Foo - If dimpled bike wheels are more aero (i.e. Zipps), why not...

Why not the body panels on cars?

Because it would look "ugly"?

Yes.



It would look like it just came out of a hailstorm. Besides, car aerodynamics are way different than a rotating mass. Different means to lower the cd I bet.

Iono if the Zipps advantage is more about the dimples, or just having a faired wheel. Wheel fairings work the same for cars mas o menos.

I think some race cars (or maybe luxury cars) have dimpled underbodies.

And yeah, it would look fugly as hell on the body of a car.

Can you imagine the "Is your car ribbed for her pleasure?" jokes?

The reason we do not see dimples on other shapes, like wings, is that these particular forms of boundary layer trips only work well on a blunt body like a sphere or a cylinder. The most dominant form of drag on these kinds of shapes is caused by pressure, as we have seen throughout this discussion. More streamlined shapes like the airfoils used on wings are dominated by a different kind of drag called skin friction drag. These streamlined bodies, like that pictured above, have a teardrop shape that creates a much more gradual adverse pressure gradient. This less severe gradient promotes attached flow much further along the body that eliminates flow separation, or at least delays it until very near the trailing edge. The resulting wake is therefore very small and generates very little pressure drag.


Taken from the following website: http://www.aerospaceweb.org/question/aerodynamics/q0215.shtml

Also, on a bicycle, the wheels are much more exposed than on a car. And they represent a much larger portion of total drag than on a car. And that more exposed and larger percentage of area flows at 2x your speed at the top of the wheel. Aero-drag reductions here would pay big dividends at TT speeds. The extra 0.1hp savings on a car won't make that big of a deal.

um, what about open wheel cars. I don't remember seeing many F1 cars w/ dimpled surfaces.

yeah, I suspect the savings aren't worth it. I suppose the turbulence from the open tyres is so high, any aero smoothing/dimpling of the wheels gives no benefits. Having the wheel-surfaces open for ducting hot air away from the brakes yields more benefits than any savings from aerodynamics. F1 cars aren't about aerodynamics anyway, they're set up for +4g of downforce @200mph for maximum cornering and braking. Indy/IRL cars tend to be more aero with wheel-covers and 2-2.5g of downforce.

Anyone see the 2003 GP at Montreal where these cars could finally be compared?

hmm, no. Interesting premise. what was the result?
My money would still be on an F1 car&driver. you can always tune the downforce down to get more speed and it is the automotive equivalent of a euro-pro cycling team.

Hmmm, well my thinking would be how it would apply to "normal" cars on the road today in terms of MPG and effeciency. I do know that there's more than just aerodynamics to the overall performance for a car but the situation i'm envisioning is what the majority of a car's driving time is and that's your everyday commute at 50-75mph for 30-60 minutes a day (well, at least that's what my car gets despite being set up to handle alot more than that). :lol:

So i'm guessing if dimpling were to be used on cars it would be effective but probably not cost effective then?

The reason we do not see dimples on other shapes, like wings, is that these particular forms of boundary layer trips only work well on a blunt body like a sphere or a cylinder. The most dominant form of drag on these kinds of shapes is caused by pressure, as we have seen throughout this discussion. More streamlined shapes like the airfoils used on wings are dominated by a different kind of drag called skin friction drag. These streamlined bodies, like that pictured above, have a teardrop shape that creates a much more gradual adverse pressure gradient. This less severe gradient promotes attached flow much further along the body that eliminates flow separation, or at least delays it until very near the trailing edge. The resulting wake is therefore very small and generates very little pressure drag.


Taken from the following website: http://www.aerospaceweb.org/question/aerodynamics/q0215.shtml

That's not quite right.

The dimples are intended to cause airflow over the body to become turbulent (tiny little eddies and swirls, as opposed to laminar). Turbulent airflow is actually worse for skin drag than laminar airflow, if I remember right, but as the above explains, much better for what is called form or pressure drag.

However, the dominant type of drag in automobiles is form drag (contrary to the quoted text), so you actually want turbulent airflow.

So why don't car manufacturers do something to get this? Because it actually occurs naturally beyond a certain threshold. The threshold is related to the Reynolds number, a engineering quantity that depends on the speed, the size of an object, and the viscosity of the fluid. Although the threshold is very complex, it typically is expected at a Reynolds number somewhere around 200000 for flat surfaces. On an object the size of a car, this could happen at around 30-40 mph, or even lower depending on the surface finish and various obstructions like windshield wipers and hood ornaments.

In short, the desired effect is is already achieved without dimples on cars.

Lexus was touting a dimpled read underbody (in nice shiny chrome) a couple years back, but I haven't heard anything about it since. I would guess they found no benefit other than marketing. I also read that BMW has started putting dimples around the roots of the side-view mirrors. In that location it might actually have some benefit because of the small size and relatively blunt shape of the mirrors.

Actually, hold on a second...this other thread:
http://www.bikeforums.net/showthread.php?t=56781
includes a claim that Lexus included the dimples to reduce buffeting over isolated portions of the car. That actually sounds quite plausible.