GP
Agreed. I was referring to the Featherbed frame, for which one needs 3 main isolastic supports and one spring loaded head steady. The latter is kinematically overdefined and provides (elastic) restraint in one direction only, thereby limiting transverse rotation of the powerplant (provided the spring is stiff enough). Inertia forces from the top end heavy Norton engines are not counteracted by the main ISO supports.
On the Commando, the powerplant is actually dangling between two elastic supports, as the designers assumed a completely stiff powerplant. This precondition is not entirely fulfilled.
The head steady is needed to make the system statically determinate about the longitudinal axis. Now, the large distance between the line between the main supports and the head support ISO means that this bearing can be made much smaller than the other two. In addition, the head support ISO serves to limit transverse rotation of the powerplant due to inertia forces. The stock design doesn't transfer any of these loads well, and the only alternative head support providing restraint in two degrees of freedom is the Norvil racer head steady. The top end is the best location to counteract inertia forces arising from the powerplant.
Addendum
Here is an example of a Commando with the Norvil racer head steady (
https://andover-norton.co.uk/en/shop-details/16979). That detail is commented at 04:30 into the video.
- Knut
bringatrailer.com
