I was having trouble with this rover:
https://www.dropbox.com/scl/fi/9qc7jopoe4wg3j9r4twgx/Rover1.xml?rlkey=hc8bbvftguhdj0r06m6ule5ly&dl=1
I'd designed successful rovers before, completed several of the timed races, and traveled about 250km on Droo with them. Each wheel on my previous craft was always attached to a single vertical shock, or I skipped shocks altogether and just used the wheel's integrated suspension. Over the course of the game I tweaked the part settings - mostly through trial and error.
With the new rover, I wanted to start from a clean-slate, and try out a double-shock system like in this video: https://youtu.be/dQl4GVRvdTM?t=230
But I found a couple of the parts kept breaking off like they were made of glass. Just try driving out into the mountainous terrain near Juno Village for a few seconds, and you'll see what I mean. The Strut ("Spindle") and lower Rotator Hinge ("Spindle Hinge") were the ones getting damaged.
At first I thought it might be because these parts are very lightweight compared to the wheels and chassis, so I tried bulking them up (scaling the size and even adding deadweight in case that factored in) but it didn't help.
I tried watching what was going on in slow motion but it was kind of hard to tell. Next to the tires, those are the lowest-mounted parts and I thought maybe they were hitting the terrain. I tried moving the spindle hinges up a bit, but the spindles kept breaking. Then I simply moved the wheels down (with a silly-looking result) and found things improved.
My best guess is the internal, built-in suspension on the wheels (which defaults to Enabled when you add wheels to your craft) was causing my whole craft to bounce up and down relative to the ground, and causing the spindle and spindle hings to hit terrain.
Disabling the wheels' built-in suspension immediately corrected the problem (or at least vastly improved matters).
Sharing this tip to maybe help save someone else some time in the future!
Also, it'd be neat if there were a way to set the initial angle of a hinge. I tried adding the "angle" attribute in the XML but I guess it's ignored until the craft is in flight. I'm not the first person to pose that ask: https://steamcommunity.com/app/870200/discussions/0/1742231069938268922/
from what i've heard over on SimplePlanes is that the built-in wheel suspension is supposedly terrible, but again, this was from the previous game and it's entirely possible that the built-in suspension in JNO is actually decent or even excellent
that said, though, using both probably isn't a good idea