How 4WD (4x4 - Four Wheel Drive) Works - 2H, 4H, 4L, LSD, Centre Diff, Diff Locks, Traction Control.

hi there in this video we will cover how

a part-time and full-time four-wheel

drive system works we will cover in

depth two wheel drive high range

four-wheel drive high range four-wheel

drive low range open sensor limited slip

and lock differentials the windup

phenomenon transfer case torque

distribution torque multiplication crawl

ratios freewheeling hubs and traction

control so hopefully by the end of this

video you will better understand and

have the knowledge you need to

confidently venture off-road in your

four-wheel drive vehicle four-wheel

drive vehicles will have either

part-time or full-time four-wheel drive

systems part-time simply means that the

vehicles engine typically drives two

wheels most of the time it can be

switched to four-wheel drive for greater

traction part of the time a part-time

four-wheel drive will typically have a

too high for high and for low option a

full time full drive system simply means

that the engine typically drives all

four wheels of the vehicle all of the

time these will typically have a four

high and a full low option with the

sensor differential lock more on this

shortly let's start with the

fundamentals our vehicle's engine

produces torque that is rotational force

this is coupled to a transmission either

manual or automatic the transmissions

role is to alter the speed and torque

and send it to the differential via a

driveshaft the differential or diff

typically opens Sal then transmits this

talk to the wheels via the axles one on

each side of the differential in a

front-wheel drive vehicle the

differential is at the front in a

rear-wheel drive vehicle the

differentials at the rear a full drive

vehicle will have two differentials one

at the front and one at the rear the

differential gets its name as it allows

talk to be transmitted to the wheels

even if the will spin at different


Sayre vehicles turning to the right the

outside left wheels cover more distance

and spin faster than the inside right

wheels which cover less distance and

spin slower

the beauty with the differential is it

does this automatically with no driver

input now in a four-wheel-drive system

between the transmission and drive shaft

is a transfer case in a part-time

four-wheel drive system the transfer

case has two major functions the first

function is to multiply the talk-through

lower gears that is low range gearing

now the second role is where part-time

and full-time for drive systems are

different in a part time for drive

system either in for high or for low the

transfer case will distribute the talk

equally to the front and rear

differentials and does this by locking

the front and rear drive shafts together

however in a full-time four-wheel-drive

system a sense of differential lock is

used instead to distribute the talk

equally to the front and rear

differentials this is because a

full-time four-wheel drive also requires

a differential action between the front

and the rear wheels during normal

driving but more on this shortly so say

our vehicle is in too high or high range

two-wheel drive

100% of the engine torque via the

transmission is sent to the rear drive

shaft which in turn provides talk to the

rear differential in an ideal case the

differential split the talk 50/50

percent equally so each will via the

axles in reality however the

differential will split the talk down

the path of least resistance for example

if one wheel is in the air and has no

traction all the talk will be sensor

this wheel as it has the least

resistance essentially meaning you only

have one wheel drive this is a big

shortcoming of an open differential the

vehicle in this video is in too high and

the rear left will starts lifting from

the ground you can see that because this

wheel has the least resistance 100

percent of the talk is sense all to this

wheel if it was now a lock differential

and most modern four drives have a rear

diff lock and can be activated by the

switch of a button the two wheels or

axles are essentially locked together

and rotate at the same speed this allows

for an

50/50 percent distribution of torque

going to each will so if in our example

where one wheel is in the air the other

wheel which does have traction will

receive 50 percent of the talk and allow

the vehicle to keep moving however a

lock differential no longer works as a

differential as it doesn't allow a

difference in wheel speed so can't be

used on high traction surfaces like a

bitumen Road and will also restrict the

vehicle when turning some vehicles are

fitted with limited slip differentials

or advanced traction control systems to

help overcome the shortcomings with open

differentials more on this later let's

see a demonstration of a vehicle in

two-wheel drive and on sand note the

rear wheels which receive all of the

torque and let's see how far we can go

to H so not verifies now moving on to

for high or high range four-wheel drive

in a part-time four-wheel drive and for

highs selected the transfer case locks

the rear drive shaft to the front drive

shaft this provides an equal 50/50

percent split ins Hauk between the front

and rear drive shafts this is great for

off-roading as now the front

differential and therefore front wheels

also receive talk along with the rear

however in high traction surfaces this

is problematic if a vehicle is tends to

the rides not only are the left outside

wheels rotating faster than the inside

wheels but also the front wheels rotate

at different speeds to the rear wheels

hence we have this differential

phenomenon but no sense our differential

this is why parts are for drives

should only be driven into high on high

traction surfaces if they driven in for

high or for low the front drive shafts

will want to rotate at a different speed

to the rear drive shaft but it can't

because they are locked together this

phenomenon is called wind up essentially

a twisting force known as torsion which

stores potential energy in the drive

shafts this energy has to be released

otherwise it'll eventually damage the


in low traction surfaces like gravel

this release of energy can cause a front

or rear wheels to skip or skid and this

is perfectly okay and desirable this is

where a part-time four-wheel drive

really differs from a full-time

four-wheel drive in a full-time

four-wheel drive system since the

vehicle is designed to be driven in high

traction surfaces as well as low

traction surfaces a sensor differential

is used to accommodate the difference in

front and rear wheel speeds the center

differential can be locked by the driver

to provide an equal 50 50 percent torque

split front and rear however it must be

unlocked on the high traction surfaces

otherwise we will get the windup

phenomenon occurring in for high in a

part-time four-wheel drive the torque

split is 50% front and 50% rear in

full-time four-wheel drive the sensor

differential must be locked to provide

this fifty fifty percent torque split

otherwise it could potentially be a

hundred percent rear only now in for

high in part time full drive or center

diff locked full-time four-wheel drive

in ideal conditions each wheel receives

25% of this talk but because we still

have open differentials front and rear

the torque will still follow the path of

least resistance for example if the rear

right wheel is off the ground the torque

distribution is 25 percent each front

wheel and 50% rear Wrightsville this is

usually okay to keep the vehicle moving

however if the front left wheel is also

off the ground known as the diagonal

effect then the torque distribution is

50 percent front left wheel and 50% rear

right wheel and no talk is actually

being transmitted to the wheels which do

have traction so with open differentials

whether in for high or although their

vehicle will be at worst case two will

drive and best case four-wheel drive

let's have a look at this diagonal

effect on two different occasions


an in-demand straight-4 high in sands

note how much further the vehicle goes

compared to too high

now let's cover for low or low range

four-wheel drive it is important to note

that for low does not change the torque

distribution at all it will not overcome

shortcomings with open differentials

either for low however will multiply the

talk coming out of the transmission this

is a second role of the transfer case in

a part-time four-wheel drive or the only

role in a full-time four-wheel drive

when for low is engaged a different gear

set consisting of lower gears is used by

nature of gears low gears have low

speeds corresponding to higher torque or

torque multiplication this low range

gearing is known as the transfer ratio

and is typically between once or two and

one so for in our test vehicle the

transfer ratio is one to two point four

eight eight therefore for every two

point four eight eight terms of the

transmission the drive shafts will

rotate only once by comparison into high

or for high for every two point four

eight eight turns of the transmission

the drive shafts will also rotate to

point four eight eight times in for low

the transmission torque is multiplied by

the transfer ratio so in our case by two

point four eight eight so what does this


in first gear with the engine at 2,000

rpm the vehicle speed in for high will

be 21 kilometers an hour but in for low

will be eight point five six kilometers

an hour

this lower speed however corresponds a

higher torque at the wheels for low has

its real advantages where slow speed and

high torque is required for example

steep hill climbs

he'll descents river crossings slippery

terrain like mud snow or sand because of

this higher torque the vehicle's engine

and transmission don't have to work as

hard reducing the likeliness of the

engine and transmission from overheating

stalling or ultimately failing now

another term associated with for low is

the crawl ratio the crawl ratio is

simply the multiplication of the first

gear ratio the final driver

and the transfer ratio in our example

the cruel ratio is once a thirty three

point three for the automatic the manual

has a crawl ratio of once a forty five

point seven the higher the crawl ratio

the lower the rotation speed of the

wheels yet higher torque provided at the

wheels sayin engine produces four

hundred and thirty Newton meters of

torque at 2,000 rpm multiplying this by

the crawl ratio gives fourteen thousand

three hundred and nineteen Newton meters

in ideal circumstances whether a torque

is split evenly to all four wheels we

get three thousand five hundred and

eighty Newton meters at each wheel by

comparison in four hi we only get one

thousand four hundred and thirty nine

Newton meters that is two point four

eight eight times less you may say this

is a lot of torque which it is but once

you factor in large diameter for drive

wheels working on demanding terrain

hauling a heavy vehicle you need as much

talk as possible let's have a look at

four low in action

now we have covered in-depth part-time

and full-time for drive systems let's

take a step back and look at

freewheeling hubs these hubs are used to

engage or disengage the transmitter talk

from the axle to the wheel and a

primarily used in the front wheels of a

part-time four-wheel drive into high for

high and for low torque is always sense

of the rear so these rear hubs are

permanently locked

however into high as the vehicle moves

so to the front wheels and therefore so

did the axles and front drive shaft even

though the front drive shaft is not

actually connected at the transfer case

these additional rotating parts at

rolling resistance to the vehicle and

therefore more fuel is consumed and

components will wear unnecessarily to

prevent this a free wheeling hub is used

to physically disengage the rotating

motion from the wheel to the axle when

for high or full lower selected these

hubs have to be locked otherwise you'd

only have two wheel drive still on older

vehicles these front hubs had to be

manually locked

whereas newer vehicles that they will

automatically lock full time for drive

systems have permanently locked hubs at

the front as well as the rear going back

to the open differential problem a

manufacturer will typically utilize one

or more of these three techniques one a

locking differential to a limited slip

differential or three break traction

control as briefly discussed before a

locking differential will provide a true

fifty fifty percent torque split to each

wheel this is the best technique in

overcoming an open differential but

again it can only be used in low

traction surfaces and it's expensive for

vehicle manufacturers however only a

full drive with locked front and rear

differentials will provide true

four-wheel drive a limited slip

differential or LSD will allow a limited

amount of movement between the two

wheels and in so doing will transmit

some of the talk to the wheel with more

traction however since the differential

transmitted talked down the path of

least resistance they are not always

that effective some vehicles have

excellent limited slip differentials

which almost negate the need for a

differential lock not all LS these are

the same again it has also added costs

to the vehicle manufacturer another

technique is break traction control or

BTC by using wheel sensors and clever

electronics if one or more will spin at

different rates than the others thus

receiving more or all of the talk the

computer will apply brakes to these

wheels this increases the resistance on

the wheel and hence the differential

will start to split the talk to the

opposing wheel now some brake traction

control systems are very good which

almost negate the need for a

differential lock and some systems are

pretty ineffective because BTC is easy

to adapt into a modern vehicle since it

already has abs ESC and so on it is the

cheapest to produce and more commonly

found on modern four-wheel drives here's

a demonstration of BTC in action

so next time you drive a four-wheel

drive you are better equipped with the

knowledge of how it works what to expect

and what its shortcomings are remember

not all four drives are the same nor are

all true four drives I hope you found

this video informative and it equips you

with the knowledge you need to

understand your full drive system and

prepare you for your next off-road

adventure don't forget to like this

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channel thanks for watching