Tank Slapper WTF?

Second ride on my new to me 2007 FJR. Rollin across wal mart parking lot in second gear with my modular helmet open as I usually do till I get moving. Take both hands off the bars to close my helmet and the bike goes into the most violent try to kill me tank slapper that I have ever experienced. WTF! Please tel me this isnt a trait of this model bike?

Most definately NOT a common trait!!

1) Tires badly worn and/or cupped

2) Tires underinflated

3) Very badly torqued steering head bearing

Did I mention tires??

OK good. Tires certainly have seen better days. Also previous owner showed me shop work order where he had head bearings repacked. I will look into both items

Almost always this is the front tire.

And what kind of helmet needs both hands to close???!??!

wfooshee said:

Almost always this is the front tire.
And what kind of helmet needs both hands to close???!??!

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Modular!

I usually close my helmet before I take off on most ocasions. When I seldom don't and I am on the move I use one hand. When using two hands I am garrenteed both sides are locked in. It's only done for the anal retentive type individual. Just Sayin!

Dave

Its a Shuberth modular and my way of doing it is to use both hands and feel for the "click" on both sides to know for sure its locked. I can close it one handed, but its easier 2 handed.

KTMRIDER said:

I can close it one handed, but its easier 2 handed.

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Maybe when confronted with the option of needing to keep contact with the handlebars in a potential tank slapper or something you need to avoid.....one-handed might be the better option.

Meanwhile, you might review the NEPRT threads on wobbling. Many, many, many theories, ideas, and prognostications....worn front tire being at the top of the list.

On the helmut issue, I have had my HJC modular helmet fail to close and lock after a brief stop at a filling station.

Problem was that on this 110 degree day, I froze the sweat in my helmet too long in the ice box at the convinience mart before re-donning it.

Not sure(?), if I was within health code standards at any time while executing this manuever.

It was sure a great treatment for the potential heat stroke head ache however.

Is this a wobble issue, or did you let the bike get too slow foe second gear and it started bucking because it was lugging?? 2nd gear in a parking lot, hands off bars makes me think your bike just got too slow.

No it was for sure a text book tank slapper

Extract from Wikipedia Clicky here:

Wobble or shimmy begins when some otherwise minor irregularity accelerates the wheel to one side. The restoring force is applied in phase with the progress of the irregularity, and the wheel turns to the other side where the process is repeated. If there is insufficient damping in the steering the oscillation will increase until system failure. The oscillation frequency can be changed by changing the forward speed, making the bike stiffer or lighter, or increasing the stiffness of the steering, of which the rider is a main component.[2]While wobble or shimmy can be easily remedied by adjusting speed, position, or grip on the handlebar, it can be fatal if left uncontrolled.[4]

Since shimmy frequency is independent of bike speed, gyroscopic effects "are clearly not essential to the phenomenon."[2] The top five influences on wobble have been found to be lateral stiffness of the front tire, steering damper, height of bike center of mass, distance of bike center of mass from rear wheel, and cornering stiffness of the front tire.[3][5]

An academic paper that investigated wobble through physical experimentation and computer modeling concludes: "the influence on wobble mode of front tyre characteristics, front frame inertia and chassis stiffness were shown. In particular, it shows that [by] increasing front tire inflation, stiffness chassis, and front frame inertia about steering axis and decreasing sideslip stiffness of front tire, wobble mode damping is improved, promoting vehicle stability."[6]

As this is not a common problem on FJRs I think you can safely discount all the stuff on chassis design / stiffness etc. Which leaves you with front tyre characteristics & increasing steering stiffness (hold on to the bars with at least one hand). YMMV

All encouraging I suppose, cuz I like to text while I ride so I need both hands free....

Go get yerself some new tires...

I believe this place this has the best prices on PR2's...

https://stores.sportbiketrackgear.com/Categories.bok?category=Motorcycle+Tires%3AMichelin+Tires

Only ever had one tank slapper thank goodness. The culprit was a worn front bridgestone.

Every bike I've owned over the last 14 years developed a wobble as you described. Typically happens when the front tire is worn and when you take your hands off the bars and the throttle closes. So I check for wobble as the tire gets old by releasing the bars and see how the bike performs. Big wobble, get back on the bars and buy a new tire.

Bill

KTMRIDER said:

...Also previous owner showed me shop work order where he had head bearings repacked...

Click to expand...

So, the previous owner admits there is a problem?

So, the previous owner simply had all the maintenance done?

Head bearings shouldn't be related to the experience you had. If you put in roller bearings or torque down existing bearings so that there is a lot of drag they might hide the root cause, but the issue would still be there.

Do you know for sure that the bike was never in an accident? Have you checked the forks to see that they are true and parallel? Have you used a run-out gauge to check that the wheel is true? Just for giggles put the bike on the center stand and check the rear wheel for play and check the swing arm pivot for play.

Now, go out and put new tires on your bike and you should be all set.

My $.02: Stay away from Metzler Z6's & prior versions. The center tread zig-zags back-n-forth & the tire will follow this pattern causing the handlebars to wobble up to about 35mph. It's very annoying & will have you thinking something major is wrong w/the bike when infact, it's only a poorly designed tread pattern.

Go for some Michelin Pilot Road 3's & they transformed the bike, they're that much better!

PO claims to know nothing about it.

My FJR has always had a wobble when the front tire gets worn down. Only occurs at around 40-50mph and replacing the tire improves it (but it never really goes away). Only occurs when I take my hands off the handle bars at that low speed.

Replace the front tire and you should be good.