You'll end up cranking the preload to max with 2 riders, but with that much weight on the back the front remains light at normal cruising speeds so basically leave the front alone.
If you do this A LOT then I recommend a 60 series front tire as it will offer a better geometry and turn-in when riding with a passenger.
38/34 is close enough for street tires and 300lbs
Thanks for the good info LDH.
by "cranking the preload to max" you mean compressing the spring the maximum amount? I'm getting all noob confused about preload terms. To me it seems 2 up i would want to increase the spring travel which would mean elongating the spring (removing preload). However i also have a bit of physics understanding, and the force exerted by the spring increases based on how much it is compressed..
from doing some googling, it sounds like compressing the spring let's that force lift the rear end of the bike more. did some reading sorta get it.
we took a 1 hr twisty ride last weekend, the rear damping was set to what was comfortable for just me on the street (slightly plush) preload was not changed from the 2nd or 3rd step from the top. fun ride, but a lot of pogoing, not as confident feeling as i'm used to. for the return trip i decided to Harden the rear damping one turn. this really seemed to reduce the pogoing and gave sharper response, granted we didn't take the same cowpath road home, but still did some twisties and it just felt much more planted.
I need to get a better spanner wrench so it's not so hard to adjust that damn preload. the handle on mine is so short, so annoying how the subframe gets in the way too.. I tried to move it a few weeks ago and it wouldn't budge. Is there a specific size that works well?