Thread: Pros and cons of chassis beam construction?

The reason RHS and other rectangular sections are used so much in trailer fabrication is that they are closed shapes and about as strong as it can be for the weight.

If by universal beam, you are talking about "I" beam type shapes.....on light trailers the shape and the available sizes are simply not economical.

"I" beam shapes are used in heavy trailer fabrication such as semitrailers....but the weight is less of an issue in the style of construction...and those beams are custom fabricated.

And in light trailers.....weight is everything......and so many forget this.

If you are looking for soomething different, think about web trusses or the use of multiple angles.....another attractive thing is Rolled C channel...bit it tends to harbour water in the bottom cord.

Angle iron is not fashonable these days, but it can make a damn fine trailer, but you need to use forms that don't disadvantage it because it is not a closed section.

I have a light box trailer with an angle iron frame and its has been good to me for 30 something years....done again I would not have the drawbar out of angle.

Bothe my brother and brotherinlaw have boat trailers made out of angle..being an open section it fares better with corrosion.

I still have a coppy of a bok that was popular in the 80's....Building angle iron trailers.

cheers

Yeah, I was talking about I-beams. Main reason, I live near the beach and salty air attacks any metal in short order. I have had the best results with good paint jobs and I worry about what's going on inside things I make out of shs/rhs. I thought if I make a heavy chassis but lightweight body it would be OK. As mentioned (and suggested in my other thread on patios) I guess I could think about fabricating my own beams. It would certainly be cheaper. But for chassis rails I would want to work with proven beams, not my own unless I got it engineered correctly.

E.g. I could make 130mm x 65mm I-beams with web thickness of 5mm and flange thickness of 6mm for 11.51kg/m (+ filler).

Compared to UB-14 150mm x 75mm with web 5mm, flange 7mm at 14kg/m

And 150mm x 50mm x 3mm RHS at 8.96kg/m.

Duragal makes a C-channel profile 125mm x 65mm x 4mm but I don't have a weight and I'm not sure it's suitability for trailer chassis.

Next time I'm procrastinating I'll do some calcs on fabricated beams.

If I went 140mm x 65mm I-beams with flange and web thicknesses of 5mm it would weigh in at 10.464kg/m + filler.

150mm x 50mm x 4mm RHS comes in at 11.6kg/m.

The engineering properties are similar in the strong axis. The beam comes in about the same as 150mm x 50mm x 3.8mm RHS in that axis.

A beam is far weaker in the weak axis (more comparable to 50mm x 50mm x 2.5-4mm in that axis depending on the calculation of interest). That is probably OK given the requirements.

The biggest problem would be making a great beam, out of 130mm x 5mm and 65mm x 5mm flat bar and aligning it all correctly and welding it up without distortion. Also, any flat bar I've bought hasn't been the straightest so starting with wonky stock wouldn't make it easier.

A bonus would be it might be easier to create my own chassis rail/drawbar bend while constructing a beam instead of paying someone $150/bend to do it.

What you are doing is turning a realavly simple and economical process into one that is very expensive, time consuming and complicated.

What you will end up with is a readiculously heavy trailer that took 4 times as long to build.

If you are looking at some sort of alternative construction that is viable..I beams aint it.

cheers

Thanks for the smack down soundman.

I do a lot of dumb things and it's good to be reminded to think first rather than go down useless tangents.

Really I'm procrastinating and not even in the design stage yet, just throwing around ideas. There are a bunch of other projects I need to complete before being able to justify spending significant time on this.

Legion mate if you stic with the simple basics and the ordinary, well exicuted, rather than flirting with the obscure you will get more done and a better result.

cheers

I have seen some beams that were cut so that when welded together, they had a 6 sided hexagon in them, 100 into 150 I beam, if that is of any help. It would certainly be a bit heavier but with a light weight body on it weight should probably work out about equal.
Kryn

I have heard of trailers made from RHS which are sealed when welded and some oil put in before closing off the chassis. You could incorporate a plug and sqirt oil in later. Pressurise the frame to check it is sealed. As long as the oil stays in there it will coat the insides and prevent corrosion. If a problem does occur it should be pretty easy to spot with regular inspections.

Dean

The logic of filling chassis rails with oil is flawed on many levels.

1/ if the chassis rails where properly sealed there would be no water in the rails and therfore no corrosion...if the railes where not sealed the oil would leak out making an unholy mess.

2/ oil is a very poor corrosion inhibiter.....it works where it stops water from sitting on a surface, but where oil and water are combined in a space the water remains, gets access to the steel and corrosion occurs regardless

3/ should there be repair of modification required on that chassis, the oil all over and impregnated into the steel makes the whole job a PITA.

4/ the idea fails to account for at least half of the corrosion occuring on the outside of the chassis

Corrosion in trailers in particular boat trailers comes down to 3 simple factors.

First and above all...care an maintenance......most trailer corrosion can be put down to neglect and gross neglect at that......boat trailers need to be washed promptly after every use.......moderately built box trailers even if left out in the weather will last decades as long as rubbish is no left in them and they are stored in a way that allows them to drain.

Second Paint is the best anti-corrosion measure we have....it beats any sort of plating hands down...but unfortunately most trailers get little more than a brief slash with some cheap paint.
Laying up a good thorough paint job built upon a good quality metal primer goes a very long way.
But you have to get the primer and the top coats right into every little nook and cranny.
Lots of people are mad keen on hot dip galvanising.....but all platings are sacrificial coatings and in time they will be erroded...best to let the galvanising "go off" and paint over it.....that will result in a very durable product.


Third is competent costruction, that does not leave places vunerable...all too aften there are places that water and crud can accumulate.....welds that are not complete and leave fissures, voids and pin holes where paint cat get in but corrosion can start.

On boat trailers making the drain holes big enough so that.
The galvanising can get in, the water can flow out and accumulated crud can not block them easily.

As far as tubular sections....If galvanising either or leaving the ends open or the large drain holes so the gall can get in.
getting pait as far up the sections as possible and after its all settled getting a good dose of lanolin spray right thru every section.

People get all uppity about corrosion and try to come up with all sorts of wild schemes to deal with it.....get back to simple basics and it will not be a problem...ignore the simple basics and the attention to detail and no matter what you do or what crazy scheme you come up with, corrsion WILL be a problem.

cheers

Hi Legion, the standard rule of design is "Fitment for purpose". The hardest issue I find in design is establishing the design load you are going to use for your design work and then establishing what the SWL will be used..ie Safe Working Load. And in accordance with the appropriate Australian Design Standards.
Once you have worked out those loads, shape of section that you want use will follow. "I" section are the stiffer in bending about x and y axis. But do fall short when taking into account of torsional stiffness. Cost per Kilo of steel Hot rolled section like "I" are the cheapest but there are many other issues to take into account that have been mentioned very well by others. Getting your section rolled doesn't matter if it is SHS,RHS or Hot rolled section. The thing to remember when getting SHS or RHS rolled is they like the wall section to be generally a little heavier. For example if you designed the section to be say 3mm wall section they may ask you if you can use 5 or 6mm wall as the heavier wall is easier to predict with string back on short runs. The price of the steel is pretty linear so go up in wall section thickness and the cost goes up proportionally. One issue with trailers and using light wall RHS say, is 9 times out 10 the section itself won't fail but the welds do. To keep the cost and weight down light wall sections are used. There are some relatively light "I" section available around 100 x 100 and these are not 'I" beam section but actually Universal Column section and are much stiffer than I beam for equivalent size. Angle i think is a really bad choice for any trailer the stiffness for the section is just not the right choice of material and its easy to see as time has gone by SHS or RHS has taken over for good reason. Might be a good idea you read the Galvanic Scale i think before you say statements like Paint is better than Plating..foolish
Sinjin

SHS Square Hollow Section
RHS Rectangular Hollow Section

PS Soundman I think your view statement. "If the chassis rails where properly sealed there would be no water in the rails and therfore no corrosion...if the railes where not sealed the oil would leak out making an unholy mess". There your words Soundman
What do you think steel is made of? Inter granular corrosion occurs because all metals are made up of different elements and they all corrode And unless you are going to magically create some sort of vacuum inside the SHS or RHS your being foolish to say something like that least. To say to someone seal the ends and you won't get corrosion i think it a misguided statement to make and i really think statements like that are fool hardy at best. Galvanizing is cathodic protection design to minimize the effect of Inter granular corrosion. paint is nothing more than a mechanical layer, chip the paint and corrosion will start

I appreciate everyone's input, despite the turn this thread has taken. Some informative stuff from my perspective.

Can I offer my experience of 40 years of Boilermaking / fabricating. Make your trailer from rhs black material (unpainted)if you can get it.Leave your large ends open and drill or blow large air vent/gal vent holes in all hollow sections and get it hot dipped after you clean all welds carefully.If you use the trailer on bitumen roads get it quality Powder coated. If you use it on gravel roads then prime in an etch primer and paint as you can touch up paint as required.Wash it down properly and store it inclined.It should still be in great nick when you are too old to use it.

and the original correct expansion of RHS Is rolled hollow section being square or rectangular,little rant over.