Thanks a lot to JunkstyleGio et al, who identified the friction-pins as cause for the stiff steering of the Unimog.
I found a solution, where I keep the pins. It works surprisingly well, so here we go:
- pick up those 4 red friction-pins and try out several black axle joiners (6536). Some will stuck very tightly on the pins, while some can be moved quite well.
If you have a lot of Lego Technic around, you should not have any problem to find 4 "loose" axle joiners.
If the Unimog is your only Lego set, I suggest that you start with building the front axle first (i.e. instruction book 2), which allows you to select the 4 best matching joiners out of the 40 that came with the Unimog.
- once you've found 4 "loose" axle joiners, put Silicone grease on the friction part. I prefer Silicone grease to lubricate my Lego, but other lubricants should work as well, as long as they are nonaggressive towards plastics.
Do not overdo it, though. Be careful that you get no grease inside the part of the pin that will hold the axle.
- and that's it! The friction-pins should now be happily move inside the axle-joiners and the beams