i just seemed to have developed some pretty nasty speed wobbles that start at about 75-75mph. any insight on this? its freakin' scary and potentially dangerous. also, i put new tires on months ago, but this hasn't happened til now.

Uneven road or rain grooves?

Do you have a fork brace?

Do your handlebars sit slightly "crooked" when you are going down the road in a straight line?

If so, this can indicate that your rear wheel is cocked off to the side (the marks on the axle are notoriously inaccurate) and this can contribute to a harmonic "wobble" that is rear wheel driven.  To correct, move the rear wheel as if you were correcting the error shown in your "crooked" handlebars.

Another test to perform -- when the wobble begins lean forward and shift your center of mass (combined mass, you and the bike) forward a bit.  If the wobble immediately begins to stop you may well have a rear wheel related issue.   Have your wheels been balanced and TRUED (spoke tuned) lately?

Next test, when the wobble begins steer the bike to the smooth hump in the dead center of the road -- this removes the "rut groove" that exists in most tarmac roads from playing against your bike's natural harmonics.

I have become a bit of an expert in fighting them evil wobbles since I got my bike smacked in the rear tire by a truck in a parking lot ...

To tighten the head bearings you don't need a special tool, a screwdriver will do.  I think I tightened mine hand tight only using the screwdriver to over come a sticky spot.

for our reference, what tire do you have?
how much tread?
even wear?

How many miles on those "new" tires? You said you put them on "months ago." If you have put a few thousand miles on them they aren't new any longer. And don't forget to check the rear tire, too. Both tires the same brand and type (radial/bias)? There have been reports (though I've not seen any on this forum) that some combinations of tires can cause odd problems. Do you have a windshield? If so, check its hardware. If not, it is just conceivable your grip on the bars fighting the wind may be the problem - the hint being that the wobble reduces when you remove your left hand from the bars. Try leaning forward to counter the wind (difficult on a cruiser), so you are not pulling back on the bars.

Edited to add: You might also check the swing arm bearings. That probably involves lifting the rear and attempting to feel for any play in the swingarm. At least on the 250 Ninja, the swing arm bearings are supposed to be greased every 6000 miles, and greasing them calls for disassembly.

Oldfeller--FSO wrote on 07/04/10 at 14:05:56:


That's just one of the ways them trucks will try to kill you.

Phelonius

You can true a wheel, Just takes some time & patience & a bit of ingenuity. I wont type the methiod, cuz that would take a typist like me about as long as truing the wheel. I guess it would require some ability to assemble tools from junk, which I can do.

groupus wrote on 07/04/10 at 12:06:27:


I had the same on my trusty old Moto Guzzi V50, the front fork stems had worn and the oil/compressed gas in the individual stems were at different levels.
In other words, the two stems of the front fork had different stiffness, so any time you bump into anything bigger than an ANT the front fork will twist and the wheel will wobble.

Else, I have Avon Venom front/rear, have ridden a mere 5000 miles in 4 years (been stationary far too long) BUT they are excellent value for money (IMHO)
because they are high performance tires for high-performance bikes (100++bhp), fit them on a 30bhp bike and they should last a long time and a half.  
My tires look like new, and hold the air pressure quite nicely.

PS small tip for that rear tire air valve, I bought an "L" adapter from  British supplier called "Holden"

cost GBP 3.95+VAT = US$ 8 more or less

http://www.holden.co.uk/default.asp?mode=bike

but now checking for air isn't the pain it used to be !