How to secure the fork when there is no stem On my half recumbent tandem, there is no stem on the front steerer tube. I would like to change the upper cup of the headset (related question Play in headset with no upper bearings on a tandem bicycle). But the stack for my new headset is a little higher than the previous one, and there is not much clearance between the top of the headset and the front seat.


Since there is no stem to secure the fork in place, there is a kind of lock ring with a bolt to do it. This is my understanding of how this system works. Am I right ? Is this lock ring really necessary ? (I'm pretty sure it is so but it adds to the height of the headset).
What would be the proper name for this kind of lock ring ? This design is not rare on cargo bikes, but when I look online to buy that kind of lock ring, I find only one model, and my use case (securing the fork when there is no stem) is not listed in the product description (https://problemsolversbike.com/products/headset/locking_spacer_-_6788).
Yes, this "lockring" is important. It functions like the stem on a regular bike (with a threadless fork) to hold the headset together. The cap on top is used during adjustment, but is not intended to hold everything together. See this description from Park for more details of headset adjustment: https://www.parktool.com/blog/repair-help/threadless-headset-service.

With regards to finding alternative lockrings, as Carel pointed out in the comments, there are seat clamps with the proper internal diameter, and it appears that is what your headset has instead of a stem. I can't tell from the picture for sure what your diameter is, but it is presumably either 1" (25.4 mm) or 1 1/8" (28.6mm). Seat clamps are available in both sizes. Just search for "seatpost clamp 28.6 mm". See if you can find one that is not as tall as what you have now. 

One final detail to note: In order to keep the headset together properly, the clamp must extend to just above the top of the steering tube, so if you are already very close to that height, then it is the steering tube not the clamp that is determining the height. To reduce the overall height once you have a shorter clamp, you will need to cut off a small bit of the steering tube so that it is below the top of the clamp.