Steering Damper install? i.e. survived a tankslapper

[carsick]

So, hit some bumps about 55 in a slight corner today and FUUUCCKK the bars just about beat me to death. I'm talking chirp chirp chirp as the tire flipped side to side, not a little wobble or wiggle. Grabbed as hard as I could and then some, it stopped in maybe less than 2 seconds, long enough to think everybody was right that bikes are going to be the end of me. I did have the presence of mind to note there was no runoff area, jes' trees and boulders. Almost scared me enough to slow down, I did finally stop to see if anything was missing like a fork tube or my brain. Went another 1-1/2 hours without a hint of it happening again.
So, anybody care to share similar experiences or fixes or hunches as to what the fuck that was all about? I won't be trying to re-enact the symptom. Ever. Hands and arms still ache.
As a checklist of not the problem-
fork tubes straight
fork bushings A-OK
oil new 15W @130mm
steering head bearings no play no notchiness
all bolts on front and rear susp torqued to spec
newish Michelin 17" on front
front sag at 35mm
yesterday swapped 1.05 for.90 springs (coincidence?)
Doug

[JCainFJ]

 Rear sag?

[Marsh White]

I just attended the Keith Code Superbike School and can give you a little tip - if that ever happens again LOOSEN up on the bars and NOT tighten.  Let the handlebars shake (actually - you should do this ALL the time) - it will let the bike settle itself.  I know that it seems counter intuitive to our human nature - but it is the right thing to do on a bike.  Also - holding the bars tight in a corner can actually cause that to happen...

My best guess is that your rebound damping is too high (meaning your forks rebounded too slow).  How much do you weigh?  You might want to go down to 10 or 12wt oil.

[carsick]

Rear sag?

Yeah, It does now!
Ho Ho.
Rear sag is 25mm, spring preload 2 notches from max, damping on max, stock shock w/52K. Rider 195 lbs with gear.

[the fan]

sag too tight at both ends coupled with mis-matched springs. I suspect that the rear is settling too much under load in relation to the front and transferring too much of the load to the back of the wheel and lightening/choppering the forks. The tightest you should go is 38-40mm ride sag and 22-25mm free sag with the stock fork. (even modified) Out back the tightest I would want to see is 30mm of ride sag and 16-18mm of free sag.

Running smaller sag numbers than this does not gain you any performance and can lead to several chassis issues, including the tank slapper you experienced. Back in the days of big, heavy, flexy bias ply tires It was not uncommon to run tighter numbers, but with modern tires its not a good idea.

I suspect that your issue is a combination of too stiff springs mated to an overly soft shock. It is possible to get ride sag numbers with preloading, but if the free sag numbers are not there, or the bike shock is topping out (no free sag). you will have issues. Eibach makes a spring that will fit the stock shock http://www.le-suspension.com/catalog/product_info.php?cPath=21&products_id=37

you will most likely need a 1200-1300 lb/in (21.5-23.5 kg/mm) 7" spring to work with that fork setting.

The FJ is inherently stable. I have seen bikes with some fairly crazy set-up that don't shake.

[andyb]

A worn tire on the front end also makes the FJ want to twitch a bit.  Still, it's such a conservative geometry that it shouldn't shake it's head much.

[RichBaker]

Have you lowered the triple clamps/raised the forks to steepen the fork angle?  that was my 1st guess...

[carsick]

if that ever happens again LOOSEN up on the bars and NOT tighten.  Let the handlebars shake (actually - you should do this ALL the time) - it will let the bike settle itself.

Marsh-
thanks for the reply. Perhaps I understated the magnitude of the shake despite my profanity. The whole bike was bucking side to side. If I had loosened my grip at that point I would have been chucked off. I did have the death grip thing when I first started riding but now I'm just resting my palms on the bars, if anything a bit too loose. It wasn't like a vibration that builds in intensity, my Honda does that and I let go just for fun to see how much it will shake. Enough to knock the mirrors loose.  Put my hand on the grip and it smooths out. This deal with the FJ was like I hit a concrete block and bent the wheels square. It has always been the smoothest, most stable bike I've ridden so not only was it a surprise, it was as if my Mom tried to kill me for drug money.
I can see overdamped making it skate wide on bumps, i.e tire not touching on rebound, but the shake?

[andyb]

Full-on tankslappers and a weave are rather different.  A tankslapper beats the steering stops up, going full lock to lock.  A little weave isn't uncommon and has a bunch of various sources, but generally if you pin the gas and don't fight it they die out quickly enough.  What you had sounds like the real deal, though.

My own guess is that it was your own fault (sorry!).  With the new springs being softer in the fork, your static sag is a bit lower in the travel.  So after giving it some gas, the front end of the bike was able to gain a bit of inertia that you weren't ready for, and did a mild wheelie, enough to get the wheel out of alignment.  When it came back down, the trail immediately tried to put things back into alignment and slammed the wheel the other direction, overcorrected due to inertia going sideways, and back and forth you go.

[UJM]

I used to manage the service department at a Honda motorcycle dealership, and I had two customers come in with the same complaint -

"When slowing down, if I take my hands off the handlebars, they start flopping back and forth violently!"

One was a VTX1300 and the other, a GL1800, I gave them both the same advise -

KEEP YOUR HANDS IN THE HANDLEBARS! 

On another note, I did notice my FJ exhibits the same tendancy, and I'm taking my own advise.....

[carsick]

Hey guys, I really appreciate all your responses, despite my disagreeing with some of them. It scared some caution into me, but now I'm pissed and want to eradicate the problem by applying some of your hard earned knowledge. I can only shake my head at the collective net wisdom (outside this forum!) on this subject, everybody's opinions wildly contradict each other.

UJM- that's funny now, bet the customers didn't laugh though! The FJ is smooth as silk no handed, even done some Keith Code no BS style turns without touching the bars.

Andyb- I know it's my fault. I'm not the litigious sort, I just want to know WHY it happened dammit! Find it and kill it. If I had done one of the many obvious things (run wide, locked brake, not looking for hazards, bein' a stuntah) and crashed I would be understanding about it. I think this is a suspension tuning error combined with a very unfortunate road condition. This totally wasn't a wheelie, 55 in 5th running downhill at steady throttle. BTW wheelie touchdowns have not been a problem, as of yet.

Rich Baker- Ah, yes, I did lower the triples recently about 5/8" (15.875mm for you communists). Figured with the much stiffer springs I wouldn't need all that travel every time I braked. Quicker turn in and all that. Didn't expect this quick though!!

the fan- Lots of info from you and some questions from me. I followed Race Tech's instruction sheet for setting preload and sag (for the forks).  I need some definitions- Is free sag = fully extended - bike's weight on wheels? Ride sag = fully extended - rider on bike?

[fj1289]

I used to manage the service department at a Honda motorcycle dealership, and I had two customers come in with the same complaint -

"When slowing down, if I take my hands off the handlebars, they start flopping back and forth violently!"

One was a VTX1300 and the other, a GL1800, I gave them both the same advise -

KEEP YOUR HANDS IN THE HANDLEBARS! 

On another note, I did notice my FJ exhibits the same tendancy, and I'm taking my own advise.....

THAT IS NORMAL BEHAVIOR!!!  THAT IS NOT A TANK SLAPPER!!!  That is also NOT what Doug posted about.  What he described was a tank slapper, NOT the normal single track vehicle weave oscillations or whatever term you want to use for it.  When YOU HAVE YOUR HANDS ON THE BARS like he described and the bars are swinging back and forth so violently that it is trying to rip the bars out of your hands as the TIRE IS CHIRPING as it is slamming from side to side -- is definitely NOT the typical weave oscillation that is typically damped out by the weight of your hands on the bars.      

Chris

[UJM]

THAT IS NORMAL BEHAVIOR!!!  THAT IS NOT A TANK SLAPPER!!!  That is also NOT what Doug posted about.  What he described was a tank slapper, NOT the normal single track vehicle weave oscillations or whatever term you want to use for it.  When YOU HAVE YOUR HANDS ON THE BARS like he described and the bars are swinging back and forth so violently that it is trying to rip the bars out of your hands as the TIRE IS CHIRPING as it is slamming from side to side -- is definitely NOT the typical weave oscillation that is typically damped out by the weight of your hands on the bars.      

Chris

Too much coffee?

[andyb]

This totally wasn't a wheelie, 55 in 5th running downhill at steady throttle. BTW wheelie touchdowns have not been a problem, as of yet.

Bumpy road perchance?  That'll put the wheel in the air some and start things off, possibly with some sideways motion depending on what it hits...

You've also got perhaps a more compressed front end being downhill, with attendant geometry changes then.

Ah, yes, I did lower the triples recently about 5/8" (15.875mm for you communists). Figured with the much stiffer springs I wouldn't need all that travel every time I braked. Quicker turn in and all that. Didn't expect this quick though!!

Shouldn't be enough to make a difference, unless the rear is significantly raised also.

Lots of info from you and some questions from me. I followed Race Tech's instruction sheet for setting preload and sag (for the forks).  I need some definitions- Is free sag = fully extended - bike's weight on wheels? Ride sag = fully extended - rider on bike?

Have some light reading.

[the fan]

Rich Baker- Ah, yes, I did lower the triples recently about 5/8" (15.875mm for you communists). Figured with the much stiffer springs I wouldn't need all that travel every time I braked. Quicker turn in and all that. Didn't expect this quick though!!

the fan- Lots of info from you and some questions from me. I followed Race Tech's instruction sheet for setting preload and sag (for the forks).  I need some definitions- Is free sag = fully extended - bike's weight on wheels? Ride sag = fully extended - rider on bike?

Yes, One of these days I need to update my use of terminology, Its changed over the years.
Per Race Tech:

A: Static Sag or Race Sag is the amount the bike compresses from fully extended, with the rider on board.

I learned it as "ride sag" and have a hard time breaking old habits.

Free sag is a measure of the difference between fully extended and supporting the weight of the bike.

The chart on the Race Tech site can be misleading.
Bike Type    Front %    Front mm     Rear %    Rear mm    Rear Free Sag mm
Street Bikes    28-33%    30-35mm    28-33%     30-35mm    0-5mm
Road Race Bikes    23-27%    25-30mm    23-27%    25-30mm    0-5mm

Ignore the 'front mm' and 'rear mm' and concentrate on the %. The chart is way off on an FJ fork which has 150mm of available travel. In the case of the FJ, this works out to 42mm-49.5mm (1 5/8 - 1 15/16" for the unenlightened) on a street bike, 35mm-40.5mm (1 3/8"- 1 9/16" for those with more or less than 10 fingers...) on a bike intended for RACE conditions.

Race tech doesn't spell out a range for front free sag, but I prefer to keep it between 12 and 18%, Ideally 15%. Out back I prefer to maintain 8-12% ideally 10% of free sag as opposed to RT's 0-5%.

I rarely "lower the triples" on any of the bikes I tune, preferring to raise the back until I reach a 15

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%%D swing arm angle. Lowering the tubes in the clamps really only loses ground clearance and stability by decreasing rake and trail. In 9 out of 10 cases I do not recommend doing it. Lowering the clamps is very popular and has many vehement supporters mainly because its free to do...and it makes the bike seem to handle quicker. In reality lifting the rear has much greater positive effect on the handling.

Here is a simple chart I made for a seminar showing rake, trail and swing arm angle.