Volvo C303 - The original Cross Country

PLAY SAFE - A guide to safe self recovery

[always under development!!]

I've been very busy recently with Scotland West All Terrain Club (SWATC) on the subject of safety.

After a long discussion about recoveries, winching and Kinetic ropes (KERR or bungee) we had a very serious accident that nearly killed 2 of our members. Both are long recovered but then the accident was in July 2003!  Then recently I witnessed some seriously stupid stuff and it nearly all came to blows. So, here are my views (and some received wisdom) on safety, split up by type of equipment/recovery along with technical information and background. 

Please feel free to email me with corrections, additions, extra information.

Basic Stuff

This is easy. Make sure that you vehicle is well enough maintained to cope with the stuff you're going to throw it at. 

• Clean and check brakes, filters, tyres washer fluid and coolant.

• Carry first aid (click here for a good source) and learn how to use it.

• Carry essential spares - don't rely on others.

• Carry some sugary food and drink e.g. Mars bars

• Recovery equipment -

  • Make sure it's clean (at least to start with) and in good condition.

  • Check it before you start and once you get home

  • Clean it often - dry mud will wear it out FAST

  • Pull out, check and wind in wire rope/plasma and check it CAREFULLY

Recoveries

Incidents. (in no particular order)

• Driving across winch cable/rope laid out prior to recovery - had the rope become tangled, injury and damage to both vehicles would have been certain.
• Connecting Kinetic ropes using shackles - the chances of exceeding the shackles rating are high and a bungee powered shackle in flight can kill.
• Strops/treesavers/straps being used for kinetic recoveries - Recovery equipment MUST be used only as designed or you risk damage injury or death!
• Rushing to recover - Recoveries must be carried with forethought and understanding.

Some tips. 

• Kinetic/bungee ropes

  • Attach securely with properly rated shackle taking into account BOTH vehicles weight and depth of stuckness (techy term)

  •  Fix to CHASSIS recovery points ideally using a bridle, don’t use thin tie down points especially on rustbuckets!

  • Don’t join together using shackles, the forces involved in pulling out even a Suzuki are tremendous and whilst you use a strong enough shackle, if a recovery point fails you then have lots more metal missiles heading straight for on or other of the vehicles.

  • If you have to then slide one end through the loop of the other putting a strong stick or pole in between so you can separate them afterwards

ONLY USE BUNGEES IN A STRAIGHT LINE

Winching

KEEP EVERYONE CLEAR                        WEAR GLOVES

Get 1 person to control and then LISTEN TO THEM

• Check your equipment before and after events, fix it first before it fails when you need it most.

• Only use proper safety closing hooks attached properly (not shackles) so they don’t fall off during recoveries

• Vehicles being recovered should carefully drive to assist winching

• Keep everyone well clear in case something breaks

• Try to winch in a straight line it uses less energy therefore less strain on both vehicles

• Snatch blocks (pulleys) halve the strain and the speed but need double the rope

• Pay out as much rope as you can – the less rope on the drum the lower the gearing on the winch the easier it is to get the recovery started.

• As more rope goes on the drum the faster it gets wound in and the higher the gearing gets

• Never straddle a winch rope.

• Pass the rope hand over hand (don't slide hands along it

• Always wear gloves when handling steel wire rope

• Never go behind a vehicle being recovered

• One person controls the job

• Anyone can stop it by shouting STOP if they see something going wrong

• Ensure recovery points are designed for the job

• Operate the winch controls from as far away from the rope as possible

• Don't go within 10m of a rope under tension

• Send those not involved in the winching activity to a safe area

• Always disconnect the slack rope from a single point before doing any other work

• Clear fouled equipment with a stick and not your fingers
   

A guide to estimating the pull required  (with thanks to Scott Bancroft)

Estimated Pull = Rolling Resistance +Gradient Resistance +Damage Resistance+ Safety Factor or

                                            EP= RR+GR+DR+SF

To work out rolling resistance: Weight of Vehicle (WOV) divided by ground factor

Ground Factors
                                Mud & Bog = 2
                                Soft wet sand = 3
                                Sand =4
                                Light Mud = 5
                                Hard wet Sand = 6
                                Grass = 7
                                Hard Standing = 25

So in the case of a land rover weighing 2 tons in deep mud the rolling resistance would be 1 ton.

Gradient resistance    Weight of Vehicle x Degree of slope
                                                    60

If slope is over 45^o just the weight of vehicle.

So the same 2 ton LR down a 30^o slope would = 1 ton resistance.

Damage Resistance
On a LR approx 0.5t per locked wheel.

Safety factor                 RR + GR + DR
                                            4

So in the case above we have RR of 1t + GR of 1t, no DR, add these together we have 2t then add a safety factor of 25% (0.5t) and we have a total estimated pull of 2.5t.
 

Snatch (pulley) blocks

• snatch blocks double the pulling power of a winch by halving the gearing

  • Double the power

  • half the speed = twice the control

  • double the length of rope

• winching vehicle hooks onto stuck vehicle or vice versa

• reduce strain by using 3 vehicles

  • the Vehicle 1 is stuck. Vehicle 2 parks about 40ft away with Vehicle 3 right beside.

  • rope runs from V2 through the snatch block shackled to V1 and anchored onto V3

  • The anchoring tension is shared between 2 vehicles at half the load because of the snatch block.

Kinetic Ropes aka Kinetic Energy Recovery Rope (KERR) or Bungee

A new KERR broke after 12 uses - the rear door won't open	A new KERR broke after 12 uses - the rear door won't open	A new KERR broke after 12 uses - the rear door won't open	A shackle gave out and came back and hit the door gently!

Possibly the most effective yet dangerous piece of kit we have access to. I've seen cars turned over by unsympathetic recoverers, 'recovery points' and tow hitches ripped off, so here's some information (some a bit scientific) and tips.

So we're stuck in Landie in a peat bog up to our knees and a passing Suzuki offer to help. Yeah, right. Like a 1200kg Zuk will have any kind of chance at pulling out a stuck Landie.  Out comes the KERR and is fixed to both vehicles (more on that later). the Zuk roars off (OK,OK, get over it!) and get brought to a quick stop after about 10 metres. The Landie had moved forwards by 6ft! Back up and try again and with all engines revving and mud flying out they come. Job done.

The Science Bit

What we observe is a recovery vehicle speeding off and being slowed smoothly by the stretch of the rope and the stuck car popping out. How?

A KERR is a storage device. It takes the momentum of a moving vehicle and stores it as kinetic energy which is then applied to another to overcome the inertia that has it stuck in the mud. Momentum is the tendency of a moving body to want to keep moving whilst Inertia is the energy required to get the damn thing moving in the first place. 

Weight and Mass are 2 different things. Weight is a force - i.e. the force applied by gravity on little old me is my mass x gravity. 90 x 9.81(acceleration by gravity=9.81ms^-2) = 882.9N (newtons). On Planet Earth I weigh 90N. On the Moon, however, where gravity is 1.64ms^-2 (1 sixth)  I weigh 90kgs x 1.64 = 147.6N but my mass on the moon is still 90kgs! So if I'm running at 1 metre per second on Earth you'd need a rope with a breaking strain of at least 900N to stop me, how strong would it have to be on the Moon........

Right, I know this is like being back at school but the answer is easy, STILL 900N! My mass is unchanged.

Lets apply this to MUD. A Landie with mass 2000kg is proper stuck. It will require 40,000N to get it out. Along comes 1000kg Zuki with a KERR which needs to accelerate at 4 ms^-2 to generate the necessary 40,000N of energy. 4ms^-1 = 9.09 mph. SO the Suzi must accelerate to 9mph before the rope starts to stretch.  Easy, huh. No, not yet. Now we have to take into account energy losses, e.g. friction, noise, resistance to stretch, the weight of the rope being pulled, slack taken up, stretching of metal components and, most importantly, the inefficiency of the rope itself.  These may add up to 30% losses that must compensated for. We can see, therefore, that either the Suzuki must accelerate to at least 12mph in 7 metres or take several bites at the Landie.

Phew!

Joining Forces

What do you do when you need longer than 7 metres?  Join 2 bungees together with a shackle? NO! The forces involved in a KERR recovery are so much greater than you think that if you were to use a tested 3.5 tonne shackle (ie a big one) to pull out a stuck Landie 110 with another 110 the approximate force would be somewhere near 120,000N on the basis that the stuck 110 has had it's weight increased by 2¹/₂ by the stuckness and the recovery 110 has a V8. This is the equivalent of a mass of 12 tonnes being dangled from the shackle, and even with a safety rating of 3:1 you're way past the breaking point. When it does, it'll kill someone!

General points

·        Plan your recovery – talk it through and take responsibility for listening, everyone knows their vehicle best

·        Keep everyone clear

·        It should be the person being recovered who is ultimately responsible for deciding what should happen but if the recoverer disagrees they’re entitled not to do it!

·        Plan it through again and make sure everyone knows what going on

·        Listen to others, you may just learn something

There are few hard and fast rules, sometimes you have take a calculated risk but in 99% of recoveries slow is safer and better. Take your time and think safe and we’ll not kill anyone, break our cars or fall out with each other.