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Old 03-03-2020, 12:48 PM   #1 (permalink)
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Aerodynamics and stability

I believe I've heard on this forum that aerodynamic cars have stability issues. Is that true? How so? What can be done about it?

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Old 03-03-2020, 01:16 PM   #2 (permalink)
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All moving vehicles have stability issues. I presume the assertion is that making aerodynamic improvements decreases stability? I'd need to see a citation on that.

Lowering will improve stability, barring ground strikes. Airdams anecdotally improve stability. NASCAR roof fences spoil lift when you're sideways.

I can't think of a counterexample.
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Old 03-03-2020, 07:43 PM   #3 (permalink)
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The only thing I've noticed with my car (not really that aero actually) is that it gets shaken very easily. If I'm behind a pickup or full size van at speed I can feel as I enter the turbulent wake, its not subtle either, and seems to have gotten worse after I put my tail on. The car is only ~2300 lbs though so being light(er) definitely affects it.

Theoretical talk though there is too many variables. Just like if you simply ask if xxx will improve on my XXXX car, you can't really apply general rules and expect great results (mediocre-good at best). If it is built in a wind tunnel it will probably be planted on the road and be fine. If you do it like most of us do on this forum I think there is always a chance of making the car handle worse.

Maybe a question in a question: If your vehicle is already turbulent and you pull into a turbulent wake from a semi, will you feel it as much since your car was already in turbulence?
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At 55 mph, a 10% drag reduction translates to a 5% increase in fuel economy. At 70mph,a 10% drag reduction translates into a 6% increase -Phil Knox (Aerohead), Aerodynamics Seminar #2
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My other hobbies have no ROI (making things go fast). So I'm having fun modding my car for better mileage and the side effect is that I save money on gas day to day.
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Old 03-03-2020, 10:28 PM   #4 (permalink)
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Originally Posted by M_a_t_t View Post
The only thing I've noticed with my car (not really that aero actually) is that it gets shaken very easily. If I'm behind a pickup or full size van at speed I can feel as I enter the turbulent wake, its not subtle either, and seems to have gotten worse after I put my tail on. The car is only ~2300 lbs though so being light(er) definitely affects it.
Touching on only this point, my retired Yaris would shake all over the place when entering the wake of a large vehicle, my Nissan Leaf (Almost a full US ton heavier) seems mostly unperturbed and unfussed. The Yaris was definitely more streamlined than the boxy Leaf is, but also much much lighter, and I am pretty sure it was solely the weight that made the difference.

I would be willing to bet in a hand-wavy sort of way that having a vehicle lighter in mass per unit length will cause it to experience greater aerodynamic perturbations. So though the mass of your vehicle is the same, you are lighter per foot of length.

It's also worth noting that a boattail increases the length of the vehicle that can be affected from sideways winds, such as the vortexes and messy spirals of air. That extra few feet on the back end of the vehicle add a lot of "lever" to the fulcrum of your rear tires and probably contributes a lot to your lateral stability too.

These are my thoughts, and full disclaimer I might be completely full of it!
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Old 03-03-2020, 10:35 PM   #5 (permalink)
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I believe I've heard on this forum that aerodynamic cars have stability issues. Is that true? How so? What can be done about it?

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I mean if you look at a regular bicycle vs a velomobile, you'll find that the aerodynamic fairing on the bike makes it vulnerable to lateral winds, which is why most velomobiles are tricycles instead of bicycles. Here's a quote from the wikipedia article on them:
"The fairing on a velomobile sometimes makes it more subject to cross-winds than a similar unfaired cycle. The effect of cross-winds is complicated because the force of the wind can act as a steering force, as-if the rider had tried to steer the cycle. "Wind steer" can be a safety issue and may also hurt performance, as a serpentine path is longer and thus slower than a straight line. Thus, a design with inferior aerodynamics may be better overall. For example, it is common for time-trial bicycles to use a solid disk rear wheel for best aerodynamics, and a spoked front wheel that has worse aerodynamics than a disk, but is less likely to steer the bicycle in a cross-wind."
Something like a boattail that increases the length of the vehicle substantially could very well affect lateral stability from crosswinds, but I feel like the majority of mods centered on streamlining will not have a negative effect, since most of them focus on the front profile of the vehicle and decreasing the trailing wake.
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Old 03-04-2020, 01:02 AM   #6 (permalink)
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Originally Posted by Flakbadger View Post
I mean if you look at a regular bicycle vs a velomobile, you'll find that the aerodynamic fairing on the bike makes it vulnerable to lateral winds, which is why most velomobiles are tricycles instead of bicycles. Here's a quote from the wikipedia article on them:
"The fairing on a velomobile sometimes makes it more subject to cross-winds than a similar unfaired cycle. The effect of cross-winds is complicated because the force of the wind can act as a steering force, as-if the rider had tried to steer the cycle. "Wind steer" can be a safety issue and may also hurt performance, as a serpentine path is longer and thus slower than a straight line. Thus, a design with inferior aerodynamics may be better overall. For example, it is common for time-trial bicycles to use a solid disk rear wheel for best aerodynamics, and a spoked front wheel that has worse aerodynamics than a disk, but is less likely to steer the bicycle in a cross-wind."
Something like a boattail that increases the length of the vehicle substantially could very well affect lateral stability from crosswinds, but I feel like the majority of mods centered on streamlining will not have a negative effect, since most of them focus on the front profile of the vehicle and decreasing the trailing wake.
Dan Empfield (founder of Quintana Roo) has a lot to say about TT bikes--specifically, it's area in front of the steering axis that makes bikes susceptible to cross-wind instability. Even then, it's not as dramatic as people make it out to be; I run a 90mm deep rim--the deepest wheel you can buy today, surpassed only by the old Blackwell 100--on my TT bike here in the windy Midwest and have never had any problems.

Back to cars: Adding length at the back of the car with a boattail should improve stability--that added area is moving the center of pressure backward, which increases the tendency of the car to self-correct by steering back toward the direction of a crosswind. Bonneville cars incorporate tall fins (where class rules allow) for just that reason:







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Old 03-04-2020, 11:46 AM   #7 (permalink)
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stability

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Originally Posted by sgtlethargic View Post
I believe I've heard on this forum that aerodynamic cars have stability issues. Is that true? How so? What can be done about it?

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Hucho basically says not to concern yourself with it,if you're driving posted speeds.
'Streamline' cars look like a wing section from the side.Urban legend and street mythology has it that these shapes are inherently unstable at speed.What's not known is,that all wings have a zero-lift angle-of-attack.You can verify this looking at Abbott and Von Doenhoff's book,Theory of Wing Sections.There's not a wing known which does not have a zero-lift condition.And they all occur at the 'drag minimum.'
For cars though,were talking about streamline bodies of revolution.Typically,they're so 'rounded' in cross-section,the pressures bleed such that uniform pressures exist for the entire length of the vehicle.
Spirit of Ecomodder.com developed 30-lbs front downforce and 22-lbs rear lift at 130-mph.Lift-related instability is essentially meaningless! I've never experienced any issues with yaw either.At 108-mph she's rock solid.Bam ZipPow's been even faster with Dark Aero.
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Old 03-04-2020, 08:11 PM   #8 (permalink)
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Originally Posted by aerohead View Post
Hucho basically says not to concern yourself with it,if you're driving posted speeds..
Chapter 5, "Directional Stability," is 70 pages long in the 4th edition of Aerodynamics of Road Vehicles, and covers a lot of ground.

I vehemently disagree with this assessment of what Hucho* "says" (and if you think he does say that--produce it!). Quite the opposite: the chapter begins--

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The flow around a vehicle not only leads to drag but also causes other aerodynamic forces and moments (as components of the resulting wind force and wind moment) which affect driving stability. At high road speeds, their influence on driving comfort can be felt and, in extreme cases, safety is affected.
*Since Hucho edited Aerodynamics of Road Vehicles and most of the book, including the chapter excerpted here, are not his words, I'm using his name as I assume you are--as a stand-in for the book itself.
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Old 03-04-2020, 08:47 PM   #9 (permalink)
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Yeah, I'd say aero stability is a factor at street legal speeds. Drive an old Microbus in some stout gusty Midwest crosswinds, throw in some passing semi's too and you'll agree. Then put some ballast near the front, to get the Cp aft of the Cg (think of shooting an arrow backwards) and like waving a magic wand, the 'Bus settles right down! In the case of Microbusses that occurs at SUB-legal speeds!
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Old 03-04-2020, 09:36 PM   #10 (permalink)
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Agreed. What worked for me was an 1 1/8th inch sway bar on the back, but none on the front.

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