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Tracks Offroad.
Low ground pressure
is important offroad
in snow, sand or muddy conditions.
Nothing can equal the low ground pressure
achieved by tracks offroad, not even the widest swamper tyres (tires).
Halftracks
The advantages of tracks in snow, sand and mud has
been realised since at least the first World War
when tracks were fitted over the rear wheels of 6x4 trucks,
and of course the first tanks were also fitted with tracks.
Citroen
built a number of successful halftracks
which took part in major expeditions in the 1920's and 1930's.
This M14 halftrack (left) was built by
International
for the American Army during World War Two.
Halftracks are usually based on standard trucks or four wheel drives
and so can share mechanical components with them.
They steer like ordinary trucks and so do not need the complex
transmission of a fully tracked vehicle.
This also reduces the need for special driver training.
Cuthbertson Track Conversion
A special purpose tracked vehicle is
obviously an expensive proposition while
conversions of regular four wheel drives can be cheaper.
In the 1950's and 1960's, the Cuthbertson
tracked Land Rover
conversion took a standard Land Rover and fitted demountable tracks to it.
The tracks are carried on a substantial subframe to take the
stresses and strains off the standard wheel hubs and axles.
A back of the envelope calculation shows
the low ground pressure advantage of tracks.
Assuming a four wheel drive weighing three tons (6000 lbs),
a wheelbase of 90" and a track width of perhaps 20"
gives a ground pressure of about 1.5 lbs/sq";
1.9 lbs/sq" is claimed for the Cuthbertson.
A 12-stone man exerts a ground pressure of
about 6 lbs/sq" in comparison
and the best that a multi-wheeled vehicle such
as the 8x8 Esarco
can manage is something like 9 lbs/sq".
Although the tracks give impressively low ground pressure
this is not everything offroad.
The step-up ability of a wheeled vehicle depends largely on the
tyre diameter.
For something like the Cuthbertson this translates to the diameter of
the leading wheels on a bogie - about the same as a regular tyre.
Army tanks in contrast
achieve a huge step-up ability by having the leading edge of the tracks
slope upwards from the first load-carrying wheel
to a drive sprocket or idler.
This is not practical on a small four wheel drive.
Bolt-on Tracks
While the Cuthbertson's tracks were, in principle, removable
this was not some five minute operation.
The idea of simple bolt-on track units is attractive
as they can be towed in a trailer behind a standard four wheel drive
to the snow or the swamp where the switch to track power is made.
Manufacturers such as Mattracks have made such gadgets.
These tracks (left) were being demonstrated at the
1996 Perth 4WD Show
on a Toyota LandCruiser 80series.
The tracks are under test by Toyota Accessories division.
They weigh in at just 85kgs each
and can be fitted in about an hour by one person with a good jack.
The main question about these devices must concern
the ability of the four wheel drive's standard steering and wheel hubs
to withstand the forces generated by the tracks.
- /4WD.html
Full Tracks
Where snow is deep, soft and permanent, a fully tracked vehicle
is the only way to go.
Volvo make the BV202
which is used by the British Army.
The Australian Antarctic division uses
Hagglund
machines.
History
Gernot writes in with some more history:
The German Daimler truck company had been contracted to built a 4WD heavy
car for the colonial army then fighting in German south-west Africa.
Since the vehicle was found suitable a 4WD truck version was commercially
built.
These 4WD trucks were well liked by the Allieds - the victorious Allies
took nearly all 4WD Daimlers (and Ehrhardts) in 1919.
The Portuguese bought one of these trucks for their Guinea. And all went
fine until the rainy season came. The truck sank in the mire.
It was sent back to the manufacturer who built the chains of a Holt tractor
onto each side in place of the back wheels. Afterwards the buyer took it
back - the arrangement worked. But there were no further orders.
The Daimler company even mislaid the plans - so when they began to build
half-tracks again in 1916 they had to start anew.
The French experimented with tracks, like the Germans, but with
"rubber bands".
The deserts of Tunesia quickly ruined the rubber chains of a Peugeot car.
The inventor of the French halftrack system, Armand Kegresse, went to
Russia. There he became chief chauffeur of the Czar. The Czar had a
fleet of cars - but could not use them because of the mud and snow.
Kegresse transformed the car pool of the Czar into halftracks - much to the
delight of the owner.
The bigwigs of Russia like the rubber halftracks way down the ages.
Kegresse must be counted as the first successful cross-country automobilist
since he perfected a design, that permitted a single chauffeur to
operate a car in difficult terrain.
- Gernot [11/'00]
See
Snowtrac
and for a bit of fun, see
[this]!
