1. How do I know if the preload of my rear strut is too soft or firm?

2. How do I know if fork oil viscosity is optimal?

For a basic setup on a basic bike just sit on the bike and bounce up and down..if the rear moves the same speed as the front, its about right..they should be fairly equal..

Fork oil as per manufactures specs....or as in the rear one experiment until you feel comfortable with the bikes movement...different strokes for different folks..

You need to take unladen and laden (rider off/rider on) measurements to check sag and properly adjust it- it's a bit of an involved process, but you won't need to do it everyday. You can go by feel, but you may as well do it right. This video is pretty informative, but there are loads of others re: setting up your suspension.

You need to take unladen and laden (rider off/rider on) measurements to check sag and properly adjust it- it's a bit of an involved process, but you won't need to do it everyday. You can go by feel, but you may as well do it right. This video is pretty informative, but there are loads of others re: setting up your suspension.

This is the correct way to do it. Any other advice is just nonsense. As above use oem oil to start.

You need to take unladen and laden (rider off/rider on) measurements to check sag and properly adjust it- it's a bit of an involved process, but you won't need to do it everyday. You can go by feel, but you may as well do it right. This video is pretty informative, but there are loads of others re: setting up your suspension.

This is the correct way to do it. Any other advice is just nonsense. As above use oem oil to start.

Unless papa has seriously upgraded his bikes ,not just nonsense at all as i posted specifically to papa al..all the bikes i have seen regards to him are basic bikes and the basic method is good enough for that....For more high spec bikes, yes the method shown is the way to go..

Why papa asks:

On my recently acquired [very basic] Lifan dual-sport [single centered rear strut], my German friend who has been riding 30+ years was examining it and after straddling it and thrusting down on handlebars/front end declared that I needed more viscosity.

I can see how this would be more resistant to bottoming out.

I really hate bottoming out but hasn't been an issue on this ride, yet.

A front end this accomidating would be great on my cbr150.

He also declared that I needed more rear preload, but really I have no clue why.

Seems to me like minimum preload would yield the cushiest ride. +

The thing seems fine as configured, but papa naive.

German really liked the bike and said he might buy a motard version for his tall 16-year-old daughter.

...or...

"Preload is perhaps the most misunderstood facet of suspension tuning. Many people think that by adjusting preload that they are stiffening or softening their suspension. Nothing could be further from the truth."

per

http://www.sonicsprings.com/catalog/preload_tech_article.phphttp://www.sonicsprings.com/catalog/preload_tech_article.php

...what the sag!

...or...

"Preload is perhaps the most misunderstood facet of suspension tuning. Many people think that by adjusting preload that they are stiffening or softening their suspension. Nothing could be further from the truth."

per

http://www.sonicsprings.com/catalog/preload_tech_article.phphttp://www.sonicsprings.com/catalog/preload_tech_article.php

...what the sag!

Link wouldn't open, but what you say is correct...its the spring strength/rating that is the main influence on suspension and the fluid/gas just dampens the movement so it doesn't act like a ''pogo-stick''....more viscosity as you say ,will not stop the bike bottoming out...preload just initially sets the bike up so the suspension doesn;t either,top out or bottom out with your own weight ...I should have added to your OP that basically if the bike tops out when bouncing on it, your preload is obviously too much...bottoms out preload is too little.. AFAIK spring preload cannot affect spring stiffness ,only affect ride height....