|Differentials (Open, Locked, Limited
Slip, EDL, Quattro)
The Open Differential
|This all started with an e-mail exchange about the Mk3 GTI-VR6. Here's a
>>Yes we have an open diff.
>>The open diff plus EDL (aka electronic traction control) has the same
>>effect as a limited slip differential. Yes it uses the ABS but the
>>end result is the same, in that both wheels turn roughly the
>>same speed, even when only one side has traction.
>I'm curious. Can you explain the way LSD works and the way ours works
>compared to LSD? Thanks.
Well, I was going to start by trying to describe everything in words, but a differential is hard to describe in words alone. I couldn't find any diagrams on the net, so I created one myself.
That's more or less the diagram of the inside of differential for a rear wheel drive car. A front wheel drive car basically doesn't have a normal "Input Shaft", but everything else is the same. I simplified it greatly as there may be more than 2 spider gears and I didn't show the bearings where the output shafts go through the housing. A real differential is also very compact with all the gears nearly the same size and packed together. Additionally, all perpendicular gear intersections use beveled gears (cut at an angle) but that was too hard to draw. See this diagram of a center differential. (The part of the left is the differential.. part on right is a Viscous Coupling)
Anyway, I'll explain and you can look at the diagram. (Click on the image to open in a new window)
The left and right drive gears have teeth on their sides. They are attached directly to the end of the left and right output shafts, and turn freely on bearings in the ends of what I called the Outside Housing.
The thing labeled "Outside Ring Gear" is a ring gear attached directly to the housing which takes power from the input shaft which comes from the transmission (directly off the tranny's output shaft/s for a FWD car or via drive shaft for RWD). When the input shaft turns, the ring gear turns and the entire differential housing turns. But I'm getting ahead of myself. Let's see what the differential does all by itself.
When you're driving around a turn, this all works together. The inside tire must turn slower than the outside. With constant throttle the housing speed remains fixed, but the spider gears rotate very slowly which allows the wheel on the inside of the turn to rotate a little slower than it normally would and the wheel on the outside of the turn to rotate faster than it normally would. The housing rotational speed is always the average of the two sides. Great for going around turns at slow speeds.
The problem is an open diff always tries to balance the torque. That's a hard statement to get a grasp on, but it means that if the spider gears are pushing on both drive gears and one of them offers lots of resistance (tire sitting on pavement) and the other side offers no resistance (up in the air, or sitting on a patch of ice), then it will find a happy balance where both sides are receiving almost no torque at all. All the rotational energy is guided to the side with the least resistance. In the end, that side spins very fast and the pressure on each drive gear is the same.. Almost no torque is needed to spin one wheel, and since the open diff always sends the same amount of torque to both output shafts, almost no torque is going to the other side as well. Anyone who's driven on snow or ice knows this trick.
So.. Back to the original question. What's a limited slip diff?
Let's start with the opposite of the open diff first.
Go back to the diagram. Look at the spot where the line
from the label "Right Drive Gear" touches the drive gear.
Problem with a locking diff, is of course why we don't all just have solid axles. When you go to drive around a corner, both tires are forced to turn the same speed. Because the path of each tire is of a different length, either one drags while the other spins a little, or probably the outside matches the ground speed (as the weight transfers outward) and the inside tire spins on the pavement (shorter distance to travel, but it's spinning as fast as the outside tire which has farther to go in the same time). Either way it tends to make the vehicle want to go straight all the time, and puts a lot of stress on the drivetrain..
A limited slip differential is a compromise. We'd like a full locking diff in specific instances of extreme traction imbalance but otherwise would prefer operation closer to an open diff, so that the tires can turn at different speeds around corners.
Disclaimer: I'm making up some of the implementation details below to illustrate a concept. The real construction details of these devices are most certainly different. My intent is to allow you to visualize how a limited slip diff differs from an open or locking diff. Some of the designs I've heard, of but don't know where or if it's been implemented. See the AWD page for more detailed information.
Viscous Coupling: (Syncro)
Go back to the diagram and put some intermeshing fins in the space between, and attached to, the drive gears. Make the fins all move close to each other, but let them pass without touching so both drive gears can still rotate at different speeds. Now enclose the space around the fins and fill it all up with a thick fluid. Now as long as both sides are turning the same speed the fins also move at the same speed, and the difference between their speeds is zero. But go back to Example 3 where one side is held still. As you rotate the housing at speed X, the difference in speed between left and right sides is 2X. That's enough to get the fins all moving past each other through the oil and the fins of one side impart some energy to those on the other side via the liquid, which typically is designed to heat up and become more viscous in this situation.. Now some percentage of torque is transferred to the right side that you're trying to hold still.
This is a speed sensitive limited slip. If you turn it very slowly very little torque is transferred to the fixed side. As the input speed increases, it becomes harder and harder to hold the right output shaft still. As the speed difference increases, so does the torque transferred. Eventually enough torque will transfer through the viscous coupling to get your car out of whatever predicament it's in.
The downside is that you're constantly turning all these fins and liquid and if you're
driving around tight corners a lot, it is transferring torque across to the other side,
perhaps when it doesn't need it. You may still get some wheelspin on the tire on the
inside of a turn, for instance. Also, you may find, after getting stuck that the side
that's spinning ends up spinning so much that the tire digs a hole in the ground out of
which you can't climb, even when torque is transferred over to the side with traction. A
VC diff is rarely used at the front or rear axles.
Hydraulic, and Electronic):
Most of the more popular LSD systems fall into this category. I'll just get this out of the way up front. The biggest problem with all clutch systems is that they wear out over time, but it can be a very long time. And like all clutches they generally have a fixed upper limit on their torque carrying capacity.
Anyway, how do we get rid of all those fins, liquid, and weight from the VC?
EDL (often referred to as Electronic Traction Control.)
This system is used by most VW models today, many Audis and several other car models, and was first introduced for offroad use in the Mercedes ML320 (AWD) and is undoubtably used by many others today.
It's what I call the poor man's LSD. (Some people dislike my characterization of it, as it can be very finely tuned, but it's still very cheap to implement and has some reliability and torque handling problems.)
This is an electronically regulated speed sensitive system using the two independent ABS channels. Start with an open differential and go back to Example 3. You're holding onto the right output shaft while the input shaft turns. Now add a disc brake and rotor on the left input shaft.
Activate EDL system. When the computer senses a speed imbalance between left and right, (remember the left shaft is spinning at 2X and the right is not moving at all), it simply applies the brakes to the left side output shaft. The open differential immediately tries to balance the torque. You will feel it trying to turn the right side immediately. If it can grab the left harder than you're holding the right (which it most certainly can and will) then it would immediately transfer some rotation to the right output shaft and it will twist out of your hand. If you could hold onto the right side hard enough (weld it the side of a tractor trailer truck) , it would be forced to slip the left brake disc, or stall the engine.
In practice, the engine will not stall
because the maximum resistance is simply whatever it takes to move the mass of the car, or
spin the other tire. So if one side is spinning and the other side has traction, then the
EDL will brake the spinning side, and the torque transfers to the side with greater
traction and either that side spins or the car moves.
The problem with the way EDL works is that it's pretty harsh. The pulsing of the ABS
isn't progressive. If the ABS is on, it applies full braking power followed by zero
braking power.. full, zero, full, zero.. The torque from the engine that's being
transferred repeatedly all the way across the drivetrain from wheel to wheel a dozen times
a second, puts stress on everything. Brakes, rotors, axles, U joints, output shafts, and
the differential itself. The left and right halves of the differential in the VW 02A
transmission are held together with rivets which if forced to take this
pounding too long will
eventually fail. Neat huh?
So why use EDL if it's so bad?
On the other hand, if one tire is on snow, and the other is spinning on ice and the EDL
has you accelerating slowly, the engine is producing little torque and if you tried to gas
it hard, the torque transfer from the EDL meets with little resistance on the snow so the
torque "escapes" as wheelspin on the snow side. If it's ice and pavement then
yes, you want to avoid mashing on the gas, but pulling away smoothly shouldn't present a
problem as each pulse of the ABS transfers into a small bit more acceleration of the car.
|Quattro, Quaife, Torsen,
Peloquin: (Torque Biasing Differentials)
Audi's Quattro (Audi's marketting name for all their AWD systems) using Torsen and the popular aftermarket Quaife systems use a set of worm gears inside the differential in place of the simple bevelled spider gears, which bind up when there's a resistive torque imbalance. That means, as long as both sides show equal resistance then they are free to rotate at different speeds, such as when going around a turn.
The whole thing is often called a "Torsen" system as in "Torque Sensing" (Torsen is actually a registered trademark, and the more generic term is Torque Biasing Differential or TBD) because it instantly reacts to torque imbalance transferring torque to the wheels that can use it most. There's a difference between the two main types of torque biasing diffs. Quattro's Torsen diffs used something developed by Gleason called invex gearing which is is really all about worm gears. A torque imbalance causes it to *try* to turn the low traction output shaft faster than the higher traction side, but that would cause the invex spider gears to turn, and they drive worm gears which have a greater mechanical advantage (due to the angle of the teeth) than the output sun gears have on the worm gears. That means that a multiple of the torque that would have gone to the low traction side actually goes to the high traction side. So if 20 ft-lbs of traction is at the low traction side, something like 80 ft-lbs goes to the side that can actually use it. A ratio of 4 to 1 or 5 to 1 is common but changing the gear teeth angles changes the ratio. The Quaife unit uses helical gears to accomplish very much the same thing, but the actual operation is not nearly as easy to understand. The helical gears float in pockets on the inside of the housing and apply radial and axial forces generated by the angle of the gear teeth. It can be tuned just like the invex gears to vary the torque ratio. Note however that without significant preload either torque biasing diff will not work well with a wheel completely off the ground. 0 ft-lbs time 4 is still 0. A simple braking trick helps though. (Note, the EDL system discussed above, actually works pretty well with a torsen diff. It activates rarely, but allows for much greater torque transfer when it does.) For this reason, they're rarely found on offroad 4x4 vehicles, the notable exception being the original Hummer H1, which has a note in the owner's manual explaining how to use the brakes and gas at the same time, should one or more tires be off the ground (as is not uncommon while offroading).
It is capable of going from an open differential to say 60% locked differential
condition absolutely instantly (zero lag), so many would argue that it's about as close to
perfect as it gets for performance driving. There are no clutches to wear out. Several AWD
systems like the Audi Quattro system put a torsen diff in the center of the car to control
slip between front and rear wheels. This system does not have the problem the VC does with
the ABS. A torsen diff only distributes torque when it's under load. When it's
freewheeling all the wheels can turn at different speeds as the ABS may desire.
The disadvantages are that the mechanism is a bit heavier (in rotating mass, where it
counts more), more mechanically complicated than some, is expensive, and can't be tuned or
adjusted dynamically. Plus, it reacts so fast and is so even handed that it literally
makes a torque biasing diff equipped AWD car a little boring when you'd really like to
hang it all out. If you want to go 100mph in the snow nothing beats Quattro. A
system that'll let you change the torque bias between front and rear dynamically will
usually be more fun though.
So there it is.
Links to more Limited Slip Differential info
Stuff Works - Differentials
Additions and corrections welcome. firstname.lastname@example.org