There are almost as many different eccentric of four - wheel - drive systems as there are four - cycle - private road vehicle . It seems that every manufacturer has several unlike schemes for provide mightiness to all of the wheel . The terminology used by the different carmakers can sometimes be a little confusing , so before we get started explaining how they work , have ’s clear up up some terminology :
Part - time and full - time four - steering wheel - drive system can be assess using the same standard . The best system will send exactly the correct amount of torsion to each wheel , which is the maximal torque that wo n’t induce that tyre to slip .
In this article , we ’ll explain the fundamental of four - wheel drive , start with some background on traction , and look at the components that make up a four - wheel - cause arrangement . Then we ’ll take a expression at a duad of dissimilar system , including the one found on the Hummer , manufacture for GM by AM General .
We need to know a fiddling abouttorque , tractionandwheel slipbefore we can understand the different four - steering wheel - campaign system launch on cars .
Torque, Traction and Wheel Slip
Torqueis the spin force that theengineproduces . The torque from the engine is what propel your car . The variousgearsin thetransmissionanddifferentialmultiply the torque and split it up between the wheels . More torque can be send to the steering wheel in first gear than in fifth gear because first train has a largergear - ratioby which to multiply the torsion .
The stripe graph below show the amount of torque that the railway locomotive is producing . The mark on the graph indicates the amount of torque that will cause steering wheel sideslip . The cable car that makes a good start never exceeds this torsion , so the tire do n’t slue ; the car that makes a bad start exceeds this torque , so the tyre err . As soon as they begin to slip , the torsion dangle down to almost zero .
The interesting affair about torsion is that in low - traction situations , the maximal amount of torque that can be created is determined by the amount of adhesive friction , not by the engine . Even if you have aNASCARengine in your car , if the tires wo n’t lodge to the ground there is just no manner to rein in that power .
For the sake of this clause , we ’ll definetractionas the maximum amount of force thetirecan apply against the ground ( or that the ground can apply against the tyre – they ’re the same thing ) . These are the element that involve traction :
The weight on the tire– The more weight on a tire , the more traction it has . weightiness can wobble as a car drives . For case , when a motorcar makes a turn , weight shift to the away wheel . When it accelerates , weight shimmy to the rearward wheels . ( SeeHow Brakes Workfor more details . )
The coefficient of friction– This factor relate the amount of friction violence between two surfaces to the force throw the two surfaces together . In our font , it come to the amount of grip between the tyre and the road to the system of weights resting on each tire . The coefficient of detrition is mostly a subroutine of the kind of tires on the vehicle and the character of surface the fomite is drive on . For case , aNASCAR tirehas a very eminent coefficient of friction when it is drive on a dry , concrete data track . That is one of the reason whyNASCAR race carscan corner at such high speeds . The coefficient of rubbing for that same tire in clay would be almost zero . By contrast , vast , knobby , off - roadtireswouldn’t have as high a coefficient of detrition on a dry rail , but in the mud , their coefficient of detrition is extremely high .
Wheel slip– There are two kinds of contact that tires can make with the road : static and dynamic .
Quite merely , wheel eluding happens when the force enforce to a tyre surpass the grip available to that tyre . Force is applied to the tyre in two ways :
Let ’s say you have a fairly powerful bottom - wheel - campaign car , and you are driving around a curve on a wet road . Your tires have mickle of adhesive friction to utilize the lateral force needed to keep your railroad car on the route as it goes around the curve . permit ’s say you floor the gas pedal in the middle of the turn ( do n’t do this ! ) – your engine beam a lot more torque to the steering wheel , create a large amount of longitudinal force . If you add the longitudinal force ( produced by the engine ) and the lateral force created in the bout , and the substance overstep the grip useable , you just created roulette wheel slip .
Most people do n’t even do close to overstep the uncommitted traction on dry pavement , or even on flat , wet pavement . Four - steering wheel and all - wheel - drive organisation are most utilitarian in low - traction situations , such as in snow and on slippy hills .
The welfare of four - bike drive is easy to understand : If you are driving four wheel instead of two , you ’ve got the potential to double the amount of longitudinal force ( the force that makes you go ) that the tires practice to the ground .
This can aid in a variety of situations . For illustration :
There are also some berth in which four - wheel drive provides no vantage over two - wheel driving force . Most notably , four - wheel - campaign systems wo n’t help you cease on slippery surface . It ’s all up to thebrakesand theanti - lock braking system(ABS ) .
Now let ’s take a look at the parts that make up a four - roulette wheel - driving organisation .
Components of a Four-wheel-drive System
The main parts of any four - bike - drive system are the two first derivative ( front and rear ) and the transfer typesetter’s case . In addition , part - meter systems have operate hubs , and both types of systems may have advanced electronics that help them make even good use of the available adhesive friction .
DifferentialsA machine has twodifferentials , one locate between the two front wheel and one between the two rear wheels . They send the torque from the driveshaft or contagion to the effort wheel . They also let the left and right wheel to spin at different speed when you go around a turn .
When you go around a turn , the privileged wheels follow a dissimilar route than the outside wheels , and the front wheels follow a unlike route than the rear rack , so each of the roulette wheel is spin around at a different speed . The differential activate the speed divergence between the inside and outside rack . ( In all - wheel drive , the speed difference between the front and rear wheels is handled by the transference subject – we ’ll discuss this next . )
There are several different kinds of differentials used in car and truck . The case of differentials used can have a substantial effect on how well the vehicle utilizes available traction . SeeHow Differentials Workfor more details .
Transfer Case
This is the gadget that split up the exponent between the front and rear axles on a four - steering wheel - parkway automobile .
Thetransfer caseon a part - meter four - bicycle - drive organization lock the front - axle driveshaft to the rearward - axle driveshaft , so the wheels are coerce to whirl at the same stop number . This requires that the tyre slip when the car goes around a good turn . Part - time system like this should only be used in miserable -traction state of affairs in which it is relatively easy for the tires to slip . On wry concrete , it is not easy for the tires to steal , so the four - wheel cause should be disengage so as to head off jerky turns and extra clothing on the tires and drivetrain .
Some transfer of training cases , more commonly those in part - time systems , also curb an extra set of gears that give the fomite alow range . This extra appurtenance ratio gift the fomite surplus torsion and a super - slow output speed . In first paraphernalia in low stove , the vehicle might have a top hurrying of about 5 miles per hour ( 8 km/h ) , but incredible torsion is produced at the cycle . This allows drivers to slowly and smoothly pussyfoot up very steep Alfred Hawthorne .
Locking Hubs
Each wheel in a motorcar is bolted to a hub . Part - time four - steering wheel - drive hand truck usually havelocking hubson the front wheels . When four - rack crusade is not hire , the locking hub are used to unplug the front wheel from the front differential , half - shafts ( the barb that connect the differential coefficient to the hub ) and driveshaft . This allows the differential , half - shafts and driveshaft to cease spinning when the elevator car is in two - wheel driveway , saving article of clothing and charge on those parts and improving fuel - economic system .
Manual locking hubs used to be quite vulgar . To engage four - rack driveway , the driver actually had to get out of the motortruck and turn a knob on the front wheels until the hubs lock . new scheme have automatic locking hubs that take when the gadget driver switches into four - wheel thrust . This type of system can usually be engaged while the vehicle is moving .
Whether manual or automatic , these systems generally use a slide collar that lock the front half - shaft to the hub .
Advanced Electronics
On many modern four - wheel and all - roulette wheel - drive fomite , advanced electronics roleplay a fundamental role . Some gondola use theABS systemto selectively utilise the brake to bicycle that start to skid – this is calledbrake - traction control .
Others have doctor up , electronically - controlled clutches that can better ascertain the torque transfer between wheels . We ’ll take a look at one such advanced system by and by in the clause .
First , let ’s see how the most basic part - time four - wheel - drive system works .
Four-wheel Drive Differential
The eccentric of part - time system typically found on four - wheel - drive pickup and older sport utility knead like this : The vehicle is unremarkably rearward - steering wheel drive . The infection hook up directly to a transference event . From there , one driveshaft turns the front axle , and another plow the rearward axle .
When four - steering wheel drive is enlist , the transfer case lock in the front driveshaft to the rear driveshaft , so each axle receives half of the torsion add up from the railway locomotive . At the same time , the front hubs lock .
The front and rearward axle each have anopen differential . Although this arrangement supply much better adhesive friction than a two - wheel - ride vehicle , it has two main drawback . We ’ve already discussed one of them : It can not be used on - road because of the locked transfer case .
The second problem comes from the eccentric of differentials used : An open differential splits the torsion equally between each of the two cycle it is connected to ( seeHow Differentials Workfor more details ) . If one of those two wheels come off the ground , or is on a very tricky surface , the torque apply to that rack drops to zero . Because the torsion is split evenly , this imply that the other bicycle also receive zero torque . So even if the other cycle has plenty of adhesive friction , no torsion is transferred to it .
antecedently , we say that the best four - wheel - drive scheme will send precisely the correct amount of torque to each wheel , the right amount being the maximum torsion that wo n’t make that tyre to slip . This system rates fairly ill by that touchstone . It sends to both rack the amount of torque that wo n’t cause the tyre with theleasttraction to skid .
There are some way to make melioration to a system like this . supervene upon the open differential with alimited - slip rear differentialis one of the most coarse single – this makes sure that both rearward wheels are able to apply some torque no matter what . Another selection is alocking derived function , which locks the rear wheel together to ensure that each one has access to all of the torque coming into the axle , even if one steering wheel is off the ground – this improve performance in off - road atmospheric condition .
In the next section , we ’ll take a look at what could be the ultimate four - wheel - drive system : the one on the Hummer .
The Four-wheel Drive Hummer
The AM General Hummer military vehicle combines some innovative mechanical technology with sophisticated electronics to create what is arguably the best four - wheel - drive arrangement available .
The Hummer has a full - time system of rules with extra features that can be lock for enhanced off - road performance . In this system , just as in our basic organisation , the transmission is hooked up to the transfer caseful . From the transference case , one driveshaft associate to the front axle and one to the rearward axle . However , the transfer case on the Hummer does not automatically lock the front and rearward axles together . Instead , it contains a set ofopen - differentialgears that can be lock by the equipment driver . In loose mode ( not locked ) , the front and rear axles can move at unlike speed , so the vehicle can drive on dry roads with no trouble . When the differential is locked , the front and rear axle each have access to the locomotive ’s torsion . If the front bike are in quicksand , the rear wheels get all of the torque they can handle .
The front and rearward differentials are bothTorsen ® differentials . These differential coefficient have a unique gearset : As soon as it feel a decrease in torque to one wheel ( which come about when a tyre is about to splay ) , the gearset transfers torque to the other wheel . Torsen differentials can channelize from two to four times the torsion from one wheel to the other . This is a big improvement over open differentials . But if one steering wheel is off the ground , the other steering wheel still flummox no torque .
To handle this trouble , the Hummer is equipped with abrake traction ascendancy system . When one tire set off to slide , the brake traction control applies the brake system to that wheel . This accomplishes two thing :
The Hummer system encountering various combinations of terrain : For the Hummer to get puzzle , all four wheels would have to lose traction .
The Pteridium aquilinum adhesive friction control system apply significant torque to the steering wheel that wants to sneak , give up the Torsen derived function to apply two to four times that increased torque to the other bike .
Let ’s put the Hummer to the test .
The system on the Hummer is capable of send a turgid amount of torque to whichever tires have adhesive friction , even if this means post it all to a single tire . This brings the Hummer moderately close to our definition of an ideal four - wheel - crusade organisation : one that supply each tire with the maximum amount of torque it can handle .
For more information on four - bike drive and related topics , check out the link on the next pageboy .
