Drilling into chassis rails
Submitted: Tuesday, Sep 21, 2021 at 09:43
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Member - Suitcase (QLD)
What are people's thoughts on drilling into caravan chassis rails? I would like to mount a piece of aluminium checkerplate under the van as protection - the best place to attach it would be the chassis rails. Rails are 150 x 50 x 4mm steel - 13' van. Looking at maybe 3 screws over a distance of 900mm along the chassis. Is drilling into the 50mm bottom better than the 100mm side?
cheers
Reply By: RMD - Tuesday, Sep 21, 2021 at 15:18
Tuesday, Sep 21, 2021 at 15:18
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!
AnswerID:
637999
Follow Up By: Member - Suitcase (QLD) - Tuesday, Sep 21, 2021 at 15:38
Tuesday, Sep 21, 2021 at 15:38
Certainly had not considered the option of just gluing it on - will give that serious thought. Similarly your suggestion re: flat plate vs checkerplate - had already given that some thought.
cheers
FollowupID:
916199
Follow Up By: Member - shane r1 - Thursday, Sep 23, 2021 at 17:19
Thursday, Sep 23, 2021 at 17:19
My vote for sikaflex too, I’ve used it to put galv sheet on a trailer , and a floor in another trailer , good stuff.
Probably harder to get off again than if you screw it on.
FollowupID:
916232
Follow Up By: tim_c - Friday, Sep 24, 2021 at 11:12
Friday, Sep 24, 2021 at 11:12
Really? I'd have thought most of the bottom of a caravan chassis would be under COMPRESSION (unless you've got the axles at the ends, which would be rather unusual for a caravan)
FollowupID:
916243
Reply By: Erad - Tuesday, Sep 21, 2021 at 16:08
Tuesday, Sep 21, 2021 at 16:08
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
AnswerID:
638001
Follow Up By: Member - Suitcase (QLD) - Tuesday, Sep 21, 2021 at 16:13
Tuesday, Sep 21, 2021 at 16:13
Don't have a problem with considering steel sheet. The position of this thing is up under the van - only got access to one side or the bottom of the chassis and a couple of cross members.
FollowupID:
916201
Reply By: Member - Wooly - Wednesday, Sep 22, 2021 at 17:24
Wednesday, Sep 22, 2021 at 17:24
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.
AnswerID:
638018
Reply By: Erad - Wednesday, Sep 22, 2021 at 22:13
Wednesday, Sep 22, 2021 at 22:13
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.
AnswerID:
638021
Reply By: nickb - Thursday, Sep 23, 2021 at 23:22
Thursday, Sep 23, 2021 at 23:22
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….
AnswerID:
638033
Reply By: swampy - Friday, Oct 08, 2021 at 11:27
Friday, Oct 08, 2021 at 11:27
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 .
AnswerID:
638243