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This article discusses the adjustments that can be made to a front fairing with a body sock. (These observations were made using a Treklite Super Zzipper and body sock.) Particular attention is paid to how the different settings have advantages and disadvantages depending on the conditions and the riders needs.
The following adjustments can be made: the top and bottom can be raised or lowered independently; the whole fairing can be moved forward and back; and the body sock can cover more or less of the bottom and sides, or can be partially or entirely removed.
The Body Sock
The body sock, for all practical purposes, is a single stretchy piece of fabric that attaches to the edges of the fairing with Velcro. It is just long enough to be pulled over the back of the seat and stretches out to cover the rear fender. The bottom is snapped to the centerline of the bike at one point under the seat. Normally there is a gap of about 10 inches in the front so it is easy to get your feet down without having to think about it. However, the sides can be attached lower down so the gap in front is reduced to zero. This leaves only a couple of inches clearance for the front wheel, and you have to be careful to stick your foot in the opening when you come to a stop. Under normal riding conditions the body sock doubles the improvement of the front fairing. (With head winds or no wind the improvement isn't as much, with side winds it is much greater.)
Handling a bike with a fairing, and particularly with a body sock in windy conditions is dangerous. It takes practice to control the bike reliably in side winds. The effect of truck and vehicle drafts is greatly increased.
The particular adjustment that a rider chooses depends on the riding conditions and needs of the rider at a particular time.
The basic setting has the fairing far enough forward so there is no interference with the handlebars, has the top of the fairing at eye level, and the bottom several inches below the bottom bracket. Changes to this default setting can optimize different characteristics.
The most important thing is reducing the total frontal area. Thus ducking your head down and lowering the fairing accordingly is the first adjustment.
As the fairing is lowered beyond your ability to duck, more of your helmet is exposed to the wind. Assuming the top of the fairing is 18 inches wide, and your helmet is 10 inches wide, then every inch the top of the fairing is lowered reduces the frontal area by 8 sq. inches. On the other hand the helmet, protruding from the fairing's smooth wind shadow, causes more wind resistance. This resistance increases as more of the helmet is exposed. Eventually the increase in wind resistance from the helmet is greater than the reduction in resistance from the reduced frontal area of the fairing. Your best setting will be unique for you on your bike, and can only be found by testing.
In my early tests I found that about three inches of exposed helmet was optimum. So, to go my fastest, I lower the top of my fairing so that three inches of my helmet is exposed when I am hunched down as much as I can.
The adjustment of the height of the bottom of the fairing is similar. I found that with the fairing about four inches below the bottom bracket I attained my fastest speed.
Closing the bottom opening by lowering the sides of the body sock and pulling their bottoms closer together in the front reduces the turbulence (and cooling) inside the fairing, and presumably reduces drag as well.
Least resistance sitting comfortably
When you are on a long ride you aren't likely to stay hunched down all day. Sitting comfortably with your head up will expose too much of it to the wind stream, so it is worth raising the top of the fairing so the wind just hits the top of your helmet
Sometimes, in order to keep the human engine comfortable, you have to cool off. In this case you can lower the top, raise the bottom, and adjust the body sock so most of the bottom is open. In really hot weather the sides of the body sock can be partially detached, or fully detached from the fairing, tucked between the seat and the rider, and used only as a rear cover.
On other occasions keeping warm is important. In this case the top is raised, the bottom is lowered, and the body sock is lowered, so its bottom edges are pulled together with just enough opening for the front wheel. A cover can also be positioned over the top opening between your chest and the handlebars.
In these conditions a full covering would be useful, but then fogging and your ability to see become problems.
Rain and Night
These conditions put seeing at a premium. It is impossible to see clearly through a wet or fogged fairing, thus, lowering the top so you can see over it is necessary.
At night there are many internal reflections both from your own as well as external lights, and looking over the fairing is necessary.
While the different adjustments can be significant, finding the ideal setting is subtle. When two bikes have been adjusted so their coast down speeds are matched then these subtle variations can be easily compared, and their significance is obvious. (When two bikes are matched at 15 mph and one rider sticks a hand into the air stream the other bike begins to pull ahead distinctly.)
It is much more difficult to find your best settings alone. Typical bike computers may not be able to resolve the differences of minor adjustments, and normal variations between runs may be greater than the effect you are measuring, making many multiple runs necessary.
Just putting a fairing on your bike will give you a significant speed increase and will improve your comfort in cold and wet weather, but if you want to get the most out of the extra weight you are carrying around, then careful adjustment and testing is important.
Copyright by Joseph Huberman 1995
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