The Wobble, A Comprehensive Look into Cause, Effect, and Fix
The Original post was removed and replaced with this one in order to focus more on the fix. UPDATED POST:
INTRO: This post is an attempt to get this 200+ year old subject into one box and explain it from a summary level physics / engineering perspective and, most importantly, list the fixes that actually work.
TECH DESCRIPTION: The wobble is a sustained oscillation felt primarily in the front end. The conditions needed for sustained oscillation are (in a nutshell, thus avoiding a lengthy, detailed physics & engineering explanation): underdamping, multiple stored energy sources, and a mode of energy transference between these sources (a feedback loop). You cannot eliminate the sources of stored energy, which are the spinning front and rear wheels (essentially two large gyroscopes), and the bike/rider system (mass, velocity, and gravity). You cannot eliminate the feedback loop. The front and rear wheels, engine and trans, front forks and rear shocks, all need to be somehow connected—answer, the frame, which by the way needs to be as rigid as possible or it too will be a source that allows the system to oscillate. That leaves underdamping. One must eliminate or minimize all sources of underdamping, and for us, there are many sources. But first, in the context of this subject, let’s be clear on what damping is.
DAMPING DEFINTION: Within the vehicle engineering community, from motorcycles to planes, trains, and automobiles, even space vehicles, damping refers to anything designed to be in motion but also has attributes designed to dissipate or isolate energy stored in oscillations that could occur from it being motion; i.e., damping prevents it from wiggling, wobbling, swaying, fluctuating, swinging, undulating, or most simply, moving in a plane it is not suppose to move within. Now, back to eliminating underdamping.
THE FIX: You cannot completely eliminate 100% of all underdamped sources, but you can minimize them and fix the wobble, oscillation problem. Some required one fix, others (like me) had to do several (I have a 99 FLTR; my brother has an 04 FLTRI, all stock, and rides likes it’s on rails).
1. Increased tire pressure from 36 or lower to 38 or 40 psi (most common)
2. Tightened steering head bearing to spec (very common)
3. Loosened steering head bearing to spec (somewhat common)
4. Replaced steering head bearings (not common)
5. Replaced front wheel bearings (not common)
6. Re-aligned rear wheel after new rear tire installation (common)
7. Tightened loose spokes on laced wheel (not common)
8. Aligned Vehicle (very common): Adjusted belt tension, then adjusted rear wheel axle to swing arms shaft distance (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 pair of 8’ fluorescent tubes, bungee corded to the rear wheel. Line up the front wheel parallel and centered within
the two fluorescent tubes. Measure the gap between the tubes and edge of front tire at four points (assumes your rear tire
is wider than your front; if they are the same width, you should have equal contact at all four points). You want the four
gaps to be equal or as near equal as possible. When measuring the gap between the tubes and edges of front tire, the gap
can change by as much as 5 mm (~ 3/16 in) with a mere 1/2 turn on the lower engine stabilizer link; so it shouldn’t take much
to get your rear wheel lined up with your front wheel (there are exceptions – like my bike—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 (this is the biggy – it’s a weak design point not
only for HD, but others as well):
a. Replaced swing arm bushings/isolators with new ones $50 (common for bikes 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 ten different manufacturer choices ranging from $100-$400.
Not all bikes need this, including baggers, but many have done it, including me. I would make this my last resort.
11. Replaced front and rear tires with manufacturer approved set (do not mix bias ply and radials; do not put radials on a bike
that was not designed for radials—huge mistake!)
12. Flushed and serviced front forks. Fork oil level in left was not same as right (mismatched damping -- not common)
13. Replaced front forks or rear shocks (left/right side damping not equal -- this is rare)
THINGS THAT ARE NOT THE PRIMARY SOURCE OF A WOBBLE
1. Vehicle (wheel) alignment is not an underdamped source. It’s a cyclic stress source. The misalignment creates lateral forces
that act on the swing arm and rear tire (neither of which are perfectly rigid-- both have damping issues). As the frequency,
which increases with speed, approaches the natural frequency of your steering/suspension system, the wobble begins.
These small lateral forces can produce very large oscillations within seconds and can progress from mild to violent,
uncontrollable gyrations that have taken riders down.
2. Wheels out of balance - this causes vibration and can trigger a wobble but is not the source; it’s a stimulus, similar to a bump
in the road that starts 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 is not a cause. It’s just another stimulus like a grove in
the road that puts stress on the steering/suspension system.
4. Overloaded luggage, or one bag much heavier than bag on opposite side – like rider position on the bike, these affect CG and
thus stability, and also stress the steering/suspension system.
5. Windshields and fairings – these are special case items unrelated to mechanical damping but can (rarely) create a wobble
from an aerodynamics perspective. Exception – avoid cruising down the interstate in the wake of a semi that’s generating
wind vortices at 70+ mph.
6. Rake or castor - changing the original design of the bike using after-market parts affects CG, turning radius, and stability, but
is not a cause provided the new parts do not move in planes they are not suppose to move in; e.g., the forks do not flex or
twist or move laterally.
SOURCE DATA/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); and prior to that a automobile and motorcycle mechanic (5 years). Yes, I’m old ... been riding since ’64 (Honda, Triumph, and Harley). As you can deduce, the wobble, sustained oscillation, is a complex issue involving physics (matter and motion), kinematics (geometry of motion), analytical dynamics (mass and moment of inertia), and mechanics (behavior of physical bodies when subjected to forces or displacements). Solving it is not always one fix scenario. Every bike is different. Have I addressed everything? NOPE … else this post would be a text book! But hopefully I’ve managed to sum up and clarify the thousands of post on this subject and will help you fix your oscillation issues. Don’t have a wobble? Never had one? Never experienced one? You’re lucky. RIDE SAFE.
“The Guide says there is an art to flying, or rather a knack. The knack lies in learning how to throw yourself at the ground and miss.” Douglas Adams
Last edited by LoneGrey; 08-23-2014 at 11:06 AM.
Reason: Oringal Post Removed -- Replaced w/Update