I can recommend the 70mm long version from Deda Elementi. Not available in the US though, IIRC.
Also, it’s not only about having support against the clamping force of the stem. Most shorter expanders provide this as well as the stem obviously sits on top of the steerer and has a stack height of max 40mm. It’s also about providing stability against the bending forces that occur when going over bumps or pulling on the bars. Ideally, if your frame is the correct size and you’re not using a humongous tower of spacers underneath the stem, the expander will protrude all the way or almost until the upper headset bearing. This provides maximum stability.
This for instance is also why Specialized provided super long expanders in their Tarmac SL7 recall. The steerer movement under load meant that the compression ring of the upper headset ate into the carbon steerer until it snapped. The new expander limits this. In my opinion this issue is not isolated to the Tarmac SL7. It’s a problem of internal cable routing designs per se.
The end result is correct, a longer plug is helpful. The reason why isn't correct. It's sort of off topic, but it's worth thinking about in the name of safety of internal routing systems. I've been working on trying to make <2mm over bearing options possible (largely for my own use) and it's really hard because of what's involved and the potential risks of not getting it right.
The majority of the bending forces are borne by the carbon tube. Carbon is exceptionally good at managing loads like that and the expander is fit via a very small amount of friction. It's not going to do a whole lot in terms of keeping that tube rigid and preventing bending. What it does do is that it prevents the carbon tube from shearing locally as it preloads the carbon tube.
Historically a long expander wasn't as critical because the steerer made contact with the compression ring nearly 360 degrees and was held relatively tight against the upper bearing. Even if you lost preload and the bending got worse, the typical symptom was your steerer developing the "ring of death." This meant eventual failure but more often than not seems to be spotted before it led to an accident when riders checked on chronically loose headsets.
The SL7 failures also start with the loss of headset preload. This causes three things to happen. First the bending moment increases exponentially, second the steerer is now facing local impacts against the compression spacer which in the SL7s case was a relatively small surface area part, third the steerer can get worn via movement against the metal spacer and because of the design of that spacer would rapidly develop the "ring of death." These three meant that fork failures happened much faster than on other bikes. The SL7 fix therefore needed to address all three: improved plug, extension hanging off the bottom to limit the effect of those impacts, and a metal ring to be a new wear surface and further distribute forces around the steerer
This is less of a problem for other systems like the FSA ACR because their compression plugs use a different design that's heavier but less likely to lead to preload loss and their C ring has more contact area. Its even less of a problem for the current gen Deda DCR because it uses a nylon spacer that will wear the carbon less and it's designed to fit tightly to the steerer. It's also 15mm deep which has even more contact area.
With all of these systems you should check them every few thousand miles or so even if you have never had any issues with preload to look for wear. If your headset can't hold preload, I would chuck it and replace it with either the FSA or Deda system that fits.
TLDR: Headset preload is more important than ever. Don't ride with a loose headset. Using a high quality insert / expander will certainly help!
