The original post wasremoved and replaced with an update to focus more on the fix.

INTRO: This post is anattempt to get this 200+ year old subject into one box and explain it from asummary level physics / engineering perspective and, most importantly, list thefixes that have actually worked.

TECH DESCRIPTION: The wobble is a sustained oscillation felt primarily in the front end. Thereare three conditions needed for sustained oscillation:

1.Underdamping

2.Multiple stored energy sources,

3.A mode of energy transference between thestored energy sources (a feedback loop)

You cannot eliminate the sources of stored energy,which are the spinning front and rear wheels (which are basically two largegyroscopes),and the bike/rider system (mass, velocity, and gravity).

You cannot eliminate the feedback loop. The frontand rear wheels, engine and trans, front forks and rear shocks all need to besome how connected; e.g., the frame (which by the way needs to be as rigid as possibleor it too will be a source that allows the system to oscillate).

That leaves underdamping. One must eliminate or minimizeall sources of underdamping, and for us, there are many sources. But first, inthe context of this subject, let’s be clear on what damping is.

DEFINTION-DAMPING: Within the vehicle engineering community, from motorcycles to planes,trains, and automobiles, damping refers to anything designed to be in motionbut also has attributes designed to dissipate or isolate energy stored inoscillations that could occur from it being motion. Damping prevents it fromwiggling, wobbling, swaying, fluctuating, swinging, undulating, or put mostsimply, moving in a plane it is not suppose to move within. Now, back toeliminating underdamping.

THE FIX: You cannot completelyeliminate 100% of each underdamped source, but you can minimize them andprevent the wobble, oscillation from occurring. Some required one fix, others(like me) had to do several. I have a 99 FLTR. My brother has an 04FLTRI, allstock, and rides likes it’s on rails.

1. Increased tire pressure from36 or lower to 38 or 40 psi (most common)

2. Tightened steering headbearing to spec (very common)

3. Loosened steering head bearingto spec (somewhat common)

4. Replaced steering headbearings (not common)

5. Replaced front wheel bearings(not common)

6. Re-aligned rear wheel afternew rear tire installation (common)

7. Tightened loose spokes onlaced wheel (not common)

8. Aligned Vehicle (very common):Adjusted belt tension, then adjusted rear wheel axle to swing arms shaftdistance (left and right sides must be exactly the same), then aligned vehicle.The goal is to get the real wheel in plane with the front wheel. I use a pairof 8’ fluorescent tubes, bungee corded to the rear wheel. Line up the frontwheel parallel and centered within the two fluorescent tubes. Measure the gapbetween the tubes and edge of front tire at four points (assumes your rear tireis wider than your front; if they are the same width, you should have equalcontact at all four points). You want the four gaps to be equal or as nearequal as possible. When measuring the gap between the tubes and edges of fronttire, the gap can change by as much as 5 mm (~ 3/16 in) with a mere 1/2 turn onthe lower engine stabilizer link; so it shouldn’t take much to get your rearwheel lined up with your front wheel (there are exceptions – like mybike—that’s another story).

9. Replaced front motor mount–worn out mount allowed rear wheel steer (common for bikes with over 60K miles;recommend you check swing arms rubber bushings/isolators, too)

10. Stabilized the rear fork /swing arm to eliminate lateral swing arm movement (steering head bearing aside,this is the number one weak link not only for HDs, but other makes well). Thereare two approaches:

A. Replaced swing arm bushings/isolators with new ones $50 (common forbikes with over 60K miles; would be wise to replace the front motor mount,too).

B. Installed swing arm/engine/trans stabilizer kit. There’s about tendifferent manufacturer choices ranging from $100-$400. Not all bikes need this,but many have done it, including me. I would make this my last resort.

11. Replaced front and rear tireswith manufacturer approved set (do not mix bias ply and radials; do not putradials on a bike that was not designed for radials—huge mistake!)

12. Flushed and serviced frontforks. Fork oil level in left was not same as right (mismatched damping –notcommon)

13. Replaced front forks or rearshocks (left/right side damping not equal -- this is rare)


THINGS THAT ARE NOT ASOURCE OF UNDERDPAMPING:

1.Vehicle (wheel) alignment is not an underdamped source. It’s a cyclicstress source (as in cycles per second). The misalignment creates lateralforces that act on the swing arm and rear tire (neither of which are perfectlyrigid-- both have damping issues). The frequency of these cycles increase withspeed. As the frequency approaches the natural frequency of your steering/suspensionsystem, harmonic resonance sets in and the wobble begins. These small lateralforces can produce very large oscillations within seconds and can progress frommild to violent, uncontrollable gyrations that have taken riders down. It’s agut wrenching feeling and horrific to experience.

2.Wheels out of balance - this causes vibration and can trigger a wobblebut is not the source; it’s a stimulus, similar to a bump in the road thatstarts the oscillations but is not what’s allowing the wobble to occur.

3.Rider position on the bike - this affects CG and thus stability, but isnot a cause. It’s just another stimulus like a grove in the road that putsstress on the steering/suspension system.

4.Overloaded luggage, or one bag much heavier than the bag on oppositeside – like rider position, these affect CG and thus stability, and also stressthe steering/suspension system.


5.Windshields and fairings – these are special case items unrelated tomechanical damping but can (rarely) create a wobble from an aerodynamicsperspective. Exception –avoid cruising down the interstate in the wake of asemi that’s generating wind vortices at 70+ mph.



6. Rake or castor - changing theoriginal design of the bike using after-market parts affects CG, turningradius, and stability, but is not a cause provided the new parts do not move inplanes they are not suppose to move in; e.g., the forks do not flex or twist ormove laterally.


SOURCEDATA/REFERENCES: Me. I’m a mechanical engineer (30 years) in space systems (NASA,USAF, NOAA, NRO). Prior to that I was a Master Aircraft Tech(10 years); andprior to that a automobile and motorcycle mechanic (5years). Yes, I’m old ...been riding since ’64 (Honda, Triumph, and Harley). As you can deduce, thewobble, or sustained oscillation, is a complex issue involving physics (matterand motion),kinematics (geometry of motion), analytical dynamics (mass andmoment of inertia), and mechanics (behavior of physical bodies when subjectedto forces or displacements). Solving it is not always a one fix scenario. Everybike indifferent. Have I addressed everything? NOPE … else this post would be atext book! But hopefully I’ve managed to sum up and clarify the thousands ofpost on this subject and will help you fix your oscillation issues. Don’t havea wobble? Never experienced one? You’re lucky. RIDE SAFE.

* * * * * * * *

“The Guide says there is an art to flying, orrather a knack. The knack lies in learning how to throw yourself at the groundand miss.” Douglas Adams

 

https://www.hdforums.com/forum/touri...e-exposed.html

 

Great stuff but the lack of properly spacing words detracted from readability - but certainly interesting

Thanks for posting

 

And, I'll add rider position.

I can introduce wobble by trying to fight the lean while in a turn. When I was learning how to race dirt bikes many years ago, this was one of my problems. Even on dirt bikes, if you did not stay in line with the bike you could start a wobble.

I've see this with new riders as well. Go in to a good sweeper and the rider leans only "half" with the bike. Almost always, especially if the road has a bump, the rider will report a wobble feeling. Get them to lean more and the problem goes away.

 

"......the wobble willbegin as the bike approaches a specific speed....."

Back in the 70's we used to call this stability upset "speed wobble" as you note certain bikes may be more prone control issues in certain speed ranges - feed back on the systems harmonic / resonant / natural frequency. As I recall most of the bikes with the problem were off shore manufactured and the speed was usually fairly high 80 + MPH.

 

Problem significantly reduced with Rushmore front end, virtually eliminated for me.

 

It's because it was copied from road glide.org. It looks like he posted it there too.

 

It will be interesting to learn if LoneGrey copied it from somewhere else, or wrote it himself.

 

LOL, who cares??? Most informational stuff posted on here is copied from somewhere.

 

Some of us do care, to put informative and accurate stuff together such as the link at post #2. It also matters when it's poorly written!