Top down = more drag?

[Mindz]

Ok I can see how with T tops and a sun roof or windows down in a harddtop/targa, the car would go slower, but can someone explain to me why a vert has a lower top speed with the top down? Is this only with certain cars or am I just overworked and not getting the concept?

Sorry in advance for my babbling. I'd really like an answe rthough.

Thanks!

[unkbd]

The air flowing over the top of the windshield will flow over the occupants at a given speed. As the vehicle starts to go faster, the air flow changes and the disturbed air over the windshield gets "caught" in the back seat....much like the tailgate of a pick-up truck. That "stuck" air acts against the forward motion of the car. The higher the speed the more resistance.

[Mindz]

ok so it's the back seat as opposed to the rear windshield in a coupe with T-tops or the window down. I just thought the airflow would hit the rear deck/trunk area. Thanks.

[JeepinMatt]

Air flow is real funny. What a lot of people think looks aerodynamic may not be. The old VW Bus was more aerodynamic than the Beetle, but when I tell people that they look at me like I just threw up a shoe. Same thing with the whole pickup tailgate-up-or-down debate. To people's eyes, tailgate down looks more aerodynamic, but it's not because the airflow moves in a funny way. We have an '81 Corvette, a year after they added the standard aero package. People look at how swoopy the C3 Vette looks and say "man that thing must be real aerodynamic." Well, it isn't. The pre-1980 C3 Vettes has a cd of around .5 That's a lot. Even the aerodynamic '80-82 is somewhere around .44 The C6 by comparison is .28

[unkbd]

In a coupe you get some airflow through the t-tops/sunroof but a majority of it is going to follow the roof line, what your actually getting through them is residual turbulance, no significant flow. In a vert there is no major surface area to guide the flow of air.

[Silverado]

Originally Posted by JeepinMatt

Air flow is real funny. What a lot of people think looks aerodynamic may not be. The old VW Bus was more aerodynamic than the Beetle, but when I tell people that they look at me like I just threw up a shoe. Same thing with the whole pickup tailgate-up-or-down debate. To people's eyes, tailgate down looks more aerodynamic, but it's not because the airflow moves in a funny way. We have an '81 Corvette, a year after they added the standard aero package. People look at how swoopy the C3 Vette looks and say "man that thing must be real aerodynamic." Well, it isn't. The pre-1980 C3 Vettes has a cd of around .5 That's a lot. Even the aerodynamic '80-82 is somewhere around .44 The C6 by comparison is .28

Thanks for pointing that out Jeep...I was going to say the same thing about pickups....

[Mr. Wyndham]

Yep, that Truck Tailgate thing is a common argument, it's been proven - too. On TV, in articles...if your tailgate is down, it's less aerodynamic that while it's up...Why? See the Picture:

As for Vert's, I can see that, Where a hard-top would "guide" the resisting air across the roofline, a vert just lets it fall right into the back seat, where it pushes down, or back, and hampers your cars speed.


This is a bad example, because the Truck is sooo Big, but this is the Idea:

[unkbd]

Sam idea as dragon ....just took longer to do.....1st is at speed......2nd is towards top speed

[Mindz]

Ok, I figured that was the general idea, just figured the force would push the wind higher and it would hit the trunk lid. I didn't think the vacuum pulling the air back towards the front and down behind the windshield was greater at higher speed. Thanks!

[CamaroSpike23]

Originally Posted by JeepinMatt

Air flow is real funny. What a lot of people think looks aerodynamic may not be. The old VW Bus was more aerodynamic than the Beetle, but when I tell people that they look at me like I just threw up a shoe. Same thing with the whole pickup tailgate-up-or-down debate. To people's eyes, tailgate down looks more aerodynamic, but it's not because the airflow moves in a funny way. We have an '81 Corvette, a year after they added the standard aero package. People look at how swoopy the C3 Vette looks and say "man that thing must be real aerodynamic." Well, it isn't. The pre-1980 C3 Vettes has a cd of around .5 That's a lot. Even the aerodynamic '80-82 is somewhere around .44 The C6 by comparison is .28

isnt the 93-97 camaros at like .28 also? lol



and in addition to all this, are we comparing a vert with its top down to a vert with the top up? or a vert with the top down to a hardtop coupe? cus a hardtop will beat the vert just due to the extra ~300lbs in weight that the vert has.

[Mindz]

I wasn't asking about which would win. I jusst wanted to know why the airflow wasn't the way I had imagined it. I know the weight difference.

[CamaroSpike23]

i was just makin sure cus the question was askin about top speed, a hard top will always top out hiya dan teh voit cus da wate...

[Mr. Wyndham]

Originally Posted by CamaroSpike23

, a hard top will always top out hiya dan teh voit cus da wate...

Sad thing is? I followed and understood that....


MY question - having never owned a Vert...Why do they wiegh more? I would think it'd be visa-versa....the vert would wiegh less with a rag-top in place of a metal roof...

[DGthe3]

there are a couple of basic rules to aerodynamics, some have been said but i'll say em again:
1) Air likes to follow surfaces.
2) generally speaking, smooth flow is prefered to turbulent flow
3) air cannot follow sharp changes in direction, it will cause eddies (aka vortexs, whirlpools). These cause drag when moving against the airflow, reduce drag when moving against. this explains pickup trucks
4) when air hits a surface, it causes lots of drag.
5) when air makes a small change in direction, it causes drag
6) when air slides past a surface, it causes a little bit of drag