Test data on bolts, hooks, strops.

[mudgrip4]

I have just refound interesting article on hi-tensile bolts, rated hooks, and strops. It is in New Zealand 4WD magazine, March 04 edition - the one put out by modest Sam Parker. They note these are only guidelines but they are interesting reading at this time.

Under testing, hooks secured with 8.8 hi tensile bolts actually sheared off at 6395kg of pressure = 6 tonnes plus. Note the maximum strength achieved for the 8.8"s is when torqued to 57ft/lbs of tightness - too much or too little decreases the strength. To do with looseness, and elasticity of the bolts.

To test standard hooks, rated at 10,000lbs (about 5 tons), they used stronger 12.9 rating hi-tensile bolts and proceeded to put load on the safety hook. It finally opened up at a massive 9810kg - double the rating stamped on the hook.

They included also tests for an 8000kg strop (the recommended rating for most 4wdrivers) - very useful for snatch towing. You can snatch a 3 tonne vehicle (covers most all 4wd's) at up to 15km per hour, reaching a 7.65 tonne stress on the strop and theoretically safe. 5 and 10 kmh snatches - far more common events - produce stress loads of 2.55 and 5.1 tonnes respectively - safe for the strop. These figures have to be guidelines only as stresses will vary hugely if hauling a 4wd out of deep bog or deep river sand - as many of us will know. In these conditions stresses can and do open up hooks, break strops, rip towbars off and rip bolts out of the chassis. Good to avoid low rate strops (4 tonne etc ) for snatching purposes.

This might clarify usual club requirements for the strength of bolts, hooks, strops etc. If you have low rated hooks (as some Mitsis have), you could be in alot of trouble in a heavy snatch situation. Also, 10ml 8.8 bolts will have much less shear strength than 12ml 8.8's. According to my ancient maths, the surface area of the head of a 12ml diameter bolt is (pi r squared) = about 113mls of metal area. The 10ml bolts on the other hand are about 79mls area = over 30% less, and over 30% down also on shear stress, which might rate the 10ml 8.8's at about 4.4 tonnes. And that is definitely dangerous. Very. Get a 10ml equipped vehicle into deep suction mud or river sand conditions and the huge snatch required to pull it out (a 2 tonne vehicle) will certainly exceed 4.4tonnes = missile territory. Hence requirement for bigger bolt size.

I must apologise for my ancient maths - someone will calculate much clearer than I can - but the need for hi grade safety equipment came home to me again when rereading this data. Mounting rated hooks with 12ml bolts is not easy for Isuzus. Eventually got a hook with bolts right through all of the chassis under left front. For rear mounting, bolted hook onto towbar on right side. But first checked to find 4x10ml 8.8's holding this side of towbar to chassis - with towbar also welded on to chassis( 2 more 10ml bolts plus welding on the exhaust side). Put my hook onto this with 12ml 8.8's and it has proved mighty strong. You can, if you also have very strong towbar , remove towball, drill extra hole and mount hook on top of towbar tongue with 12ml bolts - very common.

Found this very interesting - hope this might offer more background understanding re safety standards we are currently looking at. Once again apologies for the maths - its been a long time... Mike A.

[geeves]

Just to frighten everyone even more There was a letter to the edditor in the March mag that stated that the final hook failure was underrated as the mounting jig had bent meaning the load was not as the base of the hook This virtualy means that the hook is not going to fail before the bolts and at these loads bolt failure at the end of what is essentialy a big elastic band is not somthing to be taken lightly I have heard of hooks going through the rear door then out through the windsreen then through the rear door on a second car. It dosent take much imagination to work out what hapens if a liveing object is in the way of the flying hook

[mudgrip4]

Ouch! I recall when I began 4wdriving (not in a club) that several clubbies told me quite emphatically to get rated hooks bolted through the chassis member with 12ml 8.8's. I confess I thought they were being very over the top - rather dictatorial about things.

Rereading this mag material, I'm of the opinion the 12mls are the minimum we need. That they could break off at less than 6000kg is fairly sobering news. That 10ml bolts(Isuzu standard) could shear at round 4 tonnes frankly scares the hell out of me. Anyone reading this who has had experience recovering 4wds from mud and riversand etc will know the massive amount of snatch grunt needed to move them at times - occasionally tandem tows/snatches also.

I suspect Andrew this is the starting point for club safety - a bit of very real life insurance. A couple of hooks, bolts, and perhaps an 8000kg strop might cost $150 or so if a shop fits them - less than a single mud tyre, but as the figures suggest - one hell of a lot more important.

I suspect also Alan that you are right - earlier comment - in that OSH (govt watchdog for occupational health and safety in NZ) and insurances cos are going to look for these standards. I know that some station owners here ask about the clubs' safety training, and one major forestry owner ( Carter Holt) actually intends doing scrutineering on vehicles before allowing access here in Canterbury. True - was at the meeting. Mike A.

[James W]

Your math is fine, but note ml = mililitres, when in fact you mean mm or milimeters, and area of course is milimeters squared (mm^2) <-- superscript 2.
And don't believe anything the papers publish and being correct. Kmh is not a velocity at all (Kilometre hours!)

But getting back on topic very interesting to see some justifications behind the standards. I'm a physicist and prefer this kind of more informed method.

[simon2]

A very interesting and thought provoking post. Important to know.

[mudgrip4]

Absolutely delighted my maths ok -about 30 years since I left uni (where I did not study maths), and at that time mls/mms/kilos etc had about as much working relevance to us as the ancient greek cubit.