The 3rd wheel on the Motor coach. It just tags along. In this video describe the purpose and function of the Tag axle and why some buses have them and some buses don't

Checkout Heavy Metal Mechanic's YouTube Channel
   / @riproaringgarage  

Link to Axle Weight limitations Website
https://ops.fhwa.dot.gov/Freight/publ...)

Clips used from other YouTube videos:
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Prevost H3-45 lifting tag axle from Bus Chaser USA
   / @buschaserusa7495