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I sit right up at the tank anyway caues thats where my reach is normal but its kinda pinching my sac a bit.
I understand your idea about posture but holding your upper body with your core on extended rides is going to get pretty tiring. I have alot of long straight stretches out here where it would be nice to set the throttle and relax a bit. Im thinking that some type of bar riser might help. Im surprised I dont see any adjustable bar setups. Your steering wheel on your car is adjustable so why not bars?
Everyone is different, and your complaints are very similiar to mine.
Seat time and body position help, but if you're not comfortable...then you're not comfortable.
The busa seems as if it was designed for someone with arms longer than their legs.
The forward slanting seat is what I don't like. My nuts get crushed against the tank.
The busa is classed as a "sport touring" bike, but definately has a sport seat.
None of the aftermarket gen2 seats are flat or curve upward into the tank either.
The gen1 Corbin has the right shape for me(Sleepless Red has one if you want to see a picture). I have the Gen2 Corbin, it's pretty good, but still slants down into the tank. I wish they wouldn't have changed the design so much.
The New Image seat slants up into the tank, but they trim it down in the middle to acheive that. That has you sitting lower and to me feels more cramped.
You also cannot slide back due to the hump purposely left there. You're just stuck in the seat.
I've trimmed and added foam and gel to several seats and have yet to get it exactly as I want.
I did buy a 2' square block of firm closed cell seat foam though. From what I have previously tried I now know the shape I will cut it to(with electric knife). 3m spray glue and a staple gun, and I have a couple different types of material to cover it with. Just an idea of something you may want to try, as seats are easy to make.
I'm 5'10" 175lbs too. The busa if fine dragging a knee, but stock was an uncomfortable bastid just cruising around, for me personally.