Desena wrote:How big of a difference can a suspension system make in combat for AFVs? I don't know much about automotive design.
A lot. It's one of the most important factors in determining a tank's safe offroad speed and mobility, alongside power to weight ratio and ground pressure.
I see that most tanks seem to use torsion bar suspension, while others like the Leclerc and Challenger 2 use hydropneumatic suspension. Do they have any advantages over the other, or is it just different means of accomplishing the same thing?
Torsion bars are by far dominant because they provide good performance while also being quite cheap. During WWII there were a whole host of different suspension types in use ranging from the volute springs used in the M4 Sherman and other American tanks to the leaf springs used in the Panzer IV, the Horstmann suspension used in British AFVs, the Christie suspension used by a number of Soviet tanks, and the torsion bar system which was just coming into use (there were other types in use too, these are just a few). Torsion bars are good because they can handle very heavy weights, are quite durable, are relatively compact, and are rather inexpensive. In particular, their ability to handle very heavy vehicles and their simplicity made them displace most other suspension types after WWII. But their disadvantages are that they eat up vertical space in the vehicle because they need to be installed in the hull and this also makes them potentially difficult to replace when damaged. At the time they were introduced, tanks also often had floor escape hatches and the run of torsion bars beneath the fighting compartment meant that these had to be eliminated, but nowadays this isn't much of a problem because such hatches would be a big liability to mines and IEDs anyway.
Hydropneumatic systems are better than torsion bars because as an active suspension, their spring rate, travel distance, and other parameters can be changed on the move. A torsion bar is a fixed spring and thus cannot have its spring rate changed (although its position can be adjusted manually), but a hydropneumatic system allows things like ride height to be changed at the push of a button by the driver without ever leaving the vehicle. Modern hydropneumatic systems allow the vehicle to adjust its suspension to "lean" forward, backward, or even side to side if desired, allowing a tank to increase its gun elevation and depression. They also provide superior ride quality and do not take up any space in the vehicle; they can be bolted to the exterior without any need for the sort of hull penetration torsion bars require. But because they're active, they require hydraulic pumps and a hydraulic system, which makes them much more expensive and more maintenance intensive.
Are there any other suspension systems for heavy military vehicles, or is that it?
There are, but torsion bars and hydropneumatic are the most relevant for modern AFVs. The Israeli Merkava still uses a helical spring suspension which by all accounts works fine but doesn't have any strong advantages over a torsion bar system.
There are some tanks out there that have mixed torsion bar and hydropneumatic suspensions, usually to get some of the advantages of both. The South Korean K1 88 has hydropneumatic units on the front and rear wheel arms and torsion bars on the middle arms, allowing it to lean forward and backward but not reduce its ride height. This is advantageous because Korea is very mountainous and good elevation and depression are important for mountain fighting. The Japanese Type 90 has a similar system for similar reasons. Both though are expected to be at least partially replaced by new tanks with all-hydrpneumatic systems.