With the conventional type of road motor
vehicle, where the power unit is commonly placed at the front,
there are certain limitations imposed which make it practically
impossible to employ any form of central drive for the steering
wheels, i.e., a drive down the centre of the chassis and under
the engine or motor.
Amongst the limitations referred to may be
mentioned (A) the need for sufficient ground clearance, both
from the point of view of Ministry of Transport regulations and
for cross-country work in order to avoid inequalities in the
ground and obstacles; (B) the necessity of the height of the
frame and of the engine from the ground to be kept as low as
possible, both for loading purposes and from the point of view
of the centre of gravity for the purposes of stability; (C) the
need for the wheelbase to be kept as short as possible in order
to obtain the minimum possible turning circle and, in the
majority of cases, except pure tractors, the minimum wheelbase
for a given body space.
All the four specifications relate to means
whereby power can be transmitted from the gearbox to the road
wheels, whether steering or non-steering, by means of bevel
gears to a cross-shaft, thence by bevel gears to two shafts
which run along the frame side members, and from these shafts by
means of worm or other gears to short shafts which in turn drive
pinions engaging with spur gears on the road wheels. Four of
these wheels can be employed as steering wheels, as power is
transmitted to them by universal joints.
Patent No. 328,647 relates mainly to
methods of driving wheels that do not steer, by means of shafts
which lie against the frame side members. In one instance the
shafts are connected to, and ride with, the frame, and are
connected to the short shafts by universally jointed shafts,
whilst in the other instance the side shafts ride with the road
wheels.
Patent No. 328,676 relates mainly to
methods of driving steering wheels. In the left-hand view the
side driving shaft rides with the frame, and drives a short
shaft by means of worm gearing, which in turn drives a
universally jointed shaft which passes through a hollow stub
axle, and drives the road wheel by means of the usual splined
plate. The right-hand view shows an arrangement resembling this,
but in this instance the side shaft rides with the axle. The
vehicles to which these devices are intended to be fitted are
evidently meant for travelling on the roughest ground. |