Why You Need a Recovery Hitch Instead of a Tow ball
People die using tow balls as recovery points for snatch recoveries. Quite a few have at this stage.
Most four wheel drivers have heard not to use tow balls as a recovery point. This article is more for those that have already been told, but aren’t yet convinced about just how dangerous it can be. Here’s a bit of a deep dive on how it happens.
The Actual Forces Involved in a Snatch Recovery
A tow ball is typically rated to 3,500kg so it’s not unreasonable to think that recovering a vehicle that’s 3.5t or less is within spec.
Mass Times Acceleration
3.5t may be the mass of the car, but it’s not the force that is exerted on the tow ball. Force equals mass times acceleration.
The acceleration in this case is the negative acceleration (or the deceleration) of the car performing the recovery.
The formula looks like this: F=(Vi−Vf)/t∗m
Vi = Initial velocity i.e. the speed of the recovery vehicle before the snatch strap went tight.
Vf =Final velocity of the recovery vehicle. Usually almost stationary.
t = Time
m = Mass
Put in English, force equals the difference between the starting and stopping speed, the resultant number is then divided by the product of mass times the amount of time it took for the difference in speed to take place. The speed is measured in metres per second.
Lets take a fairly conservative example. Let’s say the vehicle doing the snatching gets up to a very modest 20km/h or 5.6m/s (metres per second) and doesn’t come to a complete stop, but gets down to about 2km/h or 0.6m/s.
Plenty of times the stuck vehicle remains stuck during an attempt or and it renders the recovery vehicle completely stationary making it even worse.
So, we have a difference of 5m/s giving us the value 5 for the (Vi-Vf) part of the equation.
To be conservative again, lets say the vehicle goes from 20km/h to 2km/h over the space of one whole second. It doesn’t sound like a lot, but a second is quite long when watching these things happen quickly. That gives us a nice easy value of 1 for the t in the equation.
Finally, the mass we’re working with is 3,500kg.
If that sounds like some hypothetical classroom type stuff and not an actual real world force, just remember it’s the exact same force as if you had a horizontally mounted towball with a 17.5 tonne weight hanging from it.
That’s a 17.5t force launching a heavy projectile right towards your head from less than 20 metres away. That’s what kills people.
Arm Times Weight
But wait, it get’s worse.
Arm x weight is what we know as leverage. As we know, a bigger lever gives us more force. It’s why a bigger spanner rounds bolts easier than a shorter one
Consider that a snatch strap or kinetic rope doesn’t sit flush with the top of the tongue that the tow ball mounts through. If it did, the tow ball would only be fighting against a 17.5t force.
As the strap is sitting above the tongue and the force is acting through the centre of where the strap is mounted, it creates an arm (lever) between the pulling force and the shear point of the tow ball.
Let’s be conservative again and say that the arm/lever this creates is only 2cm or 20mm long.
17.5 x 20 = 350t.
That’s not to say that any force over 17.5t is in play when launching the ball, as you can’t create energy. But it does mean that it’s not just probable that the tow ball will snap, it’s even likely.
Now that we know there’s a ridiculous force acting on the tow ball, let’s look at the specifics of how it breaks.
In this context, single shear simply means that there’s a single point where it will shear or snap. Owing to there being only one mounting location (at the base of the tow ball), it creates a fulcrum at this mounting point and a lever as we have already mentioned.
Double shear just means that the load point is between two shear (breaking) points. If you can imagine the pin that goes through your receiver and hitch, it’s easy to see how the force would act on the centre of the pin, between the two holes in the receiver which are holding the pin.
This achieves two things. The first is that the load is spread across two areas, essentially halving the force at any one point. The second is that there is no arm and no fulcrum, meaning that there’s no multiplication of torque. I.e. in this example, it caps the force at 17.5t.
With a mounting point that’s double shear, we have 17.5t acting across two locations, essentially giving us 8.25t at any one point of failure.
There’s a big difference between 8.5t and 350t.
Can You Use a Shackle in a Tow Hitch?
It seems like a cost-effective solution, would simply be to put your rated shackle through the tongue, where you would normally mount the tow ball.
I’ll admit, the first time I heard people warning of the dangers of this, I thought it was just another case of the over-worrying types annoying the rest of us.
Turns out they were right. A few people have actually been seriously injured from this.
It’s surprising, as rated shackles are tested to much higher forces than what they’re rated for.
Here’s what’s happening:
As you can see from the image, the arrow labelled “resultant vector” is the force acting on the hitch, receiver and shackle due to the offset of the tongue, relative to the receiver.
This creates two hazards:
The first hazard is that the shackle is now being pulled on an angle, creating an arm and a single shear situation on the underside of the tongue. Good shackles are very strong, so the second hazard is more likely: What we can see from the bottom arrow is that the force on the hitch is primarily acting on a single weld at the bottom. A weld that’s designed for a compression load, not tension.
As it turns out, the people that have been injured from using similar setups, were struck by the tongue part of the hitch being ripped from the square tube. Crazy indeed, but the logic checks out.
Factory Hitch Points Are Usually Just for Transport
And just a final note, the majority of factory inclusions on 4WD’s that look like recovery points, are actually just tie-down points for when the car gets transported from the factory.