There have been a few folks that don't seem to have the helper springs, but typically they are either 12" or 24" long. The 12" long ones seem to work better than the longer ones since the spring rate is doubled compared to the longer ones (same wire diameter and same coils per inch).
The springs are compressed when the roof is down and help for the first few inches of lift, where some force is really needed. The way the crossbars scissor up and down requires a boatload of force to get them to lift when the roof is fully down.
The helper springs sit in the channel behind the rear crossbar pivots; the ones that slides in the tracks. If you look at the sidewall of the channel where the crossbar mounts you'll see a 10" long channel. That's the approximate distance the rear crossbar mount slides from to full up to top full down.
The springs are typically 3-4" away from the crossbar when the top is up, so their usable compression is 6"-7" based on the 10" travel of the crossbar mount.
The springs also require top plates screwed into the channel to prevent them from shooting out of the channel when compressed. This is a real danger as the spring is under 100's of pounds of force when compressed and could easily injure someone or worse.
The gas strut design is something I developed and I use it in my van..it's not something Sportsmobile ever offered.
Details of the springs are in post #73 here:
https://www.sportsmobileforum.com/fo...ed-5080-8.html