Flotation of Tanks - did it really work this way?

[istari]

One of the coolest things about Combat Mission is the new things you learn about armored warfare. I never gave much thought to kg/cm2 before, but after a mission like "Singling Shootout", I pay alot of attention to a tanks flotation numbers!

My question is this: I understand that a lower ground pressure means less chance of bogging in. That makes sense. But in Combat Mission, it also seems to impart higher speeds across off-road terrain. Is this how it really works in real life?

SPOILER FOR SINGLING SHOOTOUT IF YOU'RE PLAYING AS A GERMAN. STOP READING, FOOL.

For example, in "Singling Shootout" with all tanks set to "Fast" movement orders, my Sherman 105 quickly outdistanced the remainder of my M4A3s and M476's. I know the 105 has lower ground pressure, but is that how its supposed to work?

Thanks.

Chris Johnson

[Trooper]

Assuming that the tracks are the same on the various sherman marks, lower ground pressure equates to lower total vehicle weight. Assuming equal engines, the lighter vehicle will accelerate faster. I don't know if that's how CM handles it, but it seems like sound logic to me...

Then again, one of the lowest ground pressure AFVs (The Scorpion CVRT) had a Jaguar 6 litre engine.. It could go very fast.. :-P

NTM

[Dittohead]

Don't forget that eventhough the shermans had low ground pressure they were more prone to bogging due to their narrow tracks. Whereas the Germans had learned their lessons in Russia and their tanks were fitted with wider tracks for better performance in soft ground. Not sure how BTS models tanks which are prone to bogging, but most allied tanks suffered from this.

Tony

[istari]

Trooper,

Good point about lighter weight = faster, but I think I was being unclear. I meant kg/cm2 for approximately the +same weight+ tank. In the example I was using, the M4(105) is not appreciably lighter than the other M4 models, yet it has considerably lower ground pressure (presumably to wider tracks dispersing the weight), and thus seems to zip around the battlefield, which I'm not sure makes sense to me. I would think acceleration and top speed would be more related to the hp/ton ratio rather than anything doing with ground pressure, yet there's my M4(105)thumbing his nose at his equivalently powered comrades!

Chris

[Bullethead]

Istari said:

<BLOCKQUOTE>quote:</font><HR>My question is this: I understand that a lower ground pressure means less chance of bogging in. That makes sense. But in Combat Mission, it also seems to impart higher speeds across off-road terrain. Is this how it really works in real life?<HR></BLOCKQUOTE>

I would think so. Lower ground pressure means the vehicle doesn't sink so far into the muck. The further a vehicle sinks, the more surface area on the sides of its roadwheels is in contact with the sides of the trenches made by the tracks (and maybe even its belly is dragging). This increased contact provides nothing but friction that slows movement. So I'd expect a lower ground pressure would result in lower over-all friction, and thus higher speed for a given amount of muck.

<BLOCKQUOTE>quote:</font><HR>In the example I was using, the M4(105) is not appreciably lighter than the other M4 models, yet it has considerably lower ground pressure (presumably to wider tracks dispersing the weight)<HR></BLOCKQUOTE>

Depends on what version of the M4(105) you're talking about. There are 2 versions in CM, the straight M4(105) and the "M4A3(105) HVSS", which I assume would really be designated M4A3E8(105), due to E8 being the identifier for the HVSS suspension.

But whatever the nominclature, the HVSS makes a big difference in ground pressure. Although it added about 3 tons to the gross vehicle weight, it also increased track width considerably, resulting in a net decrease in ground pressure.

Here are some numbers from CM detail unit data. As you can see, almost all Shermans seem to be assumed to have the old VVSS, but 2 types have the HVSS.

M4: 33 tons, 13.6 psi ground pressure

M4(105): 35 tons, 14.3 psi

M4A1: 33 tons, 13.6 psi

M4A1(76)W: 35 tons, 14.3 psi

M4A3: 33 tons, 13.6 psi

M4A3(75)W: 35 tons, 14.3 psi

M4A3E8(76)W: 37 tons, 11.0 psi (HVSS)

M4A3(105) HVSS: 36 tons, 10.8 psi

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-Bullethead

It was a common custom at that time, in the more romantic females, to see their soldier husbands and sweethearts as Greek heroes, instead of the whoremongering, drunken clowns most of them were. However, the Greek heroes were probably no better, so it was not so far off the mark--Flashman

[Trooper]

--Don't forget that eventhough the shermans had low ground pressure they were more prone to bogging due to their narrow tracks. ---

I'm not totally sure I follow the logic there. Gound pressure is basically weight of vehicle divided by the surface area of the track in contact with the ground. Narrow track = high ground pressure by mathematical definition. It just happened that Shermans were naturally light enough that they could still get a low enough pressure from the smaller tracks.

Then again, I do put too much faith in maths sometimes...

NTM

[guachi]

pedantic physics!!

kg/cm^2 is NOT ground pressure. lb/in^2 IS ground pressure. kilograms are a mass while pounds are a weight.

Unfortunately, I forget exactly what kg/cm^2 IS called.

Jason

[Trooper]

Whoa there now.. I seem to recall that weight is actually mass by the force of gravity and is measured in newtons (Force (Weight) = Mass x Acceleration (A in this case = 9.81m/s^2, earth gravity). Or something like that. That said, in day to day conversation, I will use the words weight and mass interchangably, because the force of gravity is constant enough to make little difference over the various ranges that I would be generally discussing.

Both kilograms and lbs/pounds are mass, just one is metric the other imperial. (You are still talking about mass divided by a surface area.) Frankly, I also have to say that the metric system seems so much more easily interchangable between weights, volumes, measurements and so on..

Anyway, weight is the effect of the force of gravity on the mass. Hence a 5kg brick weighs less on the moon than it does on Earth, even though it still masses the same. Which of course means that technically, one would assume that ground pressure ratings should be "kg/cm^2 at sea level on the equator of Earth" in order to be consistent...

I love these inane conversations sometimes... Where's the resident physicist gone then?

Sure, while we're at it, we'll ask an old favorite of mine: Which weighs more.. a pound of feathers or a pound of gold?

This is not a trick question, you either know it or you don't.

Manic Moran

[This message has been edited by Trooper (edited 07-26-2000).]

[Guest Michael emrys]

<BLOCKQUOTE>quote:</font><HR>Originally posted by Trooper:

Both kilograms and lbs/pounds are mass, just one is metric the other imperial.<HR></BLOCKQUOTE>

Well, no. Kilograms are mass and pounds are weight. The unit of mass in the English system is the slug.

<BLOCKQUOTE>quote:</font><HR>Anyway, weight is the effect of the force of gravity on the mass. Hence a 5kg brick weighs less on the moon than it does on Earth, even though it still masses the same.<HR></BLOCKQUOTE>

That part you got right!

<BLOCKQUOTE>quote:</font><HR>Sure, while we're at it, we'll ask an old favorite of mine: Which weighs more.. a pound of feathers or a pound of gold?

This is not a trick question, you either know it or you don't.

Manic Moran<HR></BLOCKQUOTE>

I dunno, but if you're buyin' I'll have the gold, thank you.

Michael

[Formerly Babra]

My gut instinct tells me that a tank which does not sink as far into the muck as another tank will be capable of higher speeds, all other things being equal.

Sadly, my Newtonian physics isn't up to par these days.

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It's a mother-beautiful bridge and it's gonna be THERE.

[micah]

<BLOCKQUOTE>quote:</font><HR>Originally posted by guachi:

Unfortunately, I forget exactly what kg/cm^2 IS called.

Jason<HR></BLOCKQUOTE>

The unit of pressure in the metric system is called a Pascal. It is equal to N/m^2 which equals (kg m)/(s^2 m^2) or a kg/(m s^2).

[Henri]

<BLOCKQUOTE>quote:</font><HR>Originally posted by Trooper:

Both kilograms and lbs/pounds are mass, just one is metric the other imperial. (You are still talking about mass divided by a surface area.) <HR></BLOCKQUOTE>

In fact, people use kg (mass), whereas the proper term would be Newtons (weight); I have never seen a store advertise food in Newtons ; in the English system, people correctly use pounds. Since the relation between mass and weight on the Earth is a constant, it doesn't make any difference.

<BLOCKQUOTE>quote:</font><HR>

I love these inane conversations sometimes... Where's the resident physicist gone then?<HR></BLOCKQUOTE>

I'm one of them.

<BLOCKQUOTE>quote:</font><HR>Sure, while we're at it, we'll ask an old favorite of mine: Which weighs more.. a pound of feathers or a pound of gold?<HR></BLOCKQUOTE>

A pound of gold (or is it the other way around , which is measured in the old Troy system, whereas other English weight measures are in in Avoirdupoids (literally "to have weight" system).

Henri

[CRourke]

I'd have to agree that high ground pressure should increase drag and lower top speed. The vehicle has to do more work just to lift itself out of the muck its in and climb into the next patch of muck, which is subsequently sinks in (though this is more of a dynamic process). Anyone who has ever tried to ride a bicycle on beach sand with know the feeling.

Slugs are one of my favorite silly english units. Nothing like seeing air density measured in "slugs/ft^3" or specific heat as "Calories/Slug*Degree Rankine" to make you want to move to a country that dispensed with this english unit nonsense.

Chris

"My car gets 30 rods to the hogshead, and thats the way I like it!"

[Philistine]

<BLOCKQUOTE>quote:</font><HR>Originally posted by Henri:

A pound of gold (or is it the other way around , which is measured in the old Troy system, whereas other English weight measures are in in Avoirdupoids (literally "to have weight" system).

Henri

<HR></BLOCKQUOTE>

It's the other way around.

1 pound (avoirdupois) = 0.45 kilogram = 1.2 troy or apothecaries' pounds

1 troy or apothecaries'pound = 0.37 kilogram

--Philistine

[Trooper]

Hang on now.. If I understand this correctly, the above is stating that 'pounds' is actually a measurement of force, not mass, so my high school conversions of 2.2 lbs to the kilogram is actually only valid on Earth, and a 180lb man on earth is something like a 30lb man on the moon?

Why the hell didn't anyone tell my physics teacher this?! :-P

Manic Moran

[Ben Galanti]

Well, it depends whether your talking pounds-force or pounds-mass. Obviously, pounds-force is a force unit (like newtons) and pounds-mass is a mass unit (like kilograms).

No, on earth they are equivalent. This is done by adding a gravitational fudge factor into the old F=ma equation (It ends up being F=ma/G I think...) That's why I always hate figuring this stuff out in english units. That G factor gets ya everytime...

[speedy24]

OK...Here is a chart from my fluid dynamics book (Janna, 1993) which should clarify all the different types of Conventional Systems:

Mass (fundamental) - kg (SI), lbm (English Engineering), lbm (British Absolute), gram (CGS Absolute)

Force (fundamental) - lbf (British Gravitaional), lbf (English Engineering)

Mass (derived) - slug (British Gravitaional)

Force (derived) - N (SI), poundal (British Absolute), dyne (CGS Absolute)

Hey, thank god the units for time are all the same, huh?

[This message has been edited by speedy24 (edited 07-26-2000).]

[Kurtz]

<BLOCKQUOTE>quote:</font><HR>Originally posted by speedy24:

Hey, thank god the units for time are all the same, huh?

<HR></BLOCKQUOTE>

Oh, Yes...what time is now...8.41 AM..or is it PM?