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Suspension compression on its own will not change the rake and trail in any major or permanent manner unless the attitude of the bike has changed. Example: I never had a head shake problem until I added more preload to the forks. In doing so, when I put my weight on the bike the rear end compresses more relative to the forks than it did before, thus making the rear of the bike effectively lower at "ride" height. While this will technically increase the rake and resulting trail figure, it also makes an already light front end even lighter. So while the hard numbers and facts say I should have heavier steering and a more stable front wheel, the physics behind the change also work against me.
A big reason many cruisers tend to suffer from head shake is for the same reason. The weight distribution is further back, so even though they generally have high rake and trail fugures, they can still be prone to a little wobble at certain speeds and conditions.
When I say my bike has a very faint head shake, it's the ever so slightest of wobble, and not enough to be unstable. I've (bravely) let go of my bars as much as I can at 65mph without losing throttle, and they barely wiggle. It also doesn't worsen with bumps in the road or other outside conditions. I have had worse wiggle from a cruiser.
Likely what I am feeling is my wheel being laterally out of true just a tiny bit. I never had a chance to check it, but I suspect this to be the case.
In an experiment to my lighter front end theory, I attempted to basically sit on the fuel tank and lean forward at around 60...which yes I probably looked ridiculous...and let go of the bars as much as I could, and no wiggle. If the wheel has just a tiny bit too much lateral runout it can sometimes be masked by extra front end weight. Sounds kind of counter-intuitive, but it wouldn't be the first time I had this issue lol.
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