Wobble and weave

Interesting they're using the Z1. Some are more prone than others. :whistle:

Fun to watch video. I survived a tank slapper wobble on a KZ1000 in the late 70s when I was young and dumb enough to get on any bike and redline it through all the gears. I most assuredly had both my hands on the handlebars, and luckily did the right thing and did not jam on the brakes. Scared the sh#t right outa me! (figuratively)

Mike

Back in the day I ran the Dunlop's that came with the bike. The front was the "ribbed" type tire that loved rain grooves. Switched to the K81 and that stopped but the rear tire was just plain junk. Installed a Chin Shin tire and the bike was more stable at HWY speeds. The Z1's were known for the high speed wobble at 93 MPH not below or above due to frame flex among other things. Changed out the swing arm bushings and better shocks made it better but it still would at times. I gusseted the steering head, and back bone tubes which made it better but sometimes I'd get the nasty wobble again at high speeds. The front end is a weak link along with the swing arm they said was like a "wet noodle" causing poor handling in the turns. There are guys here that make some sexy swing arms or will beef up a stock one to make them more stable. The idea of longer ones is an answer with better pivot bearings and shocks is a good idea but other mods have to be done as well. Not being able to afford a custom swing arm I tried a late model KZP swing arm that's 1 1/2" longer, thicker metal and has needle pivot bearings did the trick. It's a lot more stable at high speed and doesn't want to go into a wobble as I cross the line getting back into my lane but the low speed turning radius is now little wider. Adding a cross tube under the steering head close to the motor mounts made a noticeable difference as well. Next ting I want to do is the fork mod with emulators and a fork brace. It will never be as good as a modern bike but it's better than it was and looks stock.
Steve

Sad note to your post. Murray Walker long time Formula 1 commentator and life long motorcycle fan passed away on March 13th at the age of 97.

We studied the phenomenon in engineering many moons ago. There is a relationship between weight, springiness and dampening that can result in weave when conditions are appropriate. Weight distribution is critical to enabling weave and this is why it will stop when you lie down on the tank...the weight distribution changes. The center of gravity is no longer in the critical spot.

The phenomenon is known in engineering circles as harmonic oscillation. A classic example is the Tacoma Narrows bridge. It was a suspension bridge subject to oscillations when the wind hit it at the correct velocity. The deck would begin to slap around as you can see in this video. Eventually it collapsed. When engineers studied the failure they rebuilt it with special plates hanging off the sides to add damping and the problem disappeared.

krazee1 wrote: I survived a tank slapper wobble on a KZ1000 in the late 70s

You're not alone. It was an even worse problem with the original 900's.

Dr. Gamma wrote: Sad note to your post. Murray Walker long time Formula 1 commentator and life long motorcycle fan passed away on March 13th at the age of 97.

That's where I got it. It was posted saying it was from the "late" Murray Walker on FB.
www.facebook.com/groups/EARLYZ1OC

Having rode the 500 and 750 Kawasaki triples back in the day. I had my fair share of high speed wobbles in a straight line. Then in the high speed corners the combination of wobbles and the dreaded hinge in the middle. My little 400 triple gave me the worse lock to lock wobble I ever had after the stock swing arm bushings wore out after only its second road race. I had a few wobbles out of stock framed Kz1000's. I had one that my knee broke the outer spark plug caps!!!

I would not ride ANY motorcycle without a steering damper. From experience the best way to deal with a high speed wobble is to get as much weight you can off the front wheel, and slowly roll off the gas. Found that out on my 500 triple when it went into one of its famous high speed wobbles and I slid all the way to the back of a freshly Armor Alled seat and the wobble almost went away. If you have everything right on your bike you can get up near flat out in 5th gear and wiggle the bars back and forth and the bike should not go into any type of wobble!!!!!

SPOILER ALERT: I don't know chit.

It seems to me that leaning forward would place more weight on the front tire.
I wonder if it has more to do with aerodynamics. A rider sitting mostly upright will act like a large sail, with air spilling off each side, which could create an oscillation much like a pair of large saddlebags. Leaning far forward would remove the sail effect, thus reducing or eliminating the oscillation. I can imagine how having too much weight at the rear could add to the effect, which is, I suspect, why modern sport bikes have nearly 60% front weight bias. The combination of a light front end, an upright rider, sloppy suspension and wet-noodle frame, seems to guarantee an exciting ride.

wrote:

SPOILER ALERT: I don't know chit.

It seems to me that leaning forward would place more weight on the front tire.
I wonder if it has more to do with aerodynamics. A rider sitting mostly upright will act like a large sail, with air spilling off each side, which could create an oscillation much like a pair of large saddlebags. Leaning far forward would remove the sail effect, thus reducing or eliminating the oscillation. I can imagine how having too much weight at the rear could add to the effect, which is, I suspect, why modern sport bikes have nearly 60% front weight bias. The combination of a light front end, an upright rider, sloppy suspension and wet-noodle frame, seems to guarantee an exciting ride.

Is it maybe the same as loading a trailer with the weight behind the axle. That causes trailers to wobble. But moving the weight forward of the axle will stop it. I’m assuming the same principle, you upper body weighs more then your hips and legs. By leaning over you transfer more mass toward the front. 

 

Aerodynamics is part of the problem, but the biggest problem is what I said earlier.  There is a spring/mass/damper relationship that goes into a harmonic of increasing proportion at the frequency of the tire weave.  The way to stop it is to change the center of mass of the rider plus bike.  Bending over the tank does that.

Think of a car where the damper is broken.  You push on the fender and the front end goes up and down.  If you push with the right rhythm you can really make the front end rock.  It's similar with a tank slap, only instead of the hand pushing down on the fender it's the wind pushing against the bike and rider providing the energy.

Did you check out the Tacoma Narrows bridge video?  In that case it was also the wind providing the push.

Here is a example