When it comes to four-wheel-drive (4WD) and mechanical front-wheel-drive (MFWD) tractors, front axle lead is one of the most important — yet often misunderstood — factors that affects performance, efficient steering, and tire life. Properly matching front and rear wheels ensures smooth power transfer, reduces tire wear, and prevents drivetrain strain. Let’s break down what front axle lead means and how to measure it correctly.
What is Front Axle Lead?
In a tractor with MFWD, both the front and rear axles are powered. However, the front axle is usually designed to rotate slightly faster than the rear axle. This difference is called front axle lead.
- Definition: Front axle lead is the percentage by which the front wheels travel farther than the rear wheels over the same distance.
- Why it matters:
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- If the lead is too low (or negative, called lag), the front wheels drag, causing extra fuel consumption, poor steering, and higher wear on rear tires.
- If the lead is 0%, there is no tensions in the drive train causing premature wear
- If the lead is too high, the front tires scrub against the ground, leading to premature tyre wear and stress on the drivetrain. This will result in higher fuel consumption and potentially expensive repair bills
- On many tractors, the braking system relies on the 4wd lead being correct
The ideal front axle lead usually falls within 1–5%, depending on the manufacturer’s design.
How to Calculate Front Axle Lead
Front axle lead can be calculated using tire rolling circumferences and the tractor’s gear ratio. The formula is:
- Front Axle Lead (%) = [((Front Tire Rolling Circumference × Inter Axle Ratio)÷ Rear Rolling circumference) – 1] × 100
Step-by-Step Method to Measure
- Check the gear ratio:
- Find the tractor’s front-to-rear axle gear ratio. This can usually be obtained from the manufacturer’s manual, or a wheel specialist such as BWT will be able to advise
- Measure Inter axle ratio:
- With the tractor in 4WD, measure 10 complete turns of the rear wheels and record the number of revolutions of the front wheels. Inter axle ratio = Front number of turns ÷ rear number of turns
- Measure difference in rolling circumference:
- With the tractor out of 4WD, measure 10 complete turns of the rear wheels and record the number of revolutions of the front wheels. Difference = Front number of turns ÷ rear number of turns
- Apply the formula:
- Work out the front axle lead using the formula Front axle lead% = [(Interaxle Ratio ÷ difference in rolling circumference) -1] x 100
- Evaluate the result:
- 1–5% → Acceptable lead.
- <1% or negative → Possible lag, risk of dragging rear axle.
- 5% → Too much lead, front tire scrubbing, premature wear.
Why This Matters for Farmers
- Fuel efficiency: Proper lead reduces unnecessary power loss.
- Tire longevity: Balanced rolling reduces scrubbing and wear.
- Drivetrain protection: Avoids stress on gears and shafts.
- Better traction: Correct lead improves pulling power in the field.
- Better steering: Correct lead will help pull a tractor round a corner
Tip: Always use the manufacturer’s recommended tire combinations for MFWD tractors. If changing tire sizes or brands, recheck front axle lead to avoid costly mistakes.
