Part 4 of my Running Gear series, today looking at sprockets. The series is looking at all the bits of 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…
The sprocket is the toothed wheel that transfers drive from the transmission and final drives to the track itself. Teeth on the sprocket engage with the track links at each end – holes in the body of the track for single-pin, and between the end connectors for double-pin.
The friction between sprocket and track is very high, and one of the limiting factors of track life. As the teeth engage into the body of single-pin track, excessive wear can mean whole link replacement, regardless of the condition of the rest of the link
In the case of double-pin track, excess wear requires just the replacement of the end-connectors, and means the body of the link can be made from lighter, less durable materials, creating weight savings (often ~20%).
The main consideration for the sprocket itself is ensuring it interfaces tightly with the track to maximise efficiency of power transference from retransmission to track and thus the ground. Ice, mud, rocks and other debris can be significant issues to proper engagement
Location of sprocket is significant, it can be front or rear. Broadly speaking rear is better (front can be 50% worse rolling resistance), but practical realities may intervene – passengers and access reqs drive IFV and associated AFV to place engine and drive at the front
Engine location constrains drive location as you want the simplest, shortest and most space efficient means to connect transmission to sprocket. That said there have been vehicles with engines not co-located, incl. WW2 favourites Sherman, PzIV and Tiger, amongst others
Sprocket location affects track tension distribution. With front sprocket in forward motion track segments between sprocket & idler and between idler & rear roadwheel are subject to higher tension, whereas track segments between front roadwheel & sprocket are subject to lower
With rear sprocket only the track segments between rear sprocket & rear roadwheel are subject to higher tension, with track segments between front roadwheel & idler and those between idler & sprocket subject to lower tension.
Steel track normally utilises rubber bushings which elongate when subject to tension. With front sprocket total elongation of track is larger than for a vehicle with rear sprocket. W/greater elongation more track can deflect and track segments become looser and support less load
This manifests in a few ways. Most overt is the higher likelihood of track skipping and consequent detracking of the vehicle, especially at high speed or low radii turns. In a military context this is potentially disastrous and thus keenly avoided where possible.
It also has a surprisingly big impact on mobility. Rear sprocket decreases mean maximum pressure, reducing track sinkage and resistance and thereby increasing tractive performance. Drawbar pull can be 8-14% better with rear vs front on normal clay-type soil terrain.
In snow or other soft terrain the effect is even greater, as the hull is bearing onto the surface too and so friction is a vastly more impactful factor. Drawbar pull in these circumstances can be 50-110% better with rear vs front.
# roadwheels have a big part to play but that was covered yesterday, however more is generally better from this specific viewpoint (in the above charts Vehicle A has 5 roadhweels, Vehicle A (8W) has 8 and always has higher mobility in these models.
So rear is always better from tractive mobility perspective, and thats why essentially all tanks use this configuration. But as all things vehicle design, its about tradeoffs. Put engine at the rear of a vehicle carrying passengers and its a nightmare to get out. Mobility vs role
Compare here Bradley (front engine) with BMP-3 (rear engine). Getting out of BMP requires combination of rear and roof hatches to give clearance and climb over engine housing. Could be suggested to be an unacceptable tradeoff. Bradley meanwhile is a standard door and ramp affair
Electric drives offer potential flexibility with motors able to be located away from power generation and option of rear drive IFV possible. That said, only hybrid drive IFV to date, the GCV prototypes, had front drive because the drives ran the width of vehicle as a single unit
I consolidated a prior thread on drive location (bit.ly/3eV0pz5) into this topic, in case anyone got déjà vu. Next in the running gear series will be Idlers and track tension. /end #miltwitter #tanktwitter #AFVaDay
