The Holy Grail of Air Management
Since the first motorcycles, riders have searched for the holy grail in touring comfort. Riding a bare bike is fun but fighting rain, bugs and vehicle wake…not so much. Adding a shield generates its own problems such as buffeting, noise, heat and back-drafts. Often these problems are severe and the rider ends up trying different shields and deflectors in the search for capsule nirvana. Before discussing cause-effect and solutions let’s define shield categories and design fundamentals.
Fairings and shields fall into two categories, frame or handlebar mounted. The obvious difference is handlebar mounted units turn with the bars and the frame mounted fairings don’t. The subsets of these categories ranges from small sport to large touring. In general the purpose of the sport class is to take pressure off the rider’s chest and provide a low drag minimal capsule. The touring class fairings/shields however protect most of the rider. For now we will mostly focus on touring-class fairings and shields.
Touring class shields are designed to either look-over or look-through. Look-over shields gives the rider a less obstructed view of the road but in many cases lacks the quality of helmet protection offered by look-through shields. The more one moves from look-through to look-over the more critical aerodynamics are or, to put it bluntly, even a billboard makes a still envelope. For look-through riders, air management is more of an issue for their passenger… step seats and being further back can create major issues. The difficulty for sport touring riders comes in making smaller shields perform well.
Design fundamentals are the basics ingredients that go into generating a capsule. They include, but are not limited to, angle of attack, rider/shield proximity, shape or form, internal dynamics and finally external dynamics. I’m not going to deal at this time with boundary layer, laminar flow, wet surface area and parasitic drag. Though such things can be important they are not, for the most part, essential in generating a quality air pocket. It is always good to keep in mind that fairing and bike design should be mission oriented. Often this means one thing to the rider and something else to the OEM design team who’s primary purpose is to sell motorcycles from showroom floors. The industrial purpose of most designs is not to follow function but to give the buyer a perceived value or function. Whether it is a design flaw or mission creep that has gravitated you to this page… I want to thank you for reading this far and welcome you as we continue our search for the Holy Grail of Air Management.
1) Angle of attack: Like a kite or underside of a wing the angle of attack is the deflection angle between the flight surface and the air stream. Many touring shields have pitch adjustment. It seems logical the more upright the shield, the greater deflection and the quieter the capsule but this often is not so. When a shield is pitched forward it does push the air-stream higher but it also moves away from the rider meaning the rider is further back in the capsule. One might not think this is a big deal but anyone who has experienced a capsule close up knows how much quieter it can be. Which brings us to…
2) Proximity: There is an old saying… there is no replacement for displacement. Meaning a little engine doesn’t have the punch of its big brother. With all things being equal this is true but there are always playoffs. And as far as proximity and capsule quality the old truism is inverse…. There is no replacement for less displacement… Meaning the less distance between the rider’s helmet and the shield’s trailing edge the quieter and calmer the capsule.
3) Shape and Form: Angle of attach is usually talking about one axis the underside of a wing for example. Fairings and shields are double and often triple axis forms. Consider the air stream a fluid and it is these ratios of curves that determine how the air is stretched, compressed and deformed in its journey to reunite with its fellow molecules. The process of generating an air capsule is a dirty battle and has no similarity to aircraft that are trying to slip through the air mass while causing as little disturbance/drag as possible. Touring fairings have to open a capsule quickly and if you could see the internal dynamics you would call it more of a war than a negotiation. This is a good way to transition to the next section.
4) Internal Dynamics: Think of the internal dynamics as an engine. The high speed air traveling around and beyond the fairing generates a suction which results in a backwards or counter-flow within the capsule. The rider notices this mostly over the shoulders and neck but the counter flow is happening virtually on all sides of the capsule. This flow is not a smooth and is often punctuated by internal cross flows from induction vents, deflections up through steering wells and off secondary objects such as mirrors.
5) External Dynamics: Everything would be a lot simpler if the air mass was smooth and still but it is often far from that. The only smooth air is in a wind-tunnel or on a desolate back road at 3 in the morning. Not only is the air mass generally an intergalactic storm, the rider also has to deal with cross winds.
Cause-Effect and Solutions:
We don’t have much control over external dynamics. At times switching lanes or increased vehicle spacing lessens turbulence but often we have few if any options. Viewing the capsule and outer air stream as interrelated helps us understand why a smooth capsule is rare. What can we do to manage and improve comfort on long rides? It is not uncommon to buy a taller shield only to discover the problem was not solved by adding more shield. The purpose here is to share our perspective and hopefully help you avoid making the wrong decision.
Where to start. You know something needs to be done when helmet shake blurs your vision but you may not know mild wind noise is causing hearing loss. Our first instinct is to assume the shield is too short but is it? Some test by putting their hand on the shield or try sitting lower to see what a taller shield would do. If the test improve the capsule then a bigger shield should fix the problem… right? After the cost and effort of changing shields the answer is often no and the results disappointing. The test wasn’t conclusive because the rider changed more than one variable. Scrunching down or placing a hand on the shield’s edge usually forces one to lean forward thus reducing the rider-to-shield gap. As for the hand test the hand itself does not represent the same amount of shield extension. The contours and shapes of the hand generates Micro-Swirls which boost the deflection so ironically you may have proven, not that you need a taller shield but a Micro-Swirl generator… Micro-Swirl Edging to be specific. OK that was a shameless sales pitch and the real question should be how do I choose the best solution?
Several air management alternatives don’t relate to the shield. Changing bars or seating can put the rider and passenger in a smoother area of the capsule while adding side deflectors can stop a capsule disrupting airstream. All these tools are useful but often the solution is right in front of you… the shield’s top trailing edge. If the line of sight is six inches or more over the shield then an adjustable deflector can redirect the angle of the airstream. If the line of sight is between two and six inches then Micro-Swirl Edging is often a great solution.
We will be posting more on tuning air capsules so stay tuned. In the mean time please feel free to leave comments, suggestions or corrections via the contact page. All the best to our fellow riders.
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