How should I UV map complex object?

anHourlater

Hi this is a motorcycle i've been working on for my portfolio and I'm making a low poly out of it. I'm new in game modeling and it'll be really thankful if you give me some tips. 

I learned that when making a low poly, it's much better to incorporate each part into one combined mesh as tightly as possible, but not the nurnies like bolts and stuff? So I'm gonna make low poly of small and repetitive objects separately and overlap their UVs with each other to save texture space. (Correct me if I'm wrong)

Anyway my question is, after finishing low poly and when mapping UVs of combined mutiple objects like this bike, should I lay out all the UVs into a single map? Or use multiple maps? I'm curious because the model consists of various materials like shiny/matte metals, leathers, plastics, rubbers etc and if this is for image rendering I'd make texture individually for each material. But this is for games so.. how do you do it?

Oh and this model is for hero asset of AAA TPP game and gonna be imported into UE.

Bear with me,  English is my second language.
Thanks.

jStins

Nice model you got there. You could unwrap everything to a single texture sheet. One of the advantages of modern texturing / material workflows is that it's pretty straight forward to define multiple types of surfaces (metal, rubber, plastic, etc...) with a single set of textures. You could break this into separate texture sheets / materials if you need more resolution (reasonable if this is a hero asset), but it's not required. The one exception would be elements that need transparency, like the light covers. Surfaces that need to render with transparency tend to work better with their own shader / material, but even those elements could have their texture information mapped to the same UV space as the rest of the bike. 

HAWK12HT

Usually starts with how much texel density you want on the object in contrast to the overall environment it will be. 
You can break this into 3 (engine, body and wheels) or do it as a whole with a 4k texture. 
Issues with using one texture for all elements is that some smaller parts will get very less td or pixels, that is something you have to eyeball at times and increase those UV islands manually. 
Another issue if you strictly go with TD approach you might wanna break it into several texture sheets to match your defined td. 

Also you ll have to offset overlapping UV out of 0-1 space of your lowpoly in order for highpoly to bake normals otherwise you ll get errors. 

https://80.lv/articles/textel-density-tutorial/

https://chamferzone.com/

Polycount wiki has details too you should check out.  

anHourlater

Thank you so much for your advise jStins and Hawk12ht. I really appreciate it