With both isettas and Nobels the pin is tightened down so as to lock the centre tube, as you say, done with the wheels facing straight ahead.
Are they? I seem to recall both have shouldered steering bush bolts, like the Trienkel. The Trienkel has a castellated nut and split pin. I used to be certain of buying high quality silent blocks - readily available size on the Trienkel (For some reason the Club refused to believe this and made the part, badly, for three times the price. I bought in the real thing for a few quid. Somebody made a few bob there. Eventually they sold those off at a loss, as no one wanted them, and Dave and I, who used the right ones, bought the lot as we new what else they fitted and some installations needed no great accuracy, so I made a few bob there. Plus continuing to sell the correct bush the club still did not stock). The center tube is slightly longer and mounted with washers the bolt was done up to kiss the bloc. It therefore rotated about the shoulder bolt and center of the bloc in grease. One finger steering. Two sets of replaceable parts that were cheap to make. It works well. The Schmitt is similar, but uses a plastic material, latterly, that is more sacrificial in favour of the bolt.
If the steering bolts are shouldered and longer than the block depth and the bolt locked in some way it would suggest it should have a silent block that has a longer center than outer and the same rules apply. If the center is not proud then the washer will probably foul as the rubber flexes in use. Without the washers it will work, but I have them if I can as any failure of the rubber insert is not going to see a fast loss of the bushing material and a potential accident as I chose one large enough to cover the outside of the insert, if room. Of course if the steering is aligned badly by design than a free rotating system will not work since the excess flex will be upset. If really poor it will effect the steering and suspension geametry - see Tri Tech, despite ball joints. That is called crap engineering.
To lock up the steering and drive through rubber in torsion sound a pretty crap idea and might go part way to explain why I fail to enjoy driving Nobs and Isettas. In neither case did I ever get as far as pulling the steering apart to effect a result I wanted. I realized I did not like them and moved on. Stock came in, and went out, to MOT standard if on the road. Thereby showing the advantage, and extra asking price, of a car I have been using as such things get sorted out so they work.
Secondly by having rigid steering the input from caster settings, tracking, tyres and trike instability must be awful - thinks - yep, it was. The Nobel that went straight on at the Mini Roundabout being the worst case.
The Trienkel had two issues. The rubber mounted engine frame could move on cornering. The rear drums tended to work loose, in a bad case the short axle would have worn splines and need stuffing with compound (Again a problem the Club could simply resolve by supplying a hard steel bearing spacer, in place of the soft steel one that deforms under the loading. Its a design and materials fault that can be resolved for pennies. To date this has not happened). So resolving the issues mentioned and setting the steering up well the Trienkel handles as well as its front dampers will allow, which is fast at the risk of having an emergency and asking to much of the front suspension and rolling. So driver skill and risk assessment, but they are better than many think. The cure for that front suspension is whole other problem.
