Thread: Have I got my weight sums right?

Hardly worth the flack to allow only another 60kg in the car.

Originally Posted by GregMilner

Getting ready to head for the Kimberley, I weighed the car yesterday. Came in at 3,040kg, full of fuel (170-odd litres), partially loaded, including the bride and me.
The camper trailer nose weight is 220kg.
By my reckoning, with GVM of 3240kg, and 70kg of the trailer nose weight transferred to the car, I’ve got another 130kg of payload left before I’m overweight.
Am I correct?
Suggestions akin to strapping the bride to the top of the trailer may not be appreciated by the bride. (But yes, it would give me an extra 60kg of payload...)

did you weigh with the camper attached? if not.

basic version

3040 + 220 =3260.

more realistic version (and its approximate)... you need the axle weights but for the purpose of demonstrating I'll assume that 3040 is 50/50 so each axle has 1520 on it.

I'll say that the distance from the rear axle to the tow bar is 1/3 the distance of the front axle to the rear axle.

if you put 20Kg on the back of the towball you then have to add the same weight again (the balance mass) to the rear axle and take an amount off of the front axle at the rate of (distance to rear axle from towball)/(distance from towball to front axle)X weight on towball which gives us 1 unit / 4 units in this example 1/4 of 20 is 4..

so you go 1520 +20 (for the towball weight) then + 20 (for the balance mass) so the rear axle now has 1560 on it

now for the front axle you get to take some weight off....

1520 - (20 (towball weight)/4(reaction load number) = 1515.

It helps if you think of it as a see saw. The rear axle is the balance point and the towball is the fat kid on the short side and the front axle is the skinny kid on the long side.

Its a bit discombobulating at first when you look at the math for a Car but the very basic version works nearly enough and isnt as simple as take the same amount off of the front axle as you add to the rear because 1. Suspension. 2. the levers are different lengths 3. the mass distribution along the lever isnt simple.

when you weigh bridge the vehicle you're numbers will be slightly different but as a more accurate rough workout than simple Tare + towball method if you follow it and set yourself up so that your axle weghts are close to even (or the front axle is on limit and the rear which is usually a higher load limit is then near enough to the same) then youre in for a good towing experience. It also stops the typical cruiser towing a badly loaded van with the front wheels barely touching look.

Just wondering how a trailer nose weight of 220kg comes down to 70kg when transferred to the car? Where does the difference go? AFAIK ball weight gets transferred to the car. That is one of the reasons these 3.5T towing utes run into problems. They can tow as long as no one is in the ute!

Originally Posted by dirvine

Just wondering how a trailer nose weight of 220kg comes down to 70kg when transferred to the car? Where does the difference go? AFAIK ball weight gets transferred to the car. That is one of the reasons these 3.5T towing utes run into problems. They can tow as long as no one is in the ute!

In most cars that would be correct, but according the the Disco manual, as long as you don’t exceed max axle weight, you can load up to 150kg on the towball without affecting payload, anything above that is subtracted from payload, so by my reckoning, 220kg - 150kg = 70kg less payload in the car. That’s how I’ve read it in any case.

Having adjusted some figures on the spreadsheet, it looks like I'm right on the limit.
The bride is definitely getting strapped to the boat loader on the trailer. Disco weights.xlsxDisco weights.xlsx

See attached.

Originally Posted by Blknight.aus

did you weigh with the camper attached? if not.

basic version

3040 + 220 =3260.

more realistic version (and its approximate)... you need the axle weights but for the purpose of demonstrating I'll assume that 3040 is 50/50 so each axle has 1520 on it.

I'll say that the distance from the rear axle to the tow bar is 1/3 the distance of the front axle to the rear axle.

if you put 20Kg on the back of the towball you then have to add the same weight again (the balance mass) to the rear axle and take an amount off of the front axle at the rate of (distance to rear axle from towball)/(distance from towball to front axle)X weight on towball which gives us 1 unit / 4 units in this example 1/4 of 20 is 4..

so you go 1520 +20 (for the towball weight) then + 20 (for the balance mass) so the rear axle now has 1560 on it

now for the front axle you get to take some weight off....

1520 - (20 (towball weight)/4(reaction load number) = 1515.

It helps if you think of it as a see saw. The rear axle is the balance point and the towball is the fat kid on the short side and the front axle is the skinny kid on the long side.

Its a bit discombobulating at first when you look at the math for a Car but the very basic version works nearly enough and isnt as simple as take the same amount off of the front axle as you add to the rear because 1. Suspension. 2. the levers are different lengths 3. the mass distribution along the lever isnt simple.

when you weigh bridge the vehicle you're numbers will be slightly different but as a more accurate rough workout than simple Tare + towball method if you follow it and set yourself up so that your axle weghts are close to even (or the front axle is on limit and the rear which is usually a higher load limit is then near enough to the same) then youre in for a good towing experience. It also stops the typical cruiser towing a badly loaded van with the front wheels barely touching look.

Strewth Dave. Not sure my high school maths is up to that. But incredibly grateful you tried to explain it to me. And that you used a word like discombobulating. Good word, that.

Here’s what I believe is the calculation for how towball weight affects load on the rear axle.

Ball weight rear axle:
To calculate the actual ball weight affect on the rear axle; multiply the ball weight by the (vehicle wheelbase plus the rear overhang) and then divide the answer by the wheelbase.
e.g. Ford Ranger (sorry, the figures are for my old car) :-
w/b 3m overhang 1.4m
Total 4.4
Towball 170kg approx
170x4.4 =748/3=250kg approx

you could then subtract the 150kg that LR say can be added before affecting the payload.

Regardless of what the weight is it must still be included in the GVM.

Cheers

David

Originally Posted by GregMilner

In most cars that would be correct, but according the the Disco manual, as long as you don’t exceed max axle weight, you can load up to 150kg on the towball without affecting payload, anything above that is subtracted from payload, so by my reckoning, 220kg - 150kg = 70kg less payload in the car. That’s how I’ve read it in any case.

I really dont understand that. The car has a GVM. You cannot exceed that. And the car and the van have a GCM. and that cannot be exceeded. So the simple fact is add up the car weight, add on the van weight (in total) and then take that figure away from the car GCM. The diff is what you can load up extra in the car. It really is that simple in most cases.