Recovery points on 4wds!?

Hi Tony, If as you state a chassis rail recovery point is good for a gentle tug, Then this is what I'm on about !, whats the use of the thing?, and agree bridles are good, but how many have any idea what they are or use them ?,Most people think use the recovery point for a snatch, but as i reckon its near as dangerous as a tow ball,. Hitch receivers are good but do rely on two very high rated pins!..lol.

Cheers.

Axle posted:

"Most people think use the recovery point for a snatch, but as i reckon its near as dangerous as a tow ball,".

I agree Axle. I can't see how a 4500kg rated recovery (towing) hook is any better than a 3500kg tow ball when connected to a 9000kg snatch strap with a bloody great Cruiser or Patrol on the other end of it. You can always give it a "gentle" pull but how gentle is gentle?

The most worrying thing about recovery hooks when you are assisting a stranded vehicle is how many violent or even gentle pulls has the hook on that vehicle already been subjected to? Repeated use weakens them and their mounting points and there is ample evidence to show that countless ones have let go. Are you going to be the poor bugger that has the hook flying into the back of your car at better than 300 kph!

A few tow bars have also failed but the so called "rated" recovery points are by far the worst offenders.

Hi Splits,

"Repeated use weakens them and their mounting points and there is ample evidence to show that countless ones have let go. "

If using properly mounted 'forged' recovery hooks I disagree. Repeated use of a forged recovery hook will not weaken it. I have done engineering tests on forged recovery hooks and they simply unfold when over stressed, cast iron recovery hooks do break when they reach their limit. Some of the tie down loops (note they are NOT recovery hooks) do break welds and let go. If mounted properly with chassis crush tubes and double plated you will have no problems. If you want some specs on how to properly mount recovery hooks this let me know.

KK

Kiwi Kia posted:

"I have done engineering tests on forged recovery hooks and they simply unfold when over stressed, cast iron recovery hooks do break when they reach their limit. Some of the tie down loops (note they are NOT recovery hooks) do break welds and let go."


I have read reports on the net on testing hooks and they do pull out straight as you said. They usually hang on to about 5500 kg though and I doubt if many snatches would exceed that load. On the other hand they might but who knows? That is the big problem with snatching: the only thing rated for snatching is the strap itself so you really don't know exactly what you are doing to any attachment points.

Have you carried out any tests involving multiple pulls on forged hooks or noted how much they bend under loads of about one quarter to one half those required to straighten them? Constant bending or vibration will crack just about any metal and even car springs are no exception. It would be interesting to see what condition a forged hook is in after say 100 snatches at around 2500 kg.

No matter which way you look at it, the attachment points on the front of cars are the major cause of snatching accidents. I have even read of high tensile bolts having their threads ripped off sending the hook flying. Then there is the issue of heavy concealed rust inside the mounting points to contend with. That has been the cause of a few hooks coming off.

Tow bars seem to be ok in most cases although there have been a few examples of the bars coming off Hiluxes after the last 150 mm or so of the chassis rails tore off. There may have been a lot more but you would not know unless someone goes to the trouble of posting the details on the net or the incident attracts media attention.

The industry needs some form of regulations covering the use of these things, particularly the attachment points and nothing is going to change until that happens.

We tested all the well known available hooks (including a 'super' brand ). They all went up to the stamped rated load before unwinding. However, they did not exceed their rated load by any significant amount, that is they do not have much built in margin above their rating like shackles etc. We did strain gauge load tests on snatching to see what forces were actually incurred (if you are interested I can describe how we did it). The data collected involved recording time line markers so that we could also register the accelerations involved. Significant to say here that we were satisfied that the hooks and mounting methods below are ok with moderate to heavy snatching FROM A SAFETY POINT OF VIEW. We did damage a cruisers front bar attachment points when looking at using 2 vehicles in tandem doing a snatch - why did we do this ? Because we had the test gear set up, a cruiser really buried in loose gravel and we wanted to test to destruction :-)) (either the straps, the hooks or the mounting methods). Let me say, do not do tandem snatches !

You are right to raise the issue of high tensile bolts. The higher the tensile rating the more brittle the bolts so we require bolts used for mounting hooks etc. to be; 12 mm metric bolts grade 8.8 or 10.9, 1/2 inch bolts imperial grade 5 or 8 and - NO cap screws. We do not accept bolts of a higher tensile rating. The above is for 3mm chassis rails only and require crush tubes to be fitted. We also want a minimum distance from the end of the chassis rails and no where near any crush zones. We have figured a way of using a chain link to bridge a tension load across a crush zone but that starts to become complicated. On some chassis rails you can double plate the rail to distribute the load across a bigger area. A large part of the load is actually accounted for in the 'clamping' effect on the surface of the chassis rail so bolting on a double plating spreads the load very effectively.

KK

" I can't see how a 4500kg rated recovery (towing) hook is any better than a 3500kg tow ball"

Gday Splits,
The towball issue was being discussed at our 4wdclub last meeting. So in answer to your very common question above, it appears that the shearing force on a towball is different to a hook. The towball can LIFT and then shear while the recovery hook has at least two points of attachment so it cannot lift away from the chassis.

My concern about some rated recovery points are that many of them are stronger than the chassis they are attached to and require a 3.25 or 4.5T shackle. They can break away from the chassis.

Kiwi Kia posted:

"We did strain gauge load tests on snatching to see what forces were actually incurred (if you are interested I can describe how we did it).

The towball can LIFT and then shear while the recovery hook has at least two points of attachment so it cannot lift away from the chassis. "

Thanks KK, I would be interested in seeing the results of those tests.

Regarding the hook and tow ball: due to the shape of the hook, the strap pulls about 50 mm above or below the two bolts depending on whether the hook is mounted facing up or down. Would this not cause a combination of both sheering and pulling forces as the hook tries to sheer as well as pivot on the front bolt and sheer and try to pull the second bolt almost straight out?

This could explain why some bolts have stripped their threads. I would have thought a straight sideways pull would be very unlikely to do that.

A hook shaped like an eye with an opening in it would place the strap directly in line with the bolts and could be a better design.

Hi Splits, There is quite a bit of material so if you can let me have an email address I will send it on to you. Send your address to nzrees@yahoo.com.

When you bolt a hook onto a chassis a very large percentage of the 'gripping' force is due to the clamping effect between the flat surfaces. As long as you use the correct torque settings for the bolts that you use AND the bolts are not smaller then the holes in the hook and chassis AND the hook is forged not cast, you should not have to worry about a bolt pivoting and stripping the thread.

You may also find the following of interest;

Recovery Loads
Loads incurred when recovering a vehicle can be very high. In the following tables LW is the Loaded Weight of the vehicle being recovered.

A pull of 100% of LW is required to free a vehicle stuck up to the top of the sidewall of a tyre.

A pull of 200% of LW will be required to free a vehicle stuck up to the hubs of its wheels.

A pull of 300% of LW will be required to free a vehicle stuck up to the body.

Add the following load if a vehicle is to be recovered up a slope.
15 degrees - add 25% of LW
30 degrees - add 50% of LW
45 degrees - add 75% of LW

Also, have a look at the following web site for some interesting info on loading of hooks & slings etc.

http://www.pirate4x4.com/tech/billavista/Recovery/index.html

KK

PS get back to me soon if you can as I will be away for a week from Friday (4wding all next week).

Agree Phil, But i guess what i am saying is the manufactures should be having a better look at their recovery points?.



Cheers Axle.

4wd Manufacturers know that snatching with an elastic snatch strap is dangerous. Thats why they fit "tow" points and not "snatch" points.

Most vehicles leave the factory with front and rear "tow" points that are appropriately rated to "tow" that vehicle. Many of those tow points are removed when towbars and bullbars are fitted. The dealers chuck them in the recycle bin.

Same thing with tow bars. No manufacturer of tow bars will recommend their bar be used for snatching. But we know from experience that a the square hitch receiver is strong enough for mild snatching but no towbar manufacturer is willing to expose themselves to litigation by saying you can snatch from their bars.

Cheers
phil

So where does that leave us?....LOL.



Cheers .Axle.

It leave us towing or winching to recover our 4wds.......unless we want to take some responsibility for our own actions and then we can "snatch".

HMMMM!!.. Very interesting when you stop and think about it all!.


Cheers Axle.

Yep!!!!
I'm amazed in this day and age (of litigation) that snatch straps are still virtually sold in supermarkets without warning the punters of what can go wrong. There's been enough deaths and injuries from the inappropriate use of these things over many years.

I'm very keen for people to take responsibilty for what they choose to do.
But 20+ years ago I bought my first snatch strap and 10 years ago I first learnt how to use it. And today I have a lot more respect for what can go wrong, and it is fair to say I rarely snatch, and more often tow.

Axle posted:

So where does that leave us?....LOL.

Stuck in a hole that it would have been wiser not to have got into in the first place!

HeeHeeHee!

I bought a snatch strap from Nobles and it has a full page of warnings about snatching and mounting points. The strap also has extra webbing to be secured at either end in case of breakage. I think the industry is responding to the dangers of snatching but as usual the budget end is a bit slower to pick it up.

Phil G

Must be a while since you brought a strap, most (if not all) manufactures provide a warning and instruction on snatching.

I have no connection to Just Straps, its just that I know that they have all the information available on their web site.

GUIDELINES FOR SAFE USE OF SNATCH STRAPS

NEW MANDATORY SAFETY STANDARD
The Queensland government has introduced a mandatory safety standard for the supply of motor vehicle recovery (snatch) straps effective from 1 April 2008. The prescribed safety standard specifies key product and safety information that must be supplied with every Snatch Strap.

4WD PRODUCT SAFETY AND INFORMATION STANDARD JSF (AUST) 2007

Gday Tony,
That's a good 2 page guideline. Hopefully hope they package it with their products.

I buy a new strap every couple of years. I just checked out 3 brand name snatch straps, an equaliser strap, and two winch straps. Only the ARB strap purchased 12 months ago had a warning on the strap (in fine print) and it was packaged with an info booklet. The remainder were purchased in plastic bags without literature and without any useful safety warning.
Best of the rest was this (hardly a warning!)

Thanks Phil,

That is a worry when manufacturers show no care.

this may be of use as well for those who do not know the related forces.

I think this may be somewhere else in this thread, Force calculations of a bogged vehicle


again no relationship with Just Straps, but they do give all the information

Information on Snatch straps, tree protectors, winch extension straps, equalisers etc.

A good resource page.

So your saying that if you do not have a removable hitch type tow bar removing the tow ball and replacing it with a rated shackle is still a no no, or the 50% affect on stregnth is related to the snatch strap

The 50% reduction is an arbitory figure used with lifting tackle related to the safe working load of a strap that is wrapped around an object, tied or choked. Attaching a strap with a D through the hole of a tow bar that doesn't have a removable hitch would be fine in my opinion as long as the whole bar or tounge is appropiately secure. A 3 tonne rated D fails at over 20 tonne so the strap will always let go 1st.
Cheers Craig..........

I have read with interest the above comments, especially to do with High tensile bolts,

It needs to be understood that high tensile bolts are generally no stronger in shear than ordinary bolts. There advantage is that they clamp the two surfaces together much more tightly (provided they have the correct tension applied) and thus the two surfaces grip each other due to friction. A high tensile bolt not correctly tightened does nothing an ordinary bolt will do.

If using 12 mm bolts then go with grade 8.8 or 10.9. Above this grade the bolts can be assumed to be to brittle for using on hooks used for snatching.

KK