A brief primer on mobility implications of track width. I'm doing a series of these following warm reception to earlier thread on drive sprocket location (bit.ly/3aUGGNP). Spoiler: wider is better, but some interesting physics on the way #miltwitter #tanktwitter 
Usual disclaimer - this is Twitter, I dont have much space and so some things are simplified or omitted for simplicity. This is a hugely complex science, and im no expert, just giving a flavour of the considerations inherent in AFV mobility. With that out the way…
One of the greatest impacts on tracked vehicle mobility is contact area between track and surface. Increasing track length and width increases contact area, which defines your available thrust and generally decreases normal pressure and thus improves mobility as a generic concept
In most cases vehicle is supported solely on its tracks. There are cases, particularly in snow and when bogged, where track sinkage is greater than ground clearance, in which case you have a problem. Belly weight bearance and bogging is a separate topic for another day though.
Track width affects pressure exerted on track–terrain interface, with main results being track sinkage & track motion resistance. Increasing track width = decrease in sinkage and motion resistance coefficient, and increase in tractive efficiency. In short, wider = more mobility.
Overall vehicle thrust is developed by shearing action of the track on terrain & derived from frictional & cohesive components of the shear strength of the terrain. Softer the terrain, the more surface area is required to maintain tractive efficiency, drawbar pull coefficient etc
What does this all mean? For an APC type vehicle in clayey soil increasing track width from 32 to 56cm (75% increase) decreases sinkage by 28% & track motion resistance coefficient by 21%. Result is thrust coefficient increases by 16%, drawbar pull 55% and tractive efficiency 34%
The same is true in snow and other very soft terrains. Increasing track width decreases the belly loading, and therefore increases the tractive mobility of the track itself. Impact of track width is essentially the same, though the belly loading element is a factor.
The drawbacks. In practice increasing track width obviously means you have more track, and that increases unsprung weight. Worth noting that the data mentioned above did not take account of that weight increase, so these are a best case and probably marginally inflated
Another drawback is mobility outside immediate tactical "can I get through that mud". More track = more resistance & friction. That means higher wear, more drivetrain strain to move & worse mobility in operational and long distance contexts; slower & more maintenance stops
A few examples across the spectrum. At the narrow end, BMP-1. Narrow tracks @ only 300 mm, but light weight and high roadwheel density in the running gear design mitigates this and ends up at quite high mobility. Good demonstration of mobility not being a simple binary concept.
A fairly typical modern IFV - CV90, running standard 535mm track, a baseline for contemporary mobility. Interesting point to consider is just how different overtly similar vehicles can be, and that simply having a track of n diameter does not deliver a specific mobility level
Compare w/Bradley. Both similar weight & size w/~530 mm track, yet CV90 hugely more mobile. This video shows remarkable difference in snow bit.ly/3f5w3KI CV90 design optimised for snow and bogs where belly is bearing weight. More on belly weight bearing in future thread
At huge end is Russian DT-30 (bit.ly/2KPoSIs) running on simply enormous 1,100mm track. Incredible mobility but impractical for most AFV. Russia uses this for arctic oriented systems incl. Pantsir-SA, Tor-M2DT and Magnolia. Note its on rubber band tracks.
These threads address each element in isolation which is problematic as the reality is AFV design considers all factors. # roadwheels, positioning, suspension geometry, belly trim angles and much more are significant factors that are taken into consideration concurrently.
As with all things AFV, nothing is binary, everything is a compromise. Do you want high mobility in 1% of the vehicle's life in contact with live enemies, of in 99% where its not, much of which is safe going terrain. Multiple track width compatibility? Many choices. /end
