Originally Posted by grbrown

Introduction:

OK chaps, sit up straight and pay attention, we’re going back to school! There will be coffee and cake for those who stay the course.

I’ve lost count of how many times I have answered questions about ‘bagger wobble’ on HDF, so this is a description of the rubber mounting system in our Touring bikes, from an engineering perspective, in the hope of improving understanding of what the wobble is and why it happens. I make no apology for this being a long thread, but you can always print it out and read it in bed or in the small room!

It goes without saying that the following assumes a bike in tip top shape. Let us also be clear that many Harley owners have probably never experience the so-called wobble and are blissfully happy with their ride, so for them there is no problem. If you read this and are wondering what on earth it is all about, do not allow yourself to be suddenly gripped with fear!

Let us start with the original rubber-mount system introduced in 1980 and cover the 09-on bikes later.

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 allowed 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 to replace them on older bikes still fitted with cleve blocks.

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 us 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 design of rubber mount 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 the engine speed ones 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 trans. Along with the rear wheel this makes up a substantial single Assembly, which is mounted into the frame at three points, of which more shortly. 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.

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.

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, or pitch.
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 from moving sideways at that point, or yawing.
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, or rolling.

In all three cases motion is free moving, although the design of the front mount limits the total amount of vertical movement. This is an elegant engineering solution and a kinematically sound design, to be applauded. Unfortunately there is a reservation - there is no stabilizer, to support the rear rubber mounts and prevent lateral movement.

Wobble 2:

The lack of a rear stabilizer 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 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. The rubber used is stiff and only yields a little. It also is only able to compress to a small degree due to the design, but it is significant and too much.

2009-on:

While Harley continue 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 have a stabilizer.

Frankly, from an engineering perspective, Harley appears to have abandoned the fine principles of the original design. There are many reasons why Harley changed the frame, of which the engine mounting system is only one, however they could have retained the original design and simply added that vital rear stabilizer.

If you sometimes wonder about the handling of your lovely new bike, you have every right to. 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 into the frying pan!

Wobble 3:

It cannot be denied that the 2009-on set-up gives improved lateral stability, but it doesn’t cure it. Both front and rear rubber mounts can compress under lateral loads, although the degree of misalignment this causes is improved over the older design.

Stabilizer Kits:

Harley inadvertently created a market opportunity for the custom market, which many brands have filled. The better designs provide lateral support to the rear rubber mounts on earlier bikes, and both front and rear mounts on later bikes would be nice. By providing suitable lateral support, the kinematic principles of the original design are fulfilled.

On both versions of bike, the steering is made 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 suitable lateral stabilizers.

Finally:

I hope that goes some way 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.

Any questions?!