High speed oscillation

R

Rat Rice

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Location
6725 Ashwood Rd, Woodbury, 55124
It's a 1978 CB750K. It has a complete, period correct Vetter touring system on it. Mag wheels from a 1981 CB750C. Newer tires (I put them on just before my trip to New Orleans the end of August) and the chain and sprockets are also fairly new. I've checked air pressure and chain alignment everything is within spec. I didn't have the problem until just recently. When my speed gets to around 90 mph the whole bike oscillates left to right and back and forth constantly. The handle bars remain stationary and straight.
Question; what is the cause and what is the cure. And please don't lecture me about going to fast. I also ATGAT. Thank you ahead of time for your input and help.
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To tight or to loose? I have replaced the loose balls and race with tapered bearings. I would normally tighten them tight and back off a quarter to a half turn until the forks turn freely with no drag. Don't know if I'm doing that correctly or not.
 
My $0.02 - with all that weight up high in the fairing and trunk, I'd imagine it would certainly contribute to your oscilation. Have you tried taking off the trunk to see if that helps?
 
>Swingarm bushings?
>Rear tire separation?
>Try adjusting tire pressures:
*Go up 2psi in the rear and test
*With rear up 2psi drop the front by 2psi and test
*If no improvement reverse the steps going up in the front and down in the rear
Make small changes and test ride between changes. I had a vibration that would start at the front, travel down the spine of the frame, ending up with a vibration in the rear that would then smooth out only to begin again in the front and repeat. With different tire sizes (19" front, 17" rear) a harmonic imbalance was all I could figure and was able to adjust tire pressures as described and eventually eliminated the issue. "Within spec" means nothing when it comes to tire pressures since the tires that were factory installed on the bikes don't exist anymore and can only be used as a starting point.

Good luck. :cool:
 
Closed_Casket said:
My $0.02 - with all that weight up high in the fairing and trunk, I'd imagine it would certainly contribute to your oscilation. Have you tried taking off the trunk to see if that helps?
Click to expand...
No I haven't tried taking the trunk off. The bike never did it before as it sits. I'm wondering if the rear tire might be causing the oscillating. Going to pull the rear wheel off and check the tire out. Thank you for the reply.
 
The more radial the tread pattern the more it catches rain grooves in the road. I hate that more modern roads put the rain grooves in the direction of travel as opposed to perpendicular to the direction of travel that they used to do. They really catch the tire tread even on my sport bike. I see bikes wobble all the time out here no matter what it is during Sturgis. All the bikes going down the interstate just kind of wobble their way down the road. dont know if this is your cause but something to think about.
 
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