NACA ducts for your BMW R1150GS

The following was inspired by Ricky's efforts with NACA ducts on the ADVrider forums. I was lucky enough to get a secondhand R1150GS genuine BMW windscreen to experiment with, so I didn't have to worry too much about buggering up the only screen I had.

What, where and why?

99% of riders seem to complain about the stock R1150GS windscreen. I am in that 99%. I read a lot about various screens, and short of going for the full-on popemobile, the general consensus was that it's not an easy problem to fix. NACA ducts are designed to separate the boundary layer of air flowing over a surface. Get the shape and angle right, and the effect will be smooth. Drill these into your windscreen, and you'll get a second layer of boundary air flowing up the back of the windscreen. When this meets the air becoming turbulent at the top lip, it cancels it out and you get a smooth airflow. So goes the theory.

The required disclaimer

Before I start, there's a couple of disclaimers and things you ought to know.
First off, if you bugger up your windscreen doing this, obviously I'm not responsible. I'm not paying for a new item for you if you mess up the holes and ruin a $200 screen. That's your responsibility. Secondly, I'm not going to be held responsible for any medical conditions, like breathing in aerosoled perspex, drilling through your finger or getting your Harley Davidson hair wrapped in a Dremel drill.
Basically, the advice on this page is worth exactly what you're paying for it.....Sounds obvious but these days everything has to be spelled out.

What you'll need

This is a moderately easy task - it'll take you between an hour and 90 minutes. If you're a klutz, it will take longer.
Before you start, this is what I recommend you get:

A spare windscreen
Duh!
A corded Dremel tool with flexible extension and router attachment
The flexible attachment means you don't have the bulk of the drill waving around when you're doing the delicate bits. The router attachment will make the initial hole-cutting a hell of a lot easier. Dremel tools spin fast, but don't have much torque and will bite into just about anything, skipping off course if you're not careful.
A Dremel drill bit, rotary saw bit, sander, buffer and polisher bits
The drill bit will be used to punch through the screen, the rotary saw will be used for just about everything else, and the sander, buffer etc will be used to clean up your mess.
Safety gear
$4.00 from Home Depot will get you a pair of goggles, earplugs and a mask. I didn't use the earplugs, but the mask and goggles are essential. You're going to be melting lexan plastic at high speed and bits of swarf will go everywhere.
A template
Click here to download a .jpg image of the template I created for my ducts. It should be a 600dpi image, and when printed out, the ducts should be about 3cm across (that dimension is marked on the template).
Don't think much of these new satellite dishes...

Some techie stuff - duct size and placement

I don't have a degree in aero engineering, so this whole experiment is based on guesstimation. The end result seemed to work well, so apparently I got it right first time. Well - if not 100% right, lets say "better than a stock GS screen". NACA ducts have a zillion variables determining size, spacing, speed rations, duct angles etc etc etc. I found the diagram below and used it as the basis of my ducts. I reasoned that two ducts, 3cm wide, 3cm apart would do the trick. Why? Because I didn't want one large duct, and any smaller or larger size looked wrong. Yeah - looked wrong. Nothing more technical than that I'm afraid. Most NACA duct designs are for aircraft doing at least 100mph. At that point, changes in design and shape probably make a huge difference. But for those of us on motorbikes who will cruise between 60mph and 80mph 90% of the time, I reasoned that fairly coarse design estimations wouldn't be a real problem.
As for placement of the ducts vertically - again I eyeballed it. I looked at Rickys original posts and guesstimated how far up the screen I should place them. Effectively, I put them as low as I could without jeopardising the structural integrity of the lexan plastic by drilling right close to the edge. You'll see the right duct is quite close to the larger cutout for the headlight. Ultimately, placing them where I did worked well both aesthetically (viewed from riding position and the front) and functionally.

It's all very technical...

Step-by-step

  • Print out my template, tape it to the inside of your screen aligned with the centreline of your screen. As I said above, vertical placement is a bit of guesswork. If you want more than guesswork, and you're using a stock GS screen, align the leading edges of the ducts with the centreline of the BMW logo stamped in the plastic in the centre of your screen. Use a thin, permanent marker to mark out the section to be totally removed, on the outside of the screen.
  • On the left, the victim screen with my template taped to the backside of it. On the right, a closeup showing where I inked the main cutout directly on to the front side of the screen. You can see my ducts have their leading edge aligned with the centreline of the BMW logo in the middle of the screen.
  • I did this with the screen off the bike, propped on a conveniently-sized receiver box. I recommend you do this because if you don't, the swarf will get everywhere and you'll spend the next two days cleaning the bike. Put the drill bit into your Dremel, and drill into the top two corners of each duct template, and somewhere in the bottom corner.
  • Three pilot holes, drilled with the drill bit.
  • Take the drill bit out and put the rotary saw bit in. Attach the router collar to the Dremel and pop the bit through one of the holes. Adjust the router collar height so the lexan plastic you're cutting is about midway up the rotary saw bit. Hold the router collar at the bottom - where it's resting on your screen - and flick the Dremel up to mid-speed. Hang on tight, and trace out the lines inked on the outside of the screen. For ease, I tried to align the centre of the rotary drill bit with the line itself, and trace around the whole thing. Be careful in the corners.
  • The router collar attached, and the first line drilled.
    A closeup of the rotary saw bit poking through the screen.
  • Once you've gone all the way around, the centrepiece will flick out and hit you in the head / eye / toe. Turn off the Dremel and examine your handywork. With any luck, the initial hole will be about the same shape as that inked template you so carefully drew on to the screen.
  • Voila. One hole done, one to go.
  • Do the same for the second hole, starting in one of the drilled pilot holes and working your way around until the centrepiece pops out.
  • Both holes done. Notice how professionally they appear to be cut, with the precise shape matched from one hole to the other. Erm. Well. Sort of.
  • Leaving the rotary saw bit in the Dremel, use the router collar to straighten up any edges that should be straight, and to square off the corners as best you can. This is where you'll find the table leg effect if you're not careful. You'll do one side of one hole, then the other, then neither will match. And that hole won't match the other hole. Problem is, each time you go back at it with the saw, you're losing more and more material. So gently does it. Round the curved edges, straighten the straight edges. It doesn't have to be 100% perfect - it has to be functional.
  • Take the router collar off the Dremel, take out the chuck, the bit and the collet and fit the flexible extension drive, and the extension itself. Put the collet, chuck and rotary saw in the other end of the flexible drive, and find somewhere handy (and close) to hang the Dremel up.
  • The Dremel, hooked to the wall, with the flexible extension attached and the rotary saw in place.
  • Flick the Dremel to about mid-speed again and using the middle of the saw bit at about a 60 angle to the screen, start to chew away the 'scoop' part of the duct at the thin end, on the front side of the screen. This scoop should angle down and back as you can see in the image below. Be careful though. The bit will have a tendancy to pull and skip to the right so you'll need to restrain it. Better, is to start at the right and work to the left. There's a lot of science involved in making these ducts, but eyeballing it is much easier. You're aiming to get a wedge cut down through the screen to the leading edge of the inside of the hole. That little lip should be as thin as possible (marked in red in the second image).
  • Starting to shape the scoop with the rotary saw.
    The red line is the lip on the inside (back) of the windscreen. The yellow line is the start of the 'scoop out' on the front of the windscreen.
  • Flip the screen over and use the same technique to drill out the runout at the back of the duct, on the inside (back side) of the screen. In the diagram below, I've attempted to show the shape you're aiming for from the side, in a cutaway view. It's an extreme close-up of the area you're going to be drilling out, showing the frontside and backside of the windscreen and the basic scoopout and runout shapes you want. The photo below it shows the two holes as they appear in my screen so you'll be able to get an idea what to aim for.
  • The cutting and drilling is finished. Time to tidy them up. You can see the scoop out at the leading edge, and the runout at the trailing edge, which is on the backside of the screen.
  • The rest of the task now is back to the regular Dremel setup with a variety of polishing, sanding and buffing attachments. I went around all the edges to get off the remaining swarf and sharp little bits of lexan left over from the drilling and cutting. I used a brush sander to polish the edges and get rid of the white haze and I used a buffing bit to finish it all up.
  • Removing the sharp edges and finishing everything off.

    The finished item

    The photos were taken at night in my garage with a flash, so there's some odd light and shadow effects going on in those ducts, but you get the general idea.


    So does it work?

    I tried the newly ducted screen in all three positions, travelling into wind, with the wind and across the wind, at speeds up to 90mph. I'm 6ft 2in, 32in inseam with the seat in the high position.
    Screen fully forward. Noise still there, as expected, but turbulence and buffetting gone, High and low frequency turbulence around my lid have all vanished. Standing on the pegs, I couldn't find the turbulence anywhere.
    Screen in mid position. Noise still there, as expected, but turbulence and buffetting gone, High and low frequency turbulence around my lid have all vanished as above.
    Screen fully back. Noise still there, as expected, and higher frequency wind noise now a little more evident. As above, turbulence and buffetting all gone, High and low frequency turbulence around my lid have all vanished. If I lift my head right up and stretch my back, I get into the airstream off the top of the screen but there's still no turbulence.

    Given that my goal with this was to sort out the terrible turbulence I was experiencing, I think it's worked pretty well. The wind noise is slightly reduced, but I use earplugs most of the time anyway, and with those in, it's pretty quiet. I think the next step is to get a newer, more aerodynamic lid, but for now, I can at least ride without blurry vision and head-shaking violence.

    Interestingly, while riding, if I put my hand behind the screen now, just above the ducts, there's a superfast layer of air travelling up the inside of the screen and off the top. It feels pretty weird.

    All images, words and bodily injuries expressed and/or contained on this page are ©2003 Chris Longhurst.