Question: Ground Pressure (Vehicle Design Sequence)

K

Klaus Kipling

Mongoose
I can't square the listed ground pressure for the sample vehicles in both books to tally with the rules on p.27.

The rules state that:

Divide mass (kg) by volume (litres).

!f > 2.5, speed x0.8; > 3, speed x0.6; >4, speed x0.5, no off road; > 5 needs prepared surface.

My 70ton 100 metre cubed MBT has a ground pressure, by these rules, of 0.7.

However, the MBT listed on p43 (Mil, TL10), has a ground pressure of 2.66, yet by the rules I make it 0.53, out by a factor of precisely 5.

The offroader in Civ, p39, has a listed GP of 1.82, but by the system, 0.182, this time a factor of 10. :?:

Which is correct? We have 3 sets of results over both books. :)
 
Sometimes people forget to use the dimensions that of the parts that actually touch the ground.

Tires only have a small precent of their surface on touching the ground.

Where tracks/treads ususally have about 35 to 45% touching the ground.

That might be why the difference.

Dave Chase
 
Klaus Kipling said:
My 70ton 100 metre cubed MBT has a ground pressure, by these rules, of 0.7.
Click to expand...
In my version of Civilian Vehicles it says on page 27:
" ... by dividing the weight of the vehicle by the volume in litres (1,000
litres per M3) of the drive system."

Does your MBT really have 100 meters cubed of drive system ? :shock:
 
Thankyou! Feel like a right nerk now. :oops: :wink:

I must of read that section a hundred times, but somehow I have a blind spot for the phrase 'drive system'.

Wow. Now my tanks are faster! :D

Oh, no they're not. Mass divided by vehicle volume x5 is the same as for tanks... :(

And it's worse for my crazy wheeled super-jeeps.

Is the 'drive system' volume affected by additional kit like extra drive wheels and/or suspension...? Or is that counted separately in terms of ground pressure?
 
Klaus Kipling said:
Is the 'drive system' volume affected by additional kit like extra drive wheels and/or suspension...? Or is that counted separately in terms of ground pressure?
Click to expand...
I think it is only the drive system (page 8 ) itself, without any of the op-
tions, but this is more or less a guess, I did not find any real informa-
tion in the supplement.
 
One of the real benefits of forums - it helps us see with many more eyes!

I don't have the book yet - so I may be reading into this wrong... but this is one of those areas (real world stats) that I generally dislike to see in a game 'design system'. Trying to provide realistic calculations without enough info to calculate.

If one is specifying ground pressure - then to be useful, terrain, etc. must be tracked for strength? This just 'bogs' down play with something that is make believe and not all that realistic to begin with...

Not to say it can't be useful - just the game mechanic that I question. In other words - I might qualify (class) things based on wether they might have issues with 'ground pressure' relative aspects - but I would not try to quantify them (i.e. give specific value). So a vehicle might be - all terrain (like a Russian heavy lift cargo plane that can land without a prepared runway) versus manufactured surface only (like a U.S. plane requiring a man-made runway) and require a success check...
 
This is one case where real world values can be useful.

Pulled from that font of (insert disclaimer here) Wikipedia:

Hovercraft: 0.7 kPa (0.1 psi)
Human on Snowshoes: 3.5 kPa (0.5 psi)
Rubber-tracked ATV: 5.165 kPa (0.75 psi)
Diedrich D-50 - T2 Drilling rig: 26.2 kPa (3.8 psi)
Human Male (1.8 meter tall, medium build): 55 kPa (8 psi)
M1 Abrams tank: 103 kPa (15 psi)
1993 Toyota 4Runner / Hilux Surf: 170 kPa (25 psi)
Adult horse (550 kg, 1250 lb): 170 kPa (25 psi)
Passenger car: 205 kPa (30 psi)
Wheeled ATV: 240 kPa (35 psi)
Mountain bicycle: 245 kPa (40 psi)
Racing bicycle: 620 kPa (90 psi)
Note pressures for Man and Horse are for standing still. A walking human will exert more than double its standing pressure. A galloping horse will exert up to 3.5 MPa (500 psi)
Click to expand...

The article goes on to say that 2 psi or less is recommended for marshy ground, and that humans can get up to ~1800 psi in spiked heels. Snowshoes, at the other extreme, shouldn't get above 1 psi and may be lower than that for some types of snow.
 
Ouch - that goes exactly to what I was saying... way, way to many exceptions (and irregularities) to be even remotely playable! Much less expect a 'design system' to accommodate except in the broadest of terms.
 
A 200lb human walking could and does generate upto 200lbs of PSI

it is located at the point of the heel touching the ground first during movement.

Dave Chase
 
More than that if the human uses their muscles to actively stomp... :D

And that same human can generate quite a bit more pressure per square inch from a fingertip!

As for a surface contact vehicle - over rough terrain, where not all the wheels/treads/feet are contacting the surface - again too many variables. Then have that vehicle firing large calibre propellant weapons to boot (that can be a lot of force translated to ground - as witnessed by watching lower mass tanks firing rounds).

Allowing for the effects of ground pressure certainly can add flavor and 'believability' to roleplay (having been to mud bogs - this can certainly be entertaining :) ) - trying to quantitatively provide stats in a design system is silly. It reduces believability (or increases ignorance) - and encourages lack of situational exceptions...
 
Right now ground pressure has some broad categories that mostly just affect speed, with a number rating that could maybe made use of later.

Query: having extra wheelsets should decrease the ground pressure, yes?

As would wider tracks/tires, and big deflatable balloon tyres...?
 
Klaus Kipling said:
Query: having extra wheelsets should decrease the ground pressure, yes?
As would wider tracks/tires, and big deflatable balloon tyres...?
Click to expand...
Yes, indeed. The more surface area (of tyres, tracks or whatever) is in
contact with the ground, the lower is the ground pressure of the vehicle.
 
rust said:
Klaus Kipling said:
Query: having extra wheelsets should decrease the ground pressure, yes?
As would wider tracks/tires, and big deflatable balloon tyres...?
Click to expand...
Yes, indeed. The more surface area (of tyres, tracks or whatever) is in
contact with the ground, the lower is the ground pressure of the vehicle.
Click to expand...

To put that into simple equations

Pressure=Force/Area

Force is generally equal to weight, which is equal to Mass*g where g on earth is about 9.8065 m s-2, or 10 for most sums :lol:

So putting the same force over a greater area reduces the pressure. Which is why if you must crawl out onto thin ice, they recommend you do it flat on your belly rather than on 2 feet, or even all fours. Bigger area spreading your weight means less pressure on the ice.

LBH
 
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