Originally Posted by
njkayaker
You can induce the harmonics without a lot of effort. It's a very low frequency vibration. Essentially, the bike is making tiny back-and-forth turns around the headset axis. No "chassis flexing" or "joint expansion" is necessary.
Again, are you a chassis engineer? A harmonic cannot begin unless something is out of balance either radially or axially. Sorry, you need to understand where the orginator of the problem lies.
Also, if you are vey familiar with automotive chassis dynamics you will understand that most items cannot be felt until they hit 10 to 15 hz which is around 55 mph.
EDIT - to help the OP... you should look into the balance of both of your wheels (start with the front) and if they both are very accurate (most are not due to valve stem), then look into potential wear on your headset and runout issues.