Trailer Axle Placement Calculator
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Placement Results
Enter dimensions and click Calculate.
Guide to Trailer Axle Placement
Correct axle placement is the single most critical factor in trailer design and fabrication. It determines how the trailer handles, how stable it is at highway speeds, and how much weight rests on the tow vehicle's hitch.
Why Tongue Weight Matters
The primary goal of calculating axle placement is to achieve the correct Tongue Weight (TW). Tongue weight is the downward force that the tongue of the trailer exerts on the hitch of the tow vehicle.
- Too Little Tongue Weight (< 10%): This is the leading cause of dangerous trailer sway (fishtailing). If the axle is too far forward, the trailer becomes unstable and can swing violently out of control.
- Too Much Tongue Weight (> 15%): This puts excessive strain on the tow vehicle's rear suspension, causing the front of the tow vehicle to lift. This reduces steering traction and braking effectiveness.
The Physics of Axle Placement
Think of a trailer as a lever. The axle acts as the fulcrum. The goal is to position the fulcrum so that the Center of Gravity (CG) of the loaded trailer sits slightly ahead of the axle.
[ HITCH ] ——– [ FRONT OF DECK ] —— [ AXLE ] — [ REAR ]
To achieve a target tongue weight of 10-15%, the axle typically needs to be placed behind the geometric center of the cargo area. The standard "rule of thumb" often cited is the 60/40 rule (placing the axle 60% of the way back), but precise calculation based on moment arms is safer.
Formula Explanation
This calculator uses the moment balance formula around the axle position. Assuming a uniformly distributed load (where the Center of Gravity is at the exact center of the deck), the formula determines how far back the axle must move to shift 10-15% of the total weight onto the hitch.
Note on Custom Builds: If you plan to mount heavy equipment (like a generator, toolbox, or water tank) permanently on the front or rear of the trailer, the Center of Gravity will shift. This calculator assumes a standard, evenly distributed load.
Double Axle vs. Single Axle
For tandem (double) axle trailers, the "Axle Position" calculated usually refers to the center point between the two axles (the equalizer pivot point). You would measure half the axle spacing forward and backward from this center point to locate the actual hubs.
function calculatePlacement() {
// 1. Get Inputs
var deckLen = parseFloat(document.getElementById("deckLength").value);
var tongueLen = parseFloat(document.getElementById("tongueLength").value);
var targetPerc = parseFloat(document.getElementById("targetTw").value);
var totalWeight = parseFloat(document.getElementById("loadWeight").value);
// 2. Validation
if (isNaN(deckLen) || deckLen <= 0 || isNaN(tongueLen) || tongueLen < 0 || isNaN(targetPerc) || targetPerc 0) {
var tw = totalWeight * percentDecimal;
var aw = totalWeight – tw;
tongueWeightTxt = Math.round(tw) + " lbs";
axleLoadTxt = Math.round(aw) + " lbs";
}
// 4. Output Generation
var html = `
Measure from Front Rail:
${axleDistFromFront.toFixed(2)}"
Measure from Rear Rail:
${axleDistFromRear.toFixed(2)}"
Placement Ratio:
${deckRatio.toFixed(1)}% Back
Est. Tongue Weight:
${tongueWeightTxt}
Est. Axle Load:
${axleLoadTxt}
*Measurement is to the center of the axle tube (single) or center equalizer (tandem). Assumes uniform load distribution.
`;
document.getElementById("results").innerHTML = html;
}