Weighbridge Geometry!

[ron.]

Our local council weighbridge (Dartford) has a slope both on and off from the plate. When weighing my rear axle I set the van up so that the front axle was only just off the edge of the plate, feeling that the further forward I went some of the weight would be transferred downhill and forward in error due to the slope. However, the operater said that for accuracy I should have the edge of the plate halfway between the two axles, and indeed that is how the police had 'suspects' weighed. Right or wrong I still did it my way but would be interested to hear from any brains out there which is correct - if either.

 

Ron

[Brian Kirby]

The opposite of the advice I had. The load on the axle is a simple downforce, and will not alter whether the edge of the plate is just clear of the wheels, or at the mid-axle point of the vehicle.

 

The slope would cause some transfer of the centre of gravity downslope, leading to distortion of the recorded load where only one axle was weighed, and the other derived by subtraction from the all up weight. However, I would think the slope would need to be severe for this to register on a weighbridge. They don't really do much less than, I think, 5kg increments.

 

The argument I was given was that the axle remaining on the platform should be as near to the platform centre as possible, because the platform was optimised around its centre point, and became less accurate the closer to its end the load was applied. This seems to me much more plausible.

[colin]

Brian is correct, the way you set up van would give a more accurate figure.

For reference I've been doing weight and balance for 40 years (that was frightening when I worked that out)

[ron.]

Thanks gents for your replies. I wonder how the operator came about his version of the procedure? I'm thinking maybe the police do it their way as it would give any vehicle's back axle weighed a little less weight as a bit of leeway.

 

Ron

[George Collings]

When giving evidence in court as an investigator it strengthens your testimony if you show any potential errors are stacked in the defendants favour.

[ken the kontiki]

I've asked countless balance beam weighbridge operators if and how they can give me a weight for each axle on our AL-KO tag MH. Some have come up with various methods but the consensus seems to be that dynamic axle weighing is the only accurate way. Dynamic scales are like rocking horse droppings unless I "volunteer" my MH when DVSA (VOSA) are about.

 

So how do HGV hauliers measure their axle loads on a 3-axle tractor unit and tri-axle trailer? I think some have on-board instruments but the rest???

[Derek Uzzell]

Ken

 

This link may be of interest

 

http://www.transportcafe.co.uk/90-driving/weighbridges/181-weighbridges-for-hgv-vans-farming-fixed-portable

 

In the case of a tandem rear-axle KonTiki motorhome, taking a ‘combined’ rear-axle weight-measurement (ie. with both rear axles simultaneously on the weighbridge) and dividing the result by two should really be near enough.

 

As a KonTiki has a pair of very similar Al-Ko ‘single wheel’ rear axles, with the axles being close together and each axle having the same suspension arrangement, it’s hard to see why the load on each axle should be other than half the combined load on the pair of axles.

 

If your KonTiki had a genuine ‘tag’’ rear-axle arrangement - with a dead axle behind a drive-axle - the difference in the design of the axles and the suspension could be expected to result in individually different weights, but again probably not enough to matter.

 

Other than finding a specialised weighbridge, I guess you might find someone who had weight-measuring ‘pads’ (I think Dave Newell may have some) that would allow individual axle/wheel weight measurements to be taken.

 

Your motorhome’s VIN-plate will almost certainly show the same maximum weight-datum for axles 2 and 3 , so what’s the attraction in measuring the weight being placed on individual rear axles?

[ken the kontiki]

Thanks Derek for the info and link.

 

We feel that we carry a lot of weight behind the rearmost axle (tinned food & kitchen appliances under the island bed, gear in the rear garage and two push bikes on the rear towbar). we once tried to get three axle weights on a balance beam weighbridge by shuffling the MH around so that each individual axle was on the weighbridge and a bit of maths (weigh the two rear axles alone and then the rearmost one, then deduct one from the other to give the mid axle weight) but this showed that the mid axle was 150kg over the plated 1500kg max axle loading. Having the axles sitting on the edge of the weighbridge didn't help and having read up on weighbridges, this isn't ideal.

 

Suffice to say that having the two rear axles together plus the front on its own brings us in at 4850kg fully loaded with the front coming in at 2020kg, just 80kg below the max front loading.

 

Would still like to get the rears weighed individually accurately just to see what's going on.

[George Collings]

It would be reasonable to expect that if tyre wear on the two rear axles is about equal the loading on each will be much the same with perhaps little more wear due to sideways scrubbing on the rear ones if you like mountain passes with lots of hairpin bends.

[Derek Uzzell]

Forum-member Steve928’s experience with a single rear-axle Al-Ko chassis suggests that tyre wear may not be a reliable indicator of the loadings on each rear axle of a motorhome with a twin rear-axle Al-Ko chassis.

 

Steve’s Bailey motorhome’s rear-suspension was a mite cock-eyed and the misalignment caused the vehicle to pull to the left. If something like that occurred on a Al-Ko tandem-axle chassis, misalignment of one axle might be masked by the other, but could result in unusual tyre wear on one axle even if the loads on both rear axles were the same.

 

Paragraph 3.4.1 of this guidance document

 

https://www.gov.uk/government/publications/weighing-vehicles-for-enforcement-consolidated-code-of-practice/consolidated-code-of-practice-enforcement-weighing-of-vehicles

 

suggests that the accuracy limit (+/- 150kg per axle is quoted) when dynamically weighing a vehicle may be too large for Ken’s relatively light vehicle that has rear-axle maxima of just 1500kg per axle. Four weighing ‘pads’ (one under each rear wheel) should provide the accurate figures Ken wants, but I’m doubtful any other method will.

[alan k]

At the nearby West Yorks Standards weighbridge at Gildersome just off M62 jn 27, it only measures one axle at a time and the panel advises how you have to drive over it to get the info on the screen, its free but there's no ticket just the digital display of each axle and total weight

 

cheers alan