How much power gets split with 4X4?

[T-Bone]

I was wondering how much power goes to the rear wheels and how much to the front when 4X4 is selected?

[Mickey_D]

On our little Turtles the transfer case does NOT have a differential in it like a Subaru Legacy or WRX. You can REALLY tell this if you have it locked in 4WD and turn a really sharp 180 on a dirt road.

Power is applied 50/50 in our cars. If one wheel loses traction, the other axle takes up the slack. If you have one wheel on each axle that has no traction, you're not going anywhere.

And yes, I have VAST amounts of experience with this from this winter and last from driving my little rust bucket around on 2 inches of glare ice.
At 60+mph.
On roads barely wide enough for two bicycles to pass in opposite directions.
In blizzard conditions.

Yes, I live in the God forsaken wasteland called northern Minnesota. Ever seen the movie Fargo??? Fargo gets MILD weather in comparison to us.....

[Gasoline Fumes]

With no center differential, the torque split is always changing.

Dave Coleman of SCC wrote a pretty good description of how torque splits work. Parts pertaining to the 4WD Tercel are in "olive" colored text. But I think the whole thing is worth reading anyway.

Sport Compact Car
February 1999
by: Dave Coleman
ALL WHEEL DRIVE EXPOSED

Ask any 10 people the difference between all-wheel drive and four-wheel drive and you'll probably get ten different answers. More than likely, they'll all be partially wrong. Don't think you're going to find the definitive answer here either-frankly, I find the desire to make a distinction between the two rather amusing. How many wheels does the average car have? Four. All-wheel drive sounds like four-wheel drive to me. Semantics aside, the reason most people are wrong about all-wheel drive is that they assume it to be any one system. Most say, for example, that four-wheel drive powers all four wheels all the time, while all-wheel drive powers only two until those two slip.
In reality, there are at least seven different methods being used today to transfer power from one engine to all four wheels. Getting power to all four wheels is not, in it self, a particularly challenging task. Getting it there and making it actually improve the performance of the car is where things get tricky. When traction is limited--either by low-friction surfaces, or by the fact that the car is being driven at the limit of adhesion on regular pavement--precisely how much power is applied to each wheel can become critical to how the car performs. Let's look, then, at what all these are and how they work. Pay attention now, this could be really useful at parties.

Solid four-wheel drive

The most basic form of all-wheel drive is to simply lock the front and rear wheels together. In a solid four-wheel drive system, the front and rear wheels are mechanically connected and must turn at the same speed, no matter what. This is great for getting unstuck in muddy or snowy conditions, but it is a terrible system for pavement. In normal driving there are simply too many situations when the front and rear wheels must turn at different speeds. Any time you are going around a comer, for example, the rear wheels will follow a shorter path than the fronts, so they must turn more slowly. With a solid four-wheel drive system, this means the tires have to slip. In particularly tight, low-speed comers on a high traction surface, a solid four-wheel drive car can bog to a stop as the drivetrain binds and the tires refuse to slip. This is why solid four-wheel drive cars are typically only part-time four-wheel drive, with a lever that can shift back to two-wheel drive when traction is good.
Currently, only trucks and SUVs use such a crude system, but older Subaru's (up to the mid-'80s) had solid four-wheel drive systems as well.

Open Center Differential

The obvious solution to the problems of a solid four-wheel drive system is to put a differential between the front and rear wheels. just like a differential in a two-wheel drive car allows the inside and outside wheels to spin at different speeds during a turn, a center differential allows the front and rear wheels to turn at different speeds.
A simple, open center differential does have limited usefulness in low traction situations though. An open differential on a two-wheel drive car will allow the wheel with the least traction to spin while the other sits still. Similarly, one pair of wheels can spin with an open-differential all-wheel drive system. Since the front wheels have another differential between them, you can theoretically spin just one tire with open differentials all around.
On pavement-wet or dry-this is unlikely to be a problem. To spin one wheel on a two-wheel drive car, the differential mechanism requires that the spinning wheel turn twice as fast as if both wheels were turning. To spin one wheel on an all-wheel drive car, power is transferred though two differentials, so the spinning wheel has to turn four times as fast. On ice, spinning a wheel even four times faster than normal is still easy, but on pavement it would take a tremendous amount of power.
Because it would effectively revert to an unpredictable any-which-wheel drive system on loose surfaces, an open center differential is a poor compromise. The only time an open center differential can be used is when it can be locked solid for serious low-traction duty.
Again, there is an obvious solution here: A limited-slip center differential. Limited slip differentials, of course, come in many forms, but only two are being used with any frequency in all-wheel drive systems.

Torsen Center Differential

When performance is valued above cost, the Torsen differential is usually the answer. It would take far
more space than I have here to explain how the guts of a Torsen work, but the end result is that power can be sent where it belongs with a minimum amount of power loss.
The Torsen differential is also less prone to suddenly locking up, as some less expensive mechanisms can. Torsen limited slips, and similar gear-type limited slips, are used in the RX-7, Supra Turbo, and Type R-basically anywhere where performance is ultimately important. Using a Torsen differential in the middle of an all-wheel drive drivetrain makes the transfer of power though all four wheels virtually seamless. Audi has used this design for virtually every all-wheel drive car it has built. In fact, all-wheel drive with a Torsen center diff effectively describes every Audi Quattro built to date. That is about to change, but more on that later.

Viscous Center Differential

A slightly simpler design is the viscous limited-slip differential. With this design, several plates are packed close together in a bath of silicone-based liquid goo. The plates are alternately attached to the front or rear output of the differential such that if the front and rear wheels are turning the same speed, all the plates turn together, but if one end starts to slip, the plates rotate relative to one another. When the plates start sliding past each other, the viscous goo is designed to expand, locking them together. Depending on the thickness of the goo, this can effectively lock the front and rears wheels together, or allow only a limited amount of slippage.
These systems can react surprisingly quickly--often needing only a quarter turn to lock up--and operate very smoothly and progressively. Since the goo is quite fluid when the plate slippage is slow, there is very little power loss through a viscous center differential during normal pavement driving. Also, since a small amount of slippage is needed before the viscous diff starts transferring power, ABS is easier to implement. Independent control over each wheel is critical to making a workable ABS system; if the front and rear wheels are mechanically linked, a locked wheel at one end of the car will cause a locked wheel at the other end, confusing the ABS computer. The slight delay in lockup of the viscous diff actually makes ABS more effective. To understand why, imagine this scenario: On ice or other extremely low friction surfaces there can be a significant delay between when the ABS computer releases the brakes on a wheel and when the wheel actually begins rotating again. If the other wheels are still rotating, though, the center diff will lock up and force the locked wheel to rotate again, making it easier to maintain control. Even on higher friction surfaces this is a benefit, but on snow or ice, it could mean the difference between spinning and stopping in a straight line. All U.S.-market Subaru's with manual transmissions, including our new Impreza 2.5 RS project car, use a viscous center differential. The late, lamented Celica All-Trac, the Eclipse/Talon twins, the 300OGT and the RAV4 equipped with a manual transmission also use effectively the same system. The only difference being that the viscous coupling is not directly attached to the differential, but operates in parallel elsewhere in the transaxle.

Planetary Differential

So far all of these all-wheel drive systems have been based on a 50/50 torque split. Though technically all of these systems--even the solid four-wheel drive--are variable torque split (more on that later), they all start by divvying up torque 50 percent front and 50 percent rear. Most drivers will agree, though, that a rear-wheel drive car has more enjoyable handling characteristics than either a front-wheel, or 50/50 all-wheel drive car. Not surprisingly, if you bias the torque split more toward the rear, you can reach a good compromise between rear-wheel drive handling and all-wheel drive grip.
A simple way to do this is to use a planetary gear differential instead of a normal, spider gear differential. A planetary gearset, like those used in automatic transmissions, can be used as a compact differential simply by sending power in through the planet gears, and out through the sun gear and ring gear. The reason planetary differentials are never used in two-wheel drive cars is that they don't distribute torque evenly. Though the torque split can be changed somewhat depending on the relative sizes of the sun, planet, and ring gears, the sun gear always gets a higher percentage of the engine's torque than the ring gear. This makes a planetary differential ideal for a rear-biased all-wheel drive system.
High-performance Subaru's sold overseas (the Japan-market WRX and Legacy Twin Turbo and the European market Impreza Turbo) use a planetary differential to split torque 65 percent to the rear. As with their U.S.-market cars, the differential uses a viscous limited slip mechanism to prevent slippage at either end of the car.
The late, lamented 323 GTX also used a compact planetary differential, but instead of being limited slip, there was a driver-controlled switch that would lock the differential, making it solid four-wheel drive.

Viscous Coupling

The viscous coupling used to make a viscous limited slip differential can also be used by itself in place of a center differential.
By driving the front wheels normally, and sending power to the rear wheels through a viscous coupling, the car will behave like a front-wheel drive car until there is enough slippage to lock up the viscous coupling. Aside from solid four-wheel drive, this is probably the simplest, lightest, and cheapest four wheel drive system around. Of course, a viscous coupling could also be used on a primarily rear-wheel drive car, but to the best of my knowledge, this has never been done.
The majority of Volkswagen's Synchro models, which were available primarily in Europe, used this system, as does the new Volvo V70 XC.

Clutch Pack

So far, all the systems discussed are purely mechanical, working without any external signals, but several cars have had electronically-controlled all-wheel drive systems. These systems are remarkably simple. Take our viscous coupling example, and replace it with a clutch pack. In this case, the clutch pack is very similar to the multiple plates in the viscous coupling, but instead of being separated by goo, they actually make contact (though usually submerged in gear oil). The amount of slippage allowed between the front and rear wheels can be controlled by how hard the plates are pressed together. By employing a small, electronically-controlled hydraulic ram that can precisely load the clutch pack, torque split can be controlled by a computer.
Subaru's with automatic transmissions, the upcoming Audi TT Quattro, the Japan-only Nissan Pulsar GTi-R, the old Honda Real-Time 4-wheel drive wagons and current CR-V, and even the often forgotten Toyota Camry All-Trac, use a computer-controlled clutch pack to turn a primarily front-wheel drive car into an all-wheel drive one. The Nissan Skyline GT-R, Porsche Carrera 4 and 959 go the other way, transferring power from the rear wheels to the front with such a clutch pack.

Variable Torque Split

Most manufacturers of all-wheel drive cars boast that their system can vary torque split a certain amount. A common way to rate torque split would be to say that the upcoming Audi TT Quattro, which is primarily front-wheel drive until the computer decides to send torque to the rear, is variable from 100/0 (front/rear) when the clutch pack is disengaged, to 50/50 when the clutch pack is fully locked. it could be argued, however, that the TT Quattro actually has the capability of going to somewhere in the neighborhood of 20/80 or even 0/100 in extreme situations. According to Hideki Ishido, VP of Product Planning for Subaru, torque split actually depends on the grip available at each wheel.
To see why this is true, imagine a solid four-wheel drive car (with a fully loaded clutch pack, the TT could theoretically act as one). You would assume that solid four-wheel drive always has a 50/50 torque split, but according to Ishido, you would assume wrong. If you actually put torque sensors on each driveshaft, you would see that torque split is constantly changing. The easiest way to visualize this is to imagine that the drive-shafts are made of rubber and will twist as torque is applied. Now, imagine an extreme situation in which the rear wheels are on the ground, and the front wheels are in the air. Ignore for a moment the unlikliness of the situation, and think about how much torque is going to the front and rear driveshafts. The rear driveshaft will have quite bit of twist in it, as it takes a fair amount of torque to keep the vehicle going. The front shaft, on the other hand, should be twist-free. Since the front wheels can't rotate any faster than the rears, they are only receiving enough torque to spin them at whatever speed the car is going, and since there is no resistance to their movement, the rubber driveshaft doesn't have to twist. In this case, all of the engine's torque is going to the rear wheels. Now, put the front wheels on snow or ice, and a little more torque will go up front. Even with all four tires on the same surface, torque split will depend on how much weight is being borne by each tire, since the weight on each tire determines how much grip is available.
Believe it or not, there are even more systems out there, most being various combinations of these basic systems described here. Subaru probably has the largest number of systems; obsessed as they are with all-wheel drive. In addition to using four of the systems represented here, they have at least two other systems. one is an electro-hydraulically actuated clutch pack used in conjunction with a planetary center differential for WRX's with automatic transmissions and the Japanesemarket SVX. The other is a manually adjustable electromagnetic clutch pack and planetary differential combo used in some of the more exotic models of the WRX, including the STi and STi Type-R.
There are even some systems that I never touched on here, but they are primarily used for trucks and SUVs and other uninteresting vehicles. The systems listed here should give you plenty of ammunition next time someone tries to tell you what all-wheel drive means.

[GTSSportCoupe]

Thats an excellent quote Gasoline Fumes, thanks!

[T-Bone]

Thanks for the info on the 4WD. I tested out sharp turning one time on dry pavement and it would hardly turn. I could feel the back end dragging a bit.

[Gasoline Fumes]

Thats an excellent quote Gasoline Fumes, thanks!

Yeah, the column made quite an impression on me too. I remembered it from 7 years ago. Luckily somebody else typed it up and I was able to find it through the magic of Google.

[Typrus]

I want an SVX.... My buddy in FTC has one... DEAR GOD IS IT FUN!!!!! 6 cyl boxer, AWD (rear-bias) and something like 270 horses NA. Not to mention they have a better Co-Efficient of Aerodynamic Drag than a Ferrari Enzo!

A solid 4x4 system has a 50/50 split ONLY in an ABSOLUTE straight line, with all 4 tires EXACTLY the same pressure, treadwear, and traction conditions, all bearings of equal resistance, no brakes dragging at all (or dragging at equal amount, etc.

A solid system is made to make a 50/50 split, not a 20/80 or whoever, but that only means its made to make such. Ever made a Lego Tecknics car? I made an AWD one. If you spin the center input , make it do a wheelie, you'll see the rear parts flexing while the front just idly spins, as said by the write-up. Almost no torque goes to the front, while most goes to the back.
Torque is force to do work. If you put in 100ft/lb of "work", and no work is needed in one part, that input will find where it is needed (in a locked system). If 20ft/lb is needed in one spot, and 80 in the other, in a solid-locked system, it will go to its proportionately needed area. Does that make any sense at all?
How about a system designed to put 80% up front, and 20% in back, and it does a wheelie? Well, all that torque will try to make its way to where it is needed and it will break componants. Unless it is bias by gearing, in which case the front wheels will spin faster and the car will slow down.

Either I suck at understanding my own explanations, or I am very tired... I vote both right now lol.


You notice he mentions the Torsen system right? Did you know Humvees use Torsens? Go to http://www.Howstuffworks.com to see how a Torsen works. But essentially, even in a uber-advanced Humvee, if a wheel makes its way off the ground, it will spin while the rest sit there and do nothing. The Humvee owners manual recommends applying the brakes lol.

In a locked 50/50 setup, if both front wheels lose traction, but the rears have traction, the fronts will spin at the same speed as the rear, but won't do anything. The work all goes to the rear, where it makes the vehicle move.
In a open diff front and rear with locked center, as in our systems, in the worst of scenario's, we really only have 2wd. If the left side loses traction, it will spin while the right sits and does nothing. Or if the front left loses traction, and the rear right loses traction, they will spin, and you'll stop moving.
That is the appeal of Limited Slip. It essentially applies a clutch that makes sure that both wheels will spin, but allows the resistance from cornering to let the wheels spin at different rates. Won't matter if the front and rear are locked, as in our setup, as ALL wheels have to spin at different rates in cornering. In our setup (locked front and rear axles) the total speeds of front and rear axles have to be equal. But the rear axle needs to spin slower than the front in cornering. So it puts massive stress on the drivetrain, as some of us who have accidentally driven on dry pavement in 4wd can testify to.
So how can a 20/80 split let the axles spin at different rates than purely 20/80? I'm not entirely sure how it works, but the 20% power receiver may be spinning twice as fast as the 80%, but still only be getting 20% of the power. Don't ask me how it works, I haven't done enough research in that area.


I doth rant and grumble lol.

[takza]

Far as I know...the worst systems are those with an open diff in the middle (transfer case?)...with 2 open diffs in the axles...then you have a one wheel drive 4WD?

The locked center...like the T4WD has is considered crude...but couldn't be use on a heavier vehicle? This system is cheap, effective, and reliable? How many other 4WD drivelines could handle over 200K miles with just oil changes?

Maybe 2 times offroad I've had 2 opposite corner wheels off the ground at the same time...only then was it freewheeling...but as long as I had some momentum...on it went.