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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.
I do not claim to be an expert, but I am
impressed with the general publicís understanding of
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.
in Cars and Fuel Economy: A tip for saving gas.
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
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.
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
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
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
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.
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.
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
how it works. But it is almost as useful
as the protection I get in bad weather. Truly a win-win
The Reactions of Other Motorcyclists
Inventors must have a very frustrating life.
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
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.
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.
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.
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.
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.
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.