Optimizing Tire Slip for Best Performance

Ag tires need to maintain the right slip percentage to optimize traction in the field. Tire experts from across the country have different insights on how to get it right for your tractors.

“Slip is important in optimizing the efficiency of your tractor and protecting your drivetrain from strain and damage,” says Blaine Cox, national product manager-agriculture for Yokohama Off-Highway Tires America. “Tires slip until they have accumulated enough resistance to push against so they can move your tractor forward and pull the load behind it, whether it’s a buggy full of grain or a field cultivator running through the soil.”

Equipment manufacturers rely on slip as a way to absorb some of the energy that would otherwise cause wear on transmissions and other drivetrain components, according to Cox. 

What not to do

Greg Gilland, vice president for global agriculture at Maxam Tires, offers guidelines on what to avoid in tire slippage. 

When there’s too much slip, there can be a loss of 25% to 30% of your transmitted power, which leads to more fuel consumed, increased tire wear, and greater engine component use. Too little slip means too much weight (wheel weights, suitcase weight, or liquid ballast) is being pulled which results in a loss of efficiency, increased fuel consumption, and increased soil compaction. A farmer would only want zero slip when roading a tractor.

Getting it right

Ballast and air pressure should be used together to achieve just enough traction to transmit power to the ground without excessive slip, says Dave Paulk, manager of field technical services for BKT USA.

Gilland says slip percentage also relies on tractor type:

• 2WD tractors need 10% up to 15% slip rate
• MFWD tractors need 8% up to 12% slip rate
• 4WD tractors need 8% up to 10% slip rate

If slippage is above the 15% mark, more weight may need to be added to the tractor and air pressures may need to be adjusted, says Paulk. Duals with reduced air pressures can help. If the old tires are heavily worn, new tires may be needed for the traction.

The peak tractive efficiencies occur between 8% and 15% slip, with more power lost to both rolling resistance and slippage in tilled soil versus no tilled soil.

Tires vs. tracks

Radial ag tires operating at less than 18 psi generally have less ground compaction than tracks when used for the same field application, making for a better value in comparison, according to Gilland. Radial tires between 18 and 30 psi have about the same ground contact pressure as tracks, making for an equal value. 

Self-propelled sprayers or large flotation/load applications operating above 35 psi, tracks will transmit less ground pressure due to their longer and wider contact footprint. Tracks allow lower engine slip rate — down to 3% — due to the size of the footprint, however the weight of the tracked tractor components per axle and transmission system will consume more fuel and have a greater maintenance cost than a tire equipped tractor for the same field application.

For a track, the sweet spot is between 2% to 4% slip, according to Ken Brodbeck, VP of technology at Precision Inflation. 

“As tires have significantly fewer moving parts versus a complete track system, they are very efficient at higher slip ratios,” says Brodbeck. “For most applications this translates into more power to the ground for less cost, all with reduced soil compaction.”

A typical 400 hp four track machine weighs 10,000 pounds heavier than its tire counterpart.  More weight translates to more drawbar pull.