Go Back   EcoModder Forum > EcoModding > Aerodynamics
Register Now
 Register Now
 

Reply  Post New Thread
 
Submit Tools LinkBack Thread Tools
Old 03-03-2020, 12:48 PM   #1 (permalink)
Master EcoModder
 
Join Date: Jun 2011
Location: Earth
Posts: 591
Thanks: 27
Thanked 145 Times in 113 Posts
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?

Thanks

  Reply With Quote
The Following User Says Thank You to sgtlethargic For This Useful Post:
aerohead (03-04-2020)
Alt Today
Popular topics

Other popular topics in this forum...

   
Old 03-03-2020, 01:16 PM   #2 (permalink)
Master EcoModder
 
freebeard's Avatar
 
Join Date: Aug 2012
Location: northwest of normal
Posts: 27,562
Thanks: 7,738
Thanked 8,554 Times in 7,041 Posts
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.
__________________
.
.
Without freedom of speech we wouldn't know who all the idiots are. -- anonymous poster

____________________
  Reply With Quote
The Following User Says Thank You to freebeard For This Useful Post:
aerohead (03-04-2020)
Old 03-03-2020, 07:43 PM   #3 (permalink)
マット
 
M_a_t_t's Avatar
 
Join Date: Nov 2015
Location: Indiana
Posts: 718

The Van - '95 Chevy Astro Cl V8 Swapped
Team Chevy
90 day: 7.84 mpg (US)

The new bike - '17 Kawasaki Versys X 300 abs
Motorcycle
90 day: 71.94 mpg (US)

The Mercury - '95 Mercury Tracer Trio
Team Ford
90 day: 34.35 mpg (US)

Toyota - '22 Toyota Corolla Hatchback
90 day: 40.11 mpg (US)
Thanks: 131
Thanked 258 Times in 188 Posts
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?
__________________
1973 Fiat 124 Special
1975 Honda Civic CVCC 4spd
1981 Kawasaki KZ750E
1981 Kawasaki KZ650 CSR
1983 Kawasaki KZ1100-A3
1986 Nissan 300zx Turbo 5 spd
1995 Chevy Astro RWD (current project)
1995 Mercury Tracer
2017 Kawasaki VersysX 300
2022 Corolla Hatchback 6MT

https://www.youtube.com/channel/UC6s...LulDUQ8HMj5VKA
  Reply With Quote
The Following User Says Thank You to M_a_t_t For This Useful Post:
aerohead (03-04-2020)
Old 03-03-2020, 10:28 PM   #4 (permalink)
Always Too Busy
 
Flakbadger's Avatar
 
Join Date: Apr 2012
Location: Oregon
Posts: 566

White Lightning - '17 Nissan Leaf SV
Team Leaf
90 day: 159.47 mpg (US)
Thanks: 405
Thanked 190 Times in 134 Posts
Quote:
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!
  Reply With Quote
The Following 2 Users Say Thank You to Flakbadger For This Useful Post:
aerohead (03-04-2020), Fat Charlie (03-07-2020)
Old 03-03-2020, 10:35 PM   #5 (permalink)
Always Too Busy
 
Flakbadger's Avatar
 
Join Date: Apr 2012
Location: Oregon
Posts: 566

White Lightning - '17 Nissan Leaf SV
Team Leaf
90 day: 159.47 mpg (US)
Thanks: 405
Thanked 190 Times in 134 Posts
Quote:
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?

Thanks
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.
  Reply With Quote
The Following User Says Thank You to Flakbadger For This Useful Post:
aerohead (03-04-2020)
Old 03-04-2020, 01:02 AM   #6 (permalink)
Moderator
 
Vman455's Avatar
 
Join Date: Feb 2012
Location: Urbana, IL
Posts: 1,935

Pope Pious the Prius - '13 Toyota Prius Two
Team Toyota
SUV
90 day: 51.62 mpg (US)

Tycho the Truck - '91 Toyota Pickup DLX 4WD
90 day: 22.22 mpg (US)
Thanks: 198
Thanked 1,797 Times in 937 Posts
Quote:
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:







__________________
UIUC Aerospace Engineering
www.amateuraerodynamics.com
  Reply With Quote
The Following 2 Users Say Thank You to Vman455 For This Useful Post:
aerohead (03-04-2020), Flakbadger (03-04-2020)
Old 03-04-2020, 11:46 AM   #7 (permalink)
Master EcoModder
 
aerohead's Avatar
 
Join Date: Jan 2008
Location: Sanger,Texas,U.S.A.
Posts: 15,861
Thanks: 23,922
Thanked 7,207 Times in 4,640 Posts
stability

Quote:
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?

Thanks
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.
__________________
Photobucket album: http://s1271.photobucket.com/albums/jj622/aerohead2/
  Reply With Quote
The Following 3 Users Say Thank You to aerohead For This Useful Post:
BamZipPow (03-10-2020), Flakbadger (03-04-2020), freebeard (03-04-2020)
Old 03-04-2020, 08:11 PM   #8 (permalink)
Moderator
 
Vman455's Avatar
 
Join Date: Feb 2012
Location: Urbana, IL
Posts: 1,935

Pope Pious the Prius - '13 Toyota Prius Two
Team Toyota
SUV
90 day: 51.62 mpg (US)

Tycho the Truck - '91 Toyota Pickup DLX 4WD
90 day: 22.22 mpg (US)
Thanks: 198
Thanked 1,797 Times in 937 Posts
Quote:
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--

Quote:
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.
__________________
UIUC Aerospace Engineering
www.amateuraerodynamics.com
  Reply With Quote
The Following 3 Users Say Thank You to Vman455 For This Useful Post:
aerohead (03-21-2020), Fat Charlie (03-07-2020), Flakbadger (03-04-2020)
Old 03-04-2020, 08:47 PM   #9 (permalink)
(:
 
Frank Lee's Avatar
 
Join Date: Jan 2008
Location: up north
Posts: 12,762

Blue - '93 Ford Tempo
Last 3: 27.29 mpg (US)

F150 - '94 Ford F150 XLT 4x4
90 day: 18.5 mpg (US)

Sport Coupe - '92 Ford Tempo GL
Last 3: 69.62 mpg (US)

ShWing! - '82 honda gold wing Interstate
90 day: 33.65 mpg (US)

Moon Unit - '98 Mercury Sable LX Wagon
90 day: 21.24 mpg (US)
Thanks: 1,585
Thanked 3,555 Times in 2,218 Posts
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!
__________________


  Reply With Quote
The Following 3 Users Say Thank You to Frank Lee For This Useful Post:
aerohead (03-25-2020), freebeard (03-04-2020), wdb (03-07-2020)
Old 03-04-2020, 09:36 PM   #10 (permalink)
Master EcoModder
 
freebeard's Avatar
 
Join Date: Aug 2012
Location: northwest of normal
Posts: 27,562
Thanks: 7,738
Thanked 8,554 Times in 7,041 Posts
Agreed. What worked for me was an 1 1/8th inch sway bar on the back, but none on the front.

__________________
.
.
Without freedom of speech we wouldn't know who all the idiots are. -- anonymous poster

____________________
  Reply With Quote
Reply  Post New Thread


Thread Tools




Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2024, vBulletin Solutions Inc.
Content Relevant URLs by vBSEO 3.5.2
All content copyright EcoModder.com