Steering Damper install? i.e. survived a tankslapper

[fj1289]

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?

No.  Just wanted to get the point across that this wasn't at all the same issue.  The advice to keep your hands on the bars was misplaced and dismissive - the same magnitude as telling someone to use a bandaid for arterial bleeding. 

I appologize if I came off a bit heavy. 

Chris

[carsick]

Chris,
You're making me blush, jumping to my defense like that! Sounds like you've got a little firsthand experience with this symptom. Fun, ain't it?
I get that UJM was just tellin a funny, not telling me to hang on better.
I have repeated my point a few times because there does seem to be a divide between those that have experienced it and well, not. The blame here lies squarely with me for modifying my suspension without a thorough understanding of the physics behind it. I posed my question to learn more about suspension setup. Not looking to cause discontent between folks, we have Klavdy for that.

Andyb is right, the road was very bumpy. I was running it to see if it had been repaved yet. It is possible I got one or both ends in the air at once (overdamped front, underdamped rear, harmonic mismatch between stock rear spring and stiffer fork springs?) and one end was kicked sideways. If the fork was compressed and the rear was in the air I'm thinking it would greatly reduce my rake to an unstable point and then the front gets a sideways shove off a bump- BOOM.
I'm going to move my suspension back to more conservative settings and slow the hell down in bumpy corners.

the fan- Thanks for the clarification on terminology, got it. I never realized Race Tech's recommendations in mm were for much sportier bikes with less travel (thus RACE tech, duh!), your percentage suggestion makes more sense to me. I thought I was having to crank an awful lot of preload onto those forks! I have not raised the rear other than adding shock preload. How does increasing swingarm angle affect the bike e.g. straght line and cornering traction? Is it like raising the instant center on a car to get more bite on launch?
  This is getting to be as contentious as an oil thread, but I'm still going to keep asking questions!
Doug

[fj1289]

Only a minor occurance - only made two or three chirps.  Much scarier was the full on "death wobble" in the Jeep Cherokee - but that's a totally different issue!  But it will make you get out and see if any suspension components were torn off during the episode!  On that site you weren't even allowed to say (type) "death wobble" - you had to abbreviate it as DW since it always seemed that as soon as one person posted about it, someone else got to experience it too!

My appologies again to Steve.  I took his post advice instead of relaying a funny story.  Either way, probably not the best response.

One of the things I really like about this group is the wealth of information and advice that comes from experience and knowledge.  Most of the other sites I visit are full of opinion and outright BS, with the occaisional nugget if you bother to sift through all the crap.  I guess another thing that's good about this site is the ability for anyone to ask a question and not get flamed for not being experienced or knowledgable - this group is usually very supportive and helpful instead! 

OK, now back to our scheduled suspension lesson!

[andyb]

+1

Tankslappers aren't fun!  I've only had 2 that I can think of, and one was on super bumpy pavement on a CBR600F2.  The easy way out is of course standing the bike up on end, then setting it back down straight!  Not recommended, though..

[the fan]

the fan- Thanks for the clarification on terminology, got it. I never realized Race Tech's recommendations in mm were for much sportier bikes with less travel (thus RACE tech, duh!), your percentage suggestion makes more sense to me. I thought I was having to crank an awful lot of preload onto those forks! I have not raised the rear other than adding shock preload. How does increasing swingarm angle affect the bike e.g. straght line and cornering traction? Is it like raising the instant center on a car to get more bite on launch?
  This is getting to be as contentious as an oil thread, but I'm still going to keep asking questions!
Doug

There are a lot of things going on with swing arm angle but the most common is that when the swing arm goes 'flat' and everthing is in line and under power. The suspension will become very stiff, and in some cases actually not be able to compress further until the power is removed or acted upon by a large bump. The result is that the only suspension available at that point is the flex in the sidewall of the tire. This can easily cause the rear wheel to hop or slide.

Raising the rear to the steeper 12-15 degree angle serves 3 purposes. It gains you valuable ground clearance, It raises the center of mass of the machine, making it easier to transition right to left, and in the case of mid corner traction gives you more usable traction before the dreaded lock up. Going steeper than 15 degrees can put the linkage in a bind, I generally recommend 12 degrees for street riding, 15 for very aggressive or track riding. Keep in mind that by changing the dogbones out you are changing the leverage ratio of the link, generally making it more progressive with shorter bones. Its best to balance the lift between the shock length (with an aftermarket shock) and the shorter dogbones. I have not tried to lift an FJ to the the 12-15 degree range, and in fact have never measured one. Most simply put the bike on the center stand and make bones that allow about 1/2" clearance for changing tires.

If I remember correctly you swapped out the rear for a 17" wheel. Pretty sure the overall height of the 17" tires is 1/2 -3/4 " shorter than the stock wheel/tire. Dogbones should really help out your handling.