I had the nastiest tank slapper ever.

I was trying to check the carburetion, and I was in the middle of my attempt to see if I could reach 10k rpm.
I don't remember if it was on third or fourth gear. Doing the math is at least 75mph or 110kph
In the instant I checked the dash, some irregularity on the pavement started the wobbling.


How do you deal with shimmies?

Now I'm too tired to write the complete journey of the slapper but I needed 100 meters to recover it.
 

ATTGAT

My wife had a death wobble on the freeway once. It was in the dark, in the rain, on the freeway at 60+ mph. It shook her up for a long time. We found a big scratch on her fender, so the only thing we could think of was that a big piece of road debris upset the front of the bike and the weight of her luggage on the rear caused the death wobble. It happened so fast she had to use her legs to keep the bike up. The bike managed to correct itself but she felt like she was going to loose control in that instant.

I had something similar to a death wobble once. I was pulling a trailer with a small sailboat on it on my GS500 (yeah, I know...asking for it). I was decelerating going downhill and got the trailer death sway. I think I just let off the throttle and it went away after a few seconds. But for a moment there you feel like you don't know what caused the wobble or how to fix it.

I have had a few bikes that would just start to wobble above a certain speed like a tank slapper was brewing. My NX650 would start it above 80 mph with panniers. Probably the soft/tall front suspension plus the wind resistance on the panniers. You could tell it would get bad if you kept accelerating.

I think every bike and situation is different. You see youtube videos of sportbikes coming down from freeway wheelies and immediately going into violent tank slappers. There is enough video evidence of that to show sportbike geometry must be unstable. Otherwise they wouldn't make steering dampers.

All this is to say that death wobbles are a dark art and one of the scariest things a motorcyclist can encounter because if you happen to survive it, you have to keep riding the bike that just tried to kill you for no reason. In the moment its just luck if you get out of it. I think most of the time letting of the throttle is the best method but some people say you can accelerate out of them. And you may never get a reason or know if/when it will happen again. Try and think if anything about your riding, the road, or how the bike was set up was out of the ordinary. That's really all you can do. Do you think tire pressure was to blame for yours?

Definitely.

Well,
there are various factors that contribute to the shimmy tendency or reduce the ability of the bike to naturally recover of.


Im trying to educate myself as much as i can about the subject and still consider i dont konw much about it yet:

the shimmy originates from the disalignment of the front wheel causing a oscilation where the geometry of the bike is crucial.

Rake and trail, you can check it by rotating the handlebar standing still, look how the frame leans to the opposite side of the turn of the handlebar, the same effect that initiates counter steering


this is more pronounced with a steep geometry, eg sportbikes (although some harleys also suffer this)

very powerful bikes
hard and abrupt acceleration
low weight in the front end ( also a low rear and high front)
bad suspension adjustment, both rear and front
bad landing (front wheel not centered)of a wheelie or a jump
too much weight in the rear(luggage racks,2 up, etc
going uphill, even more if the angle suddenly changes to flat

Steering stem bearings too loose /todo much play

but the worst of it is low tire rear pressure: which reduces the self centering ability of the wheel, mainly because it loses the round or V profile and makes the flanges soft and wobbly allowing more side to side movement.

other cause is a loose rear axle or the rear wheel bearing with side to side play.


all of this can be triggered by bumps, irregularities, and exacerbated by any of the items listed before.


in my case was really low tire pressure 21psi in a 150/70  tire.
a rear wheel axle that ended crooked, chain adjuster on the brake side was sticking out.



i had a few shimmies before but nothing like this.
i had a an air leak that induced the problem, went to a tire shop and it was supossedly fixed. Nope

i recall about 5 shimmies

-2 were really soft going at less than 60 kph, and triggered by irregularities.
-the other two were moderate at the end of an underground tunnel over full throttle where the angle of the road switches back to flat. lasted 2 seconds.
then i "fixed" the tire, around 80kph

-and the last one in flat condition with some bump that initiated it.

at first i tried to maintain steady throttle hoping it would recover naturally.
While you may read online that you shouldnt do anything if you have a serious wobble you will need inputs to correct it.
-Gripped my hands tightly trying not to move the throttle and realxed the arms.
-Gripped with my knees the tank, very important not to start wobbling your body.

The bike wasnt recovering so accelerating was out of the question, i was already at full throttle(in fact i had no power left to do that, nothing, so i thought about my only option, to reduce speed.
I though about it but cant say i decided to roll of the throttle in that exact moment.
Front brake was out of the question.
Then the serious wobble started so i decided  to control the bars, mostly retaining each bar from going towards me with both hands to prevent the steering to go too far on the extremes.
Then the wobble became far worse bouncing the suspension in every wobble. Luckily the front tire started gripping laterally in each turn, i could hear the tire rubber screech. In each wobble i was at lean angle closer to the ground.
I lost some speed ( around 30 kph) and it became more manageable.
I really though this could not be the end, so  all of my mental force available emerged.
With less speed and trying to be smooth it ended.
1 block after.
The more terrifying part was not knowing when it would end, it was a complete block, around 100 meters, luckily mi idiot self was no so idiot to try this after passing all the mass of traffic so i had a clear street.
Drifted about 4 lanes to my right  from my initial position in the process.


I read that smooth rear brake can help.
Stupid me i have run out of rear pads and already have them ordered. My brake pedal goes deep and minimal brake force is applied.


i have the gsxr k8 750 Kayaba steering damper to put but i never did,mainly because:
-i have to erradicate the tendency first
-i dont know where to make the fixing point on the frame
-its electronic(2 wire) so i although it has some resistance i really dont know how effective is in a passive state.


adding rear preload also could prevent lightening to much the front end.

That sounds very scary. I'm glad you didn't go down.

There were backyard welders and fabricators I knew that wanted to build their own chopper bobber and ended up with a bike that you couldn't even ride without death wobbling.
Cool.
Buddha.

The bike continues with a slight tendency to shimmy, especially when the pavement is uneven,and i don't remember being that much.

Also think my suspension is badly adjusted, packing a bit.

@The Buddha , for a r6 shock and a 75kg rider, Which preload step should I use?
 I'm at 7 of 9. Mainly because I rode 2up.
I have to remeasure sag and try to adjust it as good as I can.
If I'm traveling and have rear load I plan to increase it 1 point.


While braking with the rear I felt a lateral offset like the bike was going sideways.
Rear wheel is a bit misaligned.
I will correct it and see if it improves the feeling.

I'm struggling to ride more lively than grandpa style for now.

I plan to make one to one adjustments and test.takes time

About the R6 shock:
The Yamaha R6 has a longer swingarm, so it has more leverage to apply force against the spring than the GS does. The R6 shock has excellent dampening, but the spring is really too much for the GS application.

I find my bike has no "static sag" past 4 and very little at 3.

When the rear will not sag, even a little, under its own weight (no rider), the suspension will transmit every bump and jolt, and all the energy from braking, right back to the front forks.

Often riders will think they need heavier fork springs, but the diving is actually caused by too much pre-load on the rear spring.

I am about 72kg. I can run my R6 shock at 1, (more "plush" like a good stock set-up) but generally leave it at 2 for a little more sportiness. If I had a passenger and luggage, I might go to 3.

Thanks for the reply, I used it at 1 for a long time, then 3 was the perfect number and used it for a long time, I recall  I was about 83kg at that time.
Then my friend(110kgs) as a pillion so I increased it and that's the setting that remained.
Mostly because I couldn't find a tool to change it comfortably.

Which year is your shock?
Mine is a 2006 red r6 shock with the reservoir integrated at the top. Compression damping adjustable with flat screws. Rebound at the bottom, also flat screw.

I guess I will have to fabricate a tool to move the preload ring notches, had no luck finding one.
What do you use?
I don't like hammering the ring and I also would like to have the tool with me and make the correction if needed,for 2up riding for example.

Well that's very good info about the shock.
I also read that r6 shock was too soft. Maybe the spring rate changes from year to year. I should investigate that.
I also have to re check the bolts because at first I saw a YouTube video that simply replaced the shock.
Now I see that you have to use some hardware from the GS and other from the r6, if not you have play in some bolts.
https://www.youtube.com/watch?v=a_0gnFaONfY&t=6s&pp=ugMICgJlcxABGAE%3D
thanks to our fellow @Slipprymongoose

If I recall correctly I used the r6 bolt to the frame connection above(it came with the r6 upper clevis) and the GS bolt on the bottom to the triple roller bearing assembly.
I have to investigate more on that subject. I am in doubt now. If you recall something about it would be of great help.

Also ,could you tell me which settings are you using for damping?
I found out that rebound has to be around the middle if not it is a sofa but it wiggles a lot on ondulations at highway high speeds, around 3 slow pogos after a bridge for example.
Compression hi and low is around the middle also.

My shock is a 2011 (red) I use the Suzuki GS tool, but a brass drift and a rubber mallet work fine.
It's easier if the bike has a center-stand.

You need to grind your swing-arm a little for the shock to clear.

I did a thread...

https://gstwins.com/gsboard/index.php/topic,73684.0.html

thanks!
i didnt grind the swimgarm, but now im thinking that maybe when it over extends, eg a pot hole, it may touch, in the centerstand it is not touching though.