Drilling into chassis rails

Submitted: Tuesday, Sep 21, 2021 at 09:43
ThreadID: 142592 Views:1564 Replies:17 FollowUps:16
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
Suitcase
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Reply By: Member - Walter B - Tuesday, Sep 21, 2021 at 09:53

Tuesday, Sep 21, 2021 at 09:53
Can you use U bolts instead and not compromise the gal coat or structure?
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Follow Up By: Member - Suitcase (QLD) - Tuesday, Sep 21, 2021 at 10:00

Tuesday, Sep 21, 2021 at 10:00
Not really.
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Reply By: Peter_n_Margaret - Tuesday, Sep 21, 2021 at 10:02

Tuesday, Sep 21, 2021 at 10:02
What are you trying to protect?
Drilling the top or bottom flanges is to be avoided.
Cheers,
Peter
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Reply By: Malcom M - Tuesday, Sep 21, 2021 at 10:18

Tuesday, Sep 21, 2021 at 10:18
Aluminium and steel. Wonderful corrosion to come...

I have a feeling that drilling the chassis is against ADR's unless engineer certified.
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Reply By: tonysmc - Tuesday, Sep 21, 2021 at 10:21

Tuesday, Sep 21, 2021 at 10:21
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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Reply By: Allan B (Sunshine Coast) - Tuesday, Sep 21, 2021 at 10:30

Tuesday, Sep 21, 2021 at 10:30
.
I cannot imagine that drilling "screw" holes of less than 5mm diameter anywhere in the chassis profile could compromise its integrity.
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Allan

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Follow Up By: AlbyNSW - Tuesday, Sep 21, 2021 at 11:04

Tuesday, Sep 21, 2021 at 11:04
I am with Alan on this, I know from experience in the construction industry that engineers have no issues in drilling much larger holes than that in structural members.
The centre third of a member is traditionally the least important area structurally
I would have no hesitation in drilling holes like you described if it was mine.
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Follow Up By: Paul W43 - Tuesday, Sep 21, 2021 at 12:18

Tuesday, Sep 21, 2021 at 12:18
Avoid drilling the holes near welds, joints or other bolt holes. As someone who has worked in the marine industry I would have no hesitation in drilling 3 x 10 mm holes either side.
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Follow Up By: RMD - Wednesday, Sep 22, 2021 at 07:58

Wednesday, Sep 22, 2021 at 07:58
Alan,
That is unless the drill hole creates localized stress and eventual fracture point, it isn't electrics and sudden, it happens over time.
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Follow Up By: AlbyNSW - Wednesday, Sep 22, 2021 at 13:52

Wednesday, Sep 22, 2021 at 13:52
RMD they put whatever holes they require when manufacturing the caravan won’t they be subject to the same stresses?
I know where you are coming from in a technical sense but don’t think it is relevant in a practical one.
We are after all just talking about a very rudimentary caravan chassis not a high tensile chassis of a vehicle.
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Follow Up By: Allan B (Sunshine Coast) - Wednesday, Sep 22, 2021 at 14:17

Wednesday, Sep 22, 2021 at 14:17
.
Alby,
A hole in itself does not cause stress as RMD suggests. If it did then a lot of aircraft sections would be in jeopardy! In fact, a hole drilled at the end of a stress fracture acts to relieve and arrest the stress.
Of course, an excessively over-sized hole may reduce the area of a section such as to weaken it. But we are talking about a three measly sized screw holes in a 150x50 RHS section.
I believe this subject is being grossly overthought.

Suitcase,
On the subject of corrosion:
Electrolytic corrosion and rust will only occur when an electrolyte such as water is present so you may be wise to coat between the plate and chassis with a sealer such as silicon to prevent water ingress. Also under the heads and washers of the securing screws. This will also enhance the affixing of the plate.
Cheers
Allan

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Follow Up By: RMD - Wednesday, Sep 22, 2021 at 19:20

Wednesday, Sep 22, 2021 at 19:20
Alby
Which vans have holes in the top and bottom of chassis members?
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Follow Up By: AlbyNSW - Thursday, Sep 23, 2021 at 04:44

Thursday, Sep 23, 2021 at 04:44
RMD , in my post above I said the middle third of the structural member not that it would matter in this case
In construction we are allowed to drill a 110mm diameter hole in a 300 deep structural member as standard practice to allow the fitting of plumbing and other services.The reason the middle section ( web) is solid is because it is more economical to do so but they can be open web or castilated beams if another purpose was required like weight reduction like on aircraft for example
A 5mm hole in an RHS section is not going to effect its structural adequacy
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Follow Up By: tim_c - Friday, Sep 24, 2021 at 11:04

Friday, Sep 24, 2021 at 11:04
Partly true Alby - though I'm sure you'll find your structural engineer is not so keen on you putting large holes in the web near the ends/supports of the beam (the integrity of the web is critical for shear stresses nearer the ends/supports, while the flanges are more critical for bending stresses towards the middle, assuming uniformly distributed load).
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Follow Up By: AlbyNSW - Friday, Sep 24, 2021 at 20:42

Friday, Sep 24, 2021 at 20:42
Tim_c , yes I am fully aware of the point of contra flexure and the shear areas of a structural member which is why I said in my earlier post traditionally but didn’t think it warranted going into more detail in light of what is being discussed here and the size of the hole
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Reply By: Michael ( Moss Vale NSW) - Tuesday, Sep 21, 2021 at 11:40

Tuesday, Sep 21, 2021 at 11:40
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
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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
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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.
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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)
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Reply By: bruce 1 - Tuesday, Sep 21, 2021 at 15:19

Tuesday, Sep 21, 2021 at 15:19
I'm with Mal M, drilling into the chassis/structure is to be avoided. A suitably qualified engineering company should be consulted.
Cheers.
AnswerID: 638000

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.
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Reply By: Member - John - Wednesday, Sep 22, 2021 at 07:10

Wednesday, Sep 22, 2021 at 07:10
Suitcase, have you considered getting a Stone Stomper, knocks down majority of stones and protects underneath of van, no drilling of drawbar required.
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Follow Up By: Member - Suitcase (QLD) - Wednesday, Sep 22, 2021 at 07:14

Wednesday, Sep 22, 2021 at 07:14
Stone stomper is on order. It stops at the front wall of the van. I’m looking at some protection further back under the van.
Cheers
Suitcase
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Reply By: Member - McLaren3030 - Wednesday, Sep 22, 2021 at 08:52

Wednesday, Sep 22, 2021 at 08:52
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.
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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.
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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: Member - Michael H54 - Thursday, Sep 23, 2021 at 16:16

Thursday, Sep 23, 2021 at 16:16
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.
AnswerID: 638028

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: RMD - Friday, Sep 24, 2021 at 19:52

Friday, Sep 24, 2021 at 19:52
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.
AnswerID: 638043

Follow Up By: Member - Suitcase (QLD) - Friday, Sep 24, 2021 at 20:21

Friday, Sep 24, 2021 at 20:21
If I choose to do anything it will not involve holes in the top or bottom of the chassis rails.
Suitcase
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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

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