Axle drive
In the beginning, belt drives with variable axle distances were used as clutch and various of these as gear stages. Final, external chains were found to the drive wheels. Intermeshing gears were sought in vain,
for
example, on the drive of the first tricycle.
It also took a while before these were designed as bevel gear drives, which were then given a housing to protect them from the dust of the road and the road from dripping lubricant. In the meantime, we have
long since
reached a lifetime filling if it remains at certain, moderate temperatures.
The final drive faces two challenges: the final determination of the transmission ratio from the engine to the drive wheels and the distribution of torque to these wheels. This means that there is usually a small
gear that meshes with the engine with a larger one connected to the wheels.
Bevel gears are required then when the drive is arranged longitudinally and spur gears for a transverse engine. It does not matter whether the engine is at the front or the back, close to the axle drive or far away
from it. The latter actually only occurs with what was for a long time known as the standard drive, which has long since been replaced in large numbers by transverse front-wheel drive.
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Why are four wheels driven instead of two, and apparently with increasing frequency? Because the argument of safety was brought into play and initially there were only vehicles in which you sat higher and
probably felt safer because of the better overview of the traffic.
No, the only way to be really safer with a 4-wheel-drive is to reach certain destinations off-road. These chassis are rather less reliable in terms of brakes and chassis, regardless of whether the
road conditions are good or bad. There is no all-clear in terms of fuel consumption either.
The industry has now reacted and is also offering 2WDs with a higher seating position (cross-over). The additional weight of all-wheel drive vehicles has also been reduced. In a normal car, this could
sometimes be reduced to well under 100 kg. Although electric cars based on large SUVs are the easiest to build, but the slightly smaller ones sell better.
Unfortunately, the 'all-wheel drive' option has been successfully carried over into the age of electric drives, again only with the argument of 'driving fun', e.g. through acceleration. For a sufficient range, such a
complete drive at the front and rear is rather counterproductive. The necessary installation space is often deducted from the 2WDs, even though it is not needed.
Let's look at the structure of an axle drive. At the top of the picture you can see the three defining elements of a normal axle drive, the transmission of torque via smaller spur gears (above) to larger ones or
through smaller bevel gears (below) to larger ones, which in this case are called 'ring gears'.
Then in the middle is the differential gear with two axle shaft gears on the left and right and two differential gears, shown here at the top and bottom, but in reality attached to the ring gear via a kind of pot and
rotating with it.
Then in the middle is the differential gear with two axle shaft gears on the left and right and two differential gears, shown here at the top and bottom, but in reality attached to the ring gear via a kind of pot and
rotating with it.
Finally, there are two joints on the left and right that, unlike previous universal joints, work homokinetically, meaning that they transmit torque completely evenly with every small angle of its rotation. They are
encapsulated, have a permanent grease filling and also ensure the necessary length compensation.
The differential gear is able to compensate different speeds of the drive wheels, e.g. when cornering, but unfortunately directs the torque to spinning wheels, which makes it impossible to move forward. A lock
is required here, either switchable 100 percent by a claw clutch, or self-locking.
The designs are called 'limited-slip differentials' and are divided into mechanically, hydraulically, pneumatically and electrically operated. A type of clutch is always used either between both drive wheels or
between one of them and the drive. As an additional benefit of ESP, the spinning wheel can also be braked.
In principle, all 2WDs can be equipped with all-wheel drive, except perhaps a transaxle. We differentiate between transverse or longitudinal front-wheel drive and the standard drive mentioned above. The latter
usually requires the most additional parts, but is still typical for large off-road vehicles today. It is generally installed between the transmission output and the axle drive.
For cost reasons, the transverse front-wheel drive (picture above) is now the most common base, where either a Visko® coupling or a Torsen differential providing a type of permanent all-wheel drive towards the
rear, or an electric clutch making it switchable. Up to three locks are possible, but the first two already contain one of them.
This would be an example of an extended standard drive, but more from the early days or suitable for trucks. It can be switched on via the transfer case in the middle, but then only has a rigid connection
between the two axles, but also the option of an additional off-road reduction.
This is actually still part of the system for difficult terrain, which is no longer necessarily the case for the rigid front axle with its enormous ability to be locked in relation to the rear. The permanent or very
quickly switchable all-wheel drive also has advantages on roads with good grip, because the drive power is distributed between the front and rear axles, thus enabling more lateral guidance there.
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