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5-Ball Bagger Project Bike -- The Infamous Harley Wobble -- Part 1

An Engineers Perspective.

By Graham R Brown
5/8/2014


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Introduction:
 
The subject of Harley-Davidson ‘bagger wobble’ is all over the internet and Harley forums. As an engineer and long-standing owner I find it frustrating that much of the discussion and comment is very wide of the mark, so what follows is a description of the rubber mounting system used in our touring bikes, from an engineering perspective, in the hope of improving understanding of what the wobble is and why it happens.
 
It goes without saying that the following assumes a bike is in tip top shape, with tires in good condition and at suitable pressures, steering head bearings correctly tightened, engine rubber mounts are sound, swingarm and wheel bearings in good condition, vehicle and rear wheel alignment good.
 
Many Harley owners (hopefully the majority) have not experienced the so-called wobble frequently and are blissfully happy with their ride. If you are one of those there is no problem, but at any minute there could be, so you need to know the following and keep your heavy, powerful touring machine properly maintained. If you read this and are wondering what on earth it is all about, please do not allow yourself to be suddenly gripped with fear!
 
Let us start with the original rubber-mount system introduced with the FLT in 1980 and cover the '09-on bikes later.
 
Stock swing arm with cleve bloc bushing (top)
Custom Cycle Engineering -- Conversion assembly with spherical bearings (bottom)
Stock swing arm with cleve bloc bushing (top) Custom Cycle Engineering -- Conversion assembly with spherical bearings (bottom)

 
Wobble 1:
 
Evos and early TC bikes not only had rubber engine mounts, they also had rubbery swingarm bushings, called ‘cleve blocks’. Some of the bagger wobble reported on these bikes can be blamed in part on them, as they allow a degree of sideways movement of the rear wheel and swingarm, relative to the rest of the bike. They were dropped by Harley in 2002 and replaced with steel bushings, which cured that aspect of the handling problem.
 
After-market bushing kits are available from several sources including Custom Cycle Engineering. Cleve blocks can be replaced at anytime.
 
Factory Cleve Block Assembly Installed
Factory Cleve Block Assembly Installed

 
 
Why Rubber Mounts?
 
Over the decades our bike engines have become progressively bigger in capacity and more powerful. As a consequence the forces inside the engine have become greater, causing more vibration, which in turn is more uncomfortable to riders and more likely to damage parts of the bike. Norton tackled this challenge back in the 1960s with their Commando, a watershed design, partly because parallel twins are more prone to vibration than our V-twins. In 1980 Harley introduced the FLTs, with their own rubber mount designed system, credited to Erik Buell, which we have all come to know and love!
 
The approach Norton used in the Commando was to isolate the engine from the frame, using concentric rubber mounts. That system is far from perfect, in part because it didn’t allow the engine to actually move very much. Essentially they addressed high frequency vibration, but not engine speed one knows so well. The beauty of the Harley system is that our engines are allowed to shake, which helps deal with engine speed vibrations, but are also rubber mounted, which helps dampen higher frequency vibrations. That gives us a more comfortable ride, but also has the potential benefit of providing high quality handling at the same time.
 
Comfort:
 
Humans are sensitive creatures, despite the tough and rugged image we riders like to have! When man started to employ science in the design of engines and vehicles, we soon learned that if we are going to sit on top of one, the least unpleasant form of vibration is up and down. So our bike engines, solid or rubber mount, are designed to shake that way.
 
When you watch your Harley engine bobbing up and down at idle, that is deliberate! You are watching the laws of physics at work, in perfect harmony with engineering, under man’s control. All is well with the World.
 
Description:
 
The engine and trans are bolted together, and the swingarm is mounted directly onto the rear of the transmission casing. Along with the rear wheel this makes up a substantial single assembly, which is mounted in, and isolated from, the frame at three points. The swingarm axle not only allows the rear wheel to move relative to the rest of the bike, but doubles up as part of the rear mounting.
 
I will continue to use the word ‘Assembly’ to refer to the combined engine/trans/swingarm/wheel.
 
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Kinematics:
 
Take a deep breath, we are about to dive into the bowels of your bike!
 
Statement: kinematics is a branch of engineering that deals with the geometry of motion of moving parts. 
 
Question: what on Earth does that mean and how does it affect your Harley? 
 
Answer: It is at the very core of it and is the basis of the original Harley rubber mount design. The following words are from one of H-D’s Patents, #4,776,423, and describe the underlying design concept of our bikes and summarizes the kinematic principle used:
"A motorcycle frame design featuring a uniplanar isolation system. This is a system for vehicles which have motors that have basically uniplanar vibrations. It provides mounting means which allow the motor to have vertical and longitudinal movement, but prevent lateral motion of the motor and rear suspension unit with respect to the main frame."
As mentioned, the Assembly described above is mounted in our bikes at three locations:
 
1. The rear rubber mounts serve several purposes. The ends of the swingarm axle are fastened to those mounts, which are retained in the frame by the passenger footboard mountings. They support a large part of the weight of the bike; locate the Assembly in the frame, both laterally, vertically and fore and aft; also allow it to rotate up and down, on that axle.
 
2. The front rubber mount supports the front of the Assembly and allows it the freedom to move up and down, rotating on the rear mounts. It is laterally supported by a stabilizer link, which prevents the unit moving sideways at that point.
 
3. The top support is a simple stabilizer link, which also has several purposes. It doesn’t bear any weight, but allows the Assembly to rock up and down, also back and forth, while preventing it from moving sideways.
In all three cases motion is free moving, although the design of the front mount limits the total amount of movement.
 
This is an elegant engineering solution and is to be applauded, if with reservations! The rear mounts are not adequately supported, to “prevent lateral motion of the motor and rear suspension unit with respect to the main frame." In the original patent, the rear mounts are provided with a lateral stabilizer, which is omitted on our Touring bikes.
 
 
Wobble 2:
 
The lack of a rear stabilizer, to provide lateral support to the rear rubber mounts, is IMHO a serious flaw in the way Harley introduced this design. That comment is entirely justified as every Buell has one, as do all rubber-mount Sportsters, so Harley knows how to do this. It is a mystery why our Touring bikes don’t have one.
 
This omission in the rubber mounting system is the main source of the infamous ‘bagger wobble.' Lateral forces compress those rear rubber mounts and cause momentary misalignment of the rear wheel relative to the front wheel.
 
Custom Cycle Engineering-- Late Model Swing Arm Retrofit - with Axle
Custom Cycle Engineering-- Late Model Swing Arm Retrofit - with Axle

 
 
2009-on:
 
While Harley continues to use a similar design of rear rubber mounting on their latest bikes, the old single front mount has been changed to a pair that are similar in design to the rear mounts. There are now four rubber mounts instead of three, but instead of leaving only the rear mounts without lateral support, neither front nor rear has a stabilizer.
 
Frankly, from an engineering perspective, Harley has abandoned the fine principles of the original design. There are many reasons why they changed the frame, of which the engine mounting system is only one, however they could have retained the original 1980 design and simply added that vital rear stabilizer.
 
If you sometimes wonder about the handling of your lovely new bike, you have every right. If you changed from a pre-'09 bike to a post-'09 one, to get a better handling bike, you simply jumped out of the fire and into the frying pan!
 
Wobble 3:
 
It cannot be denied that the 2009-on set-up gives improved lateral stability, but it doesn’t completely cure the wobble. Both front and rear rubber mounts can compress under lateral loads, although the degree of misalignment this causes is improved over the older bikes.
TRUE-TRACK TOURING
TRUE-TRACK TOURING

 
Stabilizer Kits:
 
Harley inadvertently created a market opportunity for the custom market, which many brands have filled. The better designs of stabilizer kit provide lateral support to the rear rubber mounts on earlier bikes, and both front and rear mounts on later bikes. By providing suitable lateral support, the kinematic principles of the original design are fulfilled. On both versions of bike steering is more accurate, improving the steering and feel of the bike even at slow speeds, as well as giving better handing at higher speeds. The improved stability improves the riding experience throughout the speed range. The benefits of stabilizers are for all of us, not just those who want to ride fast. However, not all kits are created equal and some designs are superior to others.
 
Summary:
 
The original rubber mount design is excellent, but was compromised by the factory, by leaving off the rear stabilizer link, for reasons we may never know. Rubber squashes, which is why we make much use of it in so many aspects of our lives. It squashes on the original bikes and it does on the latest ones as well. Both sets of bikes will ride better with lateral stabilizers. 
 
Homework:
 
Those of you keen to read further may like to refer to Wikipedia and look up both ‘Kinematics’ and ‘Six degrees of freedom.' From these you will see that the original front mount only permits vertical motion or pitch and prevents yaw: the top mount only permits pitch and prevents roll and yaw: unfortunately the rear mount, while it prevents linear motion in the vertical and fore-aft planes, only permitting pitch, it does not prevent a small degree of left-right movement and yaw.
 
Such a simple analysis cannot be made for the current bikes, as their design is not as elegant! In essence a small degree of left-right movement and yaw is permitted by the front and rear rubber mounts.
 
  
 
Finally:
I hope this leads to improving understanding of the way our rubber-mount bikes work and why the original design is actually a very good one, only let down by leaving out a vital part. Later bikes are also flawed, so none of our rubber-mount Touring bikes are perfect, or as good as they could be.
 

The End 

 
 
 

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Reader Comments


I had heard about the Motor co leaving out the third link that Buel had in his original designs.
you cant help but wonder did they think that their way was smarter or was it simply a cost saving?
God only knows how many lives have been lost due to "bean counters" and their desire to raze profits.
I look forward to more reports, keep up the good work.

Tom
ogden, UT
Monday, May 12, 2014
Editor Response Tom,

You asked for it and we delivered, Here is Part 2.

http://www.bikernetbaggers.com/pages/5Ball_Bagger_Project_Bike__The_Infamous_Harley_Wobble__Part_2.aspx
Krash Kranzler

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