towing hooks

[jonfromhamilton]

I am looking at getting some towing hooks/anchor point hooks to bolt to a front bar.
they have a capacity of 4.5tonne

is that enough for such an anchor point that will probably be used with snatch straps etc.

[gwagensteve]

4.5 tonne is probably fine, but be very careful about the strength of the bar mounting. I wouldn't recover from a bar mounted to the chassis ends of a Sierra without reinforcing the chassis.

[jonfromhamilton]

I will think about the chassis strengthening when I do the RUF.

which I am picking won't be until November now

[Mike57]

Jon

The way I look at it is that you want the snatch strap to break before the attachment point on the vehicle fails. You need to understand how the different ratings work. The rating given to a snatch strap is it's nominal breaking strain. A 6 tonne strap will break at something just above 6 tonnes (and hopefully not less). Lets say you plan to use a 6 tonne strap (rule of thumb is about 2.5 to 3 times the fully loaded weight of the vehicle I think). To be safe you would normally want the hook to fail at 2.5 times this load so that would be 15 tonnes. You might choose to use a factor of safety of 2 making it 12 tonnes as I have with my car. It is a bit arbitrary since there are no regulations as far as I know.

The next thing to understand is how the hook was rated. Is the rating of 4.5 tonnes a safe working load and what is the breaking load of the hook. Ask that question when you buy it and if they can't answer it buy from somewhere else. The breaking load is probably at least 2.5 times the rated load so at least 11.25 tonnes in this case. That is probably close enough to what you want but check anyway.

The next step is to do some calculations on the bolts that you are going to use to bolt it to the bar or chassis or whatever. How many bolts are used to hold the hook to the vehicle? Usually two or four I would think. There are plenty of bolt shear charts on the internet so find one and use the single shear force which will be in KN and convert this to kg of force and make sure that the shear force of any one bolt is at least 6,000 kg or half of the 12 tonnes that you need. (This is a simplistic view by the way but I won't bore you with any more details). You can play with the grade of the bolts and the diameter to select the right bolt but make sure that the bolt of a tight fit (no more than 1.3 mm clearance as I recall but I would have to check the standard) otherwise the formulas don't necessarily apply.

You also need to think about the material that you are bolting the hook onto. You have the hook and the bolts right but will the material it is bolted to tear away before the bolts break? Things like edge clearance of holes and the thickness of the material are important. Without doing any calculations think about how much metal is in that hook and much metal you need to bolt to so you achieve the same rating. You then just keep working back down the chain. If you bolt the hook to a bar then will the bar pull away from the car etc. This part is not so critical because a front bar flying through the air won't go far but a small hook pulled off the bar will certainly go far and fast and can kill.

Hope that gives you an idea of what to think about.

Mike

[gwagensteve]

Mike, that's an excellent summary of recovery hook/mounting selection, and your logic about the safety risk of a hook coming away from its mount is very sound. However, with Sierras as a specific example, this

Without doing any calculations think about how much metal is in that hook and much metal you need to bolt to so you achieve the same rating. You then just keep working back down the chain. If you bolt the hook to a bar then will the bar pull away from the car etc. This part is not so critical because a front bar flying through the air won't go far

Is the big problem. Having the end of the chassis torn out and the bullbar sitting on the ground isn't very tops.

Sierras have eight mounting points for an ARB bar. However, four of these have to come off when bigger tyres are fitted as they foul the tyres. This leaves four holes right at the end of the (2.5mm thick) chassis to mount the bar to.

Here's a photo of the mounts once the outrigger has been removed. (thanks AJSR)



That's all that's left to recover off of (if the stock "tie down" point is considered inadequate)

There's a few ways of making this connection as strong as possible, such as using crush sleeves, but ultimately, those holes are too close to the end of the chassis and the material is too thin to recover off of, especially when you take into account the leverage of where the hooks end up - they are never in line with the mounts, they're always well above them. Even more so with a car running a body lift, so the bar is trying to be twisted downwards.

ARB don't advise the removal of the outriggers at all, and Joe's bar design, despite housing a winch, isn't intended for recovery because it doesn't use the outriggers either.

A Jon is considering building a RUF, it's the obvious time to rework the front crossmember for a heavier duty recovery point integrated into the chassis.

Greg's car as a simple heavy plate eye:



Which you can see here under the number plate,

Whilst CJ's car has a piece of C channel drilled to the standard 15 tonne pintle pattern.



(it has a tiny winch fairlead next to it)

Whilst pintle hooks are very heavy and bulky, GQ patrols have a factory "tie down" that bolts to the rear pintle mount. They're pretty common in wreckers. We installed one on the back of Michael's trolley tug:



And (almost) as an aside, here's what we did to mount the ARB bar on CJ's car. The chassis has had a small piece added above it to compensate for the body lift, and the bar has been cut away so it mounts close to the body despite the chassis extension for the RUF. It very clearly demonstrates how little material is there to work with. Imagine how much leverage there would be on this area if the stock recovery hook positions on the ARB bar (on the risers above the main box section) were used.



Steve.

[Mike57]

Sorry, I did not mean to indicate that the bar pulling off the car was not a problem just that it was not so dangerous as the hook pulling off. The calculations to check the pull through force on those 4 holes on the end of the chassis are very simple. I have the formula at work (don't remember it in my head). I will post it up during the week.

[gwagensteve]

I understood your point, I was only pointing out that whilst a hook breaking free is the worst case scenario for personal safety, it's far more likely that the chassis end will fail and result in a messy repair and a spoilt day.

I guess there's another point to consider too, and that's the attitude of the driver (both of the stuck car and the recovery)

I try and avoid heavy snatch recoveries by driving into deep mud slowly, and pulling reverse if I feel the front start to sink/drag. That way if I'm stuck it's not normally very firmly, and it's not normally far from the entry point.

Likewise if I'm recovering someone, I'll start with 1st low/idle and communicate with the driver of the stuck car to find out if it is moving. If it isn't, I'll build up slowly until the car moves. It rarely takes much effort to get a car moving.

I get very nervous when I see someone back right up for a heavy snatch recovery when one isn't required. It's probably why I also consider the stock tie down points adequate for general (i.e non competition) recovery. In my opinion, the goal is to always try and reduce the force required to recover the car.

Steve.

[jonfromhamilton]

this hook is "4500kg max load capacity"

I will fit the hook but wether I use it for snatching is another issue.
I think the issue will be reenforcing it back to the chassis properly.
Hopefully when I gusset the RUF steel (which I already have sitting at home yay!) it will be enforced enough but then I don't want the chassis to twist so maybe it needs to be anchored back through both rails equally.
I will have 4 point on the bar which mount to the end of the chassis rails wiht probably 180mm between the top of the chassis rails are and where the anchor point is (vertical).

I'ts only a Sierra :S

[jonfromhamilton]

P.S. the hook has stamped in it 6000kg
and P.P.S that pic of Greg's car makes me want to go wheeling right now !

[Mike57]

Here is how you calculate the load capacity of a hole drilled in a piece of steel where the hole is near the edge of the steel.

The standard that covers this is AS4100. The version I have is from 1998 but I think this is the latest revision. The applicable clause is 9.3.2.4 which is on page 114. The formulas are not ones that a novice can interpret so I have shown them in plain English below.

There are actually two calculations to make and the answer if the lesser of the two figures but the one that counts is this one:

The pull through force (in Newtons) = (minimum distance between the edge of the hole and the edge of the ply + half the bolt diameter all in mm) x (thickness of the ply in mm) x (Tensile strength of the ply in Mpa)

For mild steel the tensile strength is 350Mpa. There is a table on page 13 of the standard which gives the figures to use for other materials.

The only catch is that this formula only applies if the bolt or pin in the hole is a tight fit. I think there is a clearance of something like 1.5mm allowed only (could not find the exact figure). If the bolt is much smaller than the hole the pull through force is reduced.

As an example, I calculated the pull through force of the tow point on my ARB bar like this.

Hole diameter = 24.5 mm
Distance to edge of material = 13 mm
Tensile strength 350 Mpa
Thickness is 8.9 mm

Therefore the pull through force in Newtons = (13+12.24) x 8.9 x 350 = 78,622 Newtons.

100 Newton = 10.197 kg so 78,622 Newtons is close to 8 tonnes (you can just divide by 10 as an approximation).

You then need to add a factor of safety to this of say 1.5 so the safe load to apply to this point is 8/1.5 = 5.33 tonnes. (That is assuming that the bow shackle that you use has a pin size of around 23 mm but unfortunately they don’t come in this size.)

Anyway, you can use this approach to calculate if the snatch hook will pull away from the metal it is attached to assuming the force is perpendicular to the fasteners.

To calculate the shear strength of various bolts use the chart linked here. http://www.boltmasters.com.au/webfiles/ ... _Shear.pdf

Also just note that if the fastener is in double shear the breaking force is doubled. An example of a bolt in double shear is the pin which holds the hitch receiver in a tow bar in place. It is in double shear because it has to break in two places.

I must add that I am not a structural engineer and that you should seek professional advice on any particular situation. The above is provided for guidance only.