Rear Axle Load Caused by Ball Weight

Submitted: Thursday, Apr 21, 2016 at 18:10
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This has been a topic of discussion recently in our club. I decided to have a go at working it out. I think I have it right. Many of you will have already figured it for yourselves, but here's my method. It's all about moments around the rear axle. Diagram below. Corrections welcome.

You need to know
a) vehicle wheelbase, call it W
b) overhang from rear axle to hitch, call it O
c) ball weight, call it B

First you need to know the downward moment at the rear, = O x B. Call that D

There will be an equal upward moment (loss of weight) at the front axle, which equals the O x B divided by the wheelbase, = O x B / W. Call that U

The lost weight at the front axle has to go somewhere - it goes to the rear axle. So the extra weight on the rear axle is ballweight, B, plus the lost weight at the front, = B plus U.

Example from my BT50 dual cab (measurements approximate):

Wheelbase, W is 3.3 metre
Overhang, O is 1.5 metre
Ballweight, B is 160 kg

Downward moment at rear, D, = 160 x 1.5, = 240.
Loss of weight at front, U, is D/W, = 240/3.3, = 72

Weight on rear axle attributable to 160kg ballweight is B plus U, = 160 plus 72, = 232kg.



It might be interesting to work out your own figures.

Cheers




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Reply By: Member - Rod N (QLD) - Thursday, Apr 21, 2016 at 18:41

Thursday, Apr 21, 2016 at 18:41
Have you verified your calculations on a weighbridge?
What happens when you have a load in the vehicle?
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Follow Up By: Frank P (NSW) - Thursday, Apr 21, 2016 at 19:24

Thursday, Apr 21, 2016 at 19:24
No, Rod, I only worked it out today.

This doesn't change as you load the vehicle - just adds to it.

Loads behind the rear axle will work the same way, ie add more than their own weight to the rear axle.

Loads in front of ghe rear axle will share their weight between front and rear axles.

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Reply By: Notso - Thursday, Apr 21, 2016 at 19:17

Thursday, Apr 21, 2016 at 19:17
Great explanation mate! Wish I had all that a few months back. I tried to convince a mate that his 5th wheeler was putting too much weight on his rear axle as the mount point was 30cm behind his rear axle!
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Reply By: Whirlwinder - Thursday, Apr 21, 2016 at 20:46

Thursday, Apr 21, 2016 at 20:46
I agree with your calculations as I did the same exercise with a Ford Ranger about 2 years ago. I put it over the weighbridge to prove to the boss that the Ranger was dangerous.
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Follow Up By: Frank P (NSW) - Thursday, Apr 21, 2016 at 20:54

Thursday, Apr 21, 2016 at 20:54
I believe danger exists only if you don't know what's happening or if you do, ignore it.

How was the Ranger dangerous in your boss's application?

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Follow Up By: Whirlwinder - Thursday, Apr 21, 2016 at 20:59

Thursday, Apr 21, 2016 at 20:59
It was very overweight at the rear and about 230 Kgs light at the front and the chassis flexed like it was going to bend any minute. We have all seen the result of that set up.
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Follow Up By: Frank P (NSW) - Thursday, Apr 21, 2016 at 21:07

Thursday, Apr 21, 2016 at 21:07
72 kg is one thing.
230 is quite another!
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Follow Up By: Member - Redbakk (WA) - Friday, Apr 22, 2016 at 18:57

Friday, Apr 22, 2016 at 18:57
Just 2 questions,

1. If you are following the manufacturer recommendations for example the tow ball weight max is 350 KG and you keep to that then what is the problem?
2. The above example but with a weight distribution hitch added and adjusted correctly.

I know that the GVM cannot be exceeded but as far as engineering is concerned surely following the vehicle recommendations shouldn't be a problem.
That is the towing equipment is compliant.
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Follow Up By: Member - Andrew & Jen - Saturday, Apr 23, 2016 at 13:34

Saturday, Apr 23, 2016 at 13:34
Redbakk
While the GVM might be OK, the rear axle might well be over the design limit.
Take a typical modern ute towing a caravan with a 350kg ball weight: because the hitch is ~1.5m back from the rear axle, this results in a 525kg load on the rear axle. Add the distributed load from 2 people - probably 100kg - and then a bit of gear, maybe a fridge, in the tray to take it up to GVM and it would be easy to be over the rear axle design load.
While a WDH may well help in some cases, some modern vehicles with anti-sway technology actually advise against fitting a WDH.
Cheers
Andrew
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Follow Up By: Frank P (NSW) - Saturday, Apr 23, 2016 at 14:57

Saturday, Apr 23, 2016 at 14:57
Exactly, Andrew.

And some drawbars on some vans cannot accommodate a WDH, mine included.

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Frank
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Follow Up By: The Bantam - Sunday, Apr 24, 2016 at 21:49

Sunday, Apr 24, 2016 at 21:49
One of the BIG problems in towing circles is the over simplistic view many have of towing capacities .
There is a lot more to the vehicles specified limitations than towing capacity and permitted ball weight.
The result is a great many vehicle combinations on the road operating outside of manufacturers specifcations, while the operators believe they are all good.


On the matter of weight distributing hitches.
While the WD hitch may redistribute weight, it does not reduce the weight on the ball or the stress on the coupling and the chassis that carries it. ..... in fact a WD hitch considerably increases the stress on the coupling, the tow bar and its attachments.
Many of the new vehicles either advise against WD hitches or explicity forbid them.

The specification may be rear axle loading ....... but what is that specificaton seeking to limit .......
The tyres, wheels, axle assembly and springs may well be able to carry a great deal more than the specified rear axle load limit ....... but the failures we are seeing, indicate that the chassis and its various attachments are the weak points.

So redistributing the load may produce a result within specifcations, but still be ill advised.

cheers
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Reply By: 671 - Thursday, Apr 21, 2016 at 21:12

Thursday, Apr 21, 2016 at 21:12
Frank

Your figures look about right. I noticed Land Rover's specifications on their web site for the Defender say 150kg on the ball puts 206 kg onto the axle.

That is just static weight though. A simple thing like dropping the rear wheels down off a speed hump in a car park would put a lot more than that onto the axle.

This would be one of the reasons why LR say the Defender's 3500kg towing capacity comes down to a maximum of 1500kg off road.

That is not on the specification sheet but email them and they will tell you.
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Follow Up By: Frank P (NSW) - Thursday, Apr 21, 2016 at 22:29

Thursday, Apr 21, 2016 at 22:29
Thanks 671. Schoolboy physics from 50odd years ago dragged out of dim dark recesses of memory. The brothers would be pleased that some of what they taught actually stuck :-)

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Follow Up By: Ron N - Thursday, Apr 21, 2016 at 22:50

Thursday, Apr 21, 2016 at 22:50
671 is right on the mark. Aeons ago, when Bedford trucks ruled the road, GMH went to great pains in the operator manual to explain the dangers of overloading your Bedford truck.

In essence, they pointed out that any overload translates to double the overload weight, when you shock-load the chassis and axle via hitting a pothole, a sunken culvert, an out-of-level bridge approach, and the 100 other uneven road conditions that your suspension and chassis copes with daily.

So if you overload an axle by 100kgs, that translates to a 200kg overload when any of the above are encountered. Speed exacerbates the impact.

I can recall a highly embarrassing episode for GMH when the HR Holden and the Vauxhall Viva were new, in 1967.
Lap-sash seatbelts had only recently been fitted as standard to the GMH range, and GMH were keen to get a lot of media exposure about the superb strength of their seatbelts.

To this end, they invited a media pack and all their photographers to a stunning demonstration, where they were going to lift cars by their seatbelts!

They rolled out a new HR holden and slipped the seatbelt tongues out the open front windows - the crane driver hooked onto the seatbelt tongues - and he steadily lifted the HR Holden sedan 4 feet (1.2M) off the ground!!

The media pack oohed-and-ahhhed and the photographers clicked away merrily.

Then came the piece-de-resistance! The crane driver hoisted the HR Holden up about 6 feet (1.8M) and the GMH boys rolled a brand-new Vauxhall Viva under the HR, slipped its seatbelt tongues out the open windows, and hooked them to the underside of the HR!

The crane driver then hoisted both the HR Holden AND the Vauxhall Viva a good 8 feet (2.5M) off the ground - and the media went wild!
Photographers clicked away, while the GMH boys explained how the seatbelt tongues were designed to hold a load of 4480 lbs (2033kgs) for at least 30 seconds.
The HR weighed 1180 kgs and the Vauxhall Viva weighed 700 kgs (approx.), for a combined weight of 1880 kgs.

After about 10 mins of this, the crane driver got pretty bored, and he was moving around inside the crane cabin, gawking at something else.
In doing so, he accidentally bumped the lift/lower control lever with an elbow - just a fleeting bump.

The cars dropped about 2 inches (50mm) at most, with the lever bump - but almost instantly, the HR Holdens seatbelt tongues snapped! - and both cars crashed to the ground!!

Needless to say, the GMH blokes were appalled - and the media pack took a whole lot MORE pictures!!

There was a very hurried examination and discussion by all the GMH heads and engineering chiefs - and they promptly re-assembled the media pack to explain exactly what had gone wrong.

The chief engineer of GMH explained how the 2" (50mm) sudden load drop by the crane - which seemed like virtually nothing to the onlookers - had actually nearly DOUBLED the load on the seatbelt tongues!

This seemingly-slight impact from the drop, was enough to exceed the seatbelt tongues designed and engineered strength by a large margin! - thus resulting in the seatbelt tongues failure - and a very expensive media stunt!

Virtually the same effect occurs on axles when a heavily-loaded axle receives a heavy road impact - from something regarded as relatively minor - say, in the shape of a 50mm deep pothole.

The impact can send the axle load into the danger zone - and even though engineers design in shock-loading resistance into their axle designs - if you're running at maximum axle loading - or even just near it, it doesn't take much by way of repeated heavy road impacts, to result in axle damage.

Cheers, Ron.
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Follow Up By: Member - Rosco from way back - Friday, Apr 22, 2016 at 23:05

Friday, Apr 22, 2016 at 23:05
G'day Frank

As an old dog engineer who's played with this sort of calculation for more years than would normally be considered reasonable, you've done well old son......... 10/10
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Reply By: Member - DOZER - Friday, Apr 22, 2016 at 14:18

Friday, Apr 22, 2016 at 14:18
now factor in, if you used 250kg stabilisers on your van, where does the 250kg get transfered to???? shared between front axle and trailer axle???
b4 you bag me out, walk a mile in my shoes, then your a mile away and have my shoes :)

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Follow Up By: 671 - Saturday, Apr 23, 2016 at 11:25

Saturday, Apr 23, 2016 at 11:25
" where does the 250kg get transfered to???? shared between front axle and trailer axle???"

----------------------------------

Yes they will take a lot of weight off the rear axle but unfortunately the number of places where you can use them off road is very limited.
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Follow Up By: The Bantam - Sunday, Apr 24, 2016 at 22:08

Sunday, Apr 24, 2016 at 22:08
Yes but an additional 250KG of load and stress on the coupling, drawbar and vehicle attachments.

cheers
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Reply By: The Bantam - Sunday, Apr 24, 2016 at 21:29

Sunday, Apr 24, 2016 at 21:29
THE most important thing to take away from this is, the weight resting on the ball can put more weight, .... a lot more weight on the rear axle than you may expect.

Most people this would not enter their minds.

Frank has gone to trouble the of calculating this out, the calculations matter less than the reality that they show.

For most people the weigh bridge tells the truth ....... or would if they went there.

NOW something a lot of people don't consider ...... exactly what hitch and what distance from the rear axle are the vehicle specifications written for.

Many of us have 8 foot trays on our single cabs or 6 foot trays on their dual cabs, which puts the rear of the vehicle quite some distance further back than a style side tub.

Many of us have longer hitch beams on our towbars to get the ball out from under the tray.

SO if the ball on a tray back vehicle is 200mm further back than the standard hitch as used on the style side version ...... What does that do to your ball weight limitations? ... think about this.


I dare say the majorty of travelers out there do not fully understand the specified limits of their vehicle ...... pretty much limiting their understanding to the Total towing capacity and the permitted ball weight.

But it is always more complicated than that, maximum axle loading IS always specified and may prove to be the limiting factor of what that vehicle will tow.

It may be an overloaded rear axle that makes that vehicle unsafe and illegal while the rig is well within the GVM, towing capacty and ball weight specifcations.

cheers
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