Re: [GZG] AFV ground pressure ( was Re: New to the list, and 2 questions: lift/jump infantry)

At 3:57 PM -0500 1/12/09, Tom B wrote:
>\
>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.

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 
>vertically and still only equal my car's ground 
>pressure.

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 
>turn all the way around. I don't lock one track 
>and spin the other, which probably has some 
>deleterious effects on road surfaces.

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 
>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.

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) 
>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....

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.

-- 
--
Ryan Gill	       rmgill@SPAMmindspring.com
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