Cars compared in wind tunnel

[tjts1]

Originally Posted by trebuchet03

Meh - if your driving without it to save weight..... keep it out..... Just because one thing overtakes another is no reason to say one force is insignificant (I mean, people buy tires with low rolling resistance ). Yes, weight reduction only gets you so far... But, when half the pool is closed - do you swim in the other half or bake in the sun?

Exactly. Besides Its not like I'll be stranded when I get a flat.

Full size spare + jack and tools adds up to 55lb in a 2600lb car.

[trebuchet03]

Originally Posted by tjts1

Exactly. Besides Its not like I'll be stranded when I get a flat.

Full size spare + jack and tools adds up to 55lb in a 2600lb car.

AAA I only need a card - less than a half ounce, maybe?

[boxchain]

The fuel you save is going to be burned and more when you call a tow truck.

And no goo will help a total blowout. I've had two recently, one from a deep pothole at 45, almost went into a canal, and the other backing into a parking space, I hit the curb which had a old busted iron plate that went into the side like a knife. Both times I was back in business quicker than a tow truck would have arrived.

Weight is mostly a factor in acceleration, and more is better for long glides, but worse in pulses. If you drive over 40, weight (rolling resistance) is pretty negligible.

[tjts1]

To each his own I suppose. I've never had a blow out or called a tow truck. Fix a flat got me home safely twice now.

[ArtFox]

Originally Posted by Bearleener

The German car magazine "Auto Bild" (2008-04-11 issue) tested the aerodynamics of cars in the Daimler wind tunnel.
[*]Station wagons are worse than sedans because the turbulent region at the rear is larger; so-called "lifestyle wagons" are not quite as bad, but have less cargo space (see Mercedes C 200 T vs. C 200)

I was researching the 2009 Jetta Sportwagon TDI and found it gets worse mileage than my 2003. Then I checked EPA's web page and found that the new EPA MPGs show the 2003 TDI wagon gets better mileage than the 2003 TDI sedan! In this unusual case there must be other factors in addition to Cd?

[lunarhighway]

newer cars tend to be much taller than older ones, this allows for a more upright seating position, wich eliminates the need for more legroom and thus allows for more usefull cargo space or a relative shorter vehicle although whitin the same model family sizes tend to increase.

this has the disadvantage that while drag coeficients might have slightly improved (although this is not always the case)
frontal area has increased quite a lot withing the same model range over the years giveing more overall drag.

also weight increased because of more and more safety and comfort addons some of wich take a lot of energy wich has to be supplied by the engine, creating aditional electric and mechanical load ,direct and indirect cooling requirements.

this would explain why newer models have worse FE than older ones, but not why a wagon could have better FE than a sedan.
even if they have a comparable if not a more refined engine.

it's unlikely but not impossible that it has a slightly better drag quoeficient.

perhaps certain standard features of the sedan are an option on the wagon, tire size, climate controle, airbag configuration, etc etc, might differ between the base models, since the wagon is more practical oriented, some customers might perfer a cheaper more stripped version. this might make a difference in weight and electric load and in case of the tires frontal area and Rolling resistance... this could produce the FE difference for both "standard configurations"... it's not entirely unimaginable that carmakers produce a stripped out version as stock wich would get better FE figures on paper, and than offer cheap or even free upgrade packs wich give the cusommer the impression they're getting a bargain, wihile their actual FE goes down.

also it is possible engine management en the gearbox ratio is different, since the wagon would be expected to haul more weight than the sedan, wich might be configgered more for a more sporty driveing experience.

that's about all rational explanations i can think of

[NeilBlanchard]

Hello,

The Mercedes Bionic car (aka the Boxfish) is quite tall (it is similar to the Honda Element's dimensions); and yet it achieves a Cd of 0.19.

[lunarhighway]

yes sure, but that's the catch, total aero drag is frontal area (in square feet or meter) multiplied Cd

this is gives you a CdA

a tiny car with a 0.32 Cd might have less overall drag than a prius, just because the prius has more frontal area...

frontal area determins how much air will have to be pushed around the car, drag quoeficient how smooth this will go

[MechEngVT]

The comments that wagons have higher drag because of the larger wake area assumes that in the equivalent sedan the airflow separates at the rear of the trunk lid. We all know that this isn't necessarily the case and lots of sedans can have flow separate at the rear of the roof, giving you a wake area equally as large as a wagon plus a ton of rear lift. The airflow on a wagon more than likely stays attached all the way to the rear of the roof, negating some rear lift but still giving you the large wake. Since the wake vortices alternate (ever feel that side-to-side buffeting in the draft of a tractor trailer?) a wagon has one pair of vortices alternating behind the hatch, where a sedan could have a pair of vortices behind the backlight and another pair behind the trunk that interfere with one another and increase the pressure drag.

Pure speculation of course, but it just goes to show that the article goes into gross generalizations and the 2003 Jettas appear to be an exception to the generalization.

[ArtFox]

I agree with MechEngVT, who I think also just defined the "Kamm" effect.