Every day is a school day ladies and gentlemen so it’s time to get your thinking caps on, because today we are going to learn how a vehicle differential works. If you’re reading this and are lost already, don’t fret; I’m going to take it back to basics.
The automotive differential was designed in order to deliver power to the wheels of a vehicle while allowing the wheels to rotate at different speeds. This is needed because the 4 wheels of vehicle will spin at different speeds when negotiating a turn. In order to get you where you are going, each wheel will travel a different distance through that bend. The inside wheels will travel a shorter distance than the outside wheels and thereby are required to rotate at a slower rate. Still with me? Good.
The differential mechanism basically allows each wheel to spin independently of the other while providing power to both. If the left-hand and right-hand wheels on a vehicle were joined together by a solid axle, the wheels would skid when negotiating a tight turn.
The video below created way back in 1937 explains the whole concept far better than any modern animation we’ve seen. Skip straight to 1.50 for the start of the explanation
The differential in any standard day-to-day vehicle is known as “open” or standard differential. It is capable of turning the wheels at different rpm, but it has got one major drawback. You can have a situation where one wheel of the vehicle is on a surface with good traction and the other wheel on a slippery surface. With a scenario like this, an open differential will push the majority of the power to the slippery wheel, meaning the vehicle may be unable to move. To overcome this problem along came the introduction of Limited Slip Differentials or LSD’s for short.
Limited slip differentials are generally only found on performance cars to improve handling and grip, and in some 4X4’s to maximize traction on slippery and loose surfaces as mentioned before. BMW has a tradition of equipping only its most powerful cars with limited slip differentials from the factory, generally limiting their application to its line of M cars. But if cheap and cheerful is what you are after, for some limited slip fun, Mazda has long been a fan of LSD’s in certain variants of the MX5. Don’t be like this one wheeler peeler below; smoking one tyre is never going to get you anywhere.
Limited slip differentials use various mechanisms to allow normal differential action during turns, while also solving the problem of slippage. When one powered tyre slips, the LSD transfers more torque to the non-slipping wheel. The more common LSDs achieve this with clutches or a fluid-filled housing.
Whether you drive a spritely road car or a track day weapon, you will know how vital traction is. You may have added that vital few BHP and got the stickiest tyres on the market, but how your differential distributes that power will make the difference. And just exactly how an LSD distributes its power has shaped an entire car culture.
Oversteer is more fun than understeer, well-controlled oversteer that is. Any Irish rally fan will know this from crouching at a tight bend for days of a rally just to see an RS2000 throw its rear quarter panel into the oncoming ditch and keep going. Also any fan of Top Gear and now The Grand Tour will know that no supercar road-test is not complete with some B-Roll shots of the latest hypercar perched sideways shredding a mortgage payment of tyres per turn.
This is only possible with a Limited Slip Differential. For rear wheel drive vehicles, it is caused when both rear wheels are traveling at the same speed in which the LSD is locked. From the outside it would seem as simple planting the pedal to the floor in a Clarkson-esque rage of power, but is actually quite the opposite and requires a delicate balance of throttle and steering to catch a controlled drift.
Anyone that has spent some time at their local racetrack especially before a drift competition may have heard competitors talking about the rear axle ratio or final drive ratio. This refers to the gear ratio inside the differential. What they’re on about is the ratio of gears on the diff pinion to the gears on the crown wheel. If the final drive ratio is 4:1, then for every tooth on the pinion gear there will be 4 teeth on the crown wheel. What this means in simpler terms is that the pinion gear which is attached to the prop shaft, has to rotate 4 times in order for the crown wheel to rotate once. Final drive ratio has a dramatic impact on your car’s acceleration and top speed. A higher final drive ratio such as 4:1 will give good acceleration but lower top speeds and a lower final drive ratio such as 3:1 will higher top speed but slower acceleration.
Getting the correct gearing and ratios is essential and can mean the difference in that extra MPH. This has become such an important measure in recent years that most professional drift cars will have a “quick change” system in place, so you can change final drive gearing in minute’s trackside.
In the case of all-wheel-drive or 4 wheel drive vehicles, a differential is needed between each set of drive wheels, one between the front and the back wheels. As the front wheels travel a different distance through a turn than the rear wheels. The transmission hooks up directly to a transfer case. From there, one driveshaft turns the front axle, and another turns the rear axle. When four-wheel drive is engaged, the transfer case locks the front driveshaft to the rear driveshaft, so each axle receives half of the torque coming from the engine. At the same time, the front hubs lock.
But as we already found out, an open differential splits the torque evenly between each of the two wheels it is connected to. If one of those two wheels comes off the ground, or is on a very slippery surface, the torque applied to that wheel drops to zero. Because the torque is split evenly, this means that the other wheel also receives zero torque. So even if the other wheel has plenty of traction, no torque is transferred to it. There are some ways to make improvements to a system like this. Replacing the open differential with a limited-slip rear differential is one of the most common ones. But in the case of 4X4, another option is a locking differential, which locks the rear wheels together to ensure that each one has access to all of the torque coming into the axle, even if one wheel is off the ground. This drastically improves performance in off-road conditions.
The locking differential is useful for serious off-road vehicles. This type of differential has the same parts as an open differential but adds an electric, pneumatic or hydraulic mechanism to lock the two output pinions together.
This mechanism is usually activated manually by switch, and when activated, both wheels will spin at the same speed. If one wheel ends up off the ground, the other wheel won’t know or care. Both wheels will continue to spin at the same speed as if nothing had changed and will get you out of some tricky situations.
All in all, differentials will always be essential in vehicles whether mechanical or electronic and if overlooked in the search for traction and speed, you will be back to square one.
If you have a “difference” of opinion, let us know.
Thanks for reading.