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Old 05-19-2015, 03:45 PM   #1
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Lowering for fuel economy

This is something I have always wondered about. The airflow underneath the car has a significant effect on drag. If you are lifting a jeep or a truck, you always lose MPG because it makes the car even less aerodynamic. Lowering a car seems should have the opposite effect, but I've never seen any real world results of if it works out in practice, and if so how much.

I'm curious if anyone has seen a documented before/after test on how lowering affects MPG. I would be interested in seeing the results. Thanks in advance
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Old 05-20-2015, 12:03 AM   #2
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There are similarities between cars built for performance, and those built for economy. I myself was going to make a thread to discuss such things. I have considered lowering mine by about 25 to 30 mm, but the increase in insurance would make any fuel saving benefits pointless. It would definitely make a difference though, especially if you were to fit a full underbody tray to make the undercarriage smoother. Never seen any fuel results though.
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Old 05-20-2015, 07:11 AM   #3
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Going to start with a real basic aerodynamic primer. cD is coefficient of drag and is a basic measure of how much resistance airflow over the vehicle has. The lower, the less resistance, and IIRC, a brick has a cD of 1. The other important figure is the frontal area, that is the front on profile, of the car. Multiply the two and you the coefficient of drag area(cDa). That is the number you actually want when comparing different cars' aero dynamics.

The best illustration of how these values work together is to take a motorcycle and a tractor trailer. While big and boxy, the truck and trailer actually have a lower cD than the motorcycle. This because the motorcycle has the rider's knees, elbows, and head sticking out along with its own bits. Even a rider crouched and tucked in on a sport bike still has plenty of lumps to catch the wind*.

But the truck has a much larger frontal area. With its tiny profile, the motorcycle and rider end up with a much better cDa. So, while there is plenty of spots for the airflow to 'grab' a hold of on the bike, the smaller frontal area means there is less total air it has to push out of the way. The truck is slipperier in regards to airflow, but has more air that it needs to push through. In the end, the bike uses less energy moving through the air than the truck.

Which is how lowering a car will improve its aerodynamics. In addition to reducing airflow under the car, lowering it also reduces the frontal area. That should improve fuel economy. There maybe a point of going too low is bad, but don't the eco trims of cars you guys get in Europe usually lower the car a bit from the basic model. I know most of them do use a belly pan to some degree.

The real question, if the goal is to reduce fuel use, is whether the cost of lowering the car is worth the modification? There are other benefits to doing so, and reasons beyond cost for reducing fuel use, but if you want to save money, it likely isn't worth it.

Cleaning up the airflow underneath may be a better investment, and it can be done quite cheaply with an air dam vs. the belly pan, if you don't mind some small screw holes in the plastic lip beneath and behind the front bumper. On my previous car, I took a length of that black plastic garden edging that I had laying around, cut off the tube top, and screwed it to that lip. It was an HHR. I figured I couldn't hurt the aero much, and little improvement would help.

A belly pan would be best, and even the placement of the airdam isn't ideal, but it is cheap and easy to do. Which makes it a good test on whether investing on the pan or a better dam is worth it. It is also 'low profile' in terms of appearance. If the experiment doesn't pan out, you only have some screw holes in a piece of plastic that most may not even know is there. The holes from the tiny sheet metal screws I used actually had mostly filled back in when the compressed plastic had re-expanded.
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Old 05-20-2015, 10:29 AM   #4
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Most of the eco focused cars here have small aerodynamic tweaks and additions, mine included. They might seem like insignificant details, but small changes can make big differences. You'll notice the large rear spoiler on mine, and next to it, two smaller fins on the side. Further down you have two tiny fins sticking out by the rear wheels. These actually divert air away from the car, which not only affects aerodrag, but also keeps the rear of the car very clean. The underpan only really covers the bottom of the engine, but I guess it protects from water and road debris as well as helping make the bottom of the car smoother. Not many cars come with roof bars now, they actually did some tests and found that unused roof bars can use up to 30% more fuel than a smooth roof. Huge difference there. I don't think the eco models are lower, but they certainly have smaller wheels with narrower tyres which I guess would lower them a little, and of course have less rolling resistance than a large rim.
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Old 05-20-2015, 10:32 AM   #5
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I should add that now engine technology seems to be peaking, manufacturers are focusing on vehicle weight, and are coming up with ingenious ways of making cars lighter and stronger. My car, despite being larger than the previous model is 100kg lighter, that's 220 lbs, that would be a huge improvement in a race car. As I said, performance and economy are related, the faster you make your car, the quicker it will be!
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Old 05-20-2015, 03:02 PM   #6
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Originally Posted by Draigflag View Post
There are similarities between cars built for performance, and those built for economy. I myself was going to make a thread to discuss such things. I have considered lowering mine by about 25 to 30 mm, but the increase in insurance would make any fuel saving benefits pointless. It would definitely make a difference though, especially if you were to fit a full underbody tray to make the undercarriage smoother. Never seen any fuel results though.
Lowering your vehicle increases your insurance premium/rates?
That's crazy! Better handling should cause your rates to go down! lol.
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Old 05-20-2015, 03:10 PM   #7
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Yep, insurance companies are very specific here. Anything that affects the looks or performance, they want to know about. And as soon as you start adding bits, wheels, exhaust, lowering springs, they put you in the "boy racer" group and your premium will rocket! My brother added springs to his car, it put his premium up by 400, about $600! I guess it's because if the jobs not done properly, or parts are fitted incorrectly, and you cause a major accident as a result, your insurance company will not be happy, and they will find any excuse not to pay.
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Old 05-20-2015, 11:06 PM   #8
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Interesting - much to the wifes annoyance, i have mentioned the potential of lowering the Hyundai....! I hadnt considered an increase in insurance costs. I do though have the black lip that could have a home made air damn bolted on, but as my brief attempt at touching up stone chips was less than invisible, dare i risk disfiguring my car further?!
I wouldnt know where to go for a belly pa, i have neither the skill nor facilities to attattempt it myself.
Off thread, but i did look at some beautiful half moon chrome hubcaps,but was worried they wouldnt stay on - or dilute the bland, completely stock look i am going for!
I'd also wish to blank some radiator grills, for both heat and aero benefits, but again, can't even begin to imagine how i d do it without it looking hideous.
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Old 05-21-2015, 05:39 AM   #9
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RAISING your right foot a few mm helps a lot too.
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Old 05-21-2015, 08:40 AM   #10
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RAISING your right foot a few mm helps a lot too.
We need a like button.

As to insurance rates, when I looked into a Civic in 2005 it was almost $400 more a year to insurance than my other choices. The company probably just assumed that it was going to be modified because I was male.

Yes, good mods will make it more manuveurable, but just having them let,s the insurance company assume that you'll drive riskier. That might actually have statistics to back it up.
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