noseweight:: whats the worst that could happen?

[Mel]

Before the lectures start I am VERY aware of the importance of checking noseweight and that my van is nose heavy when "empty". So it is always carefully loaded and checked. However, amazingly long threads on here testify to the pitfalls and innaccuracies of noseweight measurement. So assuming I get it hideously wrong and the nose weight of my van is 100kg, not the 87kg limit of my tow car; apart from being illegal, what is the worst that could happen. Will something snap?

mel

 

[Damian]

Mel, the problem with nose weight is the limits imposed by not only the car maker on the towball, but also the max 100kg imposed by Al-Ko on the hitch.

IF,and it is a big IF, the towball breaks and the noseweight is in excess, then the insurance could easily decline any claim.

IF the excess weight damaged the chassis then Al-Ko WILL refuse any claim.

As for driving, others will have much more input, but I guess that front end traction will be reduced making steering more hit and miss, and the rear suspension may be compromised, but as I say, others will have the tech knowledge to give more precisely I am sure.

 

[ProfJohnL]

Hello Mel,

To answer you question briefly 100/87Kg will not make a great deal of difference under static conditions but the reality is that towing conditions generate stresses that greatly in excess of the static loads, and that may have sever consequences on the structure of the tow vehicle.

Under test conditions at the MIRA test track the impulse accelerations measured at a tow hitch were typically up to 4G and with some individual readings as high as 8G.

this means your basic 87Kg load could impose an impact load as high as 696Kg for fractions of a second. If you overload you hitch by 13Kg to 100Kg the peak loads could be as high as 800Kg.

The design of the towing bracket will usually be more than capable of handling these loads, but it is what the bracket is fixed to under the car, in many cases it is perhaps only 1.5mm of sheet steel.

Car Designers are aware of such impact loads and will have applied what they consider to be the necessary safety margins to their designs, eat into those safety margins and the longevity of the product is reduced.

The failure may be progressive or it may be sudden, but eventually some part will give and the resulting detachment of the caravan from the car may be spectacular.

 

[Lutz]

Hello Mel,

To answer you question briefly 100/87Kg will not make a great deal of difference under static conditions but the reality is that towing conditions generate stresses that greatly in excess of the static loads, and that may have sever consequences on the structure of the tow vehicle.

Under test conditions at the MIRA test track the impulse accelerations measured at a tow hitch were typically up to 4G and with some individual readings as high as 8G.

this means your basic 87Kg load could impose an impact load as high as 696Kg for fractions of a second. If you overload you hitch by 13Kg to 100Kg the peak loads could be as high as 800Kg.

The design of the towing bracket will usually be more than capable of handling these loads, but it is what the bracket is fixed to under the car, in many cases it is perhaps only 1.5mm of sheet steel.

Car Designers are aware of such impact loads and will have applied what they consider to be the necessary safety margins to their designs, eat into those safety margins and the longevity of the product is reduced.

The failure may be progressive or it may be sudden, but eventually some part will give and the resulting detachment of the caravan from the car may be spectacular.

Actually, without additional reinforcements, the sheet metal thickness of the vehicle underbody is likely to be considerably less than the 1.5mm that you mention. 0.8 and 1.0mm is more like what one would actually encounter. There are very few applications on the car where you will find more than 1.0mm, and these will be very local. Relatively stress-free panels, such as door panels or the roof, can be as thin as 0.6mm.

 

[gary]

Will something snap? no highly unlikely anything will snap, not unless something has been incorrectly fitted or is loose.

The towbar itself is basically massively over engineered so any loads on that are transmitted into the chassis

The chassis members to which the towbar is attached is indeed relatively very much thinner, however, it's pressed into a box section which gives it all the rigidity it needs, because that is, as an integral part of the whole chassis, it readily dissipates the loads imposed by the attached towbar away into the surrounding metalwork.

So the metalwork is not the problem but the affect the noseweight has on the car as a whole while on the move.

Think of a 75kg, (12 stone), sledge hammer?!, as a static noseweight this nothing to the affect that weight has as a 'hammer blow' pushing and pulling on the towball, this as said is easily absorbed by the metalwork but finally has to be dissipated by the cars suspension, and this is where the real problem lies.

If the noseweight is so high that the rear suspension 'bottoms out' while on the move, then the energy left is going to try quite successfully to lift the front of the car with instant loss of control of steering.

Lesser amounts of excess noseweight will have reduced affects but at some point could be enough in what might have been just a bad situation?.... to turn it into catastrophic!!

 

[Lutz]

I wouldn't be quite as sure as Gary regarding the strength of the chassis members. Despite the thinness of the sheet metal they may be rigid enough, but that does not mean that they are necessarily adequately durable. During evaluation of towbars from various potential OEM suppliers we found that some less well engineered designs resulted in stress fractures either of the towbar itself or of the vehicle structure after only a few hundred thousand cycles on a shaker rig (the complete test comprised 6 million cycles).

 

[gary]

Fair comment Lutz, however I would have thought by now this sort of design shortfall has long since ceased to exist in main-steam manufacture.

As an example I was once very familiar with, JCB are famous for their general purpose digger which up until the mid 80's had been designed without the aid of Computers. These in their various guises up to 3C had regular fatigue relate cracking which kept me very busy repairing thank you!

That was until the 3CX arrived, being fully CAD designed and if nothing else, it shed a load of weight but put me out of work as regards welding was concerned.

Often wondered if that 'X' was for ex-welder LOL

The point being and 20 odd years on, fatigue problems surely have been designed out before destruction tests simply prove them gone?

Or at worst, long before it all has a caravan hanging on it!

 

[Lutz]

The car manufacturer will design the car primarily for use without a towbar. It will definitely be computer-optimized for this condition. Structural provisions for towbar fitment take second priority. Any additional reinforcements that may be necessary must therefore be included in the design of the towbar system and that is the responsibility of the towbar manufacturer. While the OEM towbar supplier will benefit from co-operation with the car maker, for example in defining the mounting points, others, i.e. the competition, will rely copying the lead design as much as possible and their own engineering judgment, as they will not have access to all the data that the OEM supplier has at his disposal (other than the defined mounting points, which are part of the homologation procedure). This may lead to them cutting corners in an attempt to reduce costs and be more competitive and this may result in potential structural failure.

 

[Mel]

Flippin 'eck! Noseweight will be checked and double checked now! However if there is a loud cracking noise or the front wheels of the tow car lift off the ground, I'll know that my noseweight gauge is as rubbish as I suspect!

Thanks for the detailed replies everyone.

mel

 

[Lutz]

You probably won't hear a loud cracking noise, even if failure occurs, nor will the front wheels lift off the ground, unless the noseweight is ridiculously high, but by then, the back of the car will be sitting down on the bump stops anyway.