Part 6 of my Running Gear series, today looking at tracked suspension. The series is looking at all the bits of tracked vehicle mobility and started here (bit.ly/30596QZ) if you want to follow the threads. Hope its interesting. 
Usual disclaimer - this is Twitter, I don’t have much space and so some things are simplified or omitted for simplicity. This is a hugely complex science; I’m just giving a flavour of the considerations inherent in AFV design. With that out the way…
Whilst there have been many historic suspension designs, contemporary AFV almost exclusively use either torsion bar or hydro pneumatic (hydrogas) systems, so I'm looking at those here. Historic stuff perhaps another day!
Torsion bar uses lateral circular section bars anchored on the opposite side of the hull. As the suspension arm moves it twists the bar, and this torsional resistance provides the travel and desired resistance. As a design it is simple, relatively light and inexpensive.
Bars can provide limited adjustment, but for significant adjustment the bar is swapped for one of a differing composition. This is seen when users upgrade Leopard 2A4s and radical increase in weight over the front of the vehicle necessitates stronger bars in front few stations
There are drawbacks, chiefly that travel is governed by material limits of the bars. Suspension travel is proportional to bar length and thus hull width. Additionally rebound force at full travel can be limited by linear spring characteristic, requiring advanced bump stop designs
Travel was limited to ~180mm, however newer materials/processes including electroslag refined (ESR) steel, shot preening, presetting and surface finish rolling has upped this to beyond 380mm. To combat high frictional stress at the roadwheel bars run through an oil filled chamber
A drawback is that replacing bars especially when damaged can be challenging. Mine blasts in particular can distort the bar and removing it from its usually corroded anchor point can be an emotional experience.
Recent mine protection packages have included tethers to prevent torsion bars moving excessively internally in the event of a large underbody blast detaching them from their mounts and causing significant damage and injury within the fighting and driver's compartments
A further issue is that the bars have to lie across the bottom of the hull and so raise the effective height of the vehicle as components that might be mounted to the hull floor have to sit above the bars. This is typically around 100-150mm for a tank, not inconsequential
An interesting result that isn’t immediately obvious is that torsion bars necessitate asymmetric running gear, as the bars at each station are offset longitudinally from each other Does require some consideration in the design, but is not a major issue. Side view here shows this
Hydropneumatic suspension is the alternative. Invented in the 50s, its externally mounted with each wheel station having a gas cylinder which is compressed by a piston on suspension compression . Physics of gas provide for an inherent rising rate for greater pressure at rebound
Gas is held at high pressure of ~100bar rising to 500 at full compression. Sealing is an issue so system uses an intermediary oil reservoir. Axle moves a piston compressing oil which goes through a damper valve to modulate rate and compresses the gas. Sealing oil easier than gas
Contemporary systems such as the Hortsman Hydrogas InArm allow dynamic real time adjustment per wheel station of the ride height and consequent travel. This kind of system allows the fancy tricks that the K2 and Type-10 pull off – squatting, kneeling and other antics
The difference this can make is substantial - here the MBT-70 prototype showing height range. For reverse slope positions, fine tuning a hull down position and general high angle surfaces it is a very useful capability
Individual arms can even be ‘locked’ by closing the valves in the oil reservoir, allowing extremely stable firing posiitons, or to remove any ’give’ in a vehicle conducting earthmoving or similar. It’s a very adaptable system, and also generally notably lighter than torsion bars
As it is external to the hull, there is no loss of internal volume, and most drawbacks of torsion bars are removed. It can allow more radical driveline solutions to be considered, such as the Puma’s decoupled running gear, which is almost completely isolated from the hull
Drawbacks are around mechanical complexity making maintenance & repair very intensive compared to swapping out torsion bars. It is also much more expensive to procure (>50% extra) & support – support cost for the UK Challenger 2 fleet’s hydrogas systems is over £600k a year alone
A few practical issues around storage of pressurised nitrogen and oil exist but have largely been dealt with in the single unit designs like the InArm that integrate all reservoirs and cylinders into a single unit on the wheel arm itself.
Where there is not dynamic per-wheel adjustment there can be significant variation in pressure from ambient temperature and frictional temperature from prolonged use. Oil can heat beyond 130C under continued rebound and this may not be consistent across all wheel stations
Hydrogas can be retrofitted to older vehicles, here the recent Bradley experimentation with the system versus its conventional torsion bar configuration. Most future AFV will prefer the flexibility and weight/space savings of hydrogas over conventional torsion bars
And that’s a very brief look at suspension. There are whole books on the topic, so this was admittedly cursory. Next in the running gear series will be track types – single/double pin and live/dead. /end #miltwitter #tanktwitter #AFVaDay
Next part: bit.ly/30ODYpo
