Broken Bullbar Bolts

Gday Peter,
Interesting the OKA owners have a similar problem.
The bullbar uses the existing captured nuts in the chassis, so chance of nylocs.
I used high tensile washers and lock washers.
Hadn't considered socket head bolts, not sure why that would help?
I did consider going to 10.9 or 12 but I didn't want the bolts to be more brittle
Need some more time out in the GVD to give it a workout! Our trip out there last month was cancelled so we did the Gawler Ranges instead, and the corrugations were not as bad - I suspect the bolts may have been broken for a while and the remaining 6 bolts had done a good job to keep the bar on.

Hex heads are typically only available to grade 10, not 12. Grade 12 socket head set screws will have higher tensile (and shear) strength for the same diameter and can be tightened to a higher torque. They are simply stronger.
Cheers,
Peter
OKA196 motorhome

Thanks Allan,
Yeah, I think you are right - the heads got stressed by the movement and snapped off - some were broken at the head and some were broken where the thread entered the captured nuts - they all came out easy enough - but I was surprised how easy it was to drill the bolts - might have been grade 8.8 Chinesium.

When I was considering the towbar I noticed the Toyota bar was supplied with shims to make it a tight fit and the Hayman Reese that I fitted did not - might check the bolts on it tomorrow!!!!

I fitted the bullbar when the Troopy was new - just followed the instructions to the letter. No shims.
The gap made the fitting really easy!! Slid straight on.
Yeah my first though was the bolts were no good - the head said JL brand 8.8 tensile - so I looked them up and they are a chinese bolt supplier so thats when I though I'd use an Australian brand I trusted and bought Hobson. The bolts I had to drill out were very easy to drill!!!
On my last trip we did a bit of 4wding over soem rocky tracks and I rember hearing a bang but not knowing what it was - I guess might have been a bolt letting go.

.
There was a gap at the time of fitting. Phil said... "there was a 2mm gap between bullbar and chassis when the bolts were done up tight"

I’m with you Tony,

The Off Road suspension (well respected and recognised brand) on my Off Road caravan uses high tensile bolts to hold the shocks in place. These broke on its first trip after travelling on corrugations, even though they were torqued to the recommended tension. Shocks and bolts were replaced under warranty by manufacturer, but suffered the same fate on the next trip on corrugations. I have since replaced all these bolts with galvanised bolts. Have not had a problem since. I now carry spare bolts as well as a spare shock absorber.

Macca.

Macca, my offroad caravan has I think the same suspension as yours and I solved the issue by changing out the M12 8.8 twin shock bolts to 1/2" Grade 10 bolts that are slightly bigger, higher tensile and a tighter fit through the shock eyes to reduce movement. They also have a longer thread on the Nyloc nuts. I guess there's more than one way to skin a cat!!
The suspension manufacturer says that the bolts need to be checked and retorqued to 90Nm every service which I doubt the caravan service depts ever did.

Thanks for you input Axle. Yeah I also think M10 is a bit undersized for a steel bullbar but that was the thread size available on the captured nuts on the chassis from where the bumper was mounted. I thought about upsizing to M12 but only one bolt was accessible enough to get a chisel in to knock off the captured nut. The bar is held on by 10xM10 and there is also two long M12s dropping down vertically. So there's a lot of bolts.

The bolts were 25 and 30mm long, so threads all the way to the head. Two had broken off at the head and had to be extracted before I could remove the bar and the others were broken where they entered the captured nut, so might not have made a difference.

The bullbar was a very rigid design - it didn't have any of the wobbly mounts found with say some of ARBs bars.

Anyway, its shimmed now and has better quality bolts, will go find some more corrugations and test it out!!

.
Do you need help finding some corrugations Phil? lol

South Australia has more corrugations per head of population than the rest of Australia combined :-)

.
Yeah, SA had its 'ups & downs' from way back in colonial days! :-(

I blame it on the absence of convicts.

WA makes and exports corrugations to SA just so as to share the love. LOL

Yeah, had to think about WA and your dirt roads - when they are good, they are very, very good but when they are bad they are horrid!!!

Thanks Frank,
I was surprised at the gaps in total, each mount had 4mm of gap and bolts coming in from both sides. The gaps were never going to close when tightening the bolts because the bullbar was too rigid and the steel too thick.

" it shows a reduced plastic ductility and brittle fracture compared to many forms of mild steel."

That's where I was going to suggest earlier that the problem may lie. If Phil still has the failed bolts ("machine screws"?) or at least the heads, brittle fracture has a tell-tale appearance.

[Bolts vs machine screws: when I went through engineering, machine screws were all thread, and bolts were not. Maybe the terminology has gone fuzzy or merged since then. I know calling them all bolts is easier though.]

" it shows a reduced plastic ductility and brittle fracture compared to many forms of mild steel."
I am no specialist in this subject, but if a piece of metal is being flexed to the extent that its fatigue life becomes an issue, it is too weak for the job it is doing.
A bigger bolt or a higher tensile bolt (or both) will improve its life, in my view.
Cheers,
Peter
OKA196 motorhome

I agree with Zippo, the suitability of Set screws/machine screws against actual bolts is probably the issue. If, through chassis twisting repeatedly, the bolts, not really bolts, will break seeing Phil has said the thread went to near the heads, Certain to fail. As Zippo mentioned, the fine grained fatigue break face tells a story of the type of failure. Someone mentioned about shear forces, Unlikely to be ANY shear forces unless hitting things, ie, Roos. unless it was the last few bolts flogging in the holes. The bolts certainly didn't shear. I am surprised the captive nuts held on to allow the fixing bolts to break just outside the chassis. Indicates bend forces repeatedly causing cracks and failure. He is lucky it is a heavy chassis, as many earlier BT50's and Rangers developed loose bolts for some reason and the bolts which held on simply fatigued the chassis and broke bits out of the chassis.

A conrod bolt has shank all the way through both halves and is ONLY threaded in the last mm or so where NO flex, only tension happens. It not like that they do not survive. Some I fitted to large machinery engines were not tightened to a torque, but "Free and Tightened length difference was the way they achieved uniform tension/tightness, ie, degree of Yield.

Just remembered I had two photos. And yes, shearing forces were not part of it as the intact bolts would have prevented any shearing. All broken bolts were coming from the sides. Intact bolts were pointing to the rear

Gday Phil,
yes for sure.
The manufacturers seem a bit more conscious of correct torquing of bolts these days. The instructions for the bullbar I fitted here had all the torque settings (44Nm for the M10 and 70Nm for the M12) prominent on the front page of the fitting instructions.