Chassis Uprating - Have we missed something?

[Vernon B]

The recent threads on chassis uprating have reignited a problem I have understanding what is going when I’m attempting to optimise the loading of our ‘van. As you will see this has some significance for those contemplating an uprating.

 

The problem I have is that despite having a modicum of common sense, the way I thought things work doesn’t appear to be quite the case.

 

What common sense tells me is:-

 

1. If you increase your gross weight by 350kg you increase your useable payload by 350kg.

 

2. Where you place that extra load between the two axles determines the extra load taken by each of the axles. Each axle may take a different amount of the extra weight, but the sum of those extra weights will be 350Kg.

 

3. If you place the 350kg beyond the rear axle ie in a garage or on a towbar, the additional load on the rear axle will be greater than 350kg by a factor determined by the distance between the centre of the load and the rear axle.

 

What personal experience now tells me is:-

 

1. The weight limit of the rear axle ultimately determines the additional useable payload you get from uprating a chassis. This can be much less than you thought.

 

2. Rather than each axle taking a good part of the extra payload most (or all?) of this is transferred to the rear axle with the front axle acting as a pivot point. In other words, and this is my key point, the load on the front axle barely increases at all!

 

3. Loads placed beyond the rear axle do indeed magnify the weight on that axle.

 

To illustrate the significance of this it’s worth using the data from Tracker’s “MAM with PLG or PHGV via SvTech” thread.

 

Tracker was offered two options. The first involved increasing the gross weight from 3,500kg to 3,700kg and leaving the front and rear axle loadings at 1,850kg and 2,000kg respectively. “Common sense” says that would give Tracker an additional payload of 200kg.

 

Tracker tells us that SvTech asked him for the axle weights for his ‘van in a fully laden state but he didn’t quote those figures in the thread. However, if my experience with two Fiat and one Mercedes is anything to go by I’d wager that his front axle loading was close on 1,650kg and, assuming his total loaded weight was 3,500kg (hence his interest in an upgrade) his rear axle loading would have been 1,850kg.

 

Note that although the total weight is at the maximum limit both axle weights are comfortably within their limits. The front can take another 200kg and the rear another 150kg.

 

Now, if my experience holds true and Tracker places an extra 200kg of stuff at suitable storage locations between the two axles he’ll find that the load at front axle remains at 1,650kg and the rear has increased to 2,050Kg.

 

The total vehicle weight is now the new maximum 3,700kg, but the loading on the rear axle is 50kg above its limit. If this is indeed the case then the additional useable payload resulting from the uprating is 150kg and not 200kg. And would be even less if the 200kg was placed somewhere beyond the rear axle.

 

It’s worth briefly looking at how this works out with the 3,850kg option Tracker was offered. Note that on this occasion SvTech specified increasing the rear axle to 2,240kg but were happy to leave the front at 2,000kg.

 

With this set up I’d expect Tracker to find that if he placed a load of 350kg (the new “common sense” payload) between the axles:-

 

- The load on the front axle would still remain at 1,650Kg

- The load on the rear axle would rise to 2,200kg – within the new limit

- But if the load was placed beyond the rear axle the new limit for that axle could be compromised depending upon its distance from the axle.

 

So if there is indeed some truth in what I’ve come across it would appear that you need to make sure that the upgrading of the rear axle weight is consistent with the additional payload you are paying for, having regard to what amount of it is stored beyond the rear axle. Perhaps the example of what was offered as factory fitted option on our ‘van might illustrate what to look out for:-

 

Van in standard trim:

- Max Weight 3,880kg

- Front Axle 1,800kg

- Rear Axle 2,230kg

 

Uprated Option:

- Max Weight 4,200kg

- Front Axle 2,000kg

- Rear Axle 2,400kg

 

This upgrade option appears to offer an additional payload of 320kg but the increase in the rear axle loading suggests that I would only have an additional margin and hence useable extra payload of 170kg.

 

I suspect that what might ultimately have determined the limit on the rear axle – and hence the useable payload – was the unavailability of 235/65R 16C tyres with a 118 load index.

 

As it happened I didn’t take the option although I wasn’t aware of the implications at the time.

 

Have a got this right? I’d welcome comments from folks who know their way around “moments of force” or perhaps from Tracker on whether his own “before and after” findings bear out any of my sweeping assumptions.

 

V

[Tracker]

Broadly I agree with what you say which is why I opted for new tyres and air suspension to get the extra rear axle weight that I needed.

 

Pre change the van weighed in at 1600/2150 = 3750 (250 overweight which was all rear axle) and the maximum axle weights were 1850/2000 = 3500 total.

 

These are now 1850/2240 = 3850 total so I have an extra 240 kg of payload exactly where I need it - over the back axle.

 

The absolute maximum possible all up weight of around 3800 as well as both axle weights are now within the law at 1650/2150, and although there is not much leeway we usually travel not quite fully laden with absolutely everything possible so at least I am legal and safe.

 

It also has the benefit of £50 a year less VED - crazy innit - plus a much improved ride due mainly to the Air-ride but helped by fatter tyres!

 

All in all I'm a happy bunny now that I have enough payload where I need it and the vehicle is more pleasant to drive and travel in - especially as the supplying dealer kindly supplied the five new Michelin M&S (that's Mud and Snow in case anyone asks!) rated tyres for free.

[Tracker]

This was the first thread which gives a bit more info for those who want to know.

 

https://www.outandaboutlive.co.uk/forums/Motorhomes/Motorhome-Matters/The-perils-of-Payload-and-GVW-MAM/29029/

[colin]

Vernon B - 2013-02-15 7:24 PM

 

Note that although the total weight is at the maximum limit both axle weights are comfortably within their limits. The front can take another 200kg and the rear another 150kg.

 

Now, if my experience holds true and Tracker places an extra 200kg of stuff at suitable storage locations between the two axles he’ll find that the load at front axle remains at 1,650kg and the rear has increased to 2,050Kg.

 

The total vehicle weight is now the new maximum 3,700kg, but the loading on the rear axle is 50kg above its limit. If this is indeed the case then the additional useable payload resulting from the uprating is 150kg and not 200kg. And would be even less if the 200kg was placed somewhere beyond the rear axle.

 

V

 

As someone that does weight and balence as part of thier work I can tell you that somewhere along the line you've got somthing wrong. If a weight is placed between the axles it add load to both axles the amount of which will depend on postioning, now if you have added a load only just in front of the rear axle you will see little increase in front axle load and a large increase in rear axle load and with just a small error in weighing it will appear to only increase on the rear.

[Tracker]

Page 4 on this link explains the effects of leverage on weight.

 

http://www.atoc.info/ipad/members%20tips%20pdf/motorhome%20weights%20check.pdf

[kelly58]

EXHIBITOR SPOTLIGHT - SV TECH

New exhibitors SV Tech will be offering optional Free Motorhome Weight Checks for weekend campers upon arrival to the UK Spring Motorhome & Caravan Show 2013 using VOSA approved weigh pads, plus providing advisory information.

This service will be available on Thursday afternoon and Friday morning, located at the Camping Entrance. Each check will take just 3-4 mins. Staff will ask the motorhomes to roll over the weigh pads, taking a front axle and rear axle measurement, and will arrange a follow-up chat at the event.

Options include uprating & downrating available to each specific motorhome. Type Approval queries also answered. SV Tech also have a Load Distribution Analysis program, available FREE on the SV Tech website www.svtech.co.uk

Company contact details & enquiry forms can also be found on the website

[Derek Uzzell]

colin - 2013-02-15 8:10 PM

 

Vernon B - 2013-02-15 7:24 PM

 

Note that although the total weight is at the maximum limit both axle weights are comfortably within their limits. The front can take another 200kg and the rear another 150kg.

 

Now, if my experience holds true and Tracker places an extra 200kg of stuff at suitable storage locations between the two axles he’ll find that the load at front axle remains at 1,650kg and the rear has increased to 2,050Kg.

 

The total vehicle weight is now the new maximum 3,700kg, but the loading on the rear axle is 50kg above its limit. If this is indeed the case then the additional useable payload resulting from the uprating is 150kg and not 200kg. And would be even less if the 200kg was placed somewhere beyond the rear axle.

 

V

 

As someone that does weight and balence as part of thier work I can tell you that somewhere along the line you've got somthing wrong. If a weight is placed between the axles it add load to both axles the amount of which will depend on postioning, now if you have added a load only just in front of the rear axle you will see little increase in front axle load and a large increase in rear axle load and with just a small error in weighing it will appear to only increase on the rear.

 

Colin is, of course, correct.

 

Extra weight placed anywhere within a '2-axle' vehicle's wheelbase must increase both axles' loading.

 

Extra weight placed behind the rear axle must increase the rear axle's loading and decrease the front axle's loading.

 

Extra weight placed in front of the front axle must increase the front rear axle's loading and decrease the rear axle's loading.

 

Extra weight placed directly above an axle will increase that axle's loading, but won't affect the other axle's loading.

 

How could it be otherwise?

[Vernon B]

No I can't argue with that Derek but unfortunately it isn't borne out by my experience.

 

Here's an example. I weighed my 'van at an approved weighbridge and it gave the following readings:-

 

Front Axle 1,660kg

Rear Axle 1,960kg

Total Vehicle 3,620kg

 

At that time I had a full tank of fuel, a 75% tank of fresh water and what I'd refer to as the basic bits and pieces we leave in 'van. I was in the 'van and I weigh around 75kg.

 

AS previously mentioned the 'van is plated with a maxium gross weight of 3,880kg. Perhaps I should mention here that it rides on an Alko Mercedes chassis which is a little unusual in that it uses trailing arms for the rear independent suspension rather than torsion bars.

 

I'd weighed the 'van ahead of having a towbar fitted to carry two bicycles and wanted to check that I had sufficient unused payload to take that additional load. As you can see I had a useful 260kg to play with.

 

Following the fitting of the towbar I reweighed the 'van in fully loaded trim. This meant adding personal items, clothing, and food together with the bikes of course. I did not go to the trouble of weigh these items separately but the resulting loadings - from the same weighbridge - were as follows:-

 

Front Axle 1,660kg

Rear Axle 2240kg

Total Vehicle 3,900kg - ( yes I know a shade over the plated limit but it's been dealt with)

 

Now of course its possible that when adding more items and more weights that the juxtaposition of all these various items resulted in the same front axle weight by chance/coincidence, but that takes some believing. If I were to guess I'd say of that extra 280kg perhaps 100kg was at the towbar and the remaining 180kg was positioned close to or over the rear axle. The presumption would therefore be that the load at the front axle would more than likely register something less than 1,660kg - but I accept its my guess - I haven't done the calcs. The fact is it didn't.

 

Putting this example on one side, and as implied in my original posting, I have experienced this "constant" load at the front axle with two previous Fiat based 'vans. Unfortunately I no longer have any of those records but memory suggests they too were around the 1,650kg level.

 

Unfortunately I haven't had unrestricted access to a weighbridge in order to carrying out suitable experiments for understanding what is going on, and of course I'm not suggesting that there's a flaw in the maths set out in link usefully supplied by Tracker - chances are I've missed the blinding obvious.

 

But in any event that's not really the point I was trying to make, which I think you'll agree still holds. That is, it is critical to take account of the load bearing capacity of the rear axle when calculating useable payload especially when considering a chassis uprating. Simply deducting the old maxium weight from the new maxium can be misleading.

 

That's not to say I wouldn't be interested in seeing the changes in your own front axle readings under various loading conditions.

 

V

 

 

[Tracker]

As you say front axle loadings change very little regardless of where you stow stuff, and if you look at where the front wheels are in relation to the rear and see what storage is available over or near the front end it is hardly surprising!

 

However whatever you add to the front axle load will more than likely be counter balanced and offset by anything placed behind the rear axle and so the front is likely to remain fairly constant in normal loading.

 

That said, hanging a bike rack and two bikes on the tail will certainly add more than their weight to the rear axle loading - and reduce the loading on the front axle - and maybe grip on the loose and wet stuff?

 

My own upgrade has given me no more front axle load but then it was never fully loaded to start with as it was the back end that was way over and needed attention.

 

I didn't even need to change the front tyres but I did not fancy fronts differing from rears - or carrying two spares - so I changed all five!

 

As you so rightly say - blindingly obvious when you think about it!

[Brian Kirby]

Unfortunately, Vernon, you are seeing coincidence, and trying to explain it with arithmetic. :-) In effect, a motorhome is like a see-saw, with the pivot at its rear axle. The main reason for this is that it is virtually impossible to add load in front of the front axle. So, all added load must fall between the axles, and will distribute between then pro-rata to the distance from each, or behind the rear axle, in which case, as said above, it places more than its self-weight on the rear axle, with the difference deducted from the front.

 

150kg placed 1 metre ahead of the rear axle, on a vehicle with a 3.0 metre wheelbase, it will distribute 100kg to the rear axle and 50 kg to the front axle.

If you now add another 150 kg I metre behind the rear axle, it will apply 200kg to the rear axle, but remove 50 kg from the front axle.

Thus you end up adding zero load to the front axle, and all 300kg to the rear axle.

 

You have in effect, broken the first law of engineering (as understood by a non-engineer! :-)), which is only ever change one thing at a time, and then measure it's effect before changing anything else!

 

It is probably that what you are seeing is not entirely accurate, because you are trying to measure relatively small loads on a machine designed to weigh 40+ tonnes. Were the weighbridge more sensitive with its read-outs, I suspect you would be seeing some front axle load variation. Besides all of which I assume some fuel will have been burnt between weighings, and this will (probably, depending on tank location, have reduced the front axle load by more than the rear. You really need to do this with the fuel and water tanks brimmed each time, as you otherwise introduce confusion due to unquantified variables.

[Vernon B]

Thanks Brian - clear and lucid as ever. Hopefully this thread will be of help to others who may fall into the trap of thinking that a motorhome is much the same as a single axle caravan and that any load

margin at the front axle represents useable payload.

 

Thanks too to Tracker who's contributions were appreciated.

 

V

[colin]

I should point out that on PVC's it can be the exact opposite, as one member on here found out. He ended up selling van due to front axle overloading problems whilst barely troubling the rear axle and overall maximum weights.

[Derek Uzzell]

In 2004 three long articles by the late Mel Eastburn were published in MMM. These covered motorhome loading, towing, licensing, etc.

 

I’ve infringed copyright by reproducing below the section that dealt with calculating the effect of extra loads on a motorhome’s axle-weights.

 

Add a 100kg load to a motorhome (whatever its type) and, irrespective of where the additional load is placed, the vehicle’s combined axle-loadings MUST increase by 100kg. The motorhome’s overall weight MUST also increase by 100kg.