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How to make a vehicle move
Torque
To overcome forces generated by the weight of the vehicle (rolling resistance), steep grades (gravity) and resistances created by material pushed by the tires (sand, mud, snow etc) plus resistances created by vehicle component contact with ground material (rocks, logs, sand, snow, mud etc) – best combined under “Load”, the vehicle needs to generate sufficient torque by combining engine torque and adequate gear sets (see “Gear Ratio” below).
Traction
In order to generate torque, a counter force is needed: Traction.
Without traction no torque can be generated.
The more traction is available – the more torque can be generated.
If traction in insufficient, the vehicle will not move.
You always have to have more traction than the torque needed to move the vehicle.
More Torque than Traction = slipping tires = immobile vehicle – stuck.
Reasons for Stuck
Any properly designed 4x4 vehicle can always generate sufficient torque with proper gear selection to keep the vehicle mobile. Its up to the driver to create/find the needed traction balance.
Definition:
When the “Load” is higher than the Traction needed to generate Torque, a vehicle is stuck – not able to stay mobile.
A few examples:
Steep grade combined with low traction
Medium grade with additional small steps
Soft sand with too much Ground Material Resistance
Grade with split mu µ (one side rock, one side dirt)
Rock garden with split mu µ (one wheel on rock, one on sand)
Vehicle is high centered on ground material (ground anchor resistance)
Uneven terrain with severe weight shift (diagonally opposed traction loss)
Grade approached at angle (creates severe weight shift -diagonally opposed traction loss)
Steep grade with aft weight shift - almost no traction front = equals 2WD traction
Load Model
I call the combined forces a vehicle has to overcome in order to move “Load”
Involved are:
Rolling Resistance
The weight of the vehicle will create a rolling resistance at the tires
( 4 ton vehicle has only a rolling resistance of a few hundred pounds on level ground)
more rolling resistance is created by:
increased vehicle weight
less tire pressure
rough ground surfaces
Gravity
When transitioning from level ground to a grade gravity is added to the equation
The steeper the grade the higher the forces of gravity – duhh
A rather small 6” vertical step during a 20% (easy) climb can prevent even a capable vehicle from continuing.
Ground Material Resistance
Soft materials like sand, snow and mud build up in front of the tires because the tires sink more or less in soft material. As soon as the vehicle breaks the surface it constantly pushes against the resistance of the built up ground material. The deeper the tires sink in, the higher is this resistance. Deflated tires substantially reduce ground material resistance. In sand deflated tires do not create more traction – they reduce the ground material resistance (since the pressure per square inch is reduced, the sand molecules stay more stable and don’t shear – see note in “Traction” below).
 tire pressure and soft sand/snow
turns in soft sand/snow
Ground Anchors
Any contact of vehicle parts to the ground material other than the tires create very strong ground anchors. Contact with rocks and logs are obvious points of resistance – but even contact with soft material like sand, mud and snow can easily stop a vehicle.
When momentum (with few exceptions, the use of momentum is very bad) has been used to overcome an obstacle, the vehicle could end up high on a rock – higher than without momentum. Most of the vehicle weight will then be carried by the rock, leaving very little weight for the tires = severely reduced traction. You are high centered.
Gear Ratio Model
Gear ratios are shown as a relation – for example 4:1
4:1 means that output rpm in relation to input rpm is reduced by the factor 4
For example: 800 rpm from your engine are reduced to 200 rpm at the tail shaft of your transmission when your first gear has a gear ratio of 4:1.
With other words, it takes four engine rpm to make the tranny tail shaft rotate once.
At the same time, the engine torque is multiplied by the factor 4 = substantially more torque will go to the tires than the engine produces.
Off-road vehicles have several different gear boxes involved before engine torque reaches the tires (transmission, transfer case, differentials, geared hubs).
When all involved gear ratios are multiplied with each other, the crawl ratio can be calculated.
With fewer words:
Gear ratio reduces engine rpm and increases engine torque by the same factor.
crawl ratio
Traction Model
The Friction Coefficient between Rubber compound and ground material represents the base for traction.
Rubber:
best - soft, sticky rubber compound
OK - hard durable rubber
Ground
best - dry asphalt, dry flat rock
good - dry dirt, graded dirt road
marginal - wet hard surfaces
bad - wet dirt
really bad - snow, ice
+ Weight
more weight pressing the rubber onto the ground improves traction - more pounds per square inch pressure.
Drawback: more total vehicle weight requires more vehicle torque, more weight makes rubber sink in soft ground material -
Therefore...
Weight has to be viewed as a relative factor - traction is best when all 4 tires carry about the same portion of the vehicle's weight (25% ea.)
Important: If a tire carries less than 25% (caused by weight shift and other factors) its traction is reduced = the vehicle's mobility is reduced
This is true whether you have differential lockers on your axles or not.
+ Mechanical Interlocking
mechanical interlocking of tire components with ground material improves traction,
tires with large voids between tread blocks can claw into soft material and hook up to ridges on rocks
+ Deflating Tires
deflated tires increase the chances of mechanical interlocking with ground material
(drawback: tire side wall will be become more vulnerable)
Note: decreased tire pressure reduces the pounds per square inch pressure and thus it reduces per definition traction based on pressure - however the increased mechanical interlocking is more valuable on certain materials.
Facit
Friction coefficient as the base factor is aided by balanced weight distribution, well designed tire tread and well chosen tire inflation.
Note
There are some ground materials, like rock, which provide good to decent traction – however, if rock is broken up, forming shale, or pebbles – or even worse, sand – the ground will have a low shear stability. Meaning, under torque the ground particles (shale, pebbles, sand) are kicked back by the tires rather than moving the vehicle forward.
Only very gentle throttle application might move the car. Goal is to generate just enough torque to get the vehicle moving, but not so much as to separate the ground particles.
Very important to know about traction:
Traction is used as a counterforce for torque to move the vehicle
Traction is also the counter force for the brake force needed to stop the car
Traction is further needed to create lateral stability of the vehicle
Traction for lateral stability is needed at all times
So, there are always 2 forces relying on sufficient traction
Steering and acceleration
Steering and deceleration
If the total amount of traction is used up by applying the brakes too hard, there is no more traction left for lateral stability.
With a panic stop on slippery ground, vehicles without ABS will slide without directional control.
If the total amount of traction is used up by applying the throttle too hard, there is no more traction left for lateral stability.
A powerful sports car taking off under full throttle, laying rubber, the rear will almost always slide sideways in the process (losing lateral stability) because the ground is rarely 100% level. Steering control on a 4x2 sports car is not lost, however, since the front tires use their traction only for steering. But don't count on it...
Both scenarios are very important to remember for off-road travel. Once the marginal off-pavement traction is used up by your throttle or brake actions, the vehicle becomes severely unstable and could possibly roll over.
Hand Signals for spotting
Notes on downhill dangers:
Under stress (and a steep downhill on unstable material is very stressing) most untrained drivers steer hard left or right in the process, without being aware of it.
Less than straight tires create trigger/tripping points for roll overs. It’s a good mental exercise to silently repeat: “Keep the steering straight! Keep the steering straight!”
Under stress most untrained drivers will ride their brakes in addition to a selected low gear. Once the vehicle starts sliding (they all do sooner or later) most tend to brake even harder (it a genetic imprint we have to overcome).
It’s a good mental exercise to silently repeat: “Once I slide, I will get off the brake and allow the vehicle to gain some speed and directional stability. Once I slide – I must get off the brake!”
After 25 years of off-road experience, I still do my safety mantras going downhill.
Notes on uphill dangers:
Under stress (it is stress allright, if you see only sky and clouds rather than terra firma) there is a high danger that untrained drivers increase the throttle when the vehicle stops moving uphill. Spinning, slipping tires use up all traction and there is none left for lateral stability of the vehicle – it could slip sideways quickly. A roll will be hard to avoid. So, get off the gas when the truck stops moving.
After shifting into reverse, make absolutely sure that the steering wheel stays in the straight head position.
A common mistake among untrained drivers is to turn the steering while reversing – that will get the vehicle sideways and rolled in seconds.
Picking a line
Picking a good line off-road is an art. Objective is to find a path with least resistance (less need for torque and traction) and the least possibility for vehicle damage and driver injury. Avoid stuck, damage, injury.
A line where all 4 tires are at the same level (equal traction) would be ideal.
Obstacles that could anchor the vehicle anywhere (see “Ground Anchors” in the Load section) are to be avoided.
Straight lines are best. In straight lines only the path of left and right tires are to be controlled. Once the vehicle enters a turn each tire follows a different path (very difficult to control). Add the path of the 2 differentials, you are in need of controlling 6 different paths. Without a good spotter you will likely fail.
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4x4 driving tips