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I trust the Internets: M109 SPH Ground Pressure: 11.2 psi (not stated as
combat weight or other) Combat Weight: 52461 lbs. Tread: 15" wide, 156"
contact length (the math bears out)
source: http://afvdb.50megs.com/usa/155mmsphm109.html
An explanation of continous tracks and ground pressure is:
http://en.wikipedia.org/wiki/Continuous_track
If I believe them (I'm a bit skeptical), tire pressure in a car = ground
pressure. I'm skeptical because I find it hard to imagine my car can run
anywhere from 28 - 35 PSI bearing the same weight of car on slightly
different performance tires of the same general dimensions... I didn't think
the contact patch changed that much.
But by this logic, I could pile 3 M109s stacked vertically and still only
equal my car's ground pressure.
OTOH, when my car takes a corner, the wheels turn all the way around. I don't
lock one track and spin the other, which probably has some deleterious effects
on road surfaces. I've also seen tanks driving through small European towns in
the 1980s (M1s) knocking corners off of buildings while trying to navigate
narrow streets, so from that angle, most armour has poorer fine handling and
probably (no first hand experience, just a guess) less driver visibility than
normal vehicles.
Still, ground pressure for tracklayers, even an M1, is better distributed. The
only real concern would be bridges, where aggregate load matters as much as
anything. Some small bridges may not handle 25 tons or the heavier weights of
modern MBTs.
I recall in High School (in Southern Alberta) seeing a picture of a car
destroyed a Suffield by a Challenger. The driver of the track hadn't known his
buddy had parked his car behind the track and reversed up on top of it before
he realizes his mistake and could stop. This is the sort of thing that gets
people concerned about AFVs and normal traffic. A collision between two cars
of roughly equal mass results in roughly equal damage. The collision between
an AFV and a car... well, the AFV is likely to be the
winner....
This is a total threadjack about ground pressure and AFVs.
> If I believe them (I'm a bit skeptical), tire pressure
Actually, even with the exact same tires you can have a wide range of ground
pressures (although not THAT wide, I would say). It all depends on the
internal pressure of air within the tire: Too little pressure increases the
contact surface (thus reducing ground pressure), while too much internal
pressure reduces it. And to make things even more complicated, the pressure
changes with temperature and the tires get warmer the longer you drive, so
there is also a bit of change with continuous use.
(ok, more thread-jacking) In the case of cars more air pressure is
better (of course, up to a limit) because it has some influence in the gas
mileage: more contact surface means more friction, and therefore more fuel
consumption.
As a response to Jon, and getting a bit back into topic, from what I know
tracks do distribute the weight better, but have more "damage potential":
turning or making (relatively) tight sudden maneuvers can damage the road. It
has more to do with total weight that the tracks themselves, but a chunk of
steel directly in contact with the pavement does degrade it (while the
opposite happens with tires).
While working in college for my design project we had some discussions
about the runways that C-5s and An-224 (the really really huge one)
could use (some very interesting real life examples there). And I do not know
how tanks entered into the discussion, so (naturally) some research had to be
done:)
no more threadjacking (for now) but could not resist
> On Jan 12, 2009, at 3:57 PM, Tom B wrote:
> OTOH, when my car takes a corner, the wheels turn all the way
The M-109 didn't lock a track per se -- it simply applied small bits
of break to cause the turn so both tracks would still turn but at differing
rates. Unless you cranked the wheel all the way over and hit the gas.
Don't most modern AFVs work in this fashion?
> On Mon, Jan 12, 2009 at 4:33 PM, Damo <damosan@gmail.com> wrote:
Forget modern, the french Char B1 had this in the 1930's. It is not a brake.
It is a funky application of epicyclic gearing that allows an auxillary
hydraulic motor to run the tracks in opposite directions, or the steer the
vehicle if the engine is driving the tracks. If you can picture an epicyclic
gear set, there is a sun gear in the center, smaller planet gears on a carrier
that circle the sun, and a ring gear that encircles the planet gears. If the
track is connected to the planet carrier, its movement is a summation of the
movements of the sun gear and ring gear. The Char B1 combined this with a
hydrostatic transmission that was continuously variable and allowed the driver
to
aim the bow-mounted cannon. The swedish S-tank added hydraulic rams
to the suspension to provide elevation, so the gun could be rigidly mounted to
the vehicle.
For fun, I built a tracked vehicle with a similar powertrain using two lego
differential gear sets, the lego Power Functions bits from the Creature
Creator set, and the track links off of the lego Jawa Sand Crawler. The lego
XL motor had enough torque to pop one of the gear shafts off, if it slammed
into a wall (probably a good thing, so when I rebuild it, it will retain that
feature). It was an 'Amaze your friends' toy, as it was not obvious why it the
gearing did not lock solid.
Because the tracks have a non-zero length contact patch, there will
still be scuffing at the extreme ends. Tight radius turns will have scuffing
along nearly the entire length. For heavy vehicles, replace "scuff" with
"tearing up of the pavement".
The air pressure in your tires is the ground pressure of your vehicle, if the
tires are soft rubber slicks. If they have any tread pattern, the ground
pressure is higher. High pressure is better for fuel
> On Mon, Jan 12, 2009 at 5:33 PM, Damo <damosan@gmail.com> wrote:
Essentially. Some have a specific pivot steer setting on the shifter.
> At 3:57 PM -0500 1/12/09, Tom B wrote:
Contact patch size varies with air pressure. Air pressure varies the
deformation of the tire at speed and also controls the temperature of the
tire. One of the tricks of off road performance is increasing your contact
patch as much as possible and running low air pressure. Bead locks and divided
disk rims help this as they allow a lower air pressure but prevent the tire
bead from popping off.
> But by this logic, I could pile 3 M109s stacked
Does your car weigh as much as an M109? Tracked vehicles are usually armored
in military service. You have to get that weight down on the ground and still
be mobile. Wheeled vehicles of the same weight have less surface area and have
a subsequently higher ground pressure.
> OTOH, when my car takes a corner, the wheels
You can steer by slowing a track. Brakes aren't an ON or OFF issue in Armoured
vehicles. The more progressive the brakes are, the easier it is to slow a
track for fine control. Bren carriers do it a different way, they warp the
tracks by moving one of the bogie sets side to side. This curves the tracks.
> I've also seen tanks driving through small
It's also size. I've navigated my dingo through tight spaces and didn't hit
anything. I've done the same with a Bren Carrier and a Weasel. The usual
reason for a tank knocking off a part of a building is the driver just doesn't
have the precise point to turn down because he can't see it. Drive a big truck
some time.
> I recall in High School (in Southern Alberta)
That's just a weight issue. Driver should have had a ground guide or at a
minimum, gotten out walked where he was about to back up and then done it
slowly. Modern AFVs are getting rear facing cameras for this purpose as there
are tactical moments where you want to reverse quickly.