Most modern 4 Wheel drives allow you to safely switch between 2WD and 4WD provided your speed is below 62mph (100km/h) and you are driving on a low traction surface like snow, sand, or muddy tracks. Those are only some of the suitable conditions for you to safely engage 4WD mode without causing any drivetrain damage. However, what if you forgot to remove your vehicle from 4H before heading back onto the tarmac and drove it in 4WD for quite a while? Does or your vehicle feel jerky when you corner? Is your transmission 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 sends power to all 4 wheels once 4H is engaged. Driving on a high traction surface like tarmac or concrete in 4WD mode can result in difficulty turning corners as the outside wheels need to turn faster than the inside wheels because it has to cover a larger circumference. Driving in 4WD mode for extended periods on a high traction surface can result in axle binding or drivetrain wind-up.
Extensive driving in 4H with a part-time 4WD on a high-traction surface, or with the center diff locked with a permanent 4WD, will result in driveline components such as u-joints, axles, transfer-case gears, bearings, and drive-shafts seizing up and failing.
Let’s dive more into drivetrain wind-up and why your 4WD jerks when turning
4 Wheel Drive Locks Up When Turning
So why does the 4WD system feel stiff and jerky when turning? This wind-up effect causes your vehicle to under-steer heavily, gears getting jammed, and makes steering difficult and jerky. This phenomenon is caused by the front wheels battling the rotational force coming from the front driveshaft as it tries to synchronize the front wheels, causing the massive under-steer effect. You should avoid engaging 4WD on a high traction surface for extended periods at all costs. The longer you drive in 4WD mode, you risk serious damage to your drive-train components and you’ll find it increasingly difficult to remove it from 4WD or 4H and switch back to 2WD mode.
4WD Binding – How To Fix Transmission Wind-Up
If you have forgotten to remove your 4WD from 4H before heading back onto the Bitumen/Asphalt you will definitely experience transmission wind-up. One way to determine that you have transmission windup is if your vehicle is stuck/jammed in gear. You can yank and hang on the gear lever to no avail since nothing will release that gear. This is due to the massive forces built up inside the gearbox jamming up the transmission components. So how do you reverse 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/asphalt and the other 2 wheels on a slippery surface like grass, mud, or sand. This allows the wheels to all rotate at slightly different speeds which can release the wind-up in the transmission. Once the wind-up has been released you should be able to use your gearbox/transmission as usual.
The other option is to reverse in the same direction you were driving. So if you were driving forward in a left direction, reverse in a left direction and allow the wind-up in the transmission to undo itself slowly. Make sure you do this in a safe environment void of any oncoming traffic or pedestrians.
Understanding Part-Time 4WD Systems
In part-time 4WD systems, differentials are fitted to the front and rear axle assemblies. The “pumpkin”, as it is commonly referred to, is the big round bell housing you see underneath your truck. It is also the lowest part of your vehicle, and from where you measure your ground clearance. When driving in 2H the power is transferred from engine to transmission through the rear propeller shaft/driveshaft to the rear differential then split 50/50 on the rear axle shaft to the rear wheels.
Each wheel axle is attached to a diff side gear, which meshes with the differential pinions. When driving straight, the wheels and the side gears turn at the same rotational speed. There is no relative motion between the differential side gears and pinions, and they all rotate as a unit with the case and ring gear. However, when turning, the gears inside the differential or diff allow the wheels to rotate at different speeds.
When 4WD is engaged then the transfer case connects the front and rear axles and transmits 50/50 equal torque to the front and rear driveshafts. The drive is transmitted through both the front and rear axle assemblies.
When turning, the swiveling of the outer front wheels creates a wider turning circle than the inner rear wheels and this causes a difference in the rotational speeds of the front and rear wheels. This, in turn, causes a difference in the speeds of the front and rear driveshafts.
Since there is a difference in speed between the inner and outer wheels both axle shafts also turn at different speeds. Differences in speed can also be caused by uneven tire wear between the front and rear wheels or varied inflation pressures.
Understanding Full-Time & Part-time 4WD Systems
With a traditional full-time AWD system, torque is sent to all four wheels simultaneously, which means they are always driven. Most times the power is sent equally to all 4 wheels, and other times it varies due to driving surface conditions like snow, ice, and mud.
Contrary to popular belief an AWD is only advantageous under acceleration and turning, but does not assist you with stopping on slippery surfaces. Outfitting your vehicle with good winter tires will enhance your vehicle’s stopping and turning capabilities. This upgrade will also assist your vehicle’s electronic stability and traction management systems.
Can You Turn In 4 Wheel Drive?
In a permanent 4WD, the rotational differences between the front and rear driveshafts, side-shafts, and wheels caused when turning are resolved by means of a center differential inside the transfer box. The center differential allows the front and rear propeller or drive shafts to rotate at different speeds while sending equal torque to both front and rear.
The center diff can be locked by the driver when 4H is engaged in a part-time 4WD or by locking the center diff in a permanent 4WD. This action locks the front and rear driveshafts together to function as one unit and rotate at the same speed irrespective of traction on the wheels.
In both, part-time 4WD and permanent 4WD 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, 4H is ideal since the differences in rotational speeds between the front and rear driveshafts can be absorbed through slippery conditions and tire slippage caused by uneven surfaces. So in order for it to function properly, there needs to be a small level of slippage.
On firm, dry road surfaces, only 2WD or 2H should be engaged with a part-time 4WD, or in the case of a permanent 4WD, the center diff lock should be disengaged and unlocked. This will prevent tire damage, excessive fuel consumption, and expensive damage to drive-line components.
Understanding How Differentials Work
Differentials are an integral part of every 4 Wheel drive. The wheels receive torque from the engine via a driveshaft. The main function of the differential is to allow both wheels on the axle to turn at different speeds when turning. When cornering, the wheels on the outside needs to rotate at a higher speed versus the wheels on the inside of the bend. if the wheels were joined/locked 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. Hence the term “Differential”
Torque 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 meshes 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, open differential found on most trucks. It does have a drawback of being an open diff though which means, If one wheel is on a surface with good traction and the other is on a slippery surface without traction, the standard open diff will send the majority of the power to the slippery wheel. This is because torque always follows the line of least resistance. This can make the vehicle become temporary immobile, aka STUCK! To overcome this problem, vehicle manufacturers have introduced 3 solutions.
- Differential lockers
- Traction Control (TC)
- Limited slip Diffs (LSD)
Never engage your 4WD on dry pavements. You will destroy your drive-train components and cause severe tire damage very quickly. Always remember to switch a part-time 4WD back to 2H when you leave the gravel or sand and before you get back on the tarmac. If you are driving a permanent 4WD on a high traction surface, always make sure the center diff is unlocked.
Happy 4 Wheeling and remember, Safety first!!!