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 speeds 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 drivetrain 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 [Applause] 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 mean 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 video and subscribe to my youtube channel thanks for watching

Channel: NJS Instructional

Views: 1,149,707

Keywords: Four, Wheel, Drive, Two, Traction, Axle, Diff, Differential, Torque, Distribution, Multiplication, Hub, Free, Wheeling, Locked, Open, Brake, Control, Winch, 4x4, 4WD, Off road, off, road, bf goodrich, mud, snow, hill, rock, gravel, wind, wind up, engine, transmission, force, power, system, work, crawl, ratio, gear, high, low, hi, lift, jack, maxtrax, snatch, recovery, four wheel drive, 4wd 24/7, all wheel drive, permanent, lock, centre, differential, axel, toyota, ford, raptor, everest, dmax, traction, control, gearing, speed, RPM, part, time, car

Id: 18YNpG7IAQ8

Length: 16min 48sec (1008 seconds)

Published: Fri Sep 20 2019