· KNEE DRAGGER
First, I realized my spring rate numbers were off:
Checking my notes and the Penske manual, the shock actually has a 400 spring in there, after changing from the original 475. Still... it's quite less than what you're running (500).
Also, my rear sag numbers were not accurate. I get confused... I mistakenly didn't lift the bike off the ground (as the Penske manual calls for "A"), but rather just lifted the rear gently and then lightly settled it for the first number. Then I hoisted my carcass on-board for "B".
So, (until I can get another couple of assistants), I can assume that my rear sag is likely over-limit, as with my (incorrect) method we got about 45mm.
I don't have a measurement for this. (I don't recall there being any installation problem from being too long... but this was three years ago).
Shock spacer... not familiar with this part; I have the Penske manual in front of me and can't see what this would refer to..(?)
I did buy this Penske 8975 off of Ebay, used.. so it's possible that the length or adjustments have been futzed-with... But again, I did have it rebuilt at Penske before use. Hmm.
But I DO recall one thing: last year I reduced the rebound in back to almost it's max (out)... and the handling (warble) improved noticeably.
It would be interesting to see what the eye to eye length is.
Fully extend rear and measure.
Set total sag to 30-32mm
Now measure unloaded sag. If the spring is in the zone I would expect you to see 8-12mm. If less or none, this can be a sign of spring being too light.
Having way to much total sag causes the rear linkage to operate in the higher rate region and also prone to blowing through the existing travel and hitting the bumper. This can make the bike feel extremely stiff, ride way into the travel along with wiggles that can happen in the chassis from heavy weight distribution on the rear. This is even further aggravated by a shock length that is too short. Then with such small sag in the front, makes more of an issue.
Just my 2 cents...