Drilling into chassis rails

Can you use U bolts instead and not compromise the gal coat or structure?

What are you trying to protect?
Drilling the top or bottom flanges is to be avoided.
Cheers,
Peter
OKA196 Motorhome

Aluminium and steel. Wonderful corrosion to come...

I have a feeling that drilling the chassis is against ADR's unless engineer certified.

Drilling into the side would be better than the bottom, plus water/moisture won't pool around the new exposure as it would on the bottom of the rail.

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I cannot imagine that drilling "screw" holes of less than 5mm diameter anywhere in the chassis profile could compromise its integrity.

Most chassis are RHS steel tube and the wall frames nowadays are aluminium tube. The floor is crudely attached by self drilling screws and then the frame is attached to the chassis by the same method with the plywood floor sandwiched in between. Have a good look around the chassis, there are holes drilled for all sorts of reasons. A few more small holes wont affect it. Michael

Suitcase.
The bottom of a chassis is in TENSION in some places and the top can also be in tension depending where the load and axles fixings are. "A" frames are usually in tension on the bottom faces and compression on the top faces but it changes somewhat with stresses and road travel.
I would not drill holes in the chassis unless absolutely necessary and then ONLY in the middle third of the vertical sides. A van chassis flexes and after drilling the concentration of motion stresses at the point may cause a chassis failure. Despite what some say, the floor isn't screwed to the chassis mains structure but cross beams and the chassis isn't in line with the walls unless you have a very thin van. If ONLY for bash plate why does it have to be chequer plate aluminium, won't simple sheet aluminium do? If it gets marked that is what it is for, chequer plate catches and holds dirt.
I would use some decent aluminium brackets and fix them with SIKAFLEX to the chassis. NO DRILLING needed. Holes for bolts in sheet and brackets then. Sikaflex has been used to keep Commodore firewalls in place for years and also almost ALL truck bodies have their aluminium sheets GLUED ON with SIKAFLEX or similar product. Can be flexed and cut off if need be.
Once you drill a hole it is drilled remember!

I'm with Mal M, drilling into the chassis/structure is to be avoided. A suitably qualified engineering company should be consulted.
Cheers.

As said above, the bottom face of the drawbar is mostly under tension. Drilling or welding particularly on this face is likely to induce stress concentration and any failures will start at this disturbance. Drilling in the middle of the side faces is a much better option because hopefully this would be the neutral axis ie the area where there is no tension (bottom face) or compression (top face). The top face is the preferred face if you have to drill top or bottom because when you put a screw into the hole it fills the hole up again.

I would be inclined to bend the aluminium sheeting such that it sits on top of the drawbar but drops down in the middle to provide the protection required. Even then, you are creating a moisture trap between the aluminium and the galvanising (if present) on the drawbar. Corrosion could then follow on over the years, particularly if the van is left outside. If the drawbar is only painted, corrosion is guaranteed to follow. You are probably better off by using some galvanised steel sheeting for your guard. It is not as rigid as a thicker sheet of aluminium but at least it will not create galvanic rusting conditions

Suitcase, have you considered getting a Stone Stomper, knocks down majority of stones and protects underneath of van, no drilling of drawbar required.

Hi Suitcase,

Another thing to consider is that dust, dirt and small stones will find their way onto the upper surface of the “bash” plate. This will inturn become wet when it rains, creating extra weight, and potential corrosion.

Macca.

Hi Suitcase,

Most modern caravan chassis are just duragal of various sizes welded together in a jig.

Our 12 month old single axle off roader certainly appears that way.

I screwed a few ally checker plate shields under it with some self drilling button head screws as soon as we got it.

There is no way that would affect the integrity of the structure.

Splashed some cold galv on them and they have been fine for around 5-6000 kms so far.

There is a chance of electrolysis but thats a bit of a long shot I think. I'll cross that bridge if it ever happens.

It is 99% likely that you can do what you want to do without breaking anything. Keep the screws small, use zinc rich epoxy paint in the drill holes and around the screws as well to try to prevent corrosion and try to arrange the guard such that it doesn't trap moisture - either by mud/gravel or by having low points. If the chassis rails are not close to the yield point or ultimate tensile stress, you will most likely be OK, but as a general practice this is not a good thing to do.

Years ago, I was talking to an engineer at a conference. He was a private consultant who worked for the iron ore mines in WA. Apparently the WABCO dump trucks were rated for XXX tonnes of Iron Ore. They also used the trucks to move overburden from the site, so they put hungry boards on the trucks so they could carry XXX tonnes of overburden. Next thing they were putting XXXX tonnes of Iron Ore in the trucks to fill them to the level of the hungry boards. The chassis rails were cracking under the extra loads. The iron ore in the Pilbara has a specific gravity something like 6.7 - it is nearly pure iron, compared to a SG of about 2.6 for the overburden. I wonder why the chassis rails were cracking?

He said that a WABCO truck was not particularly well designed piece of machinery, and when they made them, they put the exhaust pipes on as an afterthought. They simply welded a bracket to the underside of the chassis rails. Bingo - a stress concentrator. Cracks were forming from the welds. Now the chassis rails are under tension on the lower flanges and compression on the upper flanges, so by drilling a hole in the middle of the side webs, they are at the neutral point where theoretically there is zero stress. Volia! the trucks can now carry XXXX tonnes of iron ore without cracking the chassis rails.

Same principle for caravan drawbars. No-one knows how far off ultimate loading the material is, but by drilling the underside flange (the tension side) you are inducing potentially extra stress into the flange. How much? probably not much if you keep the holes to a minimum, and have round holes (no sharp notches ). Simply understanding what you are doing may help your judgement rather than guessing the outcome.

I used 1mm stainless mesh to protect my underneath. Stainless does not react and it is fairly light. Screwed straight into the underside of the chassis rail.

My 2005 jayco has a couple 25mm holes on the inside of the draw bar for wiring from factory (located in the top third of the inner side). I have added maybe 20-30 screws/holes where required in the chassis with no issues, including 4x 10mm holes for mounting the front toolbox. Note that I have only drilled into the side, not the top or bottom.

A few small screws won’t make any difference whatsoever to the strength of a caravan chassis. If anyone here can show where a few small screws have caused any type of chassis failure I will make a donation to your nominated charity!!!

Mountain out of a molehill comes to mind….

Suitcase.
Is it ok to drill where some say it won't cause a crack? Perhaps go and look at a few vans and 4wd vehicles and see the the large number of holes NOT in the top or bottom of the chassis. Even the Swiss cheese I buy with holes in it, cracks and rips easier than the solid stuff.

hi
Many van chassis have numerous 2--4mm holes drilled in the side including the odd 19--25mm hole for wiring.
The sky won`t fall in if u put 3 holes --selftapping over 900mm LOL

This subject sure attracts the inexperienced , like blowflys to a turd .