Most modern 4 Wheel drives allows you to safely switch between 2WD and 4WD provided your speed is below 62 MPH and driving on a slippery surface like snow, sand or muddy tracks. However, those are the only suitable conditions you should engage 4WD mode. But what if you forgot to remove your 4WD from 4H after before heading back onto the tarmac and suddenly find it hard to turn in 4WD mode or your vehicle feels jerky as you take bends or its stuck in a gear? Have you done any damage to your drive-train at this point and what causes this effect?
A part-time 4WD gives power to all the wheels once 4H is engaged. When driving on a surface with good traction like tarmac or concrete in 4WD mode, this can cause difficulty when turning corners as the inside wheel needs to turn slower than the outside wheel. This situation is called “axle binding” ,”drive-line binding” or “drive-line wind up”. Continued driving in 4-Hi with a temporary 4WD on a hi traction surface will result in drive line components like u-joints, axle and transfer gears, bearings and drive-shafts to fail.
If you would like to learn more about axle-binding, continue reading!
4 Wheel Drive Locks Up When Turning
This effect causes your vehicle to under-steer heavily, gears to get jammed and makes steering very difficult and even jerky. This phenomenon is caused by the front wheels battling the rotational force coming from the front drive shaft as it tries to slow down the front wheels, causing the massive under-steer effect. You should avoid engaging 4WD on a high traction surface at all costs. The longer you drive in that mode, you risk serious damage to your drive-train components and you will find it increasingly difficult to remove it from 4WD mode and switch back to 2WD mode.
4WD Binding – How To Fix Transmission Wind-Up
If you have forgotten to take your 4WD out of 4H after heading back on to the bitumen you will definitely experience transmission wind-up. One way to tell you have transmission windup is is your vehicle is stuck in a gear. You can yank and hang on the gear lever but nothing will release that gear due to the massive forces built up inside the gearbox. So how do you fix transmission wind-up?
The easiest way to fix bind-up is by pulling over to the side of the road with two wheels firmly on the bitumen and the other 2 wheels on a slippery surface like grass, mud or sand. This allows the wheels to all spin at different speeds releasing the wind-up in the transmission box. Once the wind-up has been released you will then be able to use your gearbox/transmission properly again.
The other option is to reverse in the same direction you where driving. So if you where driving forward in a left direction, reverse in a left direction and allow the wind-up in the transmission to reverse itself.
Understanding Part-Time 4WD Systems
In part-time 4WD systems differentials are fitted to the front and rear axle assembly’s. That is the big round bell housing you see underneath your truck and is the lowest part of your vehicle. When driving in 2H the power is transferred through the rear propeller shaft or drive-shaft to the rear final drive and the differential gears and rear axle shaft. Very similar to the system of a front mounted engine rear wheel drive car.
The gears in the rear differential allows the rear wheels to turn at different speeds when the vehicle turns left or right. While continuing to transfer equal torque to each wheel.
When 4WD is engaged then the transfer case joins the front and rear axles and transmits equal power to the front and rear drive shafts. The drive is transmitted through both the front and rear axle assemblies.
When turning, the swiveling of the front wheels creates a wider turning circle than the rear wheels and this causes a difference in the rotational speeds of the front and rear wheels. This in turn causes a difference in speeds of the front and rear propeller shafts (Drive shafts)
Since there is a difference of speed between the inner and outer wheels both axle shafts also turns at a different speeds. Differences in speed can also be caused from uneven tire ware between the front and rear wheels or varied inflation pressures.
Understanding Full-Time/AWD Systems
In a full time AWD systems power is permanently sent to all four wheels and they are always driven. Sometimes the power is sent equally and other times it varies due to driving surface conditions like snow, ice and mud.
Contrary to popular believe an AWD is only advantages under acceleration and does not assist you with stopping or turning on slippery surfaces. Only outfitting your vehicle with good winter tires will enhance your vehicles stopping and turning capabilities. This upgrade will also assist your vehicles electronic stability and traction management systems.
Can You Turn In 4 Wheel Drive?
The differences of the relative speeds of front and rear axle which occurs when turning left or right are solved by the fitment of a third differential inside the transfer box. The 3rd differential allows the front and rear propeller or drive shafts to rotate at different speeds while sending equal torque.
The 3rd differential or center diff can be locked by the driver when low range is selected or 4H is engaged by the driver which locks the front and rear differentials together.
With both part time 4WD and AWD vehicles, once the center diff-lock is engaged and the front and rear drive-shafts are connected inside the transfer box, the difference in their speeds cannot be absorbed inside the transmission. This torsional stress can cause axle binding” ,”drive-line binding” or “drive-line wind up”. This makes turning in 4WD extremely difficult and what causes the jerking effect.
In off road conditions the differences in speed can be absorbed through slippery conditions and tire slippage caused by uneven surfaces.
On firm dry road surfaces, only 2WD should be engaged with a temporary 4WD or in the case of a AWD the center diff lock should be disengaged. This will prevent excessive tire ware, high gas consumption and drive-line component damage.
Understanding How Differentials Work
Differentials are an integral part of all 4 Wheel drives. The wheels receive power from the engine via a driveshaft. The main function of the differential is to allow both wheels on the axle to turn at varied RPM or speeds when receiving power from the engine. when taking a turn the wheels on the outside needs to rotate at a higher speed to the wheels on the inside of the bend. if the wheels where connected by a solid driveshaft then the wheels will have to slip in order to successfully turn. The clever gear design inside the differential allows the left and right wheels to safely turn at different speeds.
Power from the engine is transferred to the ring gear (a component inside the diff) through a pinion gear. The ring gear is connected to a spider gear which is located at the heart of the differential.
Spider gears are allowed to make 2 kinds of rotation.
- Along with the ring gear
- on its own axis
The spider gear is meshed with 2 side gears so power flows from the engine via the driveshaft to the left and right wheels via the ring and spider gears inside the differential.
The Differential Has Other Functions
- Speed reduction at pinion gear assembly
(results in torque multiplication)
- Turn the power flow direction by 90 degrees
(transfers the torque by 90 degrees through the side shafts to the wheels)
The above mentioned explanation is of a standard or open differential found on most trucks. It does have a drawback of being an open diff though. If one wheel is on a surface with good traction and the other is on a slippery surface the standard open diff will send the majority of the power to the slippery wheel. Making the vehicle temporary immobile. This occurs because power is always sent to the wheel with the least resistance. To overcome this problem vehicle manufacturers have introduced 3 solutions.
- Differential lockers
- Traction Control (TC)
- Limited slip Diffs (LSD)
Never engage your 4WD on a dry pavement. You will destroy your drive-train components and cause severe tire damage very quickly. Always remember to switch back to 2H when you leave the gravel or sand before you get back on the tarmac. If you are driving an AWD ensure the center differential lock is unlocked.
Happy 4 Wheeling and remember, Safety first!!!