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How Do Semi Truck Differentials Work? (Explained And Solved)

Each component of a vehicle has a corresponding task to perform to keep it moving. However, it’s normal to be unsure of how these parts work.

While you are shopping for a new semi truck, you might have heard terms like “limited-slip differential” or “open differential.”

But what does differential mean in a semi truck and how does it work?

Here’s a short summary of how Semi Truck Differentials Work:

Semi truck differentials work by allowing the outer and inners wheels of the truck to move at different speeds along a curved road. It distributes the same amount of torque to each wheel. Without them, the truck may lose traction whilst turning and other issues like axle and tire wear will occur.

How do Semi Truck Differentials Work?

Differentials provide independent rotation to wheels. These components give proportional “revolutions per minute” (RPM) between the right and left wheels. 

The differentials consist of four features such as differential cage, differential gear, two output shafts, and two planetary gears.

Each feature works differently to give you a safe driving experience.

When driving off-road:  

In this situation, the outside wheel will move in long distances. Hence, the output shafts must rotate at a different speed.

To do this, the planetary gears must also turn at a different speed. This is the only way to balance the speeds of these two differential features.

When driving on a smooth road: 

It is the differential cage that will mostly work this time. The plenary gears shall stay put. As such, both the planetary gears and cage will turn at a proportionate speed.

Unlike in curves, the right and the left wheel will turn at the same distance.

A clearer picture:

To have a clear picture of how differentials work, imagine a semi-truck moving into the corner.

The outside and inside wheels will move at different speeds. The differentials will distribute the same amount of torque to each wheel. 

These differentials provide resistance to the inside wheel so that the outside wheel will rotate less. The one that has lesser resistance will rotate faster.

Through the differentials, each wheel can move independently. This will prevent driving issues and will minimize the risk of an accident. 

Check this cool video out to get a better understanding of how differentials work:

Can you Drive a Semi Truck Without Differentials?

You can drive a semi truck without differentials but expect to experience an unstable ride.

The differential will provide you control as the wheel moves at various speeds. Without differentials, the wheels will lock together and turn at the same speed. 

As a result, you will have a hard time maneuvering your semi truck. The wheels will be forced to spin and to achieve a safe turn, one wheel must slip. 

Where are the Differentials in a Semi Truck?

The location of differentials depends on whether the semi truck is an AWD, 4WD, FWD, and RWD. However, most semi-truck is 4WD (four-wheel-drive).

It has a front and back differential located either in the center, front, or rear of the axle.

As such, a semi truck is useful off-road or in dangerous driving conditions. With front and back differentials, control and traction improve.

They can also be helpful with towing and hauling.

How many Differentials does a Semi Truck have?

Many people think that a semi-truck or any other car is equipped with only one differential. But the truth is… all 4 wheels have two differentials for the front and rear axle.

These two differentials are responsible for right and left wheels to move on their respective axles. This prevents the wheels from skidding or skipping as the vehicle turns.

How do you Lock a Differential on a Semi Truck?

Locking a differential depends on whether or not your semi truck is technologically advanced. It can be locked automatically or manually.

For automatic locking differentials, as the name suggests, the vehicle will lock on its own when the circumstances warrant. 

The driver-selectable or manual differential requires activation from the driver. It can be locked through the dash and most drivers use cable and lever to activate it. 

When Should I Use Diff Lock on my Truck?

The appropriate time to use a diff lock is when the truck is caught in a dangerous off-road situation.

Knowing how to employ diff lock to your differential will help you control the vehicle in tricky situations with ease. 

Diff lock allows the tires to spin at the same speed and time for traction. The purpose of which is to prevent the wheels to move at different speeds allowing safe and tight turns. 

That said, the diff lock is only meant for poor traction conditions. When you’re not experiencing such a situation, it’s better to drive with the diff lock disengaged.

Using this in normal driving conditions will be dangerous for you and the vehicle.

How Fast can your Semi Truck Go with Diff Lock On?

In general, you can’t go as fast as 25 mph. Otherwise, you may see yourself and the semi truck being lift into the air.

It is not meant to be used for high-speed driving but in tough conditions where the vehicle finds it difficult to move.

Is the Differential Lock the Same as a Power Divider in a Semi Truck?

While both features provide power to the wheels, they perform different tasks.

A power divider known as inter-axle differential (IAD) gives power to all wheels while a diff lock only works on a set of wheels, say front or back, and not simultaneously.

Both of them allow each tire of the vehicle to move at an equal speed. The diff lock caters to the axle. The IAD, however, distributes the torque on each axle.

When the IAD is locked, this provides more stable traction and prevents spinning out of the vehicle.

How Long do Differentials Last in a Semi Truck?

Like other car parts, the differential requires maintenance.

However, since differentials are located under the car, this important task is usually taken for granted.

If differentials don’t get the star treatment they deserve, it won’t last long. But fortunately, the need for a change of oil is only required when the truck reaches from 30,000 to 50,000 miles.

How do you Know if your Semi Truck has a Bad Differential?

The best way to identify a failing differential is to check any fluid leak or the noise the vehicle produces. The leak usually comes from the axle seals. 

Have you seen a brown fluid on the ground when your semi truck is parked? Well, that could be a sign of a bad differential.

Besides a broken tire, the differential is most likely damaged when you find it hard rotating the car. If the semi truck is difficult to handle, it may cause a serious accident.

Moreover, the strange noise that a vehicle makes is also an indicator that the differential is failing.

Noise is the most common symptom but it is mostly mistaken as the normal sound of a vehicle.

If you hear a “whirring” or “rumbling” noise, the bearings are usually worn or loose. However, if you notice a “banging” sound, it may lack lubrication, or the gear is improperly set up.

Can you Fix a Bad Differential in a Semi Truck?

There are several types of damages in the differential. But most repairs fall into two categories such as replacement and sealing the differential. 

In most cases, the differential may fail due to wear and tear. However, you can prolong its life through regular lubrication and proper maintenance. 

Due to aging, some parts of a differential like bearing and gears will deteriorate. You can identify that these parts need to be fixed when you start hearing a loud and annoying sound.

You will need a mechanic to fix this problem. The process may take between 3 to 5 hours.

For leaky differentials, sealing the piston and sides of the differential is required. This will stop the fluid from leaking.

This type of repair is challenging and time-consuming. The axle shafts have to be removed and the old seals need to be replaced with new ones.

How much does a Truck Differential Cost?

Prepare a hefty amount of money for your bad differential. If you get lucky, you may find yourself paying between $200 to $400.

This is the average and cheapest cost for replacement, repair, and hours of labor.

If the differential gears get damaged, prepare at least $1500 or higher for repair. It’s expensive, isn’t it? What more if the entire differential needs to be fixed?

For this reason, most vehicle owners choose to sell their vehicle especially when the repair cost is more than the actual value of the vehicle.

How to Care for your Semi Truck Differentials

You can maintain your semi truck differentials by simply bathing them with gear oil. Lubrication prevents the bearings and gears from overheating.

The oil will break down over time. Hence, you must change the fluid from time to time.

This differential service involves replacing the fluid with a new one. To do this, the differential case has to be removed and resealed before adding a clean fluid.

Hiring a mechanic is advised.