Windshield
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Motorcycle Windshield Modifications
March 2002

I am not happy with any motorcycle windshields available today.  Is it too much to ask for a windshield that provides protection from rain, bugs, and cold, without excessive noise and turbulence, without blocking my view, and without adding so much wind resistance that my average speed is reduced by frequent gas stops?   (Update: 2007.  Apparently the state of the art has moved forward since 1992.  This year Mary Ann got a Suzuki Burgman 400 with the standard windshield.  Up to at least 120 kph, this windshield seems free of vices such as buffeting or noise or back pressure.  So at least at moderate speeds it is possible to make a good windshield.  However I cannot figure out why the Burgman works so well. Perhaps it is the combination of a narrow design and a small pressure vent at the bottom.)

 

Factory BMW Windshield Raised

My BMW K1100LT has an electric windshield. I like the adjustability, but it still has problems. I drew some airstreams to show the excessive turbulence created by the air stream coming off the top edge of the almost vertical windshield. At freeway speeds, the buffeting feels like a hockey stick hitting your helmet.  The red stream is warm engine air. When lowered, there is a bit less buffeting, but more noise. I have not shown it, but there is a back draft with this windshield.


Custom Air Foil Lowered

I replaced the factory shield with a custom screen mounted like an airfoil.  Air can pass above and below the screen, with less turbulence as a result.  When lowered, warm air from the engine is forced down and away from me.  This is almost like riding without a windshield. There is no back draft to push your head forward.


Custom Airfoil Raised

Even a very small airfoil can give excellent protection in the raised position shown in this diagram.  Again turbulence is reduced by splitting the air stream in front and behind the airfoil screen. 

More Ramblings on Windshield Design

Although other riders may not be affected the same way, I also need a windshield that does not produce the strong back draft that eventually causes a knife-like pain between my shoulder blades. 

 I donít care what it looks like - if a windshield is available that meets all my requirements, I will have it on my bike. And I would be willing to change motorcycles to get it.

Aerodynamics

I do not claim to be an expert, but I am really not impressed with the general publicís understanding  of aerodynamics.  PEOPLE, WE DONíT NEED STREAMLINED VACUUM CLEANERS AND TOASTERS!!.

Windshield design is complicated.  There are other requirements besides simply cutting through the air efficiently. Most people think they know what is streamlined and what is not.  So why then are so many trucks wasting gas with permanent fixed rooftop wind deflectors?  Those wind deflectors increase the wind resistance every time the truck is driven but not pulling a trailer as high as the deflector. 

This is what NASA said was a low drag truck, back in the seventies. Notice the pointy end is at the back of the truck. NASA and the U.S. government did more research to come up with realistically useful devices for trucks to make them more fuel efficient.
Somehow NASA's good intentions got translated to this, which is not a particularly good shape to save gas.  Actually it is wasting gas!  I wonder why so many gas wasting ideas are popular?  I am just cynical enough to think that oil companies would be involved in promoting gas wasters posing as gas savers.
And speaking of so called aerodynamic devices, has one of these things ever been useful at under 180 miles per hour?  I think it's time to kick the marketing people out of the wind tunnel.

Aerodynamics in Cars and Fuel Economy: A tip for saving gas.
Here is the latest question being debated among car drivers: do you save gas by turning off the air conditioner and lowering the windows? If you google this question, you will come up with a lot of opinions, mostly that with the windows up and the air conditioner running you will save gas.  I even saw this "fact" announced on TV "news" programs.  I don't believe in oil company conspiracies, but then I do wonder who planted this idea.  Because not only is it false, it is based on a false premise.  And you end up using more oil if you believe it.

I only saw one website that actually did the research to show the comparison of fuel efficiency.  They found that at 65 mph, both air conditioner and windows were about dead even.  But at lower speeds, windows down wins easily.  My informal study, based on many cars with no air conditioning, proves that you don't need to roll down all the windows all the way to the bottom  at 65 mph and more.  I guess about two inches would be enough.  And that it would save about 10-20 percent of your fuel bill if you just turn off the air conditioner and crack the windows open.  Better yet, don't buy an air conditioner and save over $1000 on the purchase price, and don't worry about the salesman telling you you will never be able to resell the car.

By the way, for people opening the windows at high speed, it is important to understand how the air flow works. First, opening the side windows will drop the air pressure in the car.  That follows the principle that air moving at high speed past an opening lowers the pressure (Bernouille's Principle).  The back of the car does not have high speed air moving past it, and so if you open a back window, a lot air will be sucked in the back and go out the side windows.  If you have a station wagon or hatchback with a leak in the back door, air will be sucked in there, and bring with it exhaust fumes. The fumes are potentially deadly. Roll up the windows if you smell gas and get the leak in the back door fixed. 

Next tip, just for comfort.  If you open the windows a little at high speed, you sometimes get buffeting which is uncomfortable on the eardrums (similar to what happens on a motorcycle with a big windshield).  To even out the buffeting, try rolling all the windows down a little, it's usually better than with just one window open.

Motorcycle Windshields
When the moving air meets the windshield, it has to accelerate to flow around it.  The bigger the windshield, the more the acceleration.  This aggravates the turbulence as it comes off the top of the windshield where the high pressure, high velocity air meets the relatively still, low pressure air on the other side. When riding on a windless day at a speed of 100 kph, the wind off the top edge of the windshield may be coming at 140 kph. or more.  Turbulence is caused by the air stream having to make a sharp directional change at the edge of the shield where both a high pressure differential and a speed differential exist.

Of course, all windshields are tipped backward. But if the windshield was tipped forward, there would be less turbulence at the top, as the air would try to force its way under the bottom instead.  Nobody does this because ultimately the air could lift the front end of the bike off the pavement.

Without any windshield, you get very little turbulence. However, you would still experience some turbulence as the wind  flows around your helmet (or head if youíre not wearing a helmet).

I have discarded bits of windshield lying all over my garage. It took me more than 5 years of testing to come up with a design that works for a motorcycle,  but I am a bit surprised at how easy it was to build a windshield that beats the factory products. What I did in 5 years, a factory should be able to in 5 days.

Just to fill in my background, I am a touring rider and I use a full face helmet and I always wear earplugs.  I drive in cold or hot weather. I go long distances. I drive fast but not insanely fast.  My solution simply will not work for some people like sport bike riders or Harley choppers or beanie helmeted riders. I often ride two up, but  my passenger probably does not benefit much from my custom windshield. However, she is not too annoyed by my windshield design, and still goes for long distance rides with me in hot or cold weather without complaining much. 

Turbulence problems and typical solutions.

Apparently some independent wind tunnel testing has been done on K1100LTs. Here is a link to a scientific article about noise on a K1100LT: www.isvr.co.uk/reprints/mcyclesioa.pdf

I found the above link in July 2007, but it confirms some of my ideas, and specifically that the air coming over the top of a windshield is louder than air behind the windshield or with no windshield.

The most common way to deal with the turbulence problem is to simply increase the size of the windshield which pushes the high turbulence area completely over your head. Unfortunately, this causes problems of increased wind resistance and poor visibility in bad weather.  With too much wind resistance you may find you need to make lots of gas stops when going fast against the wind.  In bad weather, a dirty windshield will be blocking your view.  And for me personally, I also have a shoulder pain to deal with behind this type of shield.

BMW's electrically adjustable windshield deals with the problem of visibility.  You can lower it in rain to come just below your line of sight.  But with their setup you get turbulence in both up and down positions.   And actually, if itís not raining or cold you donít usually have much need for a windshield.

A theoretical solution to turbulence is a Ďlaminar flowí setup with air scoops at the bottom of the windshield.  The scoops are supposed to help reduce the vacuum behind the windshield which, in turn, will help calm the turbulence.  The scoops also send air up the inside of the shield so that the speed differential is not too great at the top edge of the shield. These do not work very well in practice and I think it is because the scoops are all too small.  You need to get enough air through the scoop to almost match the speed of the air on the outside of the windshield, and itís hard to make a scoop big enough to do that.

My Solution.

As show in the pictures above, I have adapted a BMW K1100LT electric windshield to meet my needs.  I mounted a smaller shield and tilted it back so that it had a very large opening at the bottom.  Amazingly enough, the large opening does not reduce the effectiveness of the shield, but it does significantly smooth out the turbulence.  The windflow goes up over my head and I can easily see over the top of the shield.  There is only a small back draft when this  shield is raised, and none at all when itís down.

As an additional bonus, my modified windshield helps keep me cool in hot weather.  With my windshield raised on the BMW K1100LT, I feel heat coming off the engine.  Then when I lower it, I get cool air all the way down to my knees.  This was a completely accidental discovery and I still do not understand how it works.  But it is almost as useful as the protection I get in bad weather.  Truly a win-win compromise.

The Reactions of Other Motorcyclists

Inventors must have a very frustrating life. The pleasure of using a well designed windshield has to be experienced to be believed.  But not one of the three people who have ridden my BMW wanted to touch the windshield button to see the difference it made when up or down. I was puzzled at the reaction of other motorcyclists too.  They will come over to take a closer look at my windshield and inquire as to what it does.  When I explain, the reaction is a polite version of  ďIt makes your bike look like crap.Ē  Apparently most people prefer looking good to comfort (not to mention hearing loss). 

To me, what works good usually ends up looking good.  If that was not true, we would all be riding bikes covered up to resemble horses.

Fabrication

I originally wanted to make a nice looking bracket made out of machined aluminum billet to mount my windshield. But it proved too difficult for me to make it. Now I think the best way would be to replace the entire BMW bracket with a new machined part.  My prototype windshield makes no modification to the bike, and has lasted for many years and has gone though wind gusts that were almost strong enough to lift up the bike.  I am reluctant to change it now that it has worked so well for me.  Here are some pictures to help explain how I made it.  If you can find a way to make it look good, please email me some pictures.

IM001085.JPG (30099 bytes)I cut off the top part of an old motorcycle windshield, and it is 45 cm wide by 22 cm high. It is 1/8 " (3.2 mm) thick plastic.  3.2 mm is strong enough to resist flexing on this size windshield, I tried a thinner material and it vibrated alarmingly in the wind. You can see two pieces of reflective tape on the bottom of the windshield.  They have no function other than maybe increased visibility at night.  You can click on the thumbnails for a close up view.

 

topscrew.JPG (8497 bytes)This is the left top screw. I found two long screws for the top bracket holes, and two short screws for the bottom bracket holes.  All four had the correct metric thread to fit the BMW windshield bracket.  The long screws in the top went straight in through the windshield, and I used a big rubber grommet underneath the windshield to allow the windshield to pivot.  I used a washer on top to protect the windshield.

 

IM001079.JPG (77765 bytes)For the bottom hole, I fabricated a bracket out of steel by bending it. I did not do it scientifically, I just bent it until it was right. One end of the bracket bolts to the BMW bracket with my metric thread screw, the other end to the windshield with any suitable nut and bolt.  I use washers and rubber grommets to protect the windshield from cracking.

 

IM001089.JPG (30786 bytes)Bottom view with windshield in lowered position. This bracket has low wind resistance and is very strong.  The windshield itself is much lighter than the BMW shield. and I think it has also has lower wind resistance.  There is a plastic protector strip along the bottom to conceal the cut I made with a saber saw.