Idea
On behalf of the Bundeswehr, Audi developed a cross-country vehicle for VW «VW litis», which can be called a good foundation for the idea of fourth generation cars. The legend tells it this way: on one of the winter tests in northern Sweden, just such a litis left the front-wheel drive Audi far behind, faster and with a much more powerful engine. Since the then head of the chassis test was engaged in the initial ignition, thanks to which it will turn out «stable running car with all-wheel drive» as a vehicle for all occasions. And after that, as you know, nothing is stronger than an idea whose time has come (and it seems to have arrived), it was developed for so long, until finally the legendary fourth-generation Audi car was created.
The diagram of the fourth generation Audi-100 clearly shows the transmission to the rear axle with cardan shaft, rear axle drive and drive shafts to the rear axle.
All-wheel drive concepts
Until now, all-wheel drive has been almost inextricably linked to the off-road vehicle concept. The principle of all-wheel drive used in this was simple: to a standard drive mechanism (engine in front, drive mechanism in back) it was just «something added», namely, that with a drive branch and an additional cardan shaft, a second phase of the drive mechanism was added to the front axle.
Now, working with the standard drive mechanism is normal, and only if the conditions of the bullet require it, the front transmission is engaged. This type of construction with plug-in all-wheel drive is also common today in off-road vehicles. Disadvantage: All-wheel drive must be disengaged before following a hard surface. Because in turns the front wheels lie much further back than the rear wheels. Different wheel speeds lead to transmission failures. Effect: the vehicle is driven heavily, in turns it periodically wears out the wheel with the most insignificant slippage to discharge the drive mechanism - the car «slips».
With constant all-wheel drive, which is stably directed to all four wheels, it must have a balance that prevents malfunctions in both final drives. This problem is solved by the so-called mid-range differential.
The differential has been known to us since the beginning of the automotive industry. It shares the forces between the right and left drive wheels. Also, balancing is necessary, because - as you know - the wheel on the outside of the curve will always make a longer way back than the wheel on the inside. This balancing is created by an axial differential. This makes it possible in the axle drive mechanism to simultaneously rotate both drive wheels freely relative to each other.
The Torsen differential is based on the principle of a worm gear. The worm final drive may have a high or low locking ability. Designations: 1 - worm, 2 - worm wheel.
Whoever drove a car in winter knows the insidiousness of the object: when the wheel on the ice turns, the opposite wheel on more stable ground does not transmit any force and stands still. One thing is clear: the stable differential is not suitable for the differential of the middle, since such a case is undesirable. The force must be where the wheels transfer it to the ground. Exactly where there is a stable road surface. And since nothing is ever known in advance, distribution must happen automatically—that is, without delay! After various stages of development, the manually lockable mid-range differential is now equipped in the fourth generation of Audi vehicles with the so-called Torsen distribution differential.
Distribution differential Torsen
Shown here is a Torsen distribution differential as a sectional model. Designations:
1 - drive gear (drive mechanism to the front axle);
2 - hollow shaft (gearbox drive mechanism);
3 - differential box;
4 - flange / cardan shaft (for rear axle drive);
5 - worm for rear axle drive;
6 - spur gears;
7 - worm wheel bridge;
8 - worm wheels;
9 - a worm to drive the front axle.
As already mentioned, distribution (or medium) The differential serves as a balancing act between the front and rear transmissions. (From this, axle differentials front and rear work independently.)
This is appropriate - from the outside it is not possible to determine individual structural elements - from the back in a manual drive. The technician would say: it's integrated. The name Torsen comes from the Gleason drive company that invented this differential. Derived from the English words Torque Sensing, which translates as «sense of torque». At the same time, the concept is also already a description of a function. Because:
- For an axle with better ground or ground traction, this differential provides more driving force.
- This happens - under the condition of rolling - if the bridge can carry more force up to 3.5 times than the other.
- Yet with even ground of good quality, an equal distribution of the load of the drive mechanism on the front and rear axles is no unconditional preference for one axle.
The Torsen differential is based on the principle of the worm drive mechanism and uses the basic principles:
- The worm final drive can be laid out so that it has a high or low locking ability. (The locking capacity is multiplied here, which the gearbox should be equipped with.)
- The amount of blocking ability depends on the angle of the worm - when the worm has many flat or few steep turns.
- An example of a high-locking flatworm is a car jack where the crank—acting on the worm—although lifts the car, the car's entire mass never sets the crank in motion.
Optimum use with only 8 worm wheels and 12 gears is achieved in combination with differential box, hollow shaft, front output shaft and rear output shaft in the best possible way.
Driving dynamics on dry roads
Now there is no doubt that in case of poor grip on the ground or the supporting surface, the Torsen differential operates correctly and optimally. This also explains what happens on dry roads in the fourth-generation Audi transmission. As already mentioned, the front wheels travel a longer rearward path than the rear wheels when cornering, as the rear wheels describe a smaller radius. However, the rear axle Torsen differential (due to the more insignificant rotational speed) releases more driving force, which maintains the desired tendency to lean into the corner. If this is too fast and turns one of the rear wheels, more driving force is applied (in this moment) slower front wheels. The quick reaction of the system causes the front wheels to spin more strongly now, which stabilizes the car. Effect: Attitude towards cornering is greatly improved.
Operating instructions for vehicles with all-wheel drive
General:
- If the front axle or front wheel is raised, no transmission occurs.
- If the rear axle is raised, no transmission occurs.
- If one of the rear wheels is raised, no transmission occurs because the rear axle differential is not locked.
- If the propeller shaft is disassembled, no transmission occurs.
Brake Test Bench:
- The brake test can take place on a normal, slow running brake test stand (maximum 6 km/h) along the axes.
- In this case, a transfer to the test stand must take place.
- Neither gear nor differential lock can be set.
Test bench results:
- It is not possible to check the results on a conventional two-wheeled test rig.
- This should take place on a four wheel test rig.
Wheels:
- All four wheels must travel the same distance traveled by the tire in one revolution (ideally equal profile depth).
- Car wheel balancing (dynamic balancing) only possible if all four wheels are raised. The transmission must come from the vehicle's engine (not from a balancing machine). Important: hand lever for engaging the parking brake system («handbrake») let go.
The photos show parts of the differential lock for the rear axle. Designations:
1 — a head of the lowered pressure to inclusion of blocking of differential;
2 - control switch for nested differential lock.
Tire set with winter tread pattern:
- Tires with a winter tread pattern are also used in fourth-generation vehicles - to a lesser extent due to the driving mode, but rather due to the braking mode.
- Snow chains must be fitted - where prescribed - also on the fourth generation models, namely in the front axle. Also here the best argument is the braking mode.
Towing:
- When rolling when towing all four Audi wheels on the ground, there are no special instructions.
- If one of the bridges (after the crash) is no longer valid, the machine must be loaded onto a low bed trailer towing a disabled vehicle.
- If such a vehicle is not available for towing defective vehicles, the Audi can also be transported with a raised axle. Condition, no further than 50 km and no faster than 50 km/h. Differential lock cannot be set (this is so incredible). If the limit is exceeded, there is a risk of damage to the drive due to lack of lubrication.
Rear axle differential lock
The rear axle differential is equipped with fourth-generation vehicles with a speed-dependent differential lock. The lock keeps both rear wheels at the same speed, thus preventing the individual wheel from spinning. A device is mentioned that increases the grip of the wheel with the road if the car is stuck in snow, slush or mud, because the lock automatically turns off at speeds of more than 25 km / h. The differential lock will be engaged when the vehicle is stationary at the switch in the center console. The actuating element is, however, the steering device under the seatback, which releases the underpressure pipeline to the lock and locks them accordingly at speeds exceeding 25 km/h. The reduced pressure comes from the suction tube of the motor.
Also under the seat back are the working parts of the all-wheel drive. The arrow points to the control device for the differential lock.
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