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Old 08-02-2008, 08:18 PM   #1 (permalink)
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Mid-engine, Rear- engines superior for aero?

I was thinking the other day, since the grille on cars is a aerodynamic nightmare, wouldn't cars with rear engines be better, since a front grille is not needed?
Discuss...

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Old 08-02-2008, 09:07 PM   #2 (permalink)
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i think a better way to look at it is the location of the radiator which is what allows the engine coolant to release heat.
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Old 08-03-2008, 12:28 AM   #3 (permalink)
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The grille is only one part of the whole picture but yes superior aero is more easily achievable with the engine mid or rear mounted. Then again you could say the same for 2 doors versus4 doors versus wagons.

Citroen managed very respectable Cd numbers from their cars which were mainly 4 doors and front engined and front drive as well.
Check the CX , GS models for the 4 doors and the SM model for the 2 door.

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Old 08-03-2008, 10:57 AM   #4 (permalink)
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I think Tourigjm hit the nail on the head. You could theoretically design a front-engine car with the same Cd as a low drag rear engine setup, by using active cooling inlets/radiators on the side of the vehicle, and waste heat ejected into the low pressure wake. Of course the plumbing/ducting would be a nightmare for the front engine version.

Another advantage to rear engine placement is you can adopt a 100% smoth undertray since there is no hot exhaust pipe running down the center of the vehicle that needs to be left exposed.
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Old 08-08-2008, 02:24 PM   #5 (permalink)
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front/rear

Quote:
Originally Posted by RiseAbove View Post
I was thinking the other day, since the grille on cars is a aerodynamic nightmare, wouldn't cars with rear engines be better, since a front grille is not needed?
Discuss...
RiseAbove,in the phil knox aerodynamic photo albums,under Book Illustrations,second row from bottom of 1st page,is an illustration of Walter Korff's zero-drag,cooling system design.It's used in race cars,and a good model for ecomodders.---------------------------------------------------------------One thought with respect to rear engine cars,is center-of-gravity,with respect to aerodynamic center of pressure,something critical to a vehicle's directional stability.Conventional wisdom suggests that you always want the center of pressure behind the center of gravity,which yields high speed terminal oversteer.For front-engine cars,especially front-wheel-drive cars( like the Citroen as mentioned),this is especially easy to accomplish.Moving the engine to the rear can introduce terminal understeer at high speed,and as the front of the car yaws downwind,it catches even more air,increasing the yawing moment,with potentially fatal results.--------------------------------------------Modern monster mpg concept cars with rear engines, were lightweight two-strokes,which helped address the weight bias issue,and will probably never see production.Its true that the rear layout can provide an edge with respect to cooling,however,it may not necessitate moving the engine,only the cooling system as done with Ford Probe concepts,or just idealizing the front system to Korff's recommendations.
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Old 08-08-2008, 02:39 PM   #6 (permalink)
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Quote:
Originally Posted by MetroMPG View Post
Another advantage to rear engine placement is you can adopt a 100% smoth undertray since there is no hot exhaust pipe running down the center of the vehicle that needs to be left exposed.
My exhaust pipe isn't exposed, its covered by a smooth aluminum sheet. I have designed it so there is an airflow running down the exhaust pipe's tunnel for cooling the pipe and the cat.
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Old 08-08-2008, 11:10 PM   #7 (permalink)
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Quote:
Originally Posted by aerohead View Post
RiseAbove,in the phil knox aerodynamic photo albums,under Book Illustrations,second row from bottom of 1st page,is an illustration of Walter Korff's zero-drag,cooling system design.It's used in race cars,and a good model for ecomodders.---------------------------------------------------------------One thought with respect to rear engine cars,is center-of-gravity,with respect to aerodynamic center of pressure,something critical to a vehicle's directional stability.Conventional wisdom suggests that you always want the center of pressure behind the center of gravity,which yields high speed terminal oversteer.For front-engine cars,especially front-wheel-drive cars( like the Citroen as mentioned),this is especially easy to accomplish.Moving the engine to the rear can introduce terminal understeer at high speed,and as the front of the car yaws downwind,it catches even more air,increasing the yawing moment,with potentially fatal results.--------------------------------------------Modern monster mpg concept cars with rear engines, were lightweight two-strokes,which helped address the weight bias issue,and will probably never see production.Its true that the rear layout can provide an edge with respect to cooling,however,it may not necessitate moving the engine,only the cooling system as done with Ford Probe concepts,or just idealizing the front system to Korff's recommendations.
yer over- and under-steers are ternt around.

Having had several rear-engine vehicles, I can say I like the handling dynamics of them- nice, feather-light and responsive steering, not to mention superior traction. From a strictly utilitarian standpoint, though, it is better to have the trunk in the back, both for super heavy and oversize loads. And they are more sensitive to crosswind gusts.
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Old 08-09-2008, 12:20 AM   #8 (permalink)
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[QUOTE=aerohead;52060]RiseAbove,in the phil knox aerodynamic photo albums,under Book Illustrations,second row from bottom of 1st page,is an illustration of Walter Korff's zero-drag,cooling system design.I

The label actually reads "ideal low drag cooling system." I used to be inspired by the North American Mustang aircraft, on which it was claimed that the radiator enclosure had negative drag - sort of like a hot-air jet engine. However, the Voyager team did their own testing, and could not get the cooling drag to be less than 20% of the total on their whole aircraft. One has to assume that they were competent researchers.

The front end provides the most direct flow to the rad, and does not have to disturb the rest of the shape. There may be some existing cars that could be improved by relocating the rad and ducting to the rear in a combined effort to solve other problems - it is not that difficult - VW did it on the wasserboxer Vanagon, in the other direction.

However, I agree with Mr. Lee - a front engine is good for stability, which becomes much more important on a light, streamlined car.
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Old 08-13-2008, 01:55 PM   #9 (permalink)
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Quote:
Originally Posted by Frank Lee View Post
yer over- and under-steers are ternt around.

Having had several rear-engine vehicles, I can say I like the handling dynamics of them- nice, feather-light and responsive steering, not to mention superior traction. From a strictly utilitarian standpoint, though, it is better to have the trunk in the back, both for super heavy and oversize loads. And they are more sensitive to crosswind gusts.
Apologize if I goofed on terminology! How I understand the stability issue is: at road speed,on slick or icy roads,if you get popped with a frontal quartering,or side gust,its best to have the rear tires break traction first,as the car will tend to "weather-vane",turning into the wind,cancelling the yawing moment produced by the gust,while the driver applies opposite lock to correct the" oversteer".---------------------- Should the center of pressure lie ahead of the center of gravity,if popped by the same quartering or side gust,if the front were to brake away first(understeer),the car's front would catch more air,amplifying the yawing moment,tending to compete with the driver's attempts at steering corrections.In the past,when fins were added to race cars,I understand that this modification was to ensure that the C.P. would be firmly behind the C.G.-------------------------- Please help me out on this one,'cause a lot's at stake,and I don't need to be advising people with dyslexic notions.Thanks ,Phil.
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Old 08-13-2008, 03:30 PM   #10 (permalink)
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I've never seen a cross wind that will actually screech my tires sideways causing me to correct. The oversteer/understeer characteristics of a street vehicle are determined by the weight distribution and wheel alignment. 99% of new cars are front engine because all that weight at the front causes the front to break away first so the car ploughs when the driver panics and slams the breaks and cranks the wheel. It is easier to survive a front-on collision, that is why they do this.

I went for a few ride-alongs in a friends MR2 at autocross and he was always battling oversteer becaues the rear end has more sideways momentum in a turn making it more likely to break traction.

As for the original topic, you could make one hell of a sleek frontal shape, top, bottom and sides if there were no ducts there for cooling or intake. A RR car definately has more aero friendly potential. I can't think of a high-speed supercar with the engine up front.

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