Tow hook mounting

[Sadam_Husain]

Think about the dynamics of a secondary retaining rope, which is probably going to be lighter than the main rope.
The first rope breaks free under load, maybe also with some steel attached, and the stretched rope shrinks and heads for its other end. The restraining rope absorbs much of that acceleration, although not all probably. The lighter rope then fails but with a lower level of energy stored. The combined absorbing of energy should be enough to reduce impacts and any directional change would be influenced by where the restraint was attached.

The tree trunk protector we all have tucked behind the seat looped through the eye of the strop and connected to the other tow hook

[85BJ73]

Might have too much strentgh, and launch the 1st projectile just hard if not harder than original strop? I dunno, never used a tree trunk protector for a recovery other than it's intended use.....

[rangimotors]

bike tubes as second straps are great they have shit loads of stretch and will absorb a large amount of the force as they stretch but will always break before the mounting point.

[KiwiBacon]

Yes I like the winch blanket idea. Anything that can work like a parachute.

at upto 500-600kph it would need to be a pretty big blanket

Blankets work better at higher speed. It won't reach anywhere near that speed with a bag attached.

Here's a COSMOS plot of the 4500kg chinese recovery hook I have. It doesn't seem quite right so I'll check it again tomorrow when my brain doesn't hurt.

The picture is pretty wide, right click and hit "view image" to open it full size in a new window.

[85BJ73]

Okay, I am not used to looking at anything like that, but I really don't know what I am looking at.... is there any stress point references etc??

For the record, I fitted 2 rated recovery hooks to the rear of my truck today. I cut four pieces of 5mm flat steel 100mm long, two to fit inside the chassis (45mm wide), and two on the outside (56mm wide), the ones on the outside are 3mm wider then the chassis on each side, to ensure the load is taken by the webs and hence sides of the chassis, to ensure the load is spread over the maximum possible area, hopefully lessening the stress on the chassis.
I fully expext to have to re-torque them after a recovery or two.....it just seems to be the way of things, I guess the bolts stretch, or need to settle in? Anyone have to do the same? Does anyone want pic's posted? If so, I will do it as soon as I can, it will mean I will have to drop my rear bumper, which I won't do until I need to re-torque the hooks.
So that gives me two rated hooks on the front, and two rated hooks on the back, plus a centrally mounted coupling (not a pintle) in the centre, which is certed to a minimum of 6000kg, but as the rating is not stamped into it ( I never thought to get it done) I am unsure as to whether it will be accepted by some trip leaders. I know I am covered for all trips now.

[TJ]

I fully expext to have to re-torque them after a recovery or two.....it just seems to be the way of things, I guess the bolts stretch, or need to settle in?

Once stretched (or over torqued), bolts are pretty much useless. Re-torquing them after a couple of recoveries would not be wise. If they have stretched that much, they are gone. Use new ones.

[PeterVahry]

The bolts should not stretch as the load on them should be in shear. Only a loading on a strong angle to the line of mounting would generate a stretch of the bolts. TJ is right, a stretched bolt needs replacing.

[85BJ73]

Yup, I know that, maybe it is just the settling in of the plates on the chassis, as they are not welded in, they can move a little. I have not torqued them down as such, just done them up tight, but I have noticed that after the initial installation, I have had to retighten them in the past, kinda like with a new vehicle, or a new installation on a heavy machine, we ask to be contacted after x amount of hours to check the installation.

[KiwiBacon]

Okay, I am not used to looking at anything like that, but I really don't know what I am looking at.... is there any stress point references etc??

I can't make the picture display smaller and the right side with the scale has been cropped off it. If you right click and hit "view image" then it'll show the whole picture in a new window. You'll be able to see the stress scale etc.
Peak stress is roughly 1000MPa, mild steel starts to bend around 250MPa.

Basically that hook would have to be made from extremely high strength steel to not bend at the 4500kg stamped on the side of it.
Being that they're cast, my guess is pig iron and I wouldn't trust them at the rating they're sold at.

After doing that, I'll be putting in my own design recovery eyes and only using hooks for mild recovery.

[85BJ73]

Tried the right click, but nothing happened, or at least I was not given the view picture option.
I often wondered how good these "Rated" hooks really are that are insisted on, especially as I know my manufactured point is stronger, and the hooks I had on the front recovered vehicles from sitiations that I feel the "rated" hooks may have straightened, but I will admit I have no actual proof the "rated" hooks would have failed.

[NJV6]

There was a chap on there J Top who had done a lot of testing of different hooks to find their breaking points including factory hooks and pintals. I remember the pintal with 4 x 8.8 bolts stalled his test rig at 20 000kg and the best of the hooks was the chrome ones. He sucessfully broke all the rest and straightened the forged hooks.

I'll try and find the work he did.

[NJV6]

Found some of his comments.

I have carried out several hook tests on a test rig.
The bighorn hook with 10mm bolts snaps the bolts around 5 tonne
Drilled out to 12 mm, 8.8 bolts still fail but 10.9s hold till the hook breaks @ 8.18 tonnes.
The Black Rat Black hook snapped around 5 Tonne
The Black Rat Chrome hook straightened around 5 Tonne
The Supa Cheap Ridge Rider Black hook snapped around 8 Tonne
The Bush Ranger Black hook straightened around 5 Tonne
3/4 shank towball snapped around 8 Tonne
Pintle Hook/Nato Coupling stripped the 10mm mounting bolts ie Patrol, around 12 Tonne
Pintle Hook with 12mm 8.8 bolts stalled the test rig around 20 Tonne
The Snatchmaster hooks from Cookes straighten around 9.8 Tonne, this opens a new can of worms as the M12x1.75 bolts broke first.

Standard Toyota L/C hooks straighten @ 4.6 tonnes,
Hilux hooks however, by design, push the rope further away from the mounting plane of the bolts, causing the rear bolt to stretch and snap , then shearing the front bolt, 8.8 bolts failing @ 4.5 tonnes.

The Cookes hooks consistently have the highest rating straightening at arround 8 to 9 Tonnes but the bolts they supply are sometimes only 4.6 instead of 8.8 and both of these can let go before the hook straightens.
A Cookes hook with 1/2" grade 8 bolts is my answer.

INteresting to see the ones that broke vs straightening.

[kiwipete]

Found some of his comments.

I have carried out several hook tests on a test rig.
The bighorn hook with 10mm bolts snaps the bolts around 5 tonne
Drilled out to 12 mm, 8.8 bolts still fail but 10.9s hold till the hook breaks @ 8.18 tonnes.
The Black Rat Black hook snapped around 5 Tonne
The Black Rat Chrome hook straightened around 5 Tonne
The Supa Cheap Ridge Rider Black hook snapped around 8 Tonne
The Bush Ranger Black hook straightened around 5 Tonne
3/4 shank towball snapped around 8 Tonne
Pintle Hook/Nato Coupling stripped the 10mm mounting bolts ie Patrol, around 12 Tonne
Pintle Hook with 12mm 8.8 bolts stalled the test rig around 20 Tonne
The Snatchmaster hooks from Cookes straighten around 9.8 Tonne, this opens a new can of worms as the M12x1.75 bolts broke first.

Standard Toyota L/C hooks straighten @ 4.6 tonnes,
Hilux hooks however, by design, push the rope further away from the mounting plane of the bolts, causing the rear bolt to stretch and snap , then shearing the front bolt, 8.8 bolts failing @ 4.5 tonnes.

The Cookes hooks consistently have the highest rating straightening at arround 8 to 9 Tonnes but the bolts they supply are sometimes only 4.6 instead of 8.8 and both of these can let go before the hook straightens.
A Cookes hook with 1/2" grade 8 bolts is my answer.

Interesting to see the ones that broke vs straightening.

Could I assume that all this testing is done in a slow controlled manner, gradualy building up the pressure on the hooks etc?

I wonder then, how these resuts would compare in a snatch recovery scenario, or am I wrong in my thoughts?

[KiwiBacon]

Could I assume that all this testing is done in a slow controlled manner, gradualy building up the pressure on the hooks etc?

I wonder then, how these resuts would compare in a snatch recovery scenario, or am I wrong in my thoughts?

Impact loading (like snatching) will cause items to break rather than bend as they do in a slow loading situation. I'd expect several of the hooks which straightened there to break under impact loads.
Similar to chewing gum, pull it slowly and it stretches. Pull it quickly and it breaks.

The hook I drew up was "Ridge Ryder" brand. Imported for Supercheap Auto Ltd.


In firefox "view image" is what you're looking for. In explorer it viewed full width for me.

[PeterVahry]

Is "snatching" using a stretch rope an "impact" load? The full loading is spread by the stretch process and influenced by the acceleration of the tow vehicle. Unquestionably a minimal stretch rope, if used to pull while any slack is present, would create a definite impact on the attachment points.

[KiwiBacon]

Is "snatching" using a stretch rope an "impact" load? The full loading is spread by the stretch process and influenced by the acceleration of the tow vehicle. Unquestionably a minimal stretch rope, if used to pull while any slack is present, would create a definite impact on the attachment points.

The loading all happens within about a second, a lot faster than your average tensile test rig.

[TJ]

The loading all happens within about a second, a lot faster than your average tensile test rig.

What about metal fatigue due to repeated use of the recovery point.

[KiwiBacon]

The loading all happens within about a second, a lot faster than your average tensile test rig.

What about metal fatigue due to repeated use of the recovery point.

Probably more an issue of the chassis and any welds in the firing line. The hooks themselves are being stressed that highly that they'll bend or break from a single large overload. Fatigue takes thousands of cycles.

I think it needs to survive at least a thousand cycles for it to be called fatigue. Less than that and it's called a f**k up.

[sodhimanawa]

Just to add. I reckon, manufacturers put in a lot into design & crash tests. I think the question of recovery for a stuck vehicle can be very well answered by the Tow hook fitted by a manufacturer. They are I m sure welded & set up in an appropriate way so that a vehicle could be recovered using them. In case of inhibtions the best mode will be the use of a chain around the pumpkin. I ve been using a chain for years to pullout stuck tractors & feedout wagons. I could not find a tow hook in the front of a tractor ....and have always used just the front diff. I think the best mode of recovery is a strong point ie the pumpkin area & a strong rated chain !!

[KiwiBacon]

Just to add. I reckon, manufacturers put in a lot into design & crash tests. I think the question of recovery for a stuck vehicle can be very well answered by the Tow hook fitted by a manufacturer. They are I m sure welded & set up in an appropriate way so that a vehicle could be recovered using them. In case of inhibtions the best mode will be the use of a chain around the pumpkin. I ve been using a chain for years to pullout stuck tractors & feedout wagons. I could not find a tow hook in the front of a tractor ....and have always used just the front diff. I think the best mode of recovery is a strong point ie the pumpkin area & a strong rated chain !!

I too have used chains around tractor axles to extricate them, but the difference there is tractor axles are mounted near the centre and pretty solid.
Pulling hard from the centre of a 4wd diff housing could in the worst case bend it.

Manufacturers designed their hooks for towing rather than lifting or recovery, but there are exceptions with factory vehicles having recovery points.

[Bulletproof]

A lot of factory tow hook mounts have not been designed for the extreme pressures and strains that we put them through trying to recovery a vehicle stuck in a bog.
Not only do you have the weight of the vehicle but also the suction of the bog so any mount definitely needs to have backing plates and rated bolts done up to the right torque.

The point I would like to make is,it doesn't matter how strong the mounts are, there is a need for education in the best methods of recovery.
I have seen 2 nissan safaris tied together and do a snatch tow at about 100mph {exaggeration } but you get the point. It dosn't matter how strong mounts etc are, when 5 tons of weight suddenly stops something has to give no matter how strong.

I personally have been involved in 3 accidents involving recovery, and all were associated with snatch towing.

At the nelson enduro I snatch towed a cruiser, the rope broke and went through windsrceen smashing a womans face. The force so great it blew the rear windows out of the cruiser.

In the upper grey a safari was stuck in the bog hole. Once again I snatch towed, this time the hook with 10000 written on the side of it straightened and demolished the front of the nissan.

On Manuka Island a long wheelbase Nisson got totally bogged .I tried winching but the hilux was too light and nothing to tie onto.
So once again snatched , this time the hook broke and the rope demolished my tailgate. We dont know what happened to the hook, could have reached Blenheim for all we know, just lucky no one was killed.

I dont do any snatch towing anymore and believe it should be discouraged, in stead a better thing is power towing using the weight of 2 vehicles but going slowly.

Personally I believe winching is the better option where every thing is controlled.

Thats my pennys worth.

Bulletproof

[coxsy]

got to admit prefere winching or towing as well, to much force is being used to snatch trucks, and nasty things happen

[skid]

A lot of factory tow hook mounts have not been designed for the extreme pressures and strains that we put them through trying to recovery a vehicle stuck in a bog.
Not only do you have the weight of the vehicle but also the suction of the bog so any mount definitely needs to have backing plates and rated bolts done up to the right torque.

The point I would like to make is,it doesn't matter how strong the mounts are, there is a need for education in the best methods of recovery.
I have seen 2 nissan safaris tied together and do a snatch tow at about 100mph {exaggeration } but you get the point. It dosn't matter how strong mounts etc are, when 5 tons of weight suddenly stops something has to give no matter how strong.

I personally have been involved in 3 accidents involving recovery, and all were associated with snatch towing.

At the nelson enduro I snatch towed a cruiser, the rope broke and went through windsrceen smashing a womans face. The force so great it blew the rear windows out of the cruiser.

In the upper grey a safari was stuck in the bog hole. Once again I snatch towed, this time the hook with 10000 written on the side of it straightened and demolished the front of the nissan.

On Manuka Island a long wheelbase Nisson got totally bogged .I tried winching but the hilux was too light and nothing to tie onto.
So once again snatched , this time the hook broke and the rope demolished my tailgate. We dont know what happened to the hook, could have reached Blenheim for all we know, just lucky no one was killed.

I dont do any snatch towing anymore and believe it should be discouraged, in stead a better thing is power towing using the weight of 2 vehicles but going slowly.

Personally I believe winching is the better option where every thing is controlled.

Thats my pennys worth.

Bulletproof

just curious to know if these breakages occured the first time you attempted the snatch, or was after repeated efforts and were all 3 of the vehicles fairly well bogged down

[Bulletproof]

just curious to know if these breakages occured the first time you attempted the snatch, or was after repeated efforts and were all 3 of the vehicles fairly well bogged down [/quote]

Yes all three happened on the first snatch and they were over 10 years ago.


The first one was in a timed section of a competition. We had won the teams enduro for the 2 previous years and had been told we were in the lead again, so it was win at all costs.
I got through the bog with a bank because of having a good approach angle . The cruiser got stuck. Because it was a timed section the rope was quickly put on and I took off. The cruiser slammed the bank and the rope broke resulting in the accident.
In this situation the bank should have been dug down.

On the 2nd one I got through the bog . The Nissan got really boged down with its big diffs dug in.
Earlier in the day I had broken a CV so had removed the axle . Because of being handicapped I thought I would need a bit more momentum ,resulting in the hook staightening.

We should have used the weight of 2 vehicles going slower and just power towing.

In the 3rd accident I was the only truck that got through the deep bog. The long wheelbase Nissan tried and was really bogged down. I weigh about 2100 and the nissan nearly 3000 so once winching failed we tried one snatch and thats when the tow hook snapped off.
It took 2 cruisers with 8000lb winchs, both winching to pull it backwards and then the trip through the bog was abandoned.

I am probably a slow learner you are probably thinking, but I did eventually get the point and changed the way I recover trucks. I still do more extreme 4 wheel driving than most people have ever seen, and for over 10 years now have not come close to an accident.

I know of one recovery in the lagoon of big river where 5 trucks were tied together to recover a cruiser that was stuck.

My argument is we should not be putting this extreme tension on our hooks and mounts.

If I do have to tow a truck I use a 24mm nylon rope.I take the slack up slowly and then feed the power on letting the stretch of the rope to do the work.
We need to be prepared to use a shovel or high lift jacks more to make the recovery easier.
Winching needs to be used more as well.

All of my tow mounts are over engineered, but this is only gives a false sense of security because a broken flying hook is probably more lethal than a bull bar , so we need to change the way we recover trucks and think things through rather than rip, shit and bust .

bulletproof

[KiwiBacon]

In the upper grey a safari was stuck in the bog hole. Once again I snatch towed, this time the hook with 10000 written on the side of it straightened and demolished the front of the nissan.

Earlier in this thread there's a picture where I ran a chinese 10,000lb hook (possibly the same type) through the finite element program on my computer.
The results fit with your experience, these aren't strong enough to wear a "10,000lb" rating.

[Bulletproof]

Earlier in this thread there's a picture where I ran a chinese 10,000lb hook (possibly the same type) through the finite element program on my computer.
The results fit with your experience, these aren't strong enough to wear a "10,000lb" rating.[/quote]

You are probably right.
After that experience I changed all my hooks to genuine toyota and have had no problem ever since.

Another area I would like your opinion on is the ratings of the bolts.

All the research I have done shows the grade 7 Japanese rating is stronger than the metre 8.8.
All the torque figures for a grade 7 on the Hino and big trucks from japan are miles higher than the recommended torque of the 8.8 that a lot of people are using for their tow hooks instead of the factory 7

I have argued with a lot of engineers but they say I am wrong.

Richard

[rayndeb]

I agree I dont like using a hook for recovery work .. But the general concensus out there is thats the only thing to use..I prefer to attactch the strap to something solid but most guys dont like that.. maybee some of them need to see a failure to change their minds..IMO

[KiwiBacon]

You are probably right.
After that experience I changed all my hooks to genuine toyota and have had no problem ever since.

Another area I would like your opinion on is the ratings of the bolts.

All the research I have done shows the grade 7 Japanese rating is stronger than the metre 8.8.
All the torque figures for a grade 7 on the Hino and big trucks from japan are miles higher than the recommended torque of the 8.8 that a lot of people are using for their tow hooks instead of the factory 7

I have argued with a lot of engineers but they say I am wrong.

Richard

For items like recovery hooks, you're better with bigger and softer bolts than smaller, harder ones. They're more damage tolerant and have a much safer failure mode.

The hook I've mentioned above uses 2 M12 bolts (some use 1/2"). At the rated load of the hook, even grade 4.6 bolts are well within their limits, even if only one of them is taking the entire load.
It's quite easy to get carried into "specification creep". Just remember there's not much point strengthening the strongest link in the chain.

For those who are following the numbers, a M12 bolt at the threaded point has a sectional area of 314 square millimetres.
Holding the hook with one bolt, it has to take 44,000N force, that gives 140 MPa shear stress in a bolt that starts to give at 240MPa and finally breaks at a minimum of 400MPa.

[Bulletproof]

All landcruisers and hiluxs are fitted with factory 12mm Grade 7 bolts on the japanese scale which is completely different from the SAE or metric grade, when they leave the factory.

Some clubs are making their members change these bolts for other gradings.
I believe 12mm 8.8 metric is downgrading from the factory specs and it leaves a person wide open to prosecution in the event of death.
Can anyone enlighten me about this ? Is this true or not ?

bulletproof

[haynzy]

the debate about bolt sizes seems irelevent considering all three of the accidents bullet proof is refering to resulted from hook or strap failure, lets be realistic here guys the method of recovery has to match the situation and the equipment available, if you winch off a well stuck 2.5 ton vehicle with a 12000lb winch hooked to a 10000lb hook with steel cable that may only be 6000lb breaking strain the result will be the same, equipment failure!