Still trying to dial in the airflow on the front of the bike.
I've added a Gurney flap to the top of the windscreen.
My hope is that it will drag the airflow off the back of the screen down an inch or two by the time it hits me.
I know on a steep angle of attack on a traditional airfoil it will keep the airflow attached to the element decreasing stall speed. However I'm unsure how it will affect the airflow off the back of an element with a steep angle of attack but has basically no airflow on the bottom side.
I'll test it out tomorrow.
My hope is it'll behave in a manor as depicted by this expert illustration.
Any guesses?
My guess is that it will actually redirect the flow so it hits you slightly higher than it does today, and that the effect will become more pronounced as you go faster. Mostly because of this:
https://www.simscale.com/blog/2017/07/formula-student-aerodynamics/
That said, I believe it's tomorrow for you now. How did it end up working?
Robbie
UltimaDork
6/24/19 5:31 a.m.
At least it is easy to test!
If just a test ride isn't good enough you can attach some fishing line from your helmet to the front of the bike and hang some strings for a visual.
In reply to boxedfox :
Yep, you nailed it. It hit maybe an inch higher so not that much but the force of the air was greater and the dead spot behind the screen was, uh, more dead I guess. Without it the air that hit me was softer. I took it off after two hours.
The problem is at 55-60 mph I'm perfectly fine. The air flow is smooth and quiet. But being Texas I'm usually running 75-80 and it becomes loud and violent. And I was in the saddle for eleven hours yesterday. My neck is very sore today.
What kind of helmet do you wear? I had a cheap AFX or something, and was super uncomfortable at higher speeds. Then I got a Shoei and it was night and day better. It wasn't cheap, of course, but not that bad at about $250.
Can you give a bigger picture of the side profile of the bike... and maybe one of you sitting in your riding position it that’s possible?
In reply to Nick Comstock :
Looks like you have a good helmet already then! 
In reply to sleepyhead the buffalo :
I'll try to get one this evening.
I've only heard of one person that did not have a problem with the stock screen. It's really terrible. Something about the shape and/or angles of the front end just play havoc with the air.
I made that little screen out of lexan to be an exact copy of the original so I could test things out. I've played with heights and angles. Using different mounts and modifying the stock mounts. I need to make another one. I've got some more ideas I want to try out.
In the winter I have a big Givi touring screen that blocks everything. I have no wind noise/buffeting with it. Unfortunately no air hits me at all so when it warms up I pull it and the handguards off for some airflow.
I'm just trying to find a compromise.
Nick Comstock said:
In reply to sleepyhead the buffalo :
I'll try to get one this evening.
I've only heard of one person that did not have a problem with the stock screen. It's really terrible. Something about the shape and/or angles of the front end just play havoc with the air.
[...] when it warms up I pull it and the handguards off for some airflow.
I'm just trying to find a compromise.
So, you're trying to reduce the buffeting of your helmet, like you get with the big 'Givi touring screen' (I'll go look that up), while also having some 'slowish' moving air going by your neck (i.e. between your shoulders and the lower edge of your helmet) so you get some cooling flow? I'll guess that we need to keep getting some air towards the cooling inlets on the Shoei... which looks like there's two 'eyebrow vents' ?
Is the buffeting fatigue limited to helmet... or is it also because of the air hitting your shoulders?
In reply to Nick Comstock :
I'm actually wondering what would happen if you went the other way. Basically instead of having the screen flip forwards and upwards with a gurney flap, have it curve down towards the ground in a gentle slope as it comes back towards you. If you can get the flow to stay attached it would slow the airflow and direct it down a bit too.
The downside is that if it works as intended it will also generate some lift. Not sure how much, but I'm guessing it will be enough that you have to shift weight forward to compensate at higher speeds. You'll also probably want to pad the sharp edges at the back somehow so you don't catch it with your hand or something.
In reply to sleepyhead the buffalo :
I have to thank you sleepy head. If you hadn't asked for this pic I never would have realized how badly my jacket billows out making it appear that I have a massive beer belly. Looks like I'll be adding a better fitting jacket to the shopping list soon lol. Judging by the fading it's about time to replace it anyway.
Anyway here is the profile shot.
Under 60 mph I have nothing but smooth laminar airflow hitting the helmet. At about 70 It get noticeably louder but if I straighten my back up and stretch my neck up the helmet gets above the turbulent air and back into the quiet laminar flow. Which is why I was trying to trick the air into flowing more downward once it left the screen.
I don't really want to drop the screen below the instrument panel. I'd be afraid water will get forced inside at highway speeds in the rain. I currently have the bottom of the screen shimmed out about three eighths of an inch sloping the screen back more than stock. I could go a little more but that really starts to put the mounting bolts at an acute angle at the surface.
Here is a pic of my winter setup. What worked well for the old helmet doesn't work at all for the new one so I've been playing around with it.
In reply to boxedfox :
There is a metal bracket that is used to reinforce the stock windscreen that I would have to remove to curve the screen back. But maybe I'll try that.
In reply to Nick Comstock :
I'd also try some unusual solutions like extending the windscreen downwards at the front and gluing on some small vortex generators to the leading edge of the screen to keep it attached to the screen as long as possible. After seeing the size and angle of the screen, I'm not as concerned about it generating front end lift.
In reply to boxedfox :
I've thought about vortex generators. Most applications I see them mounted to the trailing edge. Never occurred to me to mount them to the leading edge.
Another thought, if you still have extra lexan, is to build up something approximating the Givi screen in height and width, and the adding some 1/2” (?) holes maybe 2” above the metal plate. That way you get the “gross blocking” of the big screen, but still get some circulation behind it to cool you off?
Nick Comstock said:
In reply to boxedfox :
I've thought about vortex generators. Most applications I see them mounted to the trailing edge. Never occurred to me to mount them to the leading edge.
I think that’s because VG’s in the auto-realm have been misunderstood/over-simplified. If you place them at the LE, they’ll attach to the surface and develop along it’s angle and will tend to resist turning to follow the free-stream more than ‘regular flow’ would.
can you tape some string to your helmet with a go pro? would like to see if it's your helmet or the air coming off the bike.
I'm interested in seeing what you come up with. My Vstrom buffets my helmet (HJC CL17) a good bit at highway speeds, while my VTX with no windshield doesn't. Vstrom is better if I get my head up a bit higher.
In reply to Nick Comstock :
It's not super common in cars but you see it in aircraft. For example, if you look at the nose of a lot of small planes there are vortex generators a few feet ahead of the windshield. They keep the flow attached to nose as it travels back towards the windshield, which reduces wind buffeting and cockpit noise. Apparently it makes a huge difference on the noise front (several dB as measured from within an already soundproofed cockpit).
Well I pulled the bracket off and bent the screen back. It's a little lower than the instrument panel now but we'll see what happens. There wasn't much room there to make a curve as it hits the mounting plate beneath it.
Well that little change made a massive difference. Much much better at 70.
In reply to Nick Comstock :
Glad it worked. What does it feel like now? Does the airflow hit you a few inches lower as expected, or did it have any other unanticipated effects?
In reply to boxedfox :
It definitely lowered it. Instead of hitting right at the bottom of the helmet it's now about mid chest. But overall it's softer. It feels more like a naked bike. You'll always get turbulence from the vehicles in front but in clean air it's now super smooth and quiet.
Picture three layers, top is undisturbed laminar flow. Middle is super high energy turbulent flow. And the bottom is dead, no flow.
It feels like the middle and the bottom layer have blended together more creating a less dramatic harsh line. Which has also lowered so it's not hitting me right at the bottom of the helmet.
Of course this was in the soft morning air, we'll see what it's like when it's in the high nineties and the wind kicks up this afternoon.
