Guzzi Practical Suspension Setup Tricks for Road Bikes: Steering Dampers


  
Ah , the lonely steering damper… It’s one of the most misunderstood components of your steering system. What the heck does it do? Do you need one if your bike doesn’t already have one? If you have one, how do you adjust the darn thing for optimum performance?

    First of all, you need to know some background on the vibration modes of a motorcycle: specifically, wobble mode (i.e. the “tank slapper”).  Wobble can be described as a self-perpetuating vibration of the front fork assembly about the axis of the steering stem. A number of variables like tires, loads, and fork geometry affect this mode. Maintaining stability while providing a quick response to steering inputs is the basic tradeoff in designing the geometry of a front end. Thus, bikes that steer more quickly are more susceptible to wobble, as they operate closer to the borderline between stability and instability of the front end. Also, wobble stability can change with speed, and load. Variables such as wind gusting, vehicle speed and wheel rotation also affect wobble, and certain combinations of these variables can cause a dramatic loss of stability.

    The problem with wobble is that it’s a very lightly damped mode. The only physical damping comes from friction in the steering head bearings, and the hysteresis of tires and other dynamic elements. This damping is very slight, thus quick-steering bikes require the use of an external steering damper.

    First of all, how does one determine the degree of wobble stability of a motorcycle at a specific speed and load? For this quantification, we will use our old friend the impulse test.
Caution: perform this test at your own risk. I claim no responsibility for your actions. Disconnect your steering damper, or adjust it to a minimal setting. Bring your bike up to a constant speed on a straight, flat surface. To hold the speed constant, I use a throttle lock of some sort: my Lemans IV has a throttle friction screw, which works fine. With the bike moving straight ahead, and with your hands off the bars, whack one handlebar very quickly in the same plane that you would normally apply force to the handlebars to steer the bike. That is, give it a not too large, yet firm and quick input *whack* in a plane perpendicular to the steering axis. It may take some experimentation to whack it just right. You are basically exciting the wobble mode with an impulse input. If the steering borders instability, a very small steering input will produce a large wobble. If the steering is moderately stable, you may see a few oscillations before they die out entirely. If the steering is very stable, there will be no wobble at all.  Note: prepare to catch the bars in case your steering is unstable! The degree at which the front-end oscillates for this test tells you your wobble stability for the test conditions.  Now repeat the test. Try varying the speed in 10 MPH increments and observe how the response changes. If you normally carry a lot of load, put it all on the bike and perform the test again.

    Why does the impulse test work? An ideal impulse input has an infinite amplitude, yet an infinitesimally small time duration. The quick “whack” to your bars approximates the impulse input. Theory dictates that an impulse input has a flat frequency response- it excites all frequencies at the same level. Associated with the wobble mode is a specific natural frequency, which is excited by the impulse. The response of the front end to this input is to oscillate at the wobble natural frequency. If this vibration mode is very stable (as it is for most Guzzis and bikes with a lot of trail for most operating conditions), the oscillations will die out on their own. If the vibration mode is marginally stable the oscillations will take a long time to dissipate. If the vibration mode is unstable the oscillations will grow until your front end oscillation hits both steering stops!

    You may also want to perform this test while decelerating. This places additional weight on the front wheel and therefore the vibration of the front-end figures more prominently in the control of the bike.
Now dial in some damping from the steering damper.  Repeat the impulse test. If your bike showed signs of instability before the damping was added, crank up the damping until your impulse test generates an output wobble that quickly dies out. I’d say that 1 or 2 oscillations of the front end are tolerable. More than that and I’d feel a little uneasy.  By adding damping in this way, you can tune the instability out of your steering systems while maintaining a quick response to steering inputs.

    So when does this steering damper tuning matter the most? How about when you’re leaned over in a canyon around a 25 MPH turn with a passenger and camping gear and you have to grab a handful of brakes to avoid a family of yaks that are crossing the road. You will say a prayer of thanks to the steering damper gods then.
If you had a data acquisition setup and a few accelerometers, you could measure the steering gain and stability numerically from the impulse test responses. This is how the major manufacturers develop their bikes to handle well. The wobble mode impulse test is standard practice for evaluating steering response.

   If more people performed this test on their bikes before hitting the back roads, we would hear fewer horror stories about accidents and near-accidents due to tank-slappers. Unfortunately, this is a litigitous society that we live in and there is some amount of skill required to perform this test. Manufacturers are not about to instruct owners to excite a potentially dangerous vibration mode of their motorcycles for fear of someone misinterpreting the information. It is, however, important for riders to be aware of the issue of adequate wobble stability.

Guzzi power.
Ed Milich