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Old 11-03-2008, 11:54 AM   #1 (permalink)
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wheeldams - ideal average on production cars

wheel dams started to appear on cars a couple of years back and now almost all cars seem to have them is some form.
they seem simple enough to retrofit, but theres little info available on how big they need to be...

i've been looking at a lot of examples on real cars as they pass me or sit in a parking lot, but afterwards it's difficulty to say how big they where. and since i don't think most people would like to find me on my knees with a tape measure stuck in their wheelwell, i took a different approach


what i did is did a google search for some cars which i remember to have these dams and which i believe all have pretty good Cd's...

i tried to find profile pictures of either the front or the side to avoid perspective distortion and than used the free inkscape vector drawing software to import the picture and use vertical lines to measure the distance of the center of the bub to the ground and measured the height of the bumper and the height of the dam from the center of the hub down.

this gave me some percentage relative to the radius of the wheel, and lo and behold, despite using different cars the figures seemed to be strangely consistent!!!

B gives the percentage of the tire radius the bumper projects below the tire axle
D gives the percentage of the tire radius the wheel dam projects below the tire axle

i did a quick and dirty measure on some pics so i don't know each model exactly... but the numbers speak for themselves


Code:
model	         B	 D
bmw1		30    	55
Bmw 7 hidrogen	38	53
audi		42	58
opel insignia	42	57
mercedes	41.5	58
prius		37	57

average		38	56


the conclusion i draw is that roughly 56% of the tire surface below the axle should be covered with a dam. or, the distance between the bottom edge and the ground should be 44% of the tire radius.

because the different shapes the bumper height is more difficult to determine, i always used what seemed to be the lowest point, so at the front of the car some of these bumpers might be quite a a little higher. also this value was measured in front of the tires and not the middle which on a lot of cars is even higher

yet reasonable to assume sticking to these figures will give you something which could be fairly well dimensioned while not running the risk of the average curb or elevation doing any damage to it

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Old 11-03-2008, 12:39 PM   #2 (permalink)
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Technically you have an average % of vertical distance covered on a selection of production vehicles, not an ideal size Still awesome work though

It'd be great to be able to find an ideal size/shape/surface area, but that'll vary from vehicle to vehicle and driver to driver. (how much of a curb do you expect to climb over anyway?) The wheeldams on my car appear all but useless. Once my MPGuino gets assembled (one of these days dangit!) and I have some time I'd love to play with the airdams to see what, if any, benefit they have before I haul off and just fair the suckers.
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Old 11-03-2008, 01:22 PM   #3 (permalink)
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Quote:
Technically you have an average % of vertical distance covered on a selection
that's right,sorry if i misuse certain terms here and there...it's just one dimention, but than again, most dams i've seen where a little less wide than the wheels... i assume that's becouse they stay withing the overall bumper shape, and because the way the air separates of the edges wauses the dams to virually enlarge themselves a bit.

i had half expected a more inconsistent picture so i'm quite pleased with this.... the relation to the tire radius got me thinking...

in the past i tried to calculate the forward speed a the tire surface in the quadrant where it's exposed to the airflow...

since the bottom of the tire touches the ground, at that point there's no forward component to the motion (the only time that happens is when you slam the breaks so hard you start slideing)

So below about 56 % of this quadrant the surface of the wheel is actually moveing slower forward that the rest of the car. A dam in front of it would always have more drag as even a better Cd would be overruled by a slower speed and therefore less subject to aero drag.

i'm sure there's other forces at work here, but i think extending a dam beyond lets say 58% is a no-go...

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