New Vibration at Highway speed

[rev79xs11]

Hello All!
I recently picked up an odd vibration in the front at highway speeds. At about 65 mph it almost feels like the bars are bobbing up and down. I also noticed that if I put any weight on my rear rack, lookout, it's scary at 65.
I've checked the obvious, new tire (dunlop d404) and it is balanced. Checked for play in the triple tree, and front wheel bearings. Also played with tire pressure as well as checking bolt torque on every accessible bolt on the bike.
Any ideas would be appreciated, was wondering about fork oil? Seems odd that any substantial weight behind the rear axle makes it far worse. HMmmmmmmmm?

Thanx,
rev79xs11

[prometheus578]

Did the play start with the new tire, or have you had the tire for awhile and the vibration just started?

[prometheus578]

Read your post last night... went to bed. Dogs HAD to go out at 4:30 A.M. and this idea was in my head when I woke up. (ever have a brain that just doesn't know when to turn off?) Had the bike sat for a long while? You may have a flat spot in the tire. ...Have seen it on new bikes that have sat in the crate for too long; tire gets a flat spot. Jack up the front end of the bike so that the tire just barely clears the floor. Spin the tire slowly. Does it rotate freely 3/4's of the way around and then slightly catch? It doesn't take much to make it out of round and useless. You mentioned that it was a new tire, but didn't say if it was just put on. (An out of round tire will still balance) Check the bead area, where the tire seats agin' the rim. Molded on the tire is a thin raised line right next to where the rim is. This is to see if the tire is seated properly. You should be able to see this raised line all the way around the rim, about an 1/8 of an inch or so from the wheel. If you can see this line most of the way around the tire, but then it seems to dip inward toward the rim, then the tire's not all the way seated. As for the weight on the rear end.... when the bike's level, there's X amount of weight on the front springs, and it takes Y amount of force to make the forks compress further. When you place weight at the rear this shifts the weight offa X and the Y is lessened, making the forks easier to compress, hence the more bouncing. This could easily be expressed, had I not failed algebra, as X=Y, and then (-X)=(+Y). Now... My dream induced solution to your problem may be all wrong. But heck, I can bluff my way through it with my impressive math skills! Ok... back to bed.

[rev79xs11]

Well, I didn't notice the problem until the tire was on for a few weeks. I did jack up the front and watch the tire for bumps or flat spots. I also compared the line around the bead to the rim to rule out improper seating and I backed off the brakes and spun the wheel numerous times to look for a heave spot.
The X-Y theory on the forks is seeming possible scince the weight on the back factor comes into play. I was wondering if the oil was low, maybe it just took that little bit of rise to pull the forks out of dampening range. Guess the next step is to change the fork oil, anyone have a suggestion on fork oil viscosity?

Thanx,
rev79xs11

[skids]

Maybe a balance weight fell off?

[John]

I believe the book calls for a 10w fork oil. I don't have it in front of me right now. Some guys will use ATF in place of fork oil.

[John]

The 79 Special takes 7.6 oz of 10w fork oil according to the Clymer manual.

[crunchbird]

Unbalanced air pressure in the forks caused a slow pitching through highway turns.

[rev79xs11]

Eureka!! Found my vibration! All indications pointed to balance, but silly me figured the bike shop was capable of balancing a wheel.....WRONG! I myself am an ASE master tech on auto's and have balanced a few thousand wheels. I had a hunch that they may have placed the weights 180 out so I moved them and now it's smooth...... Guess I've learned not to assume.


rev79xs11