Why do 250 mph (400 km/h) supercars have 0.40+ Cds?

[Xist]

I believe that we have discussed this on here, although I did not see it in a search.

But starting in the 1990s, electronic aids such as active suspension combined with aerodynamic advances to make racecars so fast and so dangerous—contributing to the death of Formula One icon Ayrton Senna in front of a television audience of 300 million people—that rule-makers began slowing cars down. They banned the most exotic electronic aids. They intentionally compromised aerodynamic efficiency.

It struck him that any spectator, no matter how clueless, could see the courage and talent of professional bikers. Racecars, on the other hand, hide their drivers' skills. Their giant wings produce so much grip that driving them looks effortless. The wings also generate a wake of turbulent "dirty air" that prevents cars from racing closely together, robbing races of drama.

Most Efficient Racecar | Popular Science

This is the video of the Toyota hybrid knocking the Deltawing off of the track, but supposedly, since the Deltawing was smaller than the rest of the car, the Toyota driver could not see it, about 32 seconds in:

https://www.youtube.com/watch?v=FJAiZVuC3YI#t=32

[NHB]

Originally Posted by chillsworld

I would disagree with cooling drag, and I would say it's primarily downforce. These cars aren't designed for normal use.

Downforce is not just about keeping a car on the ground... Nor is it all about reducing the force of wind resistance. It's about being able to transfer 500-800 horsepower to the ground.

Aerodynamic balance is real but so is cooling. When a car has about 1 MW engine power and 400 km/h top speed, it has to be able to dissipate huge amount of heat in various places. For that a supercar needs a lot air running thru cooling system. It's easy to see the effect of power. Let's check Cd-values of Mercedes CLA - one of the most aerodynamic cars available.
0,22 CLA 180 BlueEFFICIENCY Edition 90 kW (skinny tires)
0,25 CLA 180 90 kW
0,29 CLA 250 4MATIC 155 kW
0,32 CLA 45 AMG 4MATIC 265 kW

How high would the coefficient factor be if that chassis had 1 MW engine? I bet it would more that 0,4. Cooling for engine, cooling for transmission, cooling of brakes and huge tires increase drag a lot. A supercar has to be also very wide and relatively short which doesn't help.

[chillsworld]

Originally Posted by NHB

Aerodynamic balance is real but so is cooling. When a car has about 1 MW engine power and 400 km/h top speed, it has to be able to dissipate huge amount of heat in various places. For that a supercar needs a lot air running thru cooling system. It's easy to see the effect of power. Let's check Cd-values of Mercedes CLA - one of the most aerodynamic cars available.
0,22 CLA 180 BlueEFFICIENCY Edition 90 kW (skinny tires)
0,25 CLA 180 90 kW
0,29 CLA 250 4MATIC 155 kW
0,32 CLA 45 AMG 4MATIC 265 kW

How high would the coefficient factor be if that chassis had 1 MW engine? I bet it would more that 0,4. Cooling for engine, cooling for transmission, cooling of brakes and huge tires increase drag a lot. A supercar has to be also very wide and relatively short which doesn't help.

Were those numbers reached with by placing the different engines in the same vehicle? Did each vehicle have the same trim level and body styling package? The answer is probably no.

0,22 CLA 180 BlueEFFICIENCY Edition 90 kW (skinny tires)

skinny tires, low A-pillar shoulder with adapted A-pillar geometry, aerodynamically optimized exterior mirror housings and rear shape, optimized diffuser, optimized underbody and rear axle paneling, radiator shutter, aero wheel trims and serrated wheel spoilers on the front and rear wheel arches.

0,32 CLA 45 AMG 4MATIC 265 kW

Different rear diffuser, rear fascia with side air outlet openings, Large side cooling intakes with flics (functional winglets), front splitter, different air dam height, different wheels, different suspension components, different exhaust, wider/larger tires, different underbody due to AWD system (underbody is an assumption, I can't find pics).

Details on the cars taken from Wiki and the MB website.

In regards to aerodynamics, these cars aren't the "same" vehicle... To display a difference in CD and say it's due to cooling, would seem to be over simplifying to me.


~C

[Big time]

Originally Posted by NHB

Aerodynamic balance is real but so is cooling. When a car has about 1 MW engine power and 400 km/h top speed, it has to be able to dissipate huge amount of heat in various places. For that a supercar needs a lot air running thru cooling system. It's easy to see the effect of power. Let's check Cd-values of Mercedes CLA - one of the most aerodynamic cars available.
0,22 CLA 180 BlueEFFICIENCY Edition 90 kW (skinny tires)
0,25 CLA 180 90 kW
0,29 CLA 250 4MATIC 155 kW
0,32 CLA 45 AMG 4MATIC 265 kW

How high would the coefficient factor be if that chassis had 1 MW engine? I bet it would more that 0,4. Cooling for engine, cooling for transmission, cooling of brakes and huge tires increase drag a lot. A supercar has to be also very wide and relatively short which doesn't help.

Good point.
I recall that about 40% of fuel power goes wasted as radiator hot air, more than the power put to the ground so these cars vent out more than 1000 HP worth of hot air.
Meredith effect radiators are a must then.

[NeilBlanchard]

It is more like 75% of the energy wasted as heat out of the radiator.

[UltArc]

The Veyron actually has 10 radiators.

And the Koenigseggs all push past 250 with the CCX and after. I don't know much about the models before, I got into them around 2006. K labels the CCX as 245+, and the Agrera at 270+.

[serialk11r]

Originally Posted by NeilBlanchard

It is more like 75% of the energy wasted as heat out of the radiator.

??? Cooling losses are not even close to the biggest proportion of losses in the typical engine.

Even if you count the extra power needed to overcome the extra drag due to the cooling requirements you will not get 75%.

[NHB]

Originally Posted by Old Tele man

It's all about having and putting LOTs of power onto the pavement. Downforce "puts" that power onto the pavement.

I have a different opinion.Traction is a problem in relatively low speeds. Cars generate downforce at high speeds. Useful downforce is usually generated somewhere over the 200 km/h limit. For me downforce is all about cornering speed and high speed balance.

[NHB]

Originally Posted by chillsworld

Were those numbers reached with by placing the different engines in the same vehicle? Did each vehicle have the same trim level and body styling package? The answer is probably no.

In regards to aerodynamics, these cars aren't the "same" vehicle... To display a difference in CD and say it's due to cooling, would seem to be over simplifying to me.

Those numbers were taken from Mercedes website (german). Most likely they are for basic model without any extra trim or spoilers. AMG-model has a bit different shape for outer decorstion for sure, but that doesn't explain a difference that huge.

I didn't say that it's only cooling. Wide tyres have also a major effect. And of cource there are many other thing but those two are always major reasons for high Cd values.

[freebeard]

Why do 250 mph (400 km/h) supercars have 0.40+ Cds?

Whatever the reason, 400mph (644kph) cars *don't*.