Determining Ballasted Tractor Weight, Weight Split, Axle Loads and Required Tire Inflation Pressures

Example 1

The Unballasted Weight guide Chart provides front axle, rear axle and total weights for all power train and tire size options. From this and information previously provided on weights of various ballast elements (suitcase weights and wheel weights), front ballast multipliers and front hitch axle loads (if used), the

ballasted tractor weight and axle loads can be calculated. These are needed to determine if total weight and weight split recommendations are met and to look up required tire inflation pressures from table provided in this section.

 
7920 MFWD with Front Suspension and Front Hitch - 200 Eng-HP
       
  Front Tires 540/65R34  
  Rear Tires 650/85R38  
  Front Ballast 900 kg (1984 lb) Block on Front Hitch
  Rear Ballast None  
       
From the Unballasted Weight Guide Charts on following pages, the front axle weight is 3652 kg (8051 lb) and the rear axle weight is 5383 kg (11867 lb). Note that these charts assume that a front weight support is mounted and do not include effect of a front hitch.
       
The NET changes from deducting the front support and adding the front hitch are to add 476 kg (1049 lb) to the front axle and subtract 76 kg (168 lb) from the rear axle.
       
The 900 kg (1984 lb) Block weight when attached to a Front Hitch adds 900 x 1.57 = 1413 kg (3115 lb) to the front axle and subtracts 900 x 0.57 = 513 kg (1131 lb) from the rear axle due to leveraging.
       
Combine all of these as shown
  Front Axle Rear Axle Total
Base Tractor 3652 kg (8051 lb) 5383 kg (11867 lb)  
Front Hitch +476 kg (1049 lb) -76 kg (168 lb)  
900 kg (1984 lb) Block on Front Hitch +1413 kg (3115 lb) -513 kg (1131 lb)  
Totals 5541 kg (12216 lb) 4794 kg (10569 lb) 10335 kg (22784 lb)

 

So the tractor is ballasted to the level of 10335 kg (22784 lb) / 200 Eng-HP = 51.7 kg (114 lb) per Eng-HP (Medium +)

The percentage of weight on the front axle is 5541 kg / 10335 kg (12216 / 22784 lb) x 100% = 53.6%.

From the Table of Recommended Inflation Pressures (Section 75) for calculated axle loads, the required inflation pressures are:

Front Inflation Pressure 1.5 bar (22 psi)

Rear Inflation Pressure 0.6 bar (9 psi) but only if tractor is towing an implement that places very little down load on drawbar (disk or field cultivator) .

For semi-integral plow or towed implements that generate high weight transfer add approximately 0.5 bar (7 psi) for a total of 1.1 bar (16 psi) for rear tires. With the aid of an assistant, visually check tire deflection when tractor is pulling hard in the field to confirm that tires are not under-inflated with these pressures.

For trailers, tankers, or any heavy integral implement the rear inflation pressures must be increased substantially to support the extra weight at transport speeds. The exact amount depends on the extra load. Usually it will be more than double the base amount or 1.3 - 1.5 bar (19 - 22 psi) for this example. To determine rear axle load exactly, weigh rear axle of loaded tractor on a platform scale.

OU1092A,00002A2 -19-23MAR05-1/2

Example 2

 
7820 MFWD with Standard Front Weight Support, No Front Hitch and No Front Suspension - 185 Eng-HP
       
  Front Tires 540/65R30  
  Rear Tires 650/65R42  
  Front Ballast 4 Suitcase Weights on Front Support
  Rear Ballast 1 pair - 205 kg (450 lb) Wheel Weights
       
From the Unballasted Weight Guide Charts, the front axle weight is 3192 kg (7037 lb) and the rear is 5283 kg (11647 lb).
       
The 4 suitcase weights (50 kg - 110 lb each) on the standard front support adds 4 x 50 x 1.5 = 300 kg (661 lb) to the front axle and subtracts 4 x 50 x 0.5 = 100 kg (220 lb) from the rear axle due to leveraging.
       
The pair of rear 205 kg (450 lb) wheel weights adds nothing to the front axle and 410 kg (900 lb) to the rear axle.
       
Combine all of these as shown
       
  Front Axle Rear Axle Total
Base Tractor 3192 kg (7037 lb) 5283 kg (11647 lb)  
4 Front Suitcase Weights + 300 kg (6618 lb) - 100 kg (220 lb)  
1 Pair 205 kg (450 lb) Wheel Weights   + 410 kg (900 lb)  
Totals 3492 kg (7699 lb) 5593 kg (12330 lb) 9085 kg (20029 lb)

 

So the tractor is ballasted to the level of 9085 (20029 lb) / 185 = 49 kg (108 lb) per Eng-HP (Medium).

The percentage of weight on the front axle is (3492 / 9085 kg - 7699 /20029 lb) x 100% = 38%

From the Table of Recommended Inflation Pressures (Section 75) for the calculated axle loads, the required inflation pressures are:

Front Inflation Pressure 0.8 bar (12 psi)

Rear Inflation Pressure 0.9 bar (13 psi) but only if the tractor is towing an implement that places very little load on the drawbar.

This is the ideal configuration for a 7820 MFWD tractor without front suspension when it is used with towed

tillage implements such as disks, chisels and field cultivators. The front weight percentage should be approximately 35% or less to help prevent Power Hop. Although this tractor is slightly over 35 %, it is close enough. Power Hop can still occur, but it can be controlled using the procedures given in this section of the manual. If the percentage is much higher, control may not be possible.

When this tractor is used to tow large trailers, the rear inflation pressure must be increased as outlined in Example 1.

If this tractor is used with heavy integral or semi-integral implements, additional front ballast and higher rear inflation pressures will be required.

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