Differentials are one of the unsung heroes of modern car design. Tucked away right at the back of the drive train, they are a component that few people even know is there. The fact that they just quietly sit there and get on with their job, very rarely going wrong or needing replacing, has also helped them fly under the radar.
But that all changes today! In this article we are going to take a good long look at these awesome components. We’re going to talk about how they work, all the different kinds of car differentials there are, and also take a peek at some of the signs and symptoms on the few occasions they do start to go wrong.
But first, we need to answer one basic little question:
What Does a Differential Do Anyway?
Yeah, we’re going to kick everything off with this, perhaps the most asked question about differentials – just what the heck do they do anyway?
Well as you probably know the power to move the vehicle forward or backwards is generated by the engine. Frankly, if you don’t know that it may be time to take a quick 101 on auto mechanics basics before getting into differential territory.
A quick note – this example assumes a standard car or truck with rear wheel drive only. Cars with all wheel or 4-wheel drive also have differentials, which are in different layouts. To us though the best way to understand differentials though it with this simple example below.
So, the power is moved from the engine to the rear wheels via the driveshaft and transmission. The differential is the final component of the car that transfers the energy generated from the engine into the powered wheels that will drive the vehicle forward.
Simple, huh?
The Differential is placed right at the end of the power transfer from engine to wheels so that it can perform three vital roles:
- It directs that engine power into the wheels.
- It serves as the final reduction point for the rotational speed on the transmission. Without the differential, the transmission and driveshaft may spin too fast, making the vehicle impossible to control.
- It allows the rear wheels to spin at different speeds.
It is that last function that helps to make differentials the unique and highly important components that they are. It is also, in case you couldn’t guess, where they get their name.
Why Your Wheels Need to Spin at Different Speeds Part 1 – Corners and Turns
There are a whole host of reasons why your vehicle wheels need to spin at different speeds. A simple mathematic statement can explain one of the most important reasons:
Speed is equaled to the distance travelled divided by the time it takes to go that distance.
See, we literally could not give a clearer explanation here. Ok, if advanced math is not your thing (and it certainly is not ours, so don’t worry – we wrote those words out and we still don’t know what they actually mean) then let us give a solid, real world example of the above statement.
Imagine a car going around a corner or a bend. One wheel is on the inside of the car as it turns. This wheel has a whole lot less ground to cover. The wheel on the outside is turning through a much larger arc however, and so needs to cover more ground.
In other words, the inside wheel needs to go slower and the and outside wheel needs to go faster.
Another way to think of this is to think of a row of privates marching on parade in an army camp. When they move in a straight line, they are all moving at the same speed and taking the same amount of steps.
When they turn a corner though the soldiers on the inside of the column have much less distance to travel – they can take little baby steps and essentially just shuffle around the corner.
Their colleagues on the outside of the row though, those poor guys have to swing around the corner covering much more ground – they are then forced to take either more or bigger steps to cover the extra distance whilst staying in line so their drill instructor doesn’t yell at them.
Put that back in terms of the car then as it turns the corner. The inside wheel is taking tiny baby steps (it has lowered its speed). The outside wheel is taking great strides (it has increased its speed).
None of this is possible without a differential, as it allows the two wheels to spin at different speeds.
Why Your Wheels Need to Spin at Different Speeds Part 2 – Terrain and Environment
Whilst turning the vehicle is the number one reason that a differential is required (since it happens so often!) there are a few other times during driving when it can be useful to a have Differential transmission fitted.
Environmental reasons for needing a differential usually come down to road conditions that affect the spinning speed of the powered wheels. If you run over a patch of gravel for example, or a narrow patch of ice, this can affect the RPM of the wheels in different ways.
The Differential will help to compensate for this change in the RPM. This in turn helps you to keep the vehicle under control when it hits road surface hazards.
The terrain you are driving across can also have an impact, though this is an issue you will mainly encounter when off road driving. Dips or holes, elevations or depressions in road surface can see one wheel making more contact with the driving surface than the other. You could even lose traction all together as the wheel is lifted up in the air!
Again, a Differential is there to help compensate in situations like this. It will help to transfer power to the wheel that has the most traction, helping to keep the vehicle moving even when one wheel is lifted clear of the driving surface.
If you look at it in that way, you can kind of see that without a good Differential, you don’t actually have true 4 wheel or all wheel drive.
Why No Differential on Unpowered Wheels?
A quick detour to answer another question that is why is there no car Differential attached to the unpowered wheels?
Well, in our standard car with rear wheel drive, the front wheels are not connected to the transmission at all. This means they are spinning freely and, most importantly, independently of each other.
There is no need for a differential as there is no connection between them. If one wheel needs to spin faster or slower than the other, well, it just does it because there’s nothing to stop it.
How Does a Differential Work?
So that is what it does and why it does it – the next question of course has to be how it does it!
We’ll kick off by talking about the simplest style of differential – The Open Differential. This is the style of differential that is found on most standard cars with two-wheel drive.
So imagine a rounded looking contraption attached to your driveshaft. That’s your differential, Sir!
Inside the differential are a number of components, all of which are interconnected. Try to keep up as we introduce you to:
- The Input Pinion
This is a large cog that is connected at one end to the drive shaft – this makes it spin with all the power of the engine! The Input Pinion is connected to, and rotates, the:
- The Ring Gear
This component sits in the middle of the differential. It looks kind of like a cage, or a weird shaped bucket. It’s actually hard to describe. The Ring gear is connected to the wheels and makes them spin. But because of it’s hollow, bucket like shape it also contains:
- Spider Gears
These are smaller gears that sit inside the Ring Gear.
That is the basic form of the simplest style of differential, the Open Differential. When the car is driving in a straight line on flat terrain, the input pinion spins the ring gear, which in turn transfers power to the wheels to move the car forwards.
Lovely stuff.
But what happens when you turn the car around a bend or corner? What will become of us when the powered wheels needs to turn at different speeds?
Spider Man to the rescue!
Wait, no, Spider Gears to the rescue. Sorry, our mistake.
The spiders gears can spin on their own axles within the spinning ring gear. When they car is moving in a straight line the entire differential operates and spins as one solid unit.
When the car starts to turn and the wheels are spinning at different speeds, the spider gears kick into action. They rotate within the ring, bypassing the main single spinning action of the larger ring gear.
This in turn allows the rear wheels to spin at different speeds to each other.
How cool is that, huh?
What Are The Different Types?
The Open Differential is great, and offers a range of benefits. It is tough, cheap and straightforward and it very rarely goes wrong.
It is however a very simple and inflexible design and that means that it has a few drawbacks.
For example whilst is great at altering wheel speeds during a simple turn around a curve or bend it is not so great at speed transfers during the other times that a differential is required – namely environmental and terrain enforced wheel speed changes.
It just lacks the level of finesses required to cope with tuff like that.
That is why there are other types of differential available, which can vary the torque (or power) applied to each wheel in much subtler ways.
These devices are called Limited Slip Differentials, or LSD.
(Don’t get them confused with other types of LSD if you don’t want the cops knocking at your door).
- Clutch Style Limited Slip Differential
This is far and away the most popular style of LSD. That is because this design takes the good old Open Differential style we discussed above and simply adds a few tweaks. These tweaks comprise a powerful spring and set of clutches mounted right in the center of the Ring Gear.
The spring is used to push the clutches into the spider gears. In this way when one wheel is trying to spin faster, instead of simply transferring power, the wheel needs to first overcome the resistance provided by the clutch and the spring.
This allows the differential to tailor the amount of torque (or power), stopping a situation where one wheel is spinning too hard on an environmental hazard – ice for example. This stops the differential applying too much power to a wheel that doesn’t need it.
The downside is that the resistance wears away at the spring and clutch inside the clutch style LSD. This style of differential will therefore have a far shorter lifespan than the simpler but effective Open Differential.
- Viscous Coupling
No, this isn’t how aliens breed – it’s a whole different style of Differential! This style makes use of differential fluid, and it is most often found on 4 wheel or all wheel drive styles of vehicle.
This type of LSD is comprised of a set of spinning plates that are found in a sealed housing. Why is the housing sealed? Because it is filled with a very thick – or viscous, see what they did there? – fluid.
One end of the plates connects to the driveshaft whilst the other connects to the car wheels. As the car moves forward (or back) the plates happily spin in their viscous fluid filled chamber, simply transferring power from drive shaft to wheels.
If one set of wheels slips however, the driveshaft plate is now spinning at a higher rate than the drive plate connected to the wheel (as it has no or less resistance from the wheel). Because the fluid between the plates is so thick, there is a delay between the fluid response and the different spinning speeds.
This in turn helps the slower spinning plate to catch up to the faster spinning plate. The torque is then matched between drive shaft and wheels and you remain in easy control of the vehicle.
How to Look After Your Differentials
Apart from not going around every corner with tires smoking like you’re Vin Diesel in Fast and Furious 47 or whatever they are up to now, there isn’t a whole you can do to protect your Differentials.
No matter which variation has been installed in your car, you will find that every one is made of tough materials and designed to last for thousands and thousands of miles.
One piece of home mechanic work you can do is to freshen up the differential fluid, also known as Differential Oil. Fortunately this is not a task that you will need to do too often, probably somewhere in the region of every 30,000 to 50,000 miles – so only a few times over the lifespan of the vehicle.
If your car is front wheel drive then the Differential will be mounted right alongside the transmission, and will likely share the same transmission fluid in the differential and transmission casing, making it easy to replace.
Rear wheel drive vehicles will have a stand alone Differential on the rear axel. Locate your Differential and pray it has a drain plug.
If it does, then remove the plug with a big oil pan or bucket underneath. Sit back and let gravity take the strain.
Related Post: Best Oil Drain Pans
If your Differential does not come with a drain plug then you are going to need to be prepared to make a big old mess. Start undoing the casing and as you do the oil will begin to leak out of the seals. Try to catch as much as you can in a wider bucket or pan, but it would be a good idea to lay down plastic sheeting here.
Lots and lots of plastic sheeting.
Once all the old oil has been drained, removed and disposed off safely, the next step is to get to cleaning. Even with a good quality lubricant inside the Differential, there is still an element of metal on metal movement within the component. This in turn is going to generate metal shavings that can gunk up the workings of your Differential.
No matter how dirty it is in there – and if this is the first oil change for your Differential, it’s going to be dirty – resist the temptation to use any chemical cleaners or soaps. They may do a good job of getting everything clean but there is also the risk that they could leave behind chemical residues that could affect the new oil.
Instead just get into the workings with plenty of clean rags, cleaning out the goop, metal shavings and any other crap that is left behind.
Once the interior of the differential is clean, put it back together and pour in your new oil. Check the owner manual for the spec of the oil, but expect to use an 80 weight as a minimum. It also goes without saying but do buy the best quality oil that you can afford for this job.
That’s about it. It may not be the most pleasant job, but it is pretty straightforward and, once you’ve done it, you don’t need to worry about it again for thousands of miles.
Related Post: Best Synthetic Oil
Signs of Worn Differentials
Finally, we’ll take a look at some of the symptoms of a Differential that is starting to get worn. If you notice any of these symptoms and have never done an oil change on your Diff, we would recommend starting there.
If that doesn’t help then you probably have finally worn down your Differential, and it’s time for a replacement. So keep an eye out for:
- Grinding
As we mentioned above, a Differential includes a lot of metal on metal movement. It is essentially a set of gears all rotating around each other. Not only that but they operate under incredible amounts of pressure to transfer a lot of energy in a small space.
This will lead to wear and tear on the gears, and they won’t all wear at the same rate. As some gear teeth erode more than others you will begin to hear a grinding or a humming noise as the vehicle drives. This noise will likely increase in pitch as the speed of the vehicle increases too.
- Whining Noise
Exactly the same as we talked about above, but instead of a humming you will hear a nasty whining noise. No, it’s not Britney Spears singing in your trunk. It’s your Differential making that awful noise. And it’s in pain.
- Vibrations
The next symptom to look out for is vibrations, and not the good ones like the Beach Boys sang about (a lot of music references in this section so far). This symptom is slightly different from the two we looked at already. The humming and whining comes from damage to the teeth of the gears inside the Differential.
Vibrations come from a worn universal joint, which is where the driveshaft is connected to the Differential. This symptom will start as a mild vibration you feel through your foot as you press down on the gas pedal. The vibrations will get worse over time until you get your Diff checked out.
- Tire Damage
Do you remember right at the top of this article where we were talking about what a Differential actually does? Come on, it wasn’t that long ago!
They help with altering power when the vehicle is turning. If however they are unable to do that – because they are worn and need replacing – you will start to notice tire damage. Uneven wear toward the outside of the tires is a big giveaway that the Differential is in trouble and needs replacing.
That’s it folks, the end of our journey into the world of Differentials. Now you know what they do and how they do it, we hope you have a new found respect for these underappreciated but extremely important components.
Sources:
- Differential (mechanical device) – Wikipedia
- How Differentials Work – howstuffworks
