Richard,
Phil Irving was under some considerable pressure due to legal liability concerns. At one point it looked like the government might investigate the rash of crashes associated with Vincents. As a result he "doubled down" on the Girdraulics being perfectly fine. In fact, he explained his problem to the late Sid Biberman and asked him to write and submit an article to MPH that stated that the Girdraulics were fine if properly maintained. Sid did so and the article was published and became part of the mythology. Apparently, the investigation was not a priority for the Government and the potential legal issues subsided.
The Girdraulics had been type tested as required for vehicular road use. Ted Davis and another rider put 10,000 miles on a set to prove the forks. They took turns riding the bike and apparently had a local course laid out so they could run off a precise number of miles to document the testing. I suspect the course simply failed to present a situation where the forks would display the bad behavior. I doubt it was intentional, just pragmatic.
George Brown was stunned at how poorly the Girdraulic forks worked for him on Gunga Din. He had crashed at Eppynt with the Bramptons on Gunga and received some serious injuries when a young girl ended up on the track. When he was healed sufficiently to get back on Gunga, it had the prototype Girdraulics installed. George had a horrible time controlling the bike and complained immediately after the race to Cliff Brown and Cliff promised to reinstall the Bramptons. Neither one told Phil Irving or Phil Vincent. It seemed that they were both concerned about anyone questioning George having lost his nerve.
I suspect what happened instead was that someone found out that the handling of the Girdraulics would calm down considerably simply by shortening the front springs. I found that almost all the photos of the racers showed short springs that allowed the lower link to be parallel to the ground or lower. As I found out with the short springs I made, this would have been a good compromise. Ron's diagrams show that if you limit the Girdraulic to the area near full compression the axle path is very good. I think that was the "insiders" solution. However, we know that George never used Girdraulics on any special that he rode after he left the Factory.
I would also agree with you on the eccentrics, but I do not believe they have any impact on the poor handling problem. The eccentrics are fine, except on the one point you make: they did not reduce the trail enough to make the Girdraulic steering as light as it could be by getting the trail closer to zero. Thus, the need to introduce the short top link for sidecar use. The Girdraulic would shorten the trail for sidecar use, just not enough for easy steering.
That leads to a potential problem today. Sidecars rigs will experience the same handling problems and reduction in braking that the solo bikes do with the stock steering stem. However, if a sidecar rig is switched to a short top link, there is a possibility that the the new trail of the modified steering stem when added to the old reduced trail of a short top link, could add up to less than zero, or negative trail. This would not be desirable. So, there is an issue with mixing new and old parts with sidecars.
This is a problem because I don't think that the Factory made any short top links. Thus, there is not "standard" length for a short top link. Different owners made different batches that varied in length. When you consider that we have become use to using "concentrics", which reduce the trail and that few owners use or think about using sidecars, it would not be surprising that someone could assemble some parts that may have some unknown consequences. Whe Jim Young made his stem based on my drawings, Ron checked the trail of both the short top link with the new stem and made suggestions accordingly. This may be a problem of having too many choices!
David