How do you make a Jbeam file without using every vertex from the mesh?

I know how to export the jbeam but when I try to load it the game just crashes because there are way too many nodes and beams. What do?

 

Don't use the same mesh for the 3d model and the jbeam. The jbeam is supposed to be a simplified structure used for collisions.

 

Thanks, I know that now but how can I make that low poly mesh?

 

Have you looked at the wiki? https://wiki.beamng.com/Introduction_to_Vehicle_Creation
This should help you with some basic jbeaming.