Topic: Everyone had a tank slapping good time - forks?

[Lenny55]

Define "tank slapper" please.  Sounds nasty.

[Retro Rocket]

Rapid shaking from left to right of the handlebars, not good for your underware, can result in skid marks and bruised thumbs.....

Mick

[andy750]

Thanks Mick!

cheers
Andy

[MCRider]

Define "tank slapper" please.  Sounds nasty.

http://en.wikipedia.org/wiki/Bicycle_and_motorcycle_dynamics About 2/3 of the way down is tank slapper in particular. Essentially, forces travel up the fork from a bump/hole/washboard, etc. If the fork can't dampen them out and the fork bottoms and force is still entering the system it has to go somewhere, so it turns right (or left) and forces the fork to turn right or left and in the extreme it hits the fork stop, and goes back the other way, over and over if you're lucky. Often the first hit is the end and you crash and/or die. (Hence the term "death wobble")

Imagine laying your pinky on the table, then hitting it with your clutch lever hard enough to break both the pinky and the lever. And remember your pinky is back stopped by the table.

I was on my 88 hawkGT, not a cartridge fork, no emulators, I did have progresively wound springs and heavier oil. Couldn't handle it. My TS was particularly violent, the first turn was to the left. I ride with 2 fingers on the lever 1st and 2nd and the 3rd and 4th on the bar. The clutch lever hit my pinky so hard it broke (smashed) my finger, broke the lever and ripped the bars out of my hands, It slapped back and forth with me sitting back watching about 3 times before I hit the pavement at about 60mph. Threw me on the ground pretty hard, ruined my helmet and tore up my riding suit gloves and boots. Luckily I had them all on, as usual.  Sometimes I skip the pants. This time they saved me lots of otherwisd painful road rash.

Luckily no one was behind me, it was a straight road, I'd dropped into a chuckhole at speed.

There have been many threads on TS in the last year, each with at least one fatality.

[MCRider]

One question I have about the dampers - you need to lose the fork gaiters is that correct? For early forks (like 72) what do you use for fork seal covers?

If someone could post a couple of photos of how the damper is mounted that would be really helpful!

Thanks
Andy

 

Here's a shot of the bracket welded to the frame. Prior to this it was a clamp-on affair which worked fine but didn't have the permanent look.


Here's a shot from the rear. There is actually a threaded hole in the lower tree. On my 75CB750F there are untrhreaded holes on both sides of the lower tree.

Many come with a clamp to clamp right on the fork tube, but because of the geometry you'll likely limit your lock to lock movements with those. reserved for track bikes.

With this setup I have full lock to lock as a stocker.

[Lenny55]

Nasty indeed!

[ev0lve]

i'm being lazy and i dont want to read all the way through. do youhave your front fender or a brace?

Yes. Front fender.

I think my problem is going to be simply too little oil as I took the dry spec from the Haynes for a K5 and it's about an ounce too little. Just have to make the time to go add some fluid.

For me it was some wavy pavement going 50ish in a sweeper. And a chuckhole in a straight line.

Disconcerting I would call it. It recovers pretty quickly but with the right stretch of pavement... evilness I think. Rather not find out so I'd like it to stop.

So no damper with the stock ears without welding?

[754]

You can clamp the damper to the frame.

 I am surprised no one brought this up;
 Check your fork sliders for excessive wear, most were not all that great new.

 Jack up the bike so no weight is on front wheel, then try moving forks front to back with your hands. Once they get a bit of play in there, they will not handle that great.. simply too much movement.

 I can remember running our bikes up to 110 or so & watching the fork legs, moving back and forth while riding.
 
 They simply are not one of the best fork designs.. If they were sloppy back then, I dont think they tightened themselves up any..

[MCRider]

i'm being lazy and i dont want to read all the way through. do youhave your front fender or a brace?

Yes. Front fender.

I think my problem is going to be simply too little oil as I took the dry spec from the Haynes for a K5 and it's about an ounce too little. Just have to make the time to go add some fluid.

For me it was some wavy pavement going 50ish in a sweeper. And a chuckhole in a straight line.

Disconcerting I would call it. It recovers pretty quickly but with the right stretch of pavement... evilness I think. Rather not find out so I'd like it to stop.

So no damper with the stock ears without welding?

Iggy: The washboard sets up a slightly different dynamic. You have partial compression, then the fork is asked to recover quickly before the next bump hits. If it doesn't, the bike hits the next bump with slightly less travel available, then poor recovery, on and on. It may compress an inch, recover 1/2 inch, compress an inch recover 1/2 inch. Its a cumulative loss of travel effect, ending with the fork bottomed out and tank slapper imminent. Even though no single bump was strong enough to bottom the fork.

Our forks, especially the early ones, use the spring only to damp on compression. Hence we like the progressively wound springs as an upgrade. Then the spring initiates recovery and the oil dampens the rebound. If the oil is too heavy you may slow the recovery too much to handle washboard.

Always a compromise. The emulators convert to double dampening. The oil is used on compression and rebound, so the spring has less work to do. (I am still in learning mode here, so open to any corrections).

As to welding, that is the option. Most steering dampeners are clamp ons as standard. Here is a poor picture of mine with the clamp. It is a "U" bolt muffler clamp with an "L" bracket. Came with the dampener for universal installation. A little funky, but it was chrome plated. My dampener is a Dunstall unit. Don't know if they are still available.