Technical Bulletins

FIELD POWER HOP ON FOUR WHEEL DRIVE TRACTORS

F-052-X

Date: MARCH, 1994

NOTE: This bulletin replaces F-035-X dated December 1991.

COMPLAINT OR SYMPTOM:

Tractor jumps or bounces during field operation and creates a very uncomfortable ride and loss of traction. Operator may need to shift to a lower gear, reduce throttle or raise implement to continue operation.

PROBLEM:

Under certain soil conditions or operating conditions, tractor may experience "power hop." Some of those conditions are:

  • Dry to very dry soil; loose soil over hard pan
  • Pull-type implements in heavy draft conditions
  • Implement not adjusted properly
  • Improper drawbar position

SOLUTION:

Generally, the tractor must be equipped with duals or triples on both axles. To control "power hop" with 4WD tractors pulling towed implements, while maintaing or enhancing tractive performance, the tire spring rate must be softened and the static tractor weight split set as close to 51/49 (front/rear) as possible. The spring rate of the tire may be changed in several ways, but most important is the removal of liquid ballast. No tire should have more than 38 percent fluid fill (valve level fill with valve at 4 o'clock position) and the fill should be distributed evenly in all tires on an axle. The front tires should be dry – this aids in shifting the static weight split to the rear tires.

The main tool in attaining a low spring rate is setting the inflation pressure in the tires on each axle to the minimum value which will support the load on that axle. Table 1 provides the pressure and load data needed to set the inflation pressures properly. An accurate axle weight must be obtained either from weighing each axle on a set of scales or based on data tables from your tractor dealer. For some cases, a low spring rate can only be obtained by utilizing larger sized tires with lower spring rate or going to triples.

The goals in controlling hop are to have a tractor with dry tires with 51/49 static axle load. The following is a step by step procedure to develop a "power hop free" tractor.

 

IMPORTANT: Adjust inflation in front and rear tires at same time. Check tire inflation, when cold, at least once per week.

Set inflation pressure on all tires (front and rear) to minimum value to carry the new static axle loads (see Tables).

Check for hop and performance, if not adequate then continue to next step.

Remove all liquid ballast from front tires.

Set inflation pressures on all tires (front and rear) to minimum value to carry the new static axle loads (see Tables).

Check for hop and performance. If not adequate, then continue to next step.

Remove liquid ballast from all rear tires or reduce to no more than 38 percent fill (valve level with valve at 4 o'clock position).

Set inflation pressure on all tires (front and rear) to minimum value to carry the new static axle loads (see Tables).

Check for hop and performance, if not adequate, then continue to next step.

Move or add cast weight to rear of tractor (cast wheels, cast wheel weights, etc.) so that the weight distribution on front is 51% to 55% and rear is 45% to 49%.

Set inflation pressures on all tires (front and rear) to minimum value to carry the new static axle loads (see Tables).

Check for hop and performance. If not adequate, then continue to next step.

Raise the inflation pressure in the front tires 4 to 6 psi above the minimum value required to carry the static load (this works best on firm soils).

Check for hop and performance. If not adequate then continue to next step.

Increase inflation pressures on rear tires 4 to 6 psi above minimum value and set the front tires to minimum inflation required to carry the load.

Evaluate going to triples or larger size tires with lower spring rate.

Contact Firestone Sales Engineering – 1-800-847-3364.

IMPORTANT: Tire inflation pressures and ballasting may need to be changed to match changes in operating conditions. For example, hitch mounted implements place more weight on the rear axle than pull-type implements and require the inflation pressures to be changed accordingly. It may also be necessary to increase front weight split when using hitch mounted implements such as rippers which produce very high draft loads. Consult your tractor manufacturer.

HOW DOES THE OPERATOR SET THE TIRES TO THE PROPER INFLATION?

Determine the axle load for your tractor.

A grain elevator scale is best.

If and elevator scale is not available, call you tractor dealer for the tractor's front and rear axle loads.

Include any cast or liquid weights, tanks, loaders and the implement hitch weight.* 
 

Divide the axle load by the number of tires on that axle. 
 

Turn to the single, dual, or triple load table for your tire size. Begin at the left side of the table and move right until you either find the load for your tire or the next higher load. Move up the column to find the correct tire pressure. 
 

Set all the tires on that axle to that pressure.** 
 

Follow steps 1 through 4 for the second axle.

*When using heavy 3-point hitch mounted equipment, adjust rear inflation pressure based on the weight being carried on the rear axle with the implement raised. Return to the lower rear inflation pressure when using towed implements. NOTE: For towed implements which transfer heavy vertical loads to the drawbar (i.e. land scrapers, grain carts), weigh tractor with maximum axle load and adjust inflation accordingly.

**It is very important to use an accurate tire gauge. Checking with a gauge that is off by 2 psi can lead to serious tire durability problems when setting tires at those lower pressures. Tire pressure fluctuates with air temperature and should always be rechecked on a weekly basis.