Re: couple Q's for the treadheads--ON TOPIC, I promise!

A main battle tank (also called a medium tank) is a balanced design
that has enough firepower for most battlefield tasks, enough armor
protection to survive most hits, and enough mobility to be
strategically useful. It is ordinarily a fully tracked armored vehicle
with a fully rotating turret. If you compromise mobility and/or armor
protection or delete the turret in favor of higher firepower, the
vehicle is usually considered a tank destroyer (on the other hand, if
you increase both armor and firepower at the expense of mobility, you
have a heavy tank).
Any vehicle that carries a gun but lacks any of the attributes of a
tank is a self-propelled gun. It may have little armor or no armor at
all, even though it may have a turret. SP guns are ordinarily used in
the indirect fire role. Self-propelled guns that are armored like tanks
and are used in a direct fire role are termed assault guns (from the
German Sturmgeschutz).
During WW2, US TD's were essentially stripped-down tanks with slightly
oversized guns but armor only sufficient to stop rifle bullets. They
were supposed to use "scoot and shoot" tactics, as they could not
survive any kind of hit from an enemy tank. If used in place of tanks
in an assault role, they would suffer heavy casualties.
German tank destroyers were of two varieties--the early Panzerjager
were mostly recycled light tanks with their turrets replaced with high
velocity antitank guns surrounded with thin gunshields. They were at a
severe disadvantage in any role other than ambush. The later Jagdpanzer
designs were armored like tanks, but lacked a rotating turret, so all
were technically assault guns. They weren't as bad off as the
Panzerjager, but they were at a severe disadvantage in offensive ops,
as they could not fire to their flanks.
Today the role of tank destroyer is handed off to light wheeled or
tracked vehicles equipped with guided missiles.
"Bogey" is a term borrowed from railroad technology before World War
Two. The box containing the springs and axles is a bogey unit, and the
term was applied to the external vertical volute spring suspension
units used on Lee/Grant/Sherman and Stuart tanks, so the road wheels
were sometimes called bogey wheels, or bogies for short. The term came
to be applied even to vehicles that had torsion bar suspensions.
Likewise, the tracks were often called "treads," though strictly
speaking the tread would be that part of the track shoe that actually
contacts the ground. These terms are rarely heard today, however.
As for what's essential in designing a tracked suspension, it depends
on how much weight the suspension has to support. J. Walter Christie's
early high speed suspension systems from the 1920's used very few
wheels, but his vehicles didn't weigh much. Some vehicles like the
Stuart tank, had the idler wheel double as a road wheel, but designers
usually don't want the drive sprocket to carry weight, as it will put
lateral stress on the final drive gearbox. Figure on at least two road
wheels besides the driver and idler. If the track has to cover a long
distance without a wheel to support it, you run the risk of snapping it
outright. For the moon, you might be able to cheat a bit. Some arctic
vehicles don't use road wheels at all--the track actually slides along
a stainless steel skid on the bottom of the external bogey unit. If you
want some ideas, check out Fred Crismon's "US Military Tracked
Vehicles." It surveys a couple of thousand tracked vehicles from WW1 to
1991, including those snow vehicles I mentioned.
Gerald Owens

.

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