First things first, this is not some super hot must have modification nor do I have long term experience with it, take all this with a grain of salt before grinding away on your precious nipples!
The basics: the idea behind this is that if the G line handlebar could be folded just a very few mm further on the hinge, it could reduce the overall folding size by up to several cm and would also make it a tad more stable because the point of gravity slightly moves towards the center.
There are 3rd party handlebar catch nipples available for this very reason. I also remember a video from someone that filed down the stock nipple to achieve the same effect, I couldn't find it any more though.
Since I'm planning to take my G line on a long distance train next week for the first time, I thought this would be a good opportunity to try it myself.
I've got the small G line and the nipple was also backed by two washers with 0.4 mm thickness each. Removing these already had a significant effect, if I remember correctly it was a little over 1 cm at the spot where I was measuring.
Then I proceeded to grind down the nipple incrementally from initially 10.4 mm to 8.5 mm using 240 grit sandpaper in circular motions. See photos for results. My measuring was not very accurate but the effect is obvious.
Going below 8.5 mm would not make any sense because the handlebar catch already touches the frame, it wouldn't go much further anyway.
Also, the nipple has just enough wiggle room now to make a clicking sound whenever the handlebar is folded or unfolded. This could be resolved by slightly shortening the screw or installing a tiny washer between nipple and the head of the screw.
by Manamonster