# Calculating axle weights with a tag axle Motorhome

Discussion in 'Motorhome Chat' started by Jim, Jan 19, 2016.

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I'm working on an FAQ and I've found a hole in my knowledge.

I'm well acquainted with the formula for working out axle weight changes when you add a load to your overhang and I'm writing an article about it for the forum. But I need to ask you cleverer Funsters than me.

Tag axle motorhomes; where is the pivot point? Rear tag/front tag/in-between?. As you may know the calculation requires the wheelbase and overhang measurements, easy in single and twins but on a tag; where are these measured to accurately determine new axle weights when weight is added to the rear? Thanks

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3. ### funflairFunsterLife Member

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Hi Jim

I am going with in between the two rear axles, this is assuming a reasonably level van as you could argue that when it is well overloaded and the front wheels are in the air that the pivot point should be the centre of the rear axle, mind you most/some people would work out for themselves that they had too much weight in the garage in that example.

Martin

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4. ### hilldwellerFunsterLife Member

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The distance between the tag wheels is small compared to the other two distances, so if you go for middle of the two wheels and then divide the load equally you won't be totally accurate but for most purposes close enough.

If it was really on the limit at a weighbridge you could come unstuck but deserve to if loading that close to the limits.

I do not know but I wonder if the way the two axles mount on the rear springs does not share the load equally by allowing a bit of twist.

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5. ### JockandRitaFunsterLife Member

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I can't answer your question as much as I'd like to Jim, but my experience at a weighbridge with our "Tandem" axle MH, is that the intermediate axle bears more of the load than the rear axle, (not by a great deal though) as the figures show on the official VOSA ticket. This has been when loaded up for a fortnight's holiday, including bikes on the rear rack.

BTW, although we all use the term "Tag" axle, a Tag axle is one which can be raised and lowered, and in the case of a 6 x 4 or 6 x 2 Tractor unit (eg, Scania, Volvo, Renault, etc), they are often operated automatically via load sensing valves.

HTH,

Jock.

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6. ### hilldwellerFunsterLife Member

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The actual figures would be very interesting if you still have them. Is there a big difference ?

And of course, is it the same for all tandems and how dependent on you actual loading is it ?

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7. ### funflairFunsterLife Member

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Are you a bit nose down Jock as that would do it if all other things, (suspension units etc) were the same.

Martin

8. ### JockandRitaFunsterLife Member

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I've no access to them at this time Brian, as I'm at work (boring ) and the MH is in storage, but IRC, there wasn't a great difference between the two axle weights. Also, you can often see by looking at the tyres (equally inflated) that the intermediate axle is bearing more weight.

Cheers,

Jock.

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9. ### JockandRitaFunsterLife Member

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Yes Martin, even after a Goldschmitt front suspension upgrade, I am still slightly "nose down".

Cheers,

Jock.

10. ### funflairFunsterLife Member

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That will be loading the front of the tandem axles a bit more.

Martin

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I believe what you are looking for Jim is the "Fulcrum effect on loading say a scooter on a rear rack
and fulcrum point is the swivel point ie center of the rear most axle
calculated by taking measurement from the rear panel to the first swivel point it therfore follows using a load calculator would be from the front wheels to the swivel ie the rearmost axle
for the purpose you require I believe the other axle can be ignored, as it not relative to either measurement

Further confirmed by this experiment
Remove rearmost wheels and the swivel point (Fulcrum) changes to the other axle
Then put them back on
remove the wheels from the middle axle and nothing changes ergo the centre axl has no relevance at all
G

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12. ### funflairFunsterLife Member

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But that argument says that all the extra effect of loading a scooter on the tow bar would go onto the rear axle, which is not the case. I can understand your argument if the front wheels are of the floor but not sure if the van is basically level. When I first looked at it I thought rear was the answer but changed for a a level van where I believe you treat the two axles as one.

Will keep thinking.

Martin

13. ### andy63Funster

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I was thinking along the same lines as @Geo .. but wasn't sure how to get it across lol...
Anything added behind the rearmost axel will have a fulcrum effect around that axel... and lighten the load on the front axels. .
Interesting anyway...

14. ### CavsFunster

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Some recollections from when I sold trucks some 40 years ago. We had what amounted to a slide rule for calculating weight distribution and turning circles (among other things). In all cases with twin rear axles the measurements were taken from the midpoint between the two rear axles. This is only completely accurate when the van runs level, as Martin says, and when the suspension for both rear axles is identical. On mine, the rear air springs are interconnected to achieve this. With a van slightly nose down there will be more weight on the leading rear axle.

However, on most weighbridges it is not possible to measure the two rear axles independently, so they would be weighed together and their capacities added together to give the weight limit.

So when adding a scooter rack I believe it would be proper to measure the wheelbase from the centre of the front axle to the midpoint between the two rear axles, and the overhang from the midpoint of the two rear axles to centre of gravity of the scooter.

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15. ### CavsFunster

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PS - it wasn't long ago that I threw away the slide rule! Isn't it always the way - don't use something for years, chuck it away then find a use for it

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I have no technical experience of this but have witnessed a phenomenon which may have a bearing.

We have two vehicle parking areas, one for cars and one for motorhomes and the first bay has a slight slope initially

When Tag axle vehicles try to reverse on to slot one, if they turn too tight, the front off side wheel simply lifts off the ground as the nearside is on the slope.

This would leave me to think that the pivot point must be close to the leading rear wheel, if the front wheel will leave the ground will no apparent ill effect

Last edited: Jan 19, 2016
17. ### matamorosFunster

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In another life, a long........long time ago, I studied Mechanical Engineering(long forgotten now) but I vaguely remember very complicated calculations for distributed and point loads on beams, which is effectively what a chassis is.
My instinct tells me that an accurate answer would not be simple but the methods suggested would give a fairly accurate approximation.

I am sure that there are other Engineers with far more knowledge than I on this forum that could give a definitive answer.

18. ### John LaidlerFunster

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The middle wheel so to speak will carry a bit more weight but the difference won't be great.

To calculate it in the absence of a diagram imagine the level MH pivoting around the front axle. Then imagine the weight of the MH pressing down below the centre of gravity at a point somewhere between the front and rear wheels. As the MH isn't actually moving (rotating) then this "moment" as it is called is resisted by the combined moments from the rear wheels.

The rear wheel being further away, and thus having a longer lever, will have less force on it than the front tag axle.

This assumes all things are equal and the rear springs being equally compressed. If they are not then the distribution will be a bit different but I think if you assume the force lies equally between the two rear wheels it will be good enough given the other variables including variations in tyre pressure.

19. ### John LaidlerFunster

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Further to the above if you assume:

Tf = force on front tag axle
Tr = force on rear tag axle
x = distance between tag axles.
W = distance from the front wheel to the front tag axle

Then:

Tr = (Tf * W) Ã· (W + x)

So if x is only a small fraction of W the difference between Tf and Tr will be small and the mid-point between the two can be assumed for calculating axle weights.

All of this assumes there is no connection between the two tag axles, such as linkages or that they are on a "walking beam" where the load on each wheel would be identical.

Last edited: Jan 19, 2016
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I made no reference to load bearing shared or otherwise as that was not the question asked.
The question again was which axle do i use for the calculation of adding a load to the tow bar
The axle to use is the rearmost of course the load will be shared but that question was not asked
the equation reqd is the distance between the front and rearmost axle regardless of how many axles it has got, add another in the middle and use that, and you have the wrong formula

A1 = existing front axle load
A2 = existing rear axle load
L = weight of new rear load
W = wheelbase (the distance between the front and rear axle centre-lines)
O = overhang (the distance from the rear axle centre to the centre of mass of the new rear load).

Add the figure for the new rear load, and carry out the following calculations:

• New front axle load (new A1) = A1 â€“ [L x (O Ã· W)]
• New rear axle load (new A2) = A2 + L + (A1 â€“ new A1).

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