Author Topic: Against The Wind / Turbulence Is Not Our Friend (by Elvin Rivera)  (Read 6590 times)

Offline elvin315

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http://www.youtube.com/watch?v=PmrkY-EZy74# <-- site may not be work safe.

We all have to face the fact that our chosen mode of transport is inherently unstable. It balances on two very small contact patches coupled with a high center of gravity. Regardless of the pretty fairings, even on the race replica bikes, motorcycles are aerodynamically very dirty. The fairing, windshield, and rider combine to catch crosswinds and every stray gust like a sail. With that high center of gravity balanced on those 2 small contact patches, faired motorcycles tend to get upset by stray wind gusts quite easily.

Just the act of moving through still air causes an air wake just as a boat creates one in water. We make our own turbulence and pull it around with us in the form of drag. The faster we go the more drag we create. This drag acts like a parachute and tugs at the bike as it moves along. That's just the way it is. I think that turbulence is part of high speed touring and I just deal with it.
http://www.exa.com/images/applications/motorcycle_ribbons_back.jpg
http://www.exa.com/images/applications/bmw_motorcycle_touring.jpg

Even on a day when Nature's winds are calm we still get pushed around. Man-made turbulence from trucks, SUVs, vans, and cars compound our troubles. And you wonder why your Connie feels unsettled? Not having 4 contact points with the road like a car makes us more susceptible to being wobbled around our axis of motion. Below are links with animation and graphic representations depicting the turbulent wake created by a tractor/trailer rig and an automobile.
http://www.exa.com/images/animated_gif/Scania_soiling_med.gif
http://www.exa.com/gifs/horiz-vmag-6-deg-yaw_small.gif
http://www.exa.com/images/animated_gif/E39Touringsm.gif

You can see the areas of high pressure build in front of the vehicles as well as the low pressure areas behind. The jumble at the back is the drag  that can suck you in if you venture too close. Same thing in the hollow beneath the trailer. The best way to deal with a big rig is to pass it as quickly as possible. Treat SUVs and Mini-Vans the same way. The air rejoining at their rear is a mix of air pressures. While not as disturbing to a motorcycle as a big rig's multiply it by 10 cars in traffic and you have wobbles.

The type of bike is a big factor. A V-Twin cruiser with its raked out fork, and no bodywork to speak of, is stable by design and resists being steered by the wind. A faired sport bike with quicker steering has its fork set closer to vertical. This allows external steering forces like bumps and winds to upset the bike. The Concours, a touring bike with sport aspirations, lies somewhere in the middle. George Young has found a way to attach a steering damper to a Concours but before going that route I would go over the suspension setup and do all the free stuff before throwing money around.
http://web.ncf.ca/ag136/steeringDamper.htm

Free stuff like tire pressures. The Concours likes them higher (40psi front/42psi rear) than Kawasaki recommends. Put 25psi in the rear shock to begin and add or subtract until you reach the proper sag, then play with the rebound damping. By the way, the faster the tires spin, the more the contact patches shrink. Centrifugal force causes the tire's carcass to expand outward thus changing the cross section and thus the contact patch. This leaves less rubber in contact with the road. Not exactly a good comparison but think of a speedboat transitioning from slow cruising on its entire hull to high speeding on the hull's step. The boat skips and hops along the waves. The smaller contact patch of the tires does much the same as it encounters cracks, patches, and other road irregularities. This also makes the tires more liable to shift their "line" on the road when the bike is caught by crosswinds and traffic turbulence.

Some recommend removing the foot scoops but at the cost of roasting your tootsies in the summer. I refuse to believe that they are the cause of this perceived instability. They could be a factor at ultra high speeds but at legal speeds their affect can't be that great.

Everyone has a different comfort level when it comes to winds so I don't think there is any one answer for all. What everyone should do is check their bike and eliminate the bike itself as a source of this instability. I say look at the suspension setup and check for loose or broken motor mounting bolts. Tire pressures, steering head bearings, and fork & rear shock preloads and damper settings all affect the bike's stability. All the pieces are there and it doesn't cost anything to play with these adjustments. See the wobbles FAQ for details:
http://forum.concours.org/index.php?topic=45119

Riding technique comes into play too. A riding position that places too much weight on the grips amplifies steering inputs so that steering adjustments aimed at countering the turbulence from a truck you're attempting to pass could tend to exaggerate it. A light grip on the bars is often better. A white knuckle death-grip on the bars passes minute steering inputs to the front wheel. Don't lock your elbows. Keep them slightly bent. This also prevents unwanted steering. Duck down to reduce your profile to the wind. I don't mean riding like this for hours, just during those stronger gusts or when passing a truck. Just twist the throttle and bust through it.

Turbulence at helmet level not only shakes your head but the shoulders it's sitting on. Again since the shoulders are attached to your arms it will cause extra inputs to the steering. A shorter shield, believe it or not, places your helmet in a smoother flow of air. A taller one shades the helmet from the buffeting but creates a longer lever for the wind to play with. The spinning wheels create a gyroscopic balance that will help the bike cope with the outside forces trying to upset her balance. We just have to trust her.

Modifications like fork braces, stiffer fork springs, and steering dampers will help lessen, but will not eliminate, the wobbling. Wind and turbulence are just a fact of life on a motorcycle. It may sound flippant but if truck turbulence bothers you, don't follow trucks. Pass them. I find that the wind upsets the bike less if I turn up the throttle. Gets those gyroscopic forces cooking.

Something that gets overlooked is the affect of pushing that barn door of a fairing through the air. At highway speed, pressure builds up in front of the fairing and the bike can't cut though it cleanly or push it aside as quickly as it did around town. Instead of the bike pushing the air from its path, the air compresses and resists, and pushes the bike back. (Newton's Law of Action & Opposite Reaction) Like a leaf fluttering from side to side on its way to the ground, the motorcycle will shake and shudder. Its subtle but it's there.

The bags add aerodynamic drag too. My bike is equipped with a GIVI E460 trunk. I feel no wobble or buffeting with the trunk mounted that isn't there without it. You can cut the shield down, way down, to reduce your frontal area and gain some cooler air to the chest as an added bonus. The vented windshield systems like the Rifle or Cee Baily pass the high pressure air ahead of the shield into the cockpit. That reduces the violent spillover of high pressure air into the cockpit. Honda molded holes in the front of the original CBR900RR fairing for the same reason.

Sorry to say but the bags add aerodynamic drag. The link below has pictures showing the high pressure air pooling ahead the bags. Remove the bags and the airflow will smoothen. These pictures are billed as windtunnel tests but I've never heard of one with windows and curtains. Still they illustrate the airflow over and around a Honda ST1100 and a BMW K1100RS. Notice where the smoke pools.
http://staff.bath.ac.uk/ensmjc/Research/Motorcycles/aero.html

The first page contains a graphic representation of air pressures on a motorcycle/scooter (the poor eggheads couldn't make up their minds). Since motor-whatchamicallits don't ride themselves maybe next time the researchers should include a virtual pilot. Notice the turbulence in the cockpit. The cockpit turbulence could conceivably move your shoulders, inducing those movements down your arms and into the handle bars. The second page shows the turbulent air as red clouds around a BMW.
http://www.fluent.com/about/news/newsletters/02v11i2/a6.htm
http://www.exa.com/images/applications/K40-Multi-right-top-front.jpg

For more on motorcycle aerodynamics go to:
http://www.rubbermag.com/news/0509/050901_01n.html
http://www.tonyfoale.com/Articles/Aerodynamics/AERO.htm
http://www.sportrider.com/tech/146_0106_aero/index.html