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Old 01-25-2013, 06:09 PM   #431 (permalink)
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Originally Posted by hotrodfeguy View Post
This all makes a ton of sense, and I will give you the wind tunnel it would be money in the bank. All of your data is spot on gentlemen !!! But in real world how much is to much? My point is this, lets say your doing 55 and have a 30MPH side wind. It kinda goes to $hit, the long side, boat tail then becomes a negitive impact on areo. Not trying to be a killjoy. Just that I see some holes in some of this. Why double the side area or even at 25% of the car to gain, front to back areo? cause when you introduce a side wind it then becomes more area for drag. look at the car form angles like a diamond if you will, It should not be areo view from just one angle. wind soes not blow form one direction.

On my Driver metro I was going to do a kamback with no sides like a naked edged Kamback to get data then put sides on it to see if I have a real issue here with sidewind or not. I know the more improvements I have made(flat hubcaps, mirroe delete passenger side, wheel well covers, grill block, front air dam) the more the car tends to get shoved off the road by cross wind. And at the last few pages thats where this 22* may be affected, In my humble opinion it should be more agressive to cheat or be more universal to cross wind. I know there is a perfect forula for front back, but I am saying maybe in daily winds it should be more general purpose?

Maybe I am all out in left field. I could be totally wrong.
The first thing is you are not driving the vehicle sideways into a crosswind, there is no argument regarding crosswind effects, but you also need to consider the overall effects of crosswinds.
Remember, even without the tail you have a wake behind the vehicle that contributes it's own forces to vehicle handling, as soon as a crosswind hit's it blows the wake out and the vehicle rear jerks in response. There have been lots of reports of improved crosswind handling with boattails, this is probably due to the fact that the boattail has a smaller cross sectional area than the wake that was present without it.

There is a significant issue with getting too aggressive on rear taper as you may only get the added weight penalty without any aero benefit, you need to do a lot of reading around on things that have been done to get a gauge of how far you can go.

It seems to me this thing works in stages,
1 / Stuffing the wake, basically any "closed" volume in the rear, any shape within the template profile will reduce the size and intensity of vortexes, this may have some benefit to Aero by increasing the pressure in the rear.
2 / Short aggressive taper, this utilises the tendancy of the air to wrap around the vehicle, the airflow will seperate fairly quickly depending on how aggressive it is, but it will give the benefit of reducing wake area and also taking the turbulance away from the rear of the vehicle if a cavity construction is used. There is a guidline ratio of cavity depth to minimum width of around 12%, any less and the turbulance is still pulling on the rear surfaces, any more is of no further benefit.
3 / Borderline steeper template profile, can work very well, but is prone to seperation in adverse conditions, so creates wavering as air attaches and dettaches.
4 / Template, 22 and other rules observed is almost 100% guaranteed to give a positive aero result consistantly.
5 / Less taper, will perform well too, but is more than required, and some aero benefits have been forgone in not using a steeper angle, extra weight is added as well, but extra skin friction is so minimal it is a non issue.

Interesting your observations on crosswind handling, in my experience, I have gained improved handling, but the FE gains have been so minimal that I don't consider an improvement in that area yet. I feel that they are all part of a jigsaw puzzle and at some point I will have enough pieces in place and the FE benefits will fall into place.

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