Have a wierd one here hope you guys can help. Have a tremendous amount of vibration in the handle bars the vibration develops at exactly 40 mph. The vibration in my opinion is bad, if you let go of the bars at 40 mph you can literally see the bar shake approx. 1-2 " side to side. The bike is an 03 Road Glide with 9500 miles.
I've checked and adjusted the steering head, had the tire balance checked it was okay. The front tire is the original and appears to have some decent tread left with minimal cupping. I'm wondering if the belts inside the tire are breaking down and the magic number is 40 mph or maybe it's something else? Has anybody experienced a problem such as this?

 

So, your steering head is lubed and tight? The tires balanced? Well, it certainly could be the tire. Is the vibration coming from the front end moving, or is it generally in the bars. If it is the bars, you might try some new bushings. If they have not been replaced, they may need to be. How about your cables? Anything binding up there? Have you checked the air pressure on both the front and rear? Have you adjusted the rear suspension? All of this might cause the vibration. How about the fairing. Any cracks, or broken mounts? Since you have a frame mount fairing, this is probally not the issue, but worth checking. What aboutany spacers on the front wheel? Is the wheel turning smooth? Could be just about anything. But, I suspect, as you do, that it is the front tire. Have you replaced the rear tire? Are they the same? Meaning bias or radial? Could also make a difference. I would think that the vibration is probally there all along, but only really noticable as you gain speed. Let us know what you find out.

FWIW, a long time ago I installed tires on cars. Mounted them, balanced them, and did alignments. It was not uncommon, although rare, that even when something was right on, on the alignment machine, there would be vibration when on the road. Most of the time, it turned out to be a faulty tire. Swapped 'em out, and all was right with the world. Motorcycle tires are new to me (or rather, I never paid much attention), so I do not know if they are made the same way. But, it seems to me, a tire is a tire. Some are great, and some are just not.

 

Replace the tire.And check the wheel bearings. Doc