HB better gas mileage?
- Thread starter dopeSonic
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laborsmith
New member
HB may do marginally better because of a slight weight difference, but both are equally hurt by the brick-like front, which is not aerodynamic, but which appearance I like. Unless traveling at great speed, the inherent advantage of the HB over the sedan will not overcome the front. At least, that is my perception. Only one way to tell, put similarly equipped HB and sedan next to each other and drive them into the sunset and one or the other or both run out of fuel and can go no farther.
Laborsmith
Laborsmith
SonicBoy24
New member
Based on rear ends alone, the sedan would have better MPG.
The greater vertical area on the hatch creates more drag with the low pressure area. This drag makes the engine have to work harder at the same speed than the sedan, where the only low pressure area is off of the small trunk surface. But the hatch does weigh less, whether thats enough to offset the drag difference, idk.
but laborsmith, i have to disagree with you completely. These front ends are much more aerodynamic than many other cars than you will see on the road. The windshields are sloped at a greater andgle, along with the hood, which both decrease drag compared to similar cars. Obviously there is a flat area on the bumper, but thats unavoidable for safety standards, you cant have a pointed surface even though it would yield better mpg.
The greater vertical area on the hatch creates more drag with the low pressure area. This drag makes the engine have to work harder at the same speed than the sedan, where the only low pressure area is off of the small trunk surface. But the hatch does weigh less, whether thats enough to offset the drag difference, idk.
but laborsmith, i have to disagree with you completely. These front ends are much more aerodynamic than many other cars than you will see on the road. The windshields are sloped at a greater andgle, along with the hood, which both decrease drag compared to similar cars. Obviously there is a flat area on the bumper, but thats unavoidable for safety standards, you cant have a pointed surface even though it would yield better mpg.
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TIEfighter
New member
I agree with SonicBoy24, the Sonic is much more aerodynamic than other competitive cars. Take for example the car I owned before my Sonic ('96 Lumina) was not very aerodynamic compared to the Sonic I have now. My mother has an '08 Malibu and the front grill area is flat at certain places and the headlights are more squarish than round. (I don't know if that makes any sense or not, sorry)
laborsmith
New member
I stand corrected. The Sonic Drag Coefficient is 31.5 (I just looked it up) much better than I expected and almost as good as my all time favorite, the SAAB Sonett III which came in at .31, if memory serves.
Laborsmith
Laborsmith
cmoore2004
New member
Awesome, so what I've learned is the sedan is more aerodynamic but it doesn't quite matter because they both get about the same gas mileage. Thanks guys, and that makes me want to go test drive both vehicles right now! Which I might do, because It's my day off! haha.
Be careful, you might not be able to avoid buying one if you drive it. I'm liking mine more every day!
Sent via Tapatalk from my Razr Maxx
TIEfighter
New member
Awesome, so what I've learned is the sedan is more aerodynamic but it doesn't quite matter because they both get about the same gas mileage. Thanks guys, and that makes me want to go test drive both vehicles right now! Which I might do, because It's my day off! haha.
Yeah, I have the Sonic sedan and it slices through the air like a F-35
Hahah. I've driven two, I just need my money to come in already so I can go get one! I really dig the blue topaz pearl, especially since it's a rarity right now... but cyber gray metallic is beautiful as well and is my second option if I can't get the sedan in blue with a sunroof.
RichieRocket
New member
SonicBoy24, for the sedan to only have pressure drag behind the trunk rear surface, the airflow would have to stay attached all the way to the rear of the trunk. It has to be a smooth transition from the roof to rear window and the rear window has to be a shallow down angle, otherwise you'll get seperation at the roof line and the pressure drag area would be basically the same as the HB.
Here is one of the many articles I have read in the past: Browser Warning About halfway down they talk about pressure drag.
EDIT: Mods, why does my link say "Browser Warning"? The link works fine...
The weight is a minor factor for highway mpg. Sure you have to drag that ~100 lbs weight up hills, but the much bigger factor is the aero drag.
I guess if the sedan really did have ~half the pressure drag, then the rated mpg should be better. The EPA test doesn't spend a lot of time at high highway speeds, but if it were true, the sedan should show 1-2 hwy mpg better? But as far as I know, they are the same...
Anybody got a windtunnel we can use?
TIEfighter
New member
No, but if I find a free one anywhere I'll let you know. :banana:
SonicBoy24
New member
there is a smooth transition, and the rear window isn't that severe of an angle. not tryin to cause an argument, but I just finished a physics class. there have even been articles comparing the sonic hatch vs sonic sedan vs spark.
the spark gets bad mpg because it is so short with a blunt windshield and large vertical rear end that the engine has to work harder to overcome the drag. the same is true for the sonics. im not saying that there is a mpg difference, but if there was the sedan would get better if we were talking merely about the rear ends.
SteveSonic
New member
I'm with you on this, Richie. I'm confident that the sedan will deliver a few more mpg's, too. There are only so many things that the manufacturers can do with the rear of cars without scaring the buyers or freaking out the DOT. The Kahmann rear is the compromise that satisfies everyone; the Prius is the refinement of what the CRX brought to the market years ago. The hard chop-off in the Sonic HB and other cars in the segment don't fit the Kahmann formula, however.
RichieRocket
New member
SteveSonic, actually we are not in agreement. I believe the flow over the sedan is seperating at the roof/rear window transition. So the pressure drag would be the same between the HB and Sedan, so the mpg would be the same.
If the sedan really did have 1/2 the pressure drag, then it's highway mpg rating would show 2-3 mpg better? Not sure...
I'm not familiar with Kahmann rear? The prius has a Kammback, named after the German aerodynamicist Wunibald Kamm in the 1930s who researched the smooth tear-drop like shape which is cut vertical. The smooth tapering shape let's the flow stay attached to reduce the area of the pressure drag.
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RichieRocket
New member
SonicBoy24, same here, no argument, no offense, we're just having a discussion. There are many interesting articles on automobile aero out on autospeed.com
For the flow to stay attached all the way down the rear window, the angle has to be very shallow, like 7 degrees. I have not measured it, but the rear window looks much steeper than that. The optimum aero shape is the tear drop. Round on the front, and gradually tapering toward the rear. Of course the super long tail would be very impractical, so Kamm tapered as far back as it could go, then just cut it off.
I agree with you on the spark having a higher drag coefficient (Cd), it's harder to sculpt the aero shape when the car gets smaller. But the car has a smaller frontal area. Riding my motorcycle, I have an incredibly bad Cd of 0.7-0.8, but the frontal area is ~1/3 of a car...
RichieRocket
New member
I spoke to one of my company's aerodynamicists today. The 7 degrees I stated was wrong for this application. 7 degrees is for a diverging air duct (diffuser), so that the duct flow does not seperate from the walls and chokes the flow.
I'm not sure what the exact angle is, need to do more research, but let's compare the sonic sedan to a Kamm-back type rear end, like a prius. See pics attached. I imported the pics into Powerpoint and measured the angles using the line 'size and shape' tool.
Note how the prius is a continuous curve from the windshield all the way back to the tail, with no transitions. The prius rear window starts at 14 degrees and ends at 17deg. The sonic sedan rear window starts at 12deg just after the transition and 22deg at the bottom of the window. So that is a 5deg difference at the back.
Does the sonic airflow stay attached after the roof transition?
Does the flow seperate part way down the rear window?
As with most of these conversations, we end up with more questions than we started with. The proof would be in testing!
Could someone with a sedan do some 'tuft-testing'? ecomodder.com has many write ups and videos. Basically, you tape little strings of yarn all over the rear windo and trunk. Then drive along at speed, have a buddy on the back seat video the tufts. Are they all laying flat? Are some lifting and wagging around? That would be some good info.
I'm not sure what the exact angle is, need to do more research, but let's compare the sonic sedan to a Kamm-back type rear end, like a prius. See pics attached. I imported the pics into Powerpoint and measured the angles using the line 'size and shape' tool.
Note how the prius is a continuous curve from the windshield all the way back to the tail, with no transitions. The prius rear window starts at 14 degrees and ends at 17deg. The sonic sedan rear window starts at 12deg just after the transition and 22deg at the bottom of the window. So that is a 5deg difference at the back.
Does the sonic airflow stay attached after the roof transition?
Does the flow seperate part way down the rear window?
As with most of these conversations, we end up with more questions than we started with. The proof would be in testing!
Could someone with a sedan do some 'tuft-testing'? ecomodder.com has many write ups and videos. Basically, you tape little strings of yarn all over the rear windo and trunk. Then drive along at speed, have a buddy on the back seat video the tufts. Are they all laying flat? Are some lifting and wagging around? That would be some good info.
Attachments
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sonic_sedan.jpg -
prius.jpg
SteveSonic
New member
Please accept my humble apologies for my dimwit response. It was a Kammback that I had in mind when I wrote "Kahmann." A couple of years ago I learned a little bit about the critical angles of rear window/deck sweep that are required to maintain contact flow. So, without doing any more research on the subject, I'll guess that the idea is to extend the tear drop as far to the rear as car design (size/length) and public acceptance will tolerate before the vertical cut.
Steve "Kahmann" Sonic
KGB Pilot125
New member
what if we had a small "vent" cut in the center of the "spoiler" to direct air flow down the back window? Like the old suv rear spoilers that helped keep the rear window clean.
That would break up the flow by sending part of the air down and causing to things one the rear window wouldnt be completely useless after 4 miles on a wet/slushy road and it might break up the air flow and reduce drag
That would break up the flow by sending part of the air down and causing to things one the rear window wouldnt be completely useless after 4 miles on a wet/slushy road and it might break up the air flow and reduce drag
sportyrider
New member
bump
Swordsmith
242hp/252 ft-lbs BNR EFI
I am very interested in this project, I've never had a car whose rear end got so incredibly filthy so quickly, and if this would also boast a mild increase to mpg that would be a bonus. I imagine it wouldn't be cost effective, because any decent looking mod ends up costing a few hundred bucks and you'd never end up making that back at 1mpg savings... but it would still give me that smug "green" glow that any mpg mod does.
