A TALE OF DIFFERENTIALS

So it was a pleasant weather outside with a cool breeze blowing and you decided to go out for a long drive in your car. It was not minutes after you started driving, came out of a corner and one of the back tyres of your car came under a large slippery, muddy patch and argh!!! It started slipping in its place and your car came to a stop. Now you don’t have an idea what the hell went wrong with your car and how to come out of it and continue your comfortable ride!! That’s because you don’t have the right differential in your car. Wait! What’s a differential?? Don’t worry, continue reading…..

Not throwing up words like torque and power over you, a differential is simply a component in a car between two wheels at the rear or front that allows them to rotate at different speeds (see that’s how it got its name). Now depending upon the situation in which your car is, say one wheel on ice and the other on rough road or one wheel in the air and the other still in contact with the road or both wheels on ice, you have different types of differentials that are designed to work in such circumstances. Let’s dig up deeper into this.

cornering

Now just visualise what happens when a car turns round a corner. Pretty simple! The two inside wheels rotate slowly as compared to the two outside wheels that rotate comparatively faster. If you didn’t get this a simple physics formula will help you out. Speed equals distance travelled divided by the time taken (I hope you know this, my friend!). Now when the car turns around a corner, the inside wheels need to travel a shorter distance than the outside wheels in the same time and so, the formula says that the inside wheels will rotate slower than the outside wheels. Actually, the formula said right!:P

Now if you are done with this cornering logic let’s bring our car to a straight road with both the wheels over the same type of rough road surface. Of course, both wheels will rotate with the same speed and no special physics treatment is required to explain this stuff. So until now, the car just went round a corner and travelled on a straight road with the wheels experiencing the same traction. The type of differential used in such cases is the simplest “open differential”. It delivers the same amount of torque to both the drive wheels.

Okay now let’s discuss what happened to your car wheels on the slippery road that ruined your beautiful long drive. So your brain rats need to get a bit high to understand this logic. Actually, the torque that gets transferred to the two rear wheels depends on two factors- engine and traction. How? If you are driving on a road that’s got a rough surface (high traction), the wheels won’t tend to slip however high power the engine delivers to them. But, if you’re driving on a low traction road you can’t just shower all your engine power over the wheels even if you wish to cruise on the way. Now you’re wondering what will happen if I do so!! Dude, the wheels will just spin (slip) faster and faster at one place and your car won’t move a bit. You are just wasting your engine power. The engine needs to give as much power to the wheels so that they don’t slip but maintain traction and the car moves. Usually, if you encounter a slippery road, you’ll start with the 2nd or 3rd gears so that the wheels don’t slip on account of a sudden acceleration. Now if you have an issue with gears, refer to my previous blog post on “The Gearing Lingo.

slippery sign

So one wheel of your car was over a slippery patch and the other on a rough surface. You accelerated enough to get you car out of it but couldn’t help it. Now understand this. On a low-traction surface, the amount of torque delivered to the wheels is limited by the greatest amount that won’t let the wheels slip. Now which wheel will require the least amount of torque to slip? Obviously the one on slippery patch. And remember, the open differential delivers the same torque to both wheels. So what happens is that the wheel on the rough surface gets the same torque as the one on ice,i.e., very very less torque and this is the reason it will stop rotating and eventually your car won’t move any further. Your engine power is getting wasted on that one wheel on ice and the other is just thinking when the engine will show mercy and give it some power to move. But it’s not the engine at fault, it’s the differential. And the type of differential that comes to the rescue is what we call the “limited slip differential(LSD)”. As obvious by the name, it limits the speed difference between the two drive wheels, not letting them spin independently of each other.

 

lsd
Limited Slip Differential

 

This LSD has its own sub-types: clutch pack based, lockers and torque biased differentials (TBD).

What the clutch pack differential does is that whenever it senses a large speed difference between the two drive wheels, the clutch pack on the side of the wheel with maximum traction gets pressed so that the majority amount of power from the engine goes to it and lesser amount to the one on ice due to loose pressing of the clutch pack on its side. Due to this, even if the wheel on ice slips a bit, the other wheel has sufficient traction to move forward and hence the car moves too. The torque biased differential works on a similar principle except that it uses gears instead of clutches to lockup. It transfers torque from the wheel with less traction to the wheel with most traction due to binding of gears and a multiplication factor known as torque bias ratio (TBR).

You might be thinking that using the clutch pack or torque biased differentials will get your car out of any situation but the problems aren’t over yet. Basically, in the TBD the amount of torque transferred to the wheel with most traction depends on the torque that the slippery wheel experiences. If one of the wheels encounters a very very slippery surface (assume zero traction), the other wheel too won’t get any torque because zero multiplied by zero is zero (TBR)!!!

locked diff

Also, imagine you encounter a rugged terrain. Though the ride will jolt your backbones a lot but here we are interested in how the wheels will behave. Now and then, one of the wheels will come up in the air with the other still in contact with the road. The former will just spin helplessly in air with no torque at all needed to spin it. And if you are using an open differential or a TBD, you’ll get into trouble as the wheel still on the ground won’t get any torque needed to move and your car will stop. So the solution to this is the locked differential that simply locks the two wheel axles together so as to spin them at the same speed. Even if one wheel ends up off the ground, it won’t make a difference to the other and both wheels will continue to spin at the same speed as if nothing had changed. However, on cornering normally or driving on straight roads with the same traction under both wheels, this will behave just like an open differential.

So this was all about the types of differentials that are usually used in cars, though there are a lot more but understanding them requires some extra taste of physics and visualisation. Hence, we’ll confine our discussion till here and I hope you now know what caused the trouble in your car at the onset of that wonderful ride. All attributes to the invention of differentials.

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