The Tow Ball Weight & Gross Weight


Can be used with:

Jockey Wheel Weigh Scale
Jockey Wheel Weigh Scale

The tow ball weight monitor consists of a load cell with 1,5 metres of cable attached. This cable plugs into a receptacle on a plastic box housing an A/D converter. The A/D converter housing is mounted on a wall at the front of the caravan, using the mounting tape provided, in a place where to will not be damaged. From the A/D converter housing there is a 4 metre cable with a 4-pin XLR  plug at its end. The 4-pin XLR plug is inserted into a socket marked 'Tow Ball'  on the control unit. When not in use the load cell may be unplugged from the A/D converter to be stored safely.

Sensor Calibration

Load cells resister strain. When they are subjected to a force some surface dimensions change minutely and this change alters their electrical resistance by a very small amount. This change in resistance is made to unbalance a resistor network known as a Wheatstone bridge by a tiny amount (never more than 3 parts per thousand at maximum capacity). The resulting differential voltage is massively amplified and converted to a digital value which can be quantified, stored, and compared to subsequent readings. After installation the load cell's no-load output must be registered in the controller as a zero datum. This is done at the time of manufacture and should never need to be repeated. However if the output on the screen seems hard to believe it may easily be accomplished in the field:  double tap the Gas button on the screen of the mobile phone when there is no weight on the load cell. The phone will buzz briefly and the weight display will go to 0 Kgs, if it is not already doing so. A message will be briefly displayed saying that the sensor has been zeroed. The updated weight info can take a few seconds to be shown. 

Measuring the Tow Ball Mass

Measuring the mass at the actual tow bar requires a device that is tall enough to reach the tow hitch under normal use conditions, as well as being sufficiently broad-based to be stable. Measuring the weight on the jockey wheel, on the other hand, could be accomplished with a far smaller sensor, and have the added advantage of being available for use while the tow bar was poised over the tow ball immediately prior to hooking on.

The jockey wheel, being nearer the Centre of Gravity of the trailer then the towball will always register a greater weight than the towball. To convert jockey wheel weight to accurate towball weight requires a computation that is done by the app using available data, partly supplied by the user and partly measured by the load cell. The data supplied by the user is the ratio of the towball to jockey wheel vs towball to axle. This ratio is found by dividing towbar to jockey wheel (typically about 400mm) by towbar to axle (typically about 4m).  Using these typical figures that would come out as 0,1 which, when multiplied by 100 would make 10%. That is the number that must be entered by moving the slide control in the relevant programming mode. The app does the rest.

In South Africa the total weight that the trailer tow bar imposes on the tow ball may not exceed 100 Kgs, and my not be less than 25 Kgs.  On the right the Tow Bar Weight button shows that the corrected tow bar weight is 106 Kgs (the background colour turns from GREEN to RED when the weight falls outside the 25 - 100 Kg limit). 

Towball Weight as a Percentage of Gross Trailer Weight

There is no universal consensus on this subject but many believe that stability during high speed towing (above 100Km/h) requires a towball weight of up to 10% of Gross Trailer Weight, but that lower towing speeds allow this to be reduced to as low as 5%. The best ratio of towball weight to GVM is up to the individual to determine. Once the Gross Trailer Weight is specified to the App the towball weight as a percentage of Gross Trailer Weight is accurately presented in the Towball Weight button.

The Jockey Wheel Radius vs the Tow Bar Radius
Enter Reduction Factor
Enter Reduction Factor

The jockey wheel weight is not the same as the tow ball weight. Because the jockey wheel is closer to the axle (and the centre of gravity), than the tow bar, it will feel more weight. There is a simple rule-of-thumb mental calculation to correct this discrepancy:

  1. Take the distance tow bar to jockey wheel (usually about 400mm)
  2. Divide it by the distance tow bar to axle (usually about 4 metres), the result is 0,1.
  3. Multiply this by 100 to get percentage: in this case 10%.

To enter this variance into the app we tap the Tow Bar button once, and then within 3 seconds double tap the Caravan Pilot Title. The slide control you see at left will appear, and you move the slider until the Reduce number above it reads 11,7% (Max is 20%).  To Save tap the Caravan Pilot Title once more. This value will be saved until you change it. 

To be precise the distance towbar to axle should actually have been towbar to centre of gravity but, since this is located just ahead of the axle the error is not significant, generally 1%, or less, of the towbar weight

Enter the Gross Weight of the Trailer

To enter the trailer's gross weight tap the CaravanPilot logo at screen bottom once and then within 3 seconds double tap the Caravan Pilot Title. This may be easier with the phone lying flat on its back. The slider seen at right will appear on the mobile's screen. Adjust the slider until the Gross Weight figure matches that of the trailer. 

To Save tap the Caravan Pilot Title once more.

To Weigh the Axle
To Weigh the Axle




Peeking the Parameters

Tapping the Gas 1, Gas 2, or Towball weight button once, will briefly reveal the Tare(s), GVM, and Correction Factor that have are stored for the button being tapped, in place of the normal button information display. This is a convenient method of quickly confirming that the correct values have been stored.

Weighing the Tow Ball at the Jockey Wheel

The load cell can be placed under the jockey wheel jack screw, or under the axle or even the wheel itself if nothing else is 

Enter Trailer Gross Weight

available. For best accuracy the point at which the jockey wheel assembly rests on the load cell should be the same distance back from the tow bar cup as was used in the calculation above. If measuring under the wheel or the axle these should be turned to reflect the same radial distance from the tow bar as your measurement.
Allow a few seconds for the messaging between the control unit and the phone to complete. If there is a significant deviation from the desired nose weight you can now enter the caravan, phone in hand, and relocate weight until you are satisfied. Remember to stand over the axle to eliminate your own weight from the equation.

Dual Use of the Control Button

The Tow Ball weight is measured only occasionally, which means the control button on the mobile screen can be used for another purpose much of the time. In this case the button doubles as a second LPG gas monitor and control button. When connected to a suitably equipped controller pressing the Tow Ball Weight button for 5 seconds will convert it to an LPG Gas 2 display and control button.

Pressing again for 5 seconds will return it to Tow Ball Weight service.

The Main Undercarriage Load Cell

Load cell 1000kg to 2000kg
Load cell 1000kg to 2000kg

There is a heavier duty load cell available that has a 1000kg measuring capacity. This unit can be quickly adapted to fit under the jockey wheel stem to measure the towbar weight, or fitted with a platform to support a main wheel. The software recognises the greater weight of the main wheel and none of the modifiers applicable to the towbar weight are invoked - the raw weight is shown on the mobile's screen. A variety of platforms can be placed on the single-point load bearing cap.

For super-heavy caravans there is a 2000kg load cell available. 

Interchangeable Load Cells

Load cells rely on the small changes that occur in surface resistance that come with compression and stretching of surface components. The resistances are arranged into a  network known as a Wheatstone bridge and the very small voltages that measurable across this bridge is highly amplified, digitised and converted to kilograms for display.

The actual voltage levels generated vary between load cell models and each type has its own arithmetic constants to make this conversion valid and accurate. The mobile phone app has access to these constants and allows the user to select from a list of popular load cell models. The model of load cell in use can be changed at any time, so the most suitable type can be chosen for the particular function to be fulfilled.

Selecting Load Cell
Selecting Load Cell





Tandem Axles

When the weight of a tandem axle is to be measured both of the wheels on each side are weighed in turn. The load cell unit must be made tall enough to lift its tandem partner entirely off the ground while the weight is being measured. The weights on each side are then added together and the total divided by 2 to arrive at an accurate weight for that side. These two divided totals (one for the left and one for the right) are then added together with the effective towbar weight to arrive at a gross vehicle weight.

The deck angle of a tandem axle caravan greatly influences the towbar weight. When the tow coupling is raised the weight on the towbar increases significantly, and it drops sharply when the nose is lowered below horizontal, When measuring the weight on the towbar of a tandem axle trailer the towbar should match the height as that when coupled to the tow ball.

Note: If the trailer is not supported by the towbar but is standing on the jockey wheel then the raw weight under the jockey wheel stem must be used in the
above calculation.


RF Interference

The Main Wheel Load Cell is has 4 metres of trailing cable to reach the A/D converter module. This cable, while screened, can still act as an antenna. There has been an instance of the control unit losing Bluetooth connection to the mobile phone when the trailing cable from the load cell was connected to the A/D module. 

The problem was traced to an intelligent battery charger which was connected nearby. These devices use a 400kHz oscillator to generate internal voltages and this can swamp the Bluetooth receiver with disabling radiation when within a metre or 2 of the trailing cable.