So I have been doing several track days this year and have been learning a ton. One thing I haven't been able to eliminate yet is this front end chatter that I experience on the straights at 100+ mph and hard braking when coming in from high speed. The chatter I feel is an up and down, high frequency feeling in the handlebars. My first track day/s were at RA and I just went with it for the two days and would figure it out after (the chatter was at its worst at RA). Once I got the bike back in the garage I replaced fork seals, fork oil and cleaned all the parts in a parts cleaner to get any crap out. I also took my front wheel to the shop to get rebalanced, which it did need. Next day at the track was at BIR and the chatter was still present but better than at RA. I paid a suspension guy to help me out throughout the day with damping and preload and we eventually completely preloaded the springs (which are stock) and although we got the chatter to reduce I could still feel it coming into corners fast and when hitting 100+. I believe the stock springs are ~.80 so I am going to jump up to .85 or .9. My question is whether doing more suspension tune will get rid of this chatter or if I need to rebuild my front brakes as well and replace worn rotors? I have not checked the rotors with a dial indicator yet but I figured some guys may have experienced this before or have heard of it to lead me in the right direction.
For more info I am running Bridgestone S20s and haven't ruled out a tire defect or mounting defect but I would lean toward the re balancing helped reduce the amount of chatter at that speed.
Also, going to the track has improved my riding 10 fold. My skills improve with each session (jumped to intermediate almost immediately) and now I just want to be confident in the bike to keep getting better. Any tribal knowledge passed along is much appreciated.