roof bubble on most cars

[winkosmosis]

Originally Posted by Christ

You didn't answer the question, and I suspect that other than on a case by case basis, you won't be able to, either.

In that article they said it reduced the CdA even though frontal area increased.

And why wouldn't it? Drag is a product of frontal area and Cd. If you increase frontal area by 2% and decrease Cd by 5%, drag goes down.

[Christ]

Originally Posted by winkosmosis

In that article they said it reduced the CdA even though frontal area increased.

And why wouldn't it? Drag is a product of frontal area and Cd. If you increase frontal area by 2% and decrease Cd by 5%, drag goes down.

Yeah, unless 2% A is greater than 5%Cd... But I digress; didn't have time to read the article: I'm at work, and I was pre-tripping my van when I replied.

[winkosmosis]

Originally Posted by Christ

Yeah, unless 2% A is greater than 5%Cd... But I digress; didn't have time to read the article: I'm at work, and I was pre-tripping my van when I replied.

How can 2% be greater than 5%?? We're talking strict ratios here right?

If you decrease Cd by half, and double A, the drag stays the same.


Compare a car has a Cd of 0.33 and a flat disc with a Cd of 1.0... The car can have 300% the frontal area of the disc but the same drag because its shape has 33% the drag. 300% * 33% = 1

[donee]

Hi winkosmosis,

Please put a warning before any Scribd links. Something like - "WARNING Scribd Link! > " . That service is so FUBAR that it takes 10 minutes to get rid of their slow loading, useless response time browser pages that spawn over multipe browser instances and get on with something useful.

Thanks.

[Christ]

2% increase in A Of 100= 102.
5% decrease in Cd of .31 = .304

100*0.31= 31
102*0.304= 31.008

In this instance, by the numbers you suggest, you've not helped anything.

[winkosmosis]

Originally Posted by Christ

2% increase in A Of 100= 102.
5% decrease in Cd of .31 = .304

100*0.31= 31
102*0.304= 31.008

In this instance, by the numbers you suggest, you've not helped anything.

You made a mistake but you didn't realize it because your head is buried in equations and you're not applying common sense!

100*0.31 = 31
100*1.02*0.31*0.95 = 30.03900

So yes, if the Cd reduction is greater than the A increase, drag goes down. By definition.

That's the whole point of Cd. It's the ratio of an object's drag to a cylinder with the same frontal area. A car with 0.5 Cd has 2x the frontal area of a cylinder with the same drag.

[Christ]

Alright, did the math a little backward there, I did.

[capnbass91]

somthing I'd like to do to my car, but it's a convertible, and would be hard to attach anything to the roof :/

[winkosmosis]

Originally Posted by capnbass91

somthing I'd like to do to my car, but it's a convertible, and would be hard to attach anything to the roof :/

Chop off your windshield. You wouldn't have any rollover protection and will probably die a painful death though.

[Frank Lee]

Yes, because you WILL roll over or go under a semi!