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0 to 10mm of pre load is ideal.
I tossed around getting stiffer springs for my weight but i'm only 160 lbs so I stock springs should work?
Once you put in proper preload up front play with your sag and see if you can improve the bike.
Even different tracks may need different sag.
The Shop
The 05 uses two spacers on the spring. One up top around the cartridge and one down below inside the lower tube. One is aluminum the other is plastic with a metal ring.
The bottom plastic spacers is 55mm tall. The top spacer is available in 42, 59 and 125mm sizes. George at Suspension 101 can get these for you.
You can also make spacers with PVC pipe. Use schedule 80. You can also make spacers with Delrin, (my fav) a slippery yet hard plastic. If you do make spacers, the most important thing to do is to keep the angles perfect. Use a good miter saw - don't do it by hand.
With that, you need to consider pre-load. Most MX bikes run 5mm and we agree with this for most applications.
To measure pre-load, undo the rod from the rebound adjuster bolt. Mount the fork in the vice with the top facing down and allow the weight of the lower tube to settle on the spring. The rebound adjuster is removed. Then measure from two points, such as the bottom edge of the upper tube to a spot on the lug. Then re-attach the rebound adjuster bolt, screw it into the lug, then take that same measurement. The difference between the two is the pre-load.
There's more than one way to measure pre-load but this is the general premise.
That said, head shake is an oscillation between the hinge point of the steering and the main chassis. It's caused from either more energy coming into the suspension than it can handle (kinetic to heat) or it's from an inability for the fork to recover fast enough after taking a hit. The later of the two is the most common. In other words, it's not re-cycling fast enough.
With that, there are a host of things that you can do to increase stability and plenty of opinions that come with it. The bottom line, if the fork is valved and set-up correctly, it won't matter what you do with the rest of the bike (sag, fork height, links...etc.).
Yoda wisdom - "deep in the fork, stability begins".
So when you say the fork can't recover fast enough, how can this be resolved?
This can for a host of reasons, one being too short of a spring and another being a spring that tends to settle.
And yes, the 05 was a funky fork so they didn't quite hit the mark on a few things.
Additionally, there are cases where you don't want pre-load on a spring...so perhaps that matter was discussed when the 05 was being buttoned up.
As for rebound rates, you can speed things up by adding pre-load, by running a stiffer spring, by running more oil in the lower chamber, by running thinner fluid, and by changing the rebound shim stack.
The logical process is to the get the right spring, set the right pre-load, set an oil height for end-of-stroke performance, then tune the stack accordingly to match all this.
In general, (and this is my opinion) most of the YZs have too much rebound damping in their forks. Best to adjust the stack.
Obviously as you go down a hill you have a weight bias towards the front, which means the fork is working as the primary damper for the mass of the entire bike and you as the rider.
The more load or more energy that goes into a damper assembly to more likely it is to...um... crack under the pressure...so to speak.
Stability is a bubble. Stay within it, you are fine. Go outside of the line and things start to happen. Head shake is one of the things that happen when you cross the line as to how much energy a damper can withstand.
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