The Tow Ball Weight & Gross Weight


Load Cell
Load Cell

Can be used with:

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.

First Use

On first use the load cell needs to be calibrated to zero even if it already reads '0 Kgs'. To do this connect the load 

Tow Ball Weight Display
Tow Ball Weight Display

cell to the controller as normal and double tap the Tow Bar button on the mobile's screen. The phone will vibrate briefly and the weight will zero if it was not that already.

Measuring the Tow Bar Weight

Measuring the weight 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.

But there would be a measuring error since the jockey wheel is always closer to the trailer's axle than the tow ball and, since the trailer's weight pivots around the axle, the weight borne by  the jockey wheel is greater than would be felt at the tow-ball itself. For a mobile phone app it is a simple matter to correct for this error as long as the app knows the relative positions of axle, tow ball, and jockey wheel. We therefore need to enter this value into the app so that the true tow-ball weight may be derived.

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

To find the correction factor to be applied to the jockey wheel position we measure the distance from axle to the centre of the tow bar cup (a) as well as distance from middle of the jockey wheel jack screw to centre of the tow bar cup (b). 
We then divide (b) by (a) and multiply the result by 100
So, if tow bar centre to axle is 4000mm (4 metres) and tow bar centre to the jockey wheel jack screw is 470mm 

Jockey Wheel Position Compensation
Jockey Wheel Position Compensation

then 470/4000 = 0,117 and multiplied by 100 = 11,7%
To enter this 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. 

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.

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

There is a heavier duty load cell available that has a 2000kg 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.

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.



Load cell assembly for measuring tow ball weight only                              R500

Load cell assembly with 2000kg capacity to weigh entire vehicle               R3500
Shipping is usually together with a control unit, which is free.