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Field Power Hop

With the advent of the powerful family of four-wheel-drive and mechanical front-wheel-drive tractors, certain problems relating to excessive power hop have developed. One of the problems is known as "field power hop". Bridgestone Firestone has outlined a step-by-step solution to the problem.

Field power hop - on mechanical front wheel drive tractors

Complaint/symptom

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

Problem

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

  • dry to very dry soil or loose soil over hard pan
  • pull-type implements in heavy draft conditions
  • implement not adjusted properly
  • improper drawbar position.

Solution

To control power hop without changing implement depth or tractor ground speed, it is necessary to stiffen front tyres by increasing tyre inflation and/or adding liquid weight, and soften rear tyres by reducing inflation and/or removing liquid weight. Generally, the tractor must be equipped with duals or triples on the rear axle.

Use the following procedures:

  1. Check for proper ballast as recommended in the operator's manual; see ballasting recommendations for tractor weight split, total working weight and wheel slip.
  2. Check for proper adjustment of implement hitch and tractor drawbar position. (See appropriate manuals to adjust implement or drawbar position.)
    Important: Adjust inflation in front and rear tyres at the same time. Check tyre inflation, with the tyres cold, at least once a week.
  3. Adjust front and rear tyre inflation as follows:
    1.  increase front tyre inflation to 190 kPa for 2-star tyres and 230 kPa for 3-star tyres. Set the inflation pressure in all rear tyres to the minimum value needed to carry the axle load. See load and inflation tables in the Firestone Farm Tyre Data Book.
    2. if hop is still present, increase front tyre inflation to 205 kPa for 2-star tyres and 245 kPa for 3-star tyres. Leave rear tyre inflation as set in step 3.1.
  4. If hop is still present after following recommended tyre inflation guidelines, install liquid weight in front tyres to 75 percent fill (valve stem level). Inflate front tyres to 205 kPa for 2-star tyres and 245 kPa for 3-star tyres. Remove front cast weight. if necessary, to stay within ballast recommendation and/or tyre-carrying capacity. Leave rear tyre inflation as set in step 3.1.
  5. If hop is still present after all the above recommendations have been implemented, it is most likely due to liquid weight in the rear tyres. Select either step 5.1 or 5.2 below. Step 5.1 is preferred because it is more effective and does not place additional restrictions on the use of loaders or other front-mounted equipment.
    1.  if there is liquid weight in the rear tyres, reduce the liquid weight in all the rear tyres to 38 percent or less (four o'clock valve position). Inflate the rear tyres to the minimum value needed to carry the axle load. (Note: cast rear weights are preferred over liquid weight.)
    2.  increase liquid weight in the front tyres to 90 percent fill and inflate the front tyres to 205 kPa for 2-star tyres and 245 kPa for 3-star tyres. Leave rear tyre inflation as set in step 5.1.

Important

  • Never operate radial rear tractor tyres with inflation of less than 80 kPa for singles, or 40 kPa for duals/triples.
  • To operate a tractor with a loader, heavy spray tanks, front-mounted implements. or other affachments that could cause high front tyre loading, liquid front weight must be limited to 75 percent fill.
  • Tyre inflation pressures and ballasting may need to be changed when operating conditions change. For example, when a tractor is being used with hitch-mounted implements, rear tyre inflation will need to be increased from the levels recommended above for pull-type implements.

Field power hop - on mechanical four wheel drive tractors

Complaint/symptom

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

Problem

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

  • dry to very dry soil or 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 maintaining or enhancing tractive performance) the tyre 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 tyre may be changed in several ways, but removal of liquid ballast is most important. No tyre should have more than 38 percent fluid fill (valve level fill with valve at the four o'clock position) and the fill should be distributed evenly in all tyres on an axle. The front tyres should be dry; this aids in shifting the static weight split to the rear tyres.

The most effective method of attaining a low spring rate is to set the inflation pressure in the tyres on each axle to the minimum value that will support the load on that axle. 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. In some cases, a low spring rate can only be obtained by utilising larger-sized tyres with a lower spring rate or by going to triples.

The reward for controlling power hop is to have a tractor with dry tyres with a 51/49 static weight split and tyre inflation set to the lowest pressure that will carry the static axle load. Below is a step-by-step procedure whereby a tractor may be rendered free of power hop.

Important: Adjust inflation in front and rear tyres at the same time. Check tyre inflation (when tyres are cold) at least once per week.

  1. Set inflation pressure on all tyres (front and rear) to minimum value to carry the new static axle loads.
  2. Check for hop and performance. If not acceptable, continue to next step.
  3. Remove all liquid ballast from front tyres.
  4. Set inflation pressures on all tyres (front and rear) to minimum value to carry the new static loads. See load and inflation tables.
  5. Check for hop and performance. If not acceptable, continue to next step.
  6. Remove liquid ballast from all rear tyres or reduce to no more than 38 percent fill (valve level with valve at the four o'clock position).
  7. Set inflation pressure on all tyres (front and rear) to minimum value to carry the new static axle loads. See load and inflation tables.
  8. Check for hop and performance. If not acceptable, continue to next step.
  9. Move or add cast weight (cast wheels, cast wheel weights. etc.) to rear tractor so that the weight distribution on front is 51 to 55 percent and rear 45 to 49 percent.
  10. Set inflation pressure on all tyres (front and rear) to minimum value to carry the new static axle loads. See load and inflation tables.
  11. Check for hop and performance. lf not acceptable, continue to next step.
  12. Raise the inflation pressure in the front tyres 30 kPa to 40 kPa above the minimum value required to carry the static load. This works best in firm soils.
  13. Check for hop and performance. If not acceptable, continue to next step.
  14. Increase inflation pressures on rear tyres 30 kPa to 40 kPa above minimum value and set the front tyres to minimum inflation required to carry the load.
  15. Evaluate going to triples or larger tyres with lower spring rate.
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tyres in south africa
tyres in south africa