I’m hoping to get some input from the aero gurus for a for-fun side-project I’m working on (designing airfoils that attach to my square Thule roof rack cross bars to improve aerodynamics), but this also seems relevant to discussions of lots of current bike designs.
For my project, the size of the bar cross section is a fixed limitation (the fairings have to fit OVER the cross section of the bar), and for frames that conform to UCI rules the 3:1 ratio is a similar limitation. So, overall is it more aero to complete an airfoil shape that has a higher width-to-depth ratio or to truncate an airfoil shape that has a lower width-to-depth ratio? It seems from Trek’s SC white paper that they think the truncated airfoil is faster, but I would guess that Cervelo disagrees or they’d be truncating their tube shapes.
Additionally, for my project, yaw isn’t much of an issue because the fairings will we positioned horizontally and wind will never be coming from below or above. (Or should I be thinking about how the car is deflecting air and slightly adjust how the fairing is positioned to account for air not flowing perfectly horizontally?)
To restate the main question, which of the below shapes would be more aero? (The rectangles inside the airfoils represent the cross section of the bars and the truncated airfoil has a light gray tail showing the shape of the original airfoil before truncation.)
Also, obviously I could complete the truncated airfoil since my roof rack isn’t subject to any regulations, but I want to keep it reasonably sized and limit material costs (I’ll be 3d printing these in sections in ABS plastic) and I’ve decided that about 120mm depth is as much as I want to put on. They’ll be shelled out with a 3 or 4 mm wall thickness and sliced in half lengthwise with the two halves placed onto the bar and held together by inset screws.)
Full airfoil is certainly faster. Trek’s truncation is faster for shapes conforming to 3:1 ratio, which you are not bound by.
But the full airfoil of a relatively fatter airfoil? (because in order to have the thickness required for the bars and keep it within 120mm I need to use an airfoil whose width is about 28% or 30% of its depth) The truncated airfoil is an airfoil shape whose width is only about 18% of its depth. Maybe the right question to ask is how much the width-to-depth ratio of an airfoil affects its aerodynamics. How much slower is a full 3:1 airfoil compared to a full 4:1 or 5:1 airfoil?
The fact that the airflow is screwed up a ton from the air bouncing around/off the top of the car makes the environment very unstable. Classic teardrop shapes and truncated ones are probably not the most efficient. I would just make it flatter with a tapered tail and not get too obsessive over it.
By the way, John Cobb once suggested a quick fix to aerodynamics - Wrap tape around the object so that the leftover tape hangs off the back like a tail. Gives huge aero gains without much fuss. Good for bike cables.
Without having a definitive answer to the specific question (and doubting most anyone else does either, save a few posters), I’m going to go with an across-the-board engineering answer to whether X or Y is better:
It depends on the situation.
So in this case, especially with the close coupling to the boundary layer on the roof itself, any symmetric foil design may be incorrect. Given truncated or completed airfoil in this case, I’d choose the truncated. It looks less likely to break/get damaged on the rack.
Air flow over the top of the screen and along the roof line is relatively clean.
Look up wool tuft tests on line or images showing vehicle aero tests with smoke… The mess is at the rear the roof panel at the point of separation.
Doing this will not make a deal of difference although wind noise may decrease… Just due to the relative cross section of the rail bars relative to the rest of the vehicle and bike frontal area…
Also the bike with the front wheel out for example, the approach angle if the bike is all to bollocks so it’s aero is also worse than usual…
Now a tension style fabric cover for the bike from forks around bars and to the saddle be interesting … As a fairing of sorts…
All of this can be Avoided though by putting the bike in the correct spot… Inside the car…
Obviously, what is needed here are two small children, two airfoil shapes, and some bungee cords. Strap the children to the roof and have them hold the foils between them. Accelerate to the maximum speed you think you’ll go and scream out the window for them to scream back (above their pre-existing screams) which one feels like it has less drag. Use any money left over from divorce and counseling bills to implement the winner.
Yes, they’re both NACA airfoils. although the truncated one isn’t quite accurately represented (the little point at the very front is fully rounded in the actual airfoil shape. That’ll be corrected before I move forward with the design.)
Obviously, what is needed here are two small children, two airfoil shapes, and some bungee cords. Strap the children to the roof and have them hold the foils between them. Accelerate to the maximum speed you think you’ll go and scream out the window for them to scream back (above their pre-existing screams) which one feels like it has less drag. Use any money left over from divorce and counseling bills to implement the winner.
I just happen to have two children who are approximately aerodynamically equal (a really skinny 6-year-old and a chubby toddler), so this may be the most accurate approach.
Obviously, I’m a total hack here without any aerodynamics training. I hadn’t thought of using non-symmetrical airfoils, but maybe if I make them big enough I could get the car to lift off the ground.
And yes, I recognize that the effect will be small compared to the size of the car, but there is a small but noticeable difference in fuel economy with and without the roof rack, even when I’m not carrying a bike. I agree that bikes are best inside the car, but the whole reason I’m carrying a bike so much is that I drive my son to school (he goes to a school across town, no buses come out as far as we are), then park my car and bike 15 miles to work. My commuter’s a heavy steel touring bike and I’ve had it standing up inside the car (Honda Fit, half the back seat down, fork clamped into a skewer/clamp in the back) next to my son in his car seat. It freaks me out to think about us getting hit from the side and a huge heavy piece of steel flying around in the back with my son. So I put the roof rack on (it was on a previous car and we hadn’t been using it for a while). I’d like to get a hitch rack eventually to get the bike out of the wind and out of the way of garage/bank drive-through ceilings, etc. But I’ve decided not to prioritize the $ for the hitch rack for now and I need to do some tests with a few 3D printers at work, so I thought this would be a fun project to do the tests with.
Anyway, it’s been instructive to read what debate there has been. Looks like there isn’t a super clear answer, especially because, as somebody above said, the truncated one would be more sturdy.
If you’re constraining the lengths and widths of the sections to be equal, then the truncated foil will be lower drag…that’s the whole point of a truncated foil, no?
You’re not listening… Two airfoils with same length and width
Well the OP’s pic doesn’t show two airfoils of the same lenght and width. It shows a truncated airfoil that has the same truncated chord lenght as an other complete airfoil. I feel I addressed that situation in my previous post. If you take two airfoil of the same lenght and width and start truncating one, it will have more drag according to the percentages in the table I posted.
And if you take the OP’s very specific images posted, as long as the steepest angle of the airfoil doesn’t go above ~17 degrees, the flow will stay attached and the non truncated airfoil will have lower drag.
