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Measuring tow ball weight

  • Alda Matavele
    Alda Matavele on

    Is it possible for one of these devices that control caravan functions from a cell phone to tell me what my tow ball weight is?

    Fyko van der Molen
    Fyko van der Molen on

    From V3.1.9 of the app, released November last year, we are able to monitor a load cell placed under the jockey wheel to measure its weight (which is very near the tow ball weight, although there’s a correction factor available for the really fussy) and report this on the mobile phone.

    The load cell (which has a maximum capacity of 150Kgs) is easily connected to a plug leading back to the control unit. You can leave the cell permanently connected and stow it in the front compartment or you can unplug it for possible use elsewhere. It takes just a few seconds to deploy it under the jockey wheel, and the same to stow it again.

    The jockey wheel weight is shown in the green oval on the right, the gas cylinder weight is still on the left.

    Simon Tasman
    Simon Tasman on

    This is fantastic! Just what I need and portable as well!

    How can I change my No2 gas weight into tow ball weight?

    Fyko van der Molen
    Fyko van der Molen on

    I’ll trade in the gas sensor on a jockey wheel weight sensor. Just send it back with the controller for upgrade. The jockey wheel sensor is quite a bit more expensive because it is a lot more robust with a max capacity of 150Kg, but the wiring remains the same.

    To summarise, what your unit can do is this:

    • Basic ewiks mover control
    • Inch Mode micro stepping the movers
    • Battery voltage monitoring and display.
    • Dual water flow sensors and tank reserve display.
    • Gas bottle weight measure with colour-coded warnings
    • Jockey wheel weight measure with colour-coded warnings
    • Dual temperature probes.
    • Display 2 axis levelling info.

    This is what it cannot do:

    • Current ramping (SoftStart)
    • Current measuring & limiting
    • Partial mover power supply
    • Combo turns (for tandem axles)
    • Control levelling motors
    • Gas alarm detection
    • Intruder alarm detection
    • Alarm annunciation by SMS

    Let me know what you want to do.

    Deon Jacobs
    Deon Jacobs on

    The jockey wheel is always located closer to the axle than the tow ball.

    Hence the jockey wheel will always carry more weight than the tow bar when it rests on the tow ball.

    Fyko van der Molen
    Fyko van der Molen on

    Hi Deon,

    You’re quite right. You will read a greater weight at the jockey wheel than is actually felt at the tow bar.

    There are several tow ball weighing devices on the market. But they are all impractical because to be stable they need to be too large to be portable. What we tried to do was produce something that is actually useful. Which means it needs to be quick and easy to use at home as well as at camp. And for that a device that goes below the jockey wheel is ideal because it only needs to be tall enough to reach the bottom of the jockey wheel spindle, and not all the way up to the tow bar.

    The actual distance from axle to jockey wheel to tow bar varies from one caravan to the other, but generally it’s about 10%. It’s easy enough to measure – pace out the distance from axle to tow bar and measure tow bar to jockey wheel. Five metres to 500mm is about average, but measure it yourself.

    When you place our load cell under the jockey wheel spindle you will now over-read by that amount – 10% or whatever applies to your caravan. You can now either reduce the weight reading mentally by that percentage, or you can invoke the calibrator that is built  into the app to do it for you. The calibrator can reduce the weight reading from 0,1% to 20%, with 10% being mid-scale, as shown below. The calibration setting is remembered by the app for future use.

    Simon Tasman
    Simon Tasman on

    Could you post some pics of this weighing device? I don’t see any photos on your website.

    Many thanks.

    Fyko van der Molen
    Fyko van der Molen on

    This is the nose weight unit:

    It’s 150mm long and those are M8 nuts you see there. The weight is a shade over 1Kg.

    You see there are no moving parts, and hence no adjustments or maintenance needed, or even possible. The unit works by translating the bending moment applied to the beam (by weight on the stirrup) to an electrical signal which is converted to weight back in the control unit (with a better than 1% accuracy).

    The stirrup can be placed under the end of the jockey’s screw jack, or under the jockey wheel itself if the screw jack end is not available.

    You can actually make this yourself – you need 300mm of 40 x 8 flat bar, 8mm round for the stirrup and some M8 hardware to screw it together.

    This forum’s php code only allows 1 picture per post.


    Josef Konrad
    Josef Konrad on

    I’d like to know how to properly work out the difference between the weight at the jockey wheel and the weight at the tow ball.

    Fyko van der Molen
    Fyko van der Molen on

    Hi Josef,

    Imagine the distance from axle to tow ball is 5 metres.

    Imagine the weight on the tow ball is 100 Kgs.

    As seen from the axle the ‘arm’ from axle to tow bar is under a rotational force, or torque, of 500 Kg/metres. (100 kgs x 5m)

    Since it is usual to think in terms of Newton/metres when it comes to torque we convert the number of Kilograms to Newtons by multiplying by 9,8.

    So now we multiply 100 (kgs) by 9,8 (Newtons per Kg) by 5 (the length of the arm)

    100 x 9,8 = 980, 980 x 5 = 4900 Nm.

    That torque figure is immutable along the entire length of the arm – the component factors will vary but their product will always be constant.

    If you now imagine the jockey wheel to be located half a metre from the tow ball then the arm length from axle to jockey is 4,5 metres. Or 10% less than 5 metres.

    If we now divide the length of the arm into the torque on the arm we get:

    4900 / 4,5 = 1088,8 Newtons.

    Convert the Newtons back to Kilograms:

    1088,8 / 9,8 = 111,1 Kgs.

    Now 10% of 111,1 Kgs is 11,1 Kgs,

    And if we subtract this 10% from 111,1 Kgs we get back to 100 Kgs.

    So the rule is if you shorten the arm that is under rotational force you will measure a higher greater weight, but you can reduce that weight by the same percentage that the arm was shortened by to calculate the weight at the original arm’s length.

    If the jockey wheel was located 1 metre inboard of the 5 metre total length, or 20% away from the tow ball, this figures would look like this:

    4900 Nm / 4 Metres = 1225 Newtons,

    1225 Newtons / 9,8 = 125 Kgs

    125 Kgs – 20% = 100 Kgs.

    So as long as you can figure out by what percentage the jockey wheel arm is shorter than the tow ball arm, the app will correct the weight measurement for you with total accuracy.

    You do this by dividing the jockey wheel arm (eg 4,5 metres), by the tow ball arm (eg 5 metres), and multiplying the result by 100. This result you subtract from 100.

    4,5 / 5 = 0,9,

    0,9 x 100 = 90%,

    100% – 90% = 10%, which is how much the arm is shorter by.

    Josef Konrad
    Josef Konrad on

    Thank you for that. Very nicely explained, I understand perfectly.

    I have one of these spring-loaded dildos that I battled to get hold of and turned out to be useless when I eventually did.

    It has no accuracy whatsoever and the footprint is so small it will fall over and drop the caravan if the brakes are not pulled up hard on both wheels.


    Dan Perkins
    Dan Perkins on

    Can I upgrade my existing 4 year-old remote control to implement one of these units?

    What do I need to do?

    Deon Jacobs
    Deon Jacobs on

    How does that load cell work if there are no moving parts?

    Fyko van der Molen
    Fyko van der Molen on

    You can see in the photo that there is a 3-ring cavity right through the beam from one side to the other.

    If a downward force is applied to the stirrup when the beam is supported as it is in the photo, then the material above the cavity will be in tension, and the material below it under compression.

    The minute amount of stretch in the upper layer will make it slightly less conductive than the lower layer where the molecules are pressed slightly together due to the compression.

    This change of relative conductivity between the upper and lower parts of the beam is greatly amplified electronically and then converted to a 24-bit binary number.

    Downstream hardware then converts this 24-bit field into Kilograms, and this value is eventually displayed on your mobile phone.

    The maximum weight capacity of this particular beam is 150Kgs, chosen so it can measure a 7% tow bar weight ratio on any caravan with a gross weight of up to 2130Kgs.

    Fyko van der Molen
    Fyko van der Molen on

    Can I upgrade my existing 4 year-old remote control to implement one of these units?

    What do I need to do?

    You need to get the controller back to me for upgrade. New firmware and a 4-pin XLR plug fitted. No charge other than the transport and the plug.

    You then need everything pictured:

    The 150Kg load cell. Can source this yourself if you can find one.

    The white box that contains the A/D converter. It mounts in the front compartment, has a 3 metre cable (the white one) to the control unit. It has a 4-pin XLR plug input from the load cell via the black cable. This makes it easy to detach and store the load cell in a safe place, but it increases the cost by a 4-pin plug/socket combination. Worth the money I say.

    The steel work you should definitely do yourself to save some money.

    The rest may be a bit of a challenge since there are shortages of everything coming out of China just now. I can supply some of the time but not all of the time.

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