Adding curved form to roof - good or bad??

[Frank Lee]

A roof top addition such as OP described will present more "frontal" area to an x-wind than it will to a straight-on wind (which is exceedingly rare) because of the length of the roof and his stated intention to run it full length if not moreso.

[wyatt]

Let's just say he has aweful attachment on the top of his vehicle, and has to raise the roof 9" to get attached flow. Let's say the length of the roof is 9'. Total additional cross wind area is going to be on the order of 3.25 square feet. This is much less than the area that is added by raising the roof on a van (which is common, and those don't tip over in cross winds), and the edges running back will no doubt be rounded over for sideways attached flow thus mitigating much of the poor handling you are hinting at.
I added a 4' long SedanKamm partial boat tail to my little car and drove it that way for over 3 months. I experienced 60mph crosswinds and saw no ill effects. Jim has added a boat tail to his Insight and driven in 60mph crosswinds with no ill effects (actually improved his cross wind handling). How big is Aerohead's bed cover when viewed from the side? I contend, it is not an issue that makes it not worth pursuing.

[Tesla]

Good to see some interesting discussion,
The vehicle shape is much as per post #7, but mine is the previous model, so is even less streamlined.
From what I have seen in various images and discussions, the windscreen angle is likely a bit steep, so I believe that there is most likely seperation at the windscreen roof junction, but haven't done any flow testing myself.
To improve this, I was thinking a 2 stage process.
1 / build up the bonnet area a bit infront of the windscreen, this would minimise the turbulance around the wipers as well as raise the airflow stream.
2 / add a bit of curvature to the roof as mentioned in the OP
then look at the issues at the rear and adjust to suit.
I think these type of vehicles are ideal candidates for improvements, because they are so bad aerodynamically.
I am also looking at rear wheel arch covers, skirts & partial underbody lining, there is also a small airdam at the front and I may look at this area a bit too, but possibilities are limited as I do go off road a bit and approach angle & clearance are important.
So in conclusion, without appropriate testing the proposal could go either way, which is what I was thinking myself, but I do lean towards a positive result, even with the increase in the frontal area, because it lays good foundations for major improvements in the rear.
Thanks again for all the comments.

[Tesla]

Just to add to the crosswinds issue, I was also planning to have curvature from the sides, so there would not be any large flat areas exposed for major crosswinds. From experience weight on the roof is a much bigger issue than crosswinds.
I suppose to put it in context, the vehicle stands about 7ft high x 7ft wide at base and about 5ft wide on the roof, so as a percentage adding say 7" to the roof height is really not very much as a percentage, whether viewed from the front or side.
Only just occured to me, what I'm thinking of is virtually an upturned boat on the roof, just a very shallow one, maybe thats what I should be looking at as a design guide??

[Cd]

I say go for it !
How easy is this to test eh !
Just slap some thin sheet plywood or other flexible sheeting to the roof, close up the gaps, and test it.

If nothing good comes from it, just remove it.

Easy !

It won't even look all that silly either. Other drivers will just think you have something on you roof rack.

[Frank Lee]

Originally Posted by wyatt

Let's just say he has aweful attachment on the top of his vehicle, and has to raise the roof 9" to get attached flow. Let's say the length of the roof is 9'. Total additional cross wind area is going to be on the order of 3.25 square feet. This is much less than the area that is added by raising the roof on a van (which is common, and those don't tip over in cross winds), and the edges running back will no doubt be rounded over for sideways attached flow thus mitigating much of the poor handling you are hinting at.
I added a 4' long SedanKamm partial boat tail to my little car and drove it that way for over 3 months. I experienced 60mph crosswinds and saw no ill effects. Jim has added a boat tail to his Insight and driven in 60mph crosswinds with no ill effects (actually improved his cross wind handling). How big is Aerohead's bed cover when viewed from the side? I contend, it is not an issue that makes it not worth pursuing.

If the roof is 5w x 9l and the peak of the addition is 9", it could have 3.75 sq ft frontal area and just about anything for side area depending on the ascending and descending geometry... likely guess puts side area at about the same as frontal.

I did not even so much as hint about handling. I was talking about the simple addition of more "frontal" area in yaw, and unlike off the back, we don't have the opportunity to boattail that.

[wyatt]

Before spending time and money on the roof, PLEASE take the time to tape some tufts to the roof and video tape the results. If it turns out that you have poor separation, start out by getting a good baseline by doing some coast down tests as ChazInMT had suggested, that way you will know more definitively if what you have done is worth while.
My friend has a van and we tested it out (he wanted to do much the same thing) and we found that he actually had very good attachment on the roof, and extrapolating from Hucho's findings, we figured we were best off to leave well enough alone. We have tried several varients of boat tails and trailing wake tests on his van. There is much fruit to be had if you will take the time/care to test.
@FrankLee: I think I see what you are saying, the cross wind comment threw me off. And yes, frontal area is a given, but according to Hucho's findings, Cd can be measurably lowered on a sedan by raising the profile of the roof, which could mean that there is larger fruit to be had on a vehicle with a longer roof (especially if it has detached flow). Naturally, Cd and CdA are two different beasts... My friend and I found it to not be worth pursuing in his case.

[Tesla]

Think I'm starting to concur with the general opinion, that it may not be worthwile, will do the tuft test down the track. Took the spirit level out today and ran it over the roof, even though it looks flat, it does have a curve over it, not much, probably 1-1.5", but as said that may be enough to hold the airflow attached.
Working from the front at the moment, have sealed the grill, leaving 2 holes, 2x5" open for radiator. still a couple of small gaps to seal & smooth out.
I have had a variety of under bonnet (hood) temp gauges in place for a while, the ex radiator coolant temp has increased to 60C from 30C, but engine bay temps still the same, so the hot air is still being drawn out effectively under the car.
Once I finalise the front area, including tuft test on the top of the bonnet to determine if there is any value in adjusting bonnet profile, then I will look at the roof again.
Thanks for all the input, much appreciated and it made me think more deeply on all the relevant issues.

[euromodder]

Originally Posted by Frank Lee

A roof top addition such as OP described will present more "frontal" area to an x-wind than it will to a straight-on wind (which is exceedingly rare) because of the length of the roof and his stated intention to run it full length if not moreso.

Straight-on wind will be just as rare in the UK, yet Don-Bur get good results on their bulged trailers.

Properly executed, the idea certainly has merit.
Invest a bit (frontal area) to gain in Cd, and end up on the positive side.
Make the increase in area too large, and it will wash out the positive effect.

[skyking]

as far as drag reduction the math is pretty simple.
For example:
Lets say frontal of 34
Cd of 0.45
Cda of 15.3
after modding, frontal of 37.5
dividing 37.5 into 15.3, we'd need to reduce the Cd below 0.408 to break even.
If it is well done, and then extended into a super smooth short boat tail transition, I think the Cd would be much lower than that. I see money!
Frank, I understand your concerns. Consider this though:
This top will allow for some optimized radii along the sides that will Improve
crosswind performance!