What is a Torsen Anyway?
Much has been written about the superiority of either
front-wheel drive, rear-wheel drive or all-wheel drive cars. On the road and
the race track they each have their benefits depending on the situation and the
environment. Wouldn't it be great if you could have the strengths of each
system combined into one car? The Audi Quattro system comes close to that
combination and STaSIS takes it even closer.
One of the best Òbang-for-your buckÓ products that STaSIS has
developed is the center differential torque bias modification. In short, it
allows you to deliver torque where you need it and when you need it. We will
present the basics of how open and Torsen differentials work and then present
the performance characteristics of the Torsen differential and what STaSIS does
to improve it.
In order to understand the benefits of a STaSIS high-bias torsen
center differential for Quattro cars, it is necessary to first understand the
purpose and workings of a simple open differential. Open differentials are
found in the front and rear of Audi vehicles equipped with Quattro
all-wheel-drive as well as the majority of the cars on the road today.
Within the transmission of most Audi all-wheel drive cars, there
is a center differential that supplies torque to the front and rear axles. The
center differential is called a Torsen differential and while it serves a
similar purpose to the open differential, its operation is much different.
Open Differential
In the early 19th century, Marcel Pecquer, a French railroad
engineer, tackled the problem of how to deliver equal amounts of torque to two
wheels that were spinning at different speeds. The same issue is present on any
car on the road today and the most common example of this occurs when a car is
driven around a turn. Consider the example in Figure 1. Looking at the front
wheels, it should be apparent that the left wheel must turn faster than the
right wheel because it has to travel further. This is not an issue in a rear
wheel drive car because the front wheels are independent of each other. Now
consider the rear wheels and let's assume that they are connected by a solid
axle. If the same logic holds true, the left rear wheel would want to go faster
than the right rear, forcing either the right wheel to spin or the left wheel
to drag.
Figure 1 Ð diagram showing right turn of rear
wheel drive car
A differential lets each wheel turn at its own speed while
providing an equal amount of torque to each wheel Ð each wheel thus keeps its
grip with the ground or railroad tracks, as the case may be. Torque can be
defined as the force that causes rotation. The basic design for an open
differential, has not changed much since 1827 when Pecquer first fitted one to
a steam traction engine (see Figure 2). Additional benefits include: reduced
power required for making a turn, decreasing turning radius, and smooth
slip-free low-speed driving.
Figure 2 Ð diagram of open differential
For the moment, we will assume that this differential is
transmitting power from the transmission to the rear wheels of a rear wheel
drive car. The input shaft (from the transmission) transfers torque directly to
the differential housing via the outer ring gear. So-called Òspider gearsÓ
rotate on a shaft that is attached to the inside of the housing. These impart
torque to the left and right drive gears, which are rigidly attached to the
drive axles via the left and right output shafts.
The simplest example to understand how the components work
together is when both wheels are turning at the same RPM (Revolutions Per
Minute). The input shaft spins the differential case, which in turn imparts
torque from the spider gears to the drive gears. Since both drive gears are
moving at the same RPM, the spider gears are not rotating around their own axes
because they only rotate when there is a speed difference between the left and
right wheels. The RPM and torque for each wheel are thus the same.
Now, suppose we are going around a right hand turn again.
Remember, that the inside wheel, the right side in this example, travels less
distance than the left side. In this case the right drive gear will be spinning
slower than the left drive gear, thus forcing the spider gears to rotate around
their own axes. This lets the sum of the drive gears' rotational speed to
remain the same while the torque is evenly split. While this is beneficial in
many situations, more extreme circumstances highlight the weaknesses of an open
differential.
Remember your high school physics? Newton 's third law tells us
that ÒFor every action there is an equal and opposite reaction.Ó Suppose the
right wheel runs over a patch of ice while the car is traveling in a straight
line Ð suddenly the force the car is able to impart is drastically reduced. The
open differential applies the same amount of torque to each side. AND, the
torque that it can deliver is limited to the amount of torque that won't make
the wheel slip. So if the right wheel is on ice, the torque that will make that
wheel spin is very low. Remember that the torque delivered to both sides is the
same, little torque is delivered to the left side with good traction.
Since we're dealing in Audis here, let's just suppose we had an
open differential for a center differential. The maximum driving force
available would then be (as above) that available by the lesser of the front or
rear wheels. In low grip situations the car would be limited by one pair of
wheels, even though the other pair might have decent traction. Fortunately,
when Audi designed the Quattro system they realized that the front and rear
pairs of wheels would greatly benefit from having a dynamic difference in
torque.
Torsen Differential
The dynamic difference in torque in the
Audi Quattro system is achieved with a Torsen ( Tor que Sen sing)
differential as the center differential. Torsen differentials have the unique
property of handling different wheels speeds (between front and rear in the
Quattro) while delivering the available torque (defined by both the engine and
grip conditions) up to a specific ratio. In most Audi applications this bias
ratio is approximately 2:1. Thus a car with 300 ft-lbs available could deliver
200 ft-lbs to the rear wheels and 100 ft-lbs to the front, or the opposite Ð
100 ft-lbs to the rear and 200 ft-lbs to the front.
A common misunderstanding about Torsen differentials is that
there is a ÒstaticÓ ratio of 1:1. Torsen differentials are constantly adjusting
the torque bias to existing conditions. If both the front and rear set of
wheels have identical traction, this ratio would be 1:1; however, any
difference in grip between the two will lead to a torque difference.
How Torsens work
Compared to the open differential described above (Figure 2),
the Torsen differential (Figure 3) replaces the side and spider gears with
element and spur gears. The spur gears function like the open differential's
side gears, and the element (also known as satellite) gears connect with the
spur gears in a modified crossed axis helical gear mesh. Also known as an Invex
gearing arrangement, it allows for continuous contact (decreasing wear and
tear) and provides the main mechanism for achieving the previously mentioned
torque bias.
The Invex gearing also has a unique characteristic in that the
driving torque , which is delivered from the transmission to the Torsen
housing, can be transferred from element gears to the spur gear but not the
reverse. This driving torque causes the element gears and spur gears to
Òlock-upÓ, delivering the engine torque to the wheels. Conversely, the side
gears can turn the element gears but not the reverse.
Figure 3 Ð Torsen and Invex Gears
Using the same example of a right turn and assuming that the
open differential has become a Torsen, the left wheel will turn the left spur
gear faster than the Torsen housing. Conversely, the right wheel will turn the
right spur gear slower than the Torsen housing. The speed difference is taken
up in the synchromesh gear portion of the element gears.
Again suppose that the right wheel runs over a patch of ice and
traction is reduced. Torque will be sent to the right wheel causing:
-
the right wheel to turn the right side gear faster than the Torsen housing that
then,
-
turns the right element gear which tries to turn the left element gear
-
The left element gear then tries to turn the left spur gear
Refer back to the Invex gearing characteristics described above.
When the element gear tries to turn the spur gear they will Òlock-upÓ and the
torque from the housing will be imparted to the left wheel. Note that in the
above example the traction is reduced . The Torsen
differential is a torque multiplier of the torque available from the wheel with
less traction. It multiplies the available torque, by the bias ratio, to the
slower turning wheel with better traction. This is important because if the
wheel is in the air, traction is reduced to zero. Multiply by zero torque and
the result is zero and nothing will move.
In the Audi Quattro, all of the above concepts hold true except
that Quattro uses a Torsen to bias torque between the front and rear axles
versus left to right as our examples have shown.
Torsen Performance
As mentioned before, on a smooth, straight road, Torsen
performance would be identical to that of an open differential Ð equal torque
balance between front and rear. In a situation where the front and rear have
different traction available (such as in inclement weather or enthusiastic
driving) the Torsen will deliver more torque to the set of wheels with more
traction, limited only by the maximum bias ratio or the amount of traction
available to that set of wheels. This is why Audi drivers are often seen with a
smug smile as they pass SUVs and pickups in the snow.
While the Torsen differential provides great advantages in
inclement weather and every-day driving, performance disadvantages become
apparent during more aggressive driving while pushing the car to its limits.
The STaSIS World Challenge Touring Car Team encountered these limits when
originally building the B5 A4s to compete in 2001. Under hard braking (i.e.
very little engine torque) the Torsen has very light internal loading, causing
it to act much like an open differential, going into the turn with little
drama. Powering out of the turn is an altogether different story. Under
acceleration the weight is shifted rearward, giving the rear tires more grip
while removing grip from the front tires. The factory Torsen (2:1 torque bias
ratio) system compensates, sending approximately 66% of available torque to the
rear tires and 33% to the front tires. This results in front wheel spin and not
a great deal of torque to the rear wheels.
This torque bias thus handicaps the Audi in corner exit - the
car can not put down its available power effectively, it loses available
lateral acceleration due to wheel spin, and decreases tire life by spinning
wheels needlessly. Rear wheel drive cars don't have this problem, for they can
put down all of their power through the rear tires and they don't lose traction
through front wheel spin.
STaSIS High Bias Torsen
The STaSIS High Bias Torsen brings Audi enthusiasts close to
this optimal solution. By modifying the frictional and geometric constraints
within the Invex gear train, a STaSIS differential can achieve up to a 7:1
torque bias ratio. In terms of the optimal solution presented above, an AWD car
fitted with a STaSIS differential will act like a FWD car in the brake zone
(recall a Torsen acts like an open differential under small loads), while in
mid corner under throttle the Torsen will send torque nearly equally to the
front and rear wheels, and on exit up to 88% of the available engine torque
will be sent to the rear wheels. This equates to 33% more power available to
the rear wheels! The STaSIS high bias Torsen allows your car to make full use
of the available grip AND available power.
hile it's a relatively simple modification, the STaSIS torque
bias modification provides a dramatic improvement in negotiating a turn quickly
and efficiently.
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