Trailer design safety factor

[Renze]

Hello,

I have a new job, changed from drawing stainless steel kitchen cabinets to something more interesting: trailers for small trucks (Or actually Vans that are modified to 5th wheel tractor, so they can still be driven with small (B) drivers license, without the need fo a 5000 Euro truck drivers license, with all the other pains like tacho registration etcetera...

The company was used to make it overbuilt, some customers reported no damage at 200% of the certified load... Anyways, the extra iron costs about a euro per kilogram, and the bath zinc coating is also about 1 euro per kilogram. Next to that, the legal load rating goes down when the empty weight of the trailer goes up...

We need to reduce the weight on our smaller trailers, because they use the same IPE 180 ladder frame as the longer ones. The policy of the national institute for road traffic, has changed recently and prescribes no design safety margins. They make the trailer builder responsible for what they sell to their customers...
So my question is, does any engineer on this board have any idea what safety margin is used commonly in the market ? i'm talking about 5th wheel air suspended trailers.

 

[plowking]

I'm not an engineer, but for research many years ago in USA The Frito-Lay potato chip company took box vans, like UPS,FED-EX, use and enclosed behind the driver's compartment and opened the back to look like big pickup. It had a fifth wheel setup with dual rear wheels, The transmission was a turbo 400, but not sure of engine.It was built on a General Motors chassis. They are rare now as they only did this for a few years. The trailers looked like the truck body people made them . The bodies were aluminum and some have been bought by people to use as glorified pickups. plowking

 

[Renze]

I've been digging in my old school books and found something: for a jump or swell load, safety factor 1,5 is necessary. For variable load (push/pull) a safety factor of 3 is prescribed, the safety factor is to avoid fatigue failure.
last trailers i designed and calculated, had safety factors varying from 1,8 to 2,5

when we calculate a safe overload rate of 120%, the final strength should be 1,2 multiplied by the fatigue safety factor of 1,5 so that makes 1,8

 

[hsvhobbit]

If I understand you're question and basic situation correctly. You state that for the smaller trailers, you're using the SAME ladder frame as for longer trailers? Is this correct?

If so, then I would expect the smaller trailers to hold up substantial overloads, assuming that the longer ones are comfortably holding up. The shorter trailers will have less bending moment on the rails than a longer one (due to leverage). You could simply reduce the sizing of the rails for the smaller trailers. This would lead to having two different items to build/track/stock but would possibly be a more cost efficient thing to do since extra weight is such a cost burden. You'd have to do a cost/benefit analysis to see where the break point is in terms of volumn built...Sometimes just having your fabricators worry about building ONE product is a good thing. They get their process well defined and a smooth routine going for an efficient setup/build.

I have no direct personal knowledge of safety factors on road vehicles but if I were designing the frames I'd start with a SF of 2.0 given that I had a well defined dynamic environment (shock loads from potholes, etc). Sorry I couldn't directly contribute to your answer.

 

[Renze]

Yes, before i worked there, every trailer, from a 6 meter to a 12 meter, everything was built on 2 IPE 180 main frame rails.

You are right about the dynamic environment, the mass inertia causes greater forces on the frame than at standstill, when driving through a pothole.
The chassis is air suspended though, the axle is connected with a leaf spring, the front of the leaf spring is carried by a console which also holds the shock absorber, and the rear of the leaf spring carries the air bellow. If the pothole may be so deep that the air bellow ducks in its solid rubber endblock, the leaf spring will take out a big share of the impact.

About unification of design:
I wouldnt worry about that, every trailer is unique. In this business there is no such thing as a standard trailer, people dont buy any longer trailer than they need because they can only take 5 ton or so. Next to that, the market for B trailers is small, so there isnt enough volume for manufacturers to specialise in just one line, like low loaders, cooled box or canopies.
We do cut down the number of pre-made parts, because the company employs only 8 people, there is not enough work to have sheetmetal and punching machines in house. So we do have an amount of standard parts made, someone else can make it cheaper on a punch press and sheet shear, than we can with a circular saw and drill press...

Our biggest competitor, price wise, is brake pressing frames out of sheet metal and you cant find any that's not welded or reinforced in any way....