering marks on eligible axles, but that does not grant permission for any other alteration. ISBD and NDR rules are separate authorities and can change independently.
SOURCES & FURTHER READING
Check the primary material.
- National Derby Rallies rules hub Official NDR landing page, 2025 tail-weight update, rule books, and inspection links.
- ISBD Masters build plans, July 2026 Current official build plan linked from the division hub. Use it directly; this site does not reproduce it.
- New Hampshire Soap Box Derby: Tips from the locals Licensed local-program notes on alignment, weights, tuning, and driver position.
- 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.