07/10/2022
REAR AXLE LOADING - WHY WE NEED TO CHECK.
The ongoing random roadside weigh checks by Queensland Police have found some interesting statistics.
An alarming 80% of the caravan rigs weighed beside the highways were found to be overweight in some respect.
Of those overweight, a large percentage exceeded the rear axle weigh loading.
LET'S HAVE A LOOK AT THE REASON BEHIND THIS SAD STATISTIC.
Many caravanners, boaties, and horse float towers are now becoming familiar with the following terms and vehicle weight legalities that they must be under when towing.
These are set by the vehicle manufacturer.
GVM (Gross Vehicle Mass) - Total weight of the fully loaded tow vehicle. Includes tow ball weight., driver, passengers, all accessories, bull bar, canopy and 'stuff'.
GCM (Gross Combined Mass) - The total weight of the tow vehicle and trailer being towed.
GTM (Gross Trailer Mass) - Total weight 'on the wheels' of the fully loaded trailer. Excludes the tow ball weight which is transferred to the tow vehicle when attached for towing.
ATM (Aggregate Trailer Mass) - Total weight of the fully loaded trailer. It includes both weights on the wheels and the jockey wheel.
HOWEVER, many forget to measure the weight on the tow vehicle's rear and front axles.
Most tow vehicles, apart from 5th wheelers, have a hitch point approx. 1 - 1.5m behind the rear axle. This is called an 'overhung hitch.'
When you place a heavy load on the tow ball of an overhung hitch, the weight is leveraged by the rear axle acting as a pivot point, placing an increase load on the rear axle and taking weight off the front axle.
It is just like a see saw in a children's playground.
Typically, this weight transfer onto the rear axle can be 140 - 150% of the actual tow ball weight and means that a 300kg load on the tow hitch can mean a 420 to 450kg load on the rear axle.
This can often mean that the fully loaded vehicle now exceeds the manufacturers total rear axle loading and the vehicle is therefore unroadworthy and insurance voided.
ALL vehicles are affected by this leverage effect.
Now before you listen to the comments " Oh! you just need a weigh distribution hitch", do your own research on your individual rig.
A Weigh Distribution Hitch (WDH) / Load Levelers are NOT often a fix for the actual problem and many vehicle manufacturers recommend NOT using them on certain vehicles for a variety of individual model relative reasons.
Firstly, find out all the facts on your individual vehicle by having it professionally weighed, fully loaded.
Secondly, Check all weight is distributed correctly. Often this problem can be solved by moving some weight around, or leaving some out and saving your thousands of dollars on recommended purchases or work.
Thirdly, also check that the tow vehicle suspension is in good condition. If you have a vehicle more than a few years old, especially if it has been used for towing or heavy loads, the suspension may be fatigued causing towing imbalance. Fit it first.
These are common causes of the problem of rear down, front up towing where many just try to cover the problem with a WDH.
Only use ANY 'add on' accessory as a last resort if ALL else does not solve your problem.
A weigh distribution hitch can be a good accessory if actually needed, if adjusted correctly and for the right reasons. Because your mate uses one in not a valid reason for you to use one. Your rig, van and loading are completely different to his.
Just for the record, I use a WDH on my unique rig, showing I am not against their use, only their misuse.
DO NOT WASTE MONEY TO FIX A SYMPTOM OF A PROBLEM.
FIX THE ACTUAL PROBLEM.
The problem will always be there unless you fix it. A band aid fix of a symptom will not solve your towing problem.
It can be difficult to weigh the rear axle on a standard weighbridge as not all have separate axle weighing. Some do but most don't.
That is why I recommend a mobile scales company that can weight each wheel individually and give more information on weight distribution.
This is where a mobile scales companies like
http://www.checkweight.com.au/ in Newcastle can assist.
Owner Jeff De Ath has weighted many different types of vehicles. Here is his description of when he weighed a RAM 1500 and his comments on the rear axle loading.
https://checkweight.com.au/post/check-weight-should-i-buy-a-ram-1500-to-tow-my-caravan/
Automotive journalist Robert Pepper has a way of explaining things with demonstrations that helps understand this concept of weight increasing by leverage.
https://youtu.be/tyvpVv3PaWw
Robert Peppers video also help explain why weighing your caravan at ride height is so important. If weighing your tow ball weight, the van MUST be at normal towing height to get any accurate reading. This can be particularly difficult when using one of the cheap (long) spring pole type tow ball scales.
Stay safe everyone.
Cheers
Ken Wilson
