Does a GVM upgrade affect axle weights?

QUESTION

Dear John,

Your recent articles on overloading vehicles has (unless I missed it) not touched on the unmentionable - maximum axle weights.

We had a 4WD single cab tray truck with a slide on camper. It had, of course, a GVM. The camper was a bit heavy so we had an upgrade for the suspension engineered to increase the GVM.

But, the owner’s manual specified maximum axle loads. The problem was that hitting the GVM, let alone the upgraded figure, put us over the rear axle weight limit.

Part of the problem is that, say, 100kgs added behind the rear axle adds more to the axle than 100kgs. It shifts some front axle weight to the rear.

Try your numbers on a dual cab tray truck with one of those off-road super dooper canopy bodies and rear-mounted spare tyres, and see how adequate the rear axle is. Does it meet the axle limit, even if not being over the GVM?

Sincerely,

Graeme

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ANSWER

Graeme,

Engineer here: You’ve explaining to me, below, how altering the loads on a beam affects the way those loads are resolved by the supports. It’s a little ironic, don’t you think? (Seeing as I kinda studied that, in all of its different flavours, for six years, at university…)

GVM upgrades are a disgraceful joke because the chassis is the weak link, and a GVM upgrade does not strengthen the chassis - it makes the chassis even more susceptible to catastrophic failure.

Obviously increased cantilevered loads significantly increase the burden on the rear axle. (Axle load specifications are there so you don’t break the chassis.)

This whole ’the camper was a bit heavy’ thing is completely unacceptable in my view. What you mean is really that you took the decision to drive the vehicle dangerously overloaded. This cannot be interpreted any other way.

The suspension is not the problem in this situation - therefore changing the suspension does not fix the problem or eliminate the risk to you or the public. It is, however, a great way to risk your life while voiding the warranty and also your insurance.

I know this is not what you want to hear. I blame facts generally, for being so inconvenient.

Sincerely,

JC

MODIFYING YOUR VEHICLE TO CARRY MORE WEIGHT

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