Tracks vs Wheels in snow and ice.

At 9:40 PM +0100 1/16/02, Oerjan Ohlson wrote:
>
>The armoured cars (AML-90) the Argentines used in the Falklands were 
>designed in the late 1950s, and didn't have any centralized tyre 
>pressure control.

The only great benefit CTPR systems give is that you can trundle 
along on black top, then go off onto soft ground with out having to 
get out and run around. One is still limited to a certain range of 
pressure based on rim configuration and if the tire/rim combo has 
bead locks or not. But the argies very well could have climbed out 
and lowered the pressure with a gauge then pumped them up later just 
like early DUKW drivers did.

>  What the Armor article does say is that the SISUs (XA-180s) could 
>drive when even the US M113s were immobilized. (The US Bv206/SUSVs 
>were still operating, of course! :-) ) The article can be found 
>on-line at:
>
>http://knox-www.army.mil/center/ocoa/ArmorMag/ja96/4wheeled96.pdf
>
>>The Fins probably did something specific to get those to work in 
>>the cold weather and in snow.

>They added snow chains, but that's it. I would've thought that 
>tracks gave better traction than wheels+chains, but appearently this 
>isn't always the case <shrug>

Probably depends on the Freeze conditions too. Could it have been 
especially icy? If so according to the article M-113 tracks didn't 
grip very well. However, the SUSV tracks did grip just fine.

What I want to know is why the wheeled XA-180s were nice and mobile 
and yet the HEMTT wrecker wasn't so mobile. What really seems to be 
at issue is the type of tracks and availability of snow chains for 
the vehicles in question, tracked or wheeled.

>A bit heavier than the M113 IIRC... not particularly light, no. The 
>XA-200 has an empty weight of 14 tons, but it is somewhat bigger 
>than the XA-180 discussed in the above article.

But it doesn't appear to be a floatation issue. It appears to be a 
traction issue.

>*Why* the Bradleys are too heavy to swim isn't very relevant, is it? 
>All that matters to the users is that they can't swim. (The Bradley 
>isn't air-droppable either, but AFAIK no-one has ever claimed that 
>it is <g>)

One thing that effects vehicle mobility is ground pressure. If I have 
a big heavy tank with long thin tracks, then I'll have a higher 
ground pressure than if I put wider tracks on the same tank. Compare 
the difference between the HVSS and VVSS on the US tanks in WWII. 
There was a great deal of "Thank god for those wider tracks" from 
many US tankers when the newer tracks started making their rounds on 
the later makes of Sherman (and other types with that chassis).

Ground pressure applies to wheeled vehicles as well. A central tire 
pressure regulation system allows a driver to lower the pressure and 
increase the foot print of each tire for softer ground. BeadLocks 
allow for a lower pressure. More wheels along each side also affects 
this pressure (as one would expect).

The ground pressure of each is:
BV206 (SUSV) 1PSI (front) 2 PSI (rear)
M113 8.63 PSI
LAV 52.5 (nominal it goes higher...)

I cannot locate the ground pressure for the SISU XA-180, perhaps 
someone else can locate it...

Now, traction in snow and ice is such that even if I have a wheeled 4 
Wheel drive vehicle with a low ground pressure, I'll still slide if I 
hit hard snow and Ice. If I add spikes or chains, I will not slide. 
The same thing goes for tracked vehicles. Compare the tracks on the 
SUSV to those on other vehicles.

See: http://sites.netscape.net/chrisyateschris/snowcat.htm

 From reading your cited article, it appears that the M113's problem 
was in fact traction and not bogging down. Otherwise the M113 
wouldn't have performed so well in Vietnam in the jungle and in areas 
of marshland. Looking at the tracks of the CV90 (also a Hagglunds 
product) I see some grippier tracks that likely work just as well in 
snowbound conditions.

Now perhaps the US army would be better equipped in snow bound 
conditions getting some special track pads for snow and ice for the 
M113 and M2/3 series vehicles as well as investing in snow chains for 
each of the primary vehicles. A further step would be to get the 
BvS10 which is an armored version of the Bv206.

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