Differential
Differential is what i find most miraculous(jokes!) on the drive line side of the vehicle. I've just watched this hilarious video by Chevy it was sooo old presenting like the 1st generation of the diff. I then came to realize that was how they have come to invent the differential. Many years ago, wheel axle is just a bar connecting 2 wheels, but when it comes to a drive axle where 2 wheels are drivers and ability to comprehend steering, different load/terrain/ road conditions is needed, there comes the differential.
I've realized there are mainly 2 elements revolving this area: speed( rotational/ rpm) and torque.
1. Speed different:
When a car is steering left, the left wheel always travels less than the right wheel. And in order to keep equal, it has to turn faster. At the same time, both wheels are driven by the pinion/ crown wheel (final drive) set. This is the part that is miraculous about the DIFF. Examining the basic logical construction of the diff leads us right to the explanation or speed matter. Let assume that each wheel side is independent. In the diff hub there is a side gear for each wheel drive and both not locked with each other.This looks like they are gonna be able to be speed different freely, but how are they driven? So there is the final drive gear locked to one wheel. What about the other wheel how is it gonna get driven? If yes how are both of them gonna be free from each other? The answer is: there is a set of spider gears just idling freely in side the DIFF housing. They are put in mesh with the side gears but 90 deg cross-layout with them and have a shaft that keeps them idling in place. By this way, when there is a speed difference, they just be like: side gears are travelling at opposite direction so one spider spin a way one spin another way according to the difference between the side gears speeds.
Before all those miracles happen, the crown wheel which drives everything, are bolted to a cage that contains all the magical gears. Side gears are nt driven by locked in with the walls of the cage but spider gears are locked by a shaft to the cage, and the cage is the crown wheel so when the crown wheel spins, the locked spiders gear move with the housing pushing the side gears with them making them spin. This is when the car goes straight, both wheels are a@ equal speed.
This ability is so useful as it allows no tyre skidding during steering of any WD type, because every wheel is able to stick to the road on their own will.
For 4WD vehicle, there needs to be a DIFF @ front and one @ rear. But there's only 1 transmission out put then there needs to be a 3rd DIFF synchronising 2 UJs together. As far as i know this 3rd one needs to be able to switch on n off on different road condition. The reason 4 this is: when driven on plain road, steering n wheel sizes makes them turn @ different speed eventually, a 4WD without the 3rd DIFF is NO good for tar-sealed road or any concrete road because its a highly gripping road, allows no type skidding so if there is difference between front bridge n rear bridge eventually one of them is gonna get twisted. "Part-time four-wheel-drive systems don't have a differential between the front and rear wheels; instead, they are locked together so that the front and rear wheels have to turn at the same average speed. This is why these vehicles are hard to turn on concrete when the four-wheel-drive system is engaged"( how stuffs work source: http://auto.howstuffworks.com/differential1.htm)=> this makes a lot of sense.
However, on muddy, or rough terrain, cars need 2 be able to over run this so, all DIFF operation is locked so every wheel gets a chance of driving the car out of trouble. Plus there are rooms for skidding, some free " on air" time.
For any who concerns, here how the speed different works: OF course what's coming out of the final drive is constant, say...500 rpm. The car is turning left, so left wheel is turning at 375 rpm, guess how fast the right one turns....500+(500-375)=500+125=625. Now take R+L and average them: 625+375=1000; 1000/2=500= original final drive speed. Simple math huh?
2. Torque and traction
Take the maths exercise above as an example. When there is rpm obviously there is torque. Torque difference or speed difference i meant the same thing, but they might be differently illustrated. You see how torque is delivered equally 2 both wheel when going a straight line R-F-L is 500-500-500 and when turning left 625-500-375. So clearly torque delivered to left wheel decreases that torque delivered to right wheel gains the exact same amount: 125 rpm.
That was just the case with plain mechanical DIFF where both wheels get the same amount of road grip. In the case of Diff lock-up, when turning, inner wheel will spin while outer wheel will drag possessing the result of balanced amount of torque forced to both wheel=> this is the preferable case of drifting where it is encouraged to lose rear wheel traction to over-steer, skid, or power sliding etc...
What happen if there is one wheel on air while the other is turning? This is also a case of open differential operating. The wheel on air have no grip whatsoever all the torque will logically be transferred to the ground wheel, and the ar still move as usual? NO!!! The theory behind the open differential is "the open differential always applies the same torque to both wheels, and the maximum amount of torque is limited to the greatest amount that will not make the wheels slip"
(http://auto.howstuffworks.com/differential3.htm) This explains everything isn't it?
-When on dry cond there is plenty of grip, torque will only be limited by engine power, but in low grip cond, the torque provided by engine power can excess the threshold of road friction in this case is low therefore "burnouts"( when 2 wheels are locked 2gether).
-When one wheel is on good grip n the other is on thin ice, an open DIFF suffers because the wheel on ice won't get a torque that can excess the threshold of the very low friction allowed for maximum gripped acceleration, and so does the well-gripped wheel. In the end, the car won't move as much
-4WD; SUV with open diffs front n rear will also suffer on off-road, because when a wheel comes off the surface (which happens quite often) or lifts up at an amount enough to lose effectively all grip with the ground, the car might get stuck. Why? because when on air, very easy little torque will spin the wheel helplessly on the air while it is not enough to move the well on the load, so your car gets stuck.
3. Limited slip diff or positraction:
Soo with the recognized weaknesses of the open diff we can all guess out obviously what features the more advanced stuffs need to have. The problem was due to the limited grip on 1 wheel the other wheel doesn't get enough torque right? So we need more torque to the non-slipping wheel( on-load wheel) when those stinky situations happen. But how do they do that?
I'd say to gas more and make both wheels spin stronger, i might b able to move my car but then I waste gas inefficiently and my car ends up slipping. However, the solution is still making the load wheel spin with more torque when the slipping wheel is turning faster, it is exactly what these LSD are designed to achieve.
Most common LSD probably the clutch-type LSD, where there's a spring pack that pressed against the side gear to the clutch pack( actually friction disc). When the car moves straight, the clutches has no effect. But when steering, the outer wheel tries to spin faster hence resisted by the clutch pak get pressed with the outer side gear, wanting both wheels to travel at same speed, meanwhile adding more torque to the slower wheel. I think this also makes turning more effective because less slipping means more traction in turning. This special ability helps cars getting out of very stinky situations when one wheel is slipping it will try to over power the stiffness of the spring n the clutches friction to spin faster, hence the torque that is lost due to the resistance will be transferred to the slower( some case idle/ on load) wheel. In the end your car still be able to move out of the trouble but nt as much as engine power, but hey thats basically what we need.
An also popular LSD is the viscous coupling usually used between front n rear of a 4WD. It has 2 clutch paks one of the front one of the rear and stuck between all there is the thick fluid, all trapped in a box. when front drive tries to spin faster than rear drive due to steering, the front coupling will spin faster, driving the fluid with centrifugal force that also drives the rear clutch pak faster to catch up, resulting in more torque to the slower wheel. This principle is effectively giving more torque to the wheel on load as well as eliminate unnecessary spinning giving more traction steering and terrain over coming. I also see this can be popularly applied to 2 wheels drive as well.
""(Torsen( Torque- Sensing) is a torque sensitive LSD, works as an open differential when the amount of torque going to each wheel is equal. As soon as one wheel starts to lose traction, the difference in torque causes the gears in the Torsen differential to bind together. The design of the gears in the differential determines the torque bias ratio. For instance, if a particular Torsen differential is designed with a 5:1 bias ratio, it is capable of applying up to five times more torque to the wheel that has good traction. These devices are often used in high-performance all-wheel-drive vehicles. Like the viscous coupling, they are often used to transfer power between the front and rear wheels. In this application, the Torsen is superior to the viscous coupling because it transfers torque to the stable wheels before the actual slipping occurs.
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.)""http://auto.howstuffworks.com/differential6.htm) THIS MAKE HELLA LOT OF SENSE!!!
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