Part 2 of my Running Gear series, today looking at roadwheels. The series is looking at all the bits of a tracked vehicle mobility stuff 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…
An AFV is really a wheeled vehicle, it just brings its own road with it to run on. Roadwheels bear the weight of the vehicle and are the primary interface with the track and thus terrain
That said, they are very different to normal wheels. One of their primary requirements is to provide positive lateral engagement to the track, this has resulted in a dual-wheel design being standard with the track horn engaging between the wheels
They do not use pneumatic tyres, as the size required to support typical AFV weights would be enormous and not fit. Solid rubber is used, however AFV weights can result in overheating issues under prolonged usage so compound and bonding agent choice is v important
Selected compound needs to balance softness for ride comfort, vibration and noise attenuation with robustness to resist chipping, cutting and the generally huge wear and tear of an AFV. FInally, must tolerate significant temperatures generated by track runing under it.
Ideally larger diameter wheels are desired to reduce rolling resistance and ground pressure, however space constraints limit this. The standard result is a 6 or 7 roadwheel design for most contemporary AFV with diameters of ~600mm, tyre thickness of <45mm and width of <180mm
Roadwheels used to be all steel, often cast, but to save weight are more often a steel/aluminium wheel with a welded-on steel rim for high durability. The rubber tyre is then bonded to the rim. The end result is a lighter and quieter wheel.
# of roadwheels makes a big difference to ground pressure and consequent mobility. Broadly, more roadwheels is better for pure mobility. Chart shows resistance as track tension coefficient increases using 3 vehicles with differing roadwheel number configurations, 8, 6 and 5.
8W on that chart is showing up to 50% less resistance than the baseline 5W. The configurations used in this model were a 5, 6 and 8 roadwheel vehicle. For the 8, the wheels were interleaved like on a Tiger tank. The vehicles were otherwise identical in design.
Practical limitations mean most AFV sit between 5 and 7 wheels, increasing in number as the weight increases. Interleaved roadwheels like the Tiger tank are not used any more, as they bring prohibitive practical and maintenance issues.
You can see designs toying with roadwheel configurations from time to time. Most overtly of late was the German Puma evolving from 5 to 6 wheels from its late prototypes to the production version.
A light run through of roadwheels Next in the running gear series will be return rollers. /end #miltwitter #tanktwitter #AFVaDay
Part 3 on Return Rollers is here:
