Can small aero changes significantly change Cd if body shape keep the same?

[Fluido]

I will take Mercedes CLA 180 Blue Efficiency as example.

CLA 180 BE has Cd =0.22, but same car with label CLA 250 has Cd=0.30, to me this is huge difference because both has identical body shape.

Is it possible to reduce Cd by 8 points,if body shape keep the same, but only change this "minor" things?
Members who work with CFD can help here, how much CFD results change, for this "minor" changes, maybe you notice that during your CFD work at F1 cars?

(I allways thought tha body shape,especialy in cars, is 95% of aero job.You cant get 0.25 from brick, whatever you do.)


CLA 180 BE has this aero features:
large diffusor cover
aero baffles
optimized rear muffler
aero covers for spring links
cover for rear axle subframe
optimized main cover
slotted wheel house liner
optimized lower stiffener
engine cover
demand controlled cooling air, using adjustable radiator shutter
optimized front apron
large area of underbody paneling and additional cladding in the center rear axle area
serrated wheel spoilers at the front and rear wheels
slotted wheel arch linings
low A-pillar shoulder
aerodynamically optimized exterior mirror housings
aerodynamically optimized rear end design (flush sitting boot lid and fins in tail lights)
aerodynamically optimized rear silencer with downstream diffuser
wheels and tires with low air resistance

[PhillipM]

Details are massive for aero, mirrors, mudguards, wheels and arches, wiper blades, small details where the air detaches around screens or the roof are huge issues that are as much of an issue as the body shape itself.

[MadMax]

It's worth remembering that some/many aero details are as much to do with occupant comfort as efficiency.

Get the mirror shape wrong and you create a strong noise source close to the driver / front passenger. Likewise the A-pillar / windscreen interface, side window surrounds, door handles, etc.

Shaping the rear "corners" is important to give consistent airflow off the rear of the car. Get it wrong and you can have von Kármán shedding that can create a lateral movement at the rear. Not much but can cause issues for sensitive rear passengers. Also, shaping of the rear lights to improve airflow so they don't get too dirty from crud thrown up by the tyres.

Similarly the use of tuned masses to help reduce resonances caused by the suspension.

[MadMax]

I will take Mercedes CLA 180 Blue Efficiency as example.

CLA 180 BE has Cd =0.22, but same car with label CLA 250 has Cd=0.30, to me this is huge difference because both has identical body shape.

Isn't the 250 a 4wd version? Might that have an effect on underside airflow and hence drag?

[e36jon]

I defer to the actual aero experts should they weigh in.

A looong time ago I bought 'Aerodynamics of Road Vehicles' by Hucho and was surprised to learn that at one point two pretty distinct approaches existed for developing vehicle aero. VW was a champion of detail engineering and the Vanagon is used as an example. VW was able to make huge aero gains by optimizing a ton of small details while still having a brick at the end of the day.

The other end of the spectrum was the form optimization camp, where getting the overall shape right was job one. I can't remember who the mfr. was that they used as an example, but there were a lot of fairly blank forms used in the wind tunnel looking at windshield angles and trunk angles, etc...

The book is a great read (Lots of pictures!).

Jon

[Greg Locock]

Windtunnel results are notoriously misleading. The results vary between tunnels and depend on test techniques. That is, if Audi and VW measure the same car in the same tunnel, they will get different numbers. A consortium of manufacturers did a round robin sending the same cars to each windtunnel used by the consortium. I have seen that report but was asked not to take a copy.

Marketing often announce the best number that was ever measured during a program, whether it was cfd, windtunnel on a 1:5 clay, or a full size one, or panel gaps taped, grill blanked off, etc.

A surprisingly large proportion of drag comes from flow through the engine bay, and then through the gubbins under the floor.

[Fluido]

Windtunnel results are notoriously misleading. The results vary between tunnels and depend on test techniques. That is, if Audi and VW measure the same car in the same tunnel, they will get different numbers. A consortium of manufacturers did a round robin sending the same cars to each windtunnel used by the consortium. I have seen that report but was asked not to take a copy.

Marketing often announce the best number that was ever measured during a program, whether it was cfd, windtunnel on a 1:5 clay, or a full size one, or panel gaps taped, grill blanked off, etc.

A surprisingly large proportion of drag comes from flow through the engine bay, and then through the gubbins under the floor.

I didnt mean on different results from diffrent wind tunnels, I talk about Cd numbers that Mercedes write it for own models.

[Latios]

It depends on how some the change is, and what exactly the difference is.
If it stop/create separation/stall/vortex, small change may have huge impact, suct as some cars' sharp rear light edge, it actively creates seperation, to replace the original big separation at rear of the car.

[Fluido]

It depends on how some the change is, and what exactly the difference is.
If it stop/create separation/stall/vortex, small change may have huge impact, suct as some cars' sharp rear light edge, it actively creates seperation, to replace the original big separation at rear of the car.

Yes, 8 points seems to me a big change for same body shape.

[AR3-GP]

Dive planes can often have the effect you describe.

[Greg Locock]

Yes, 8 points seems to me a big change for same body shape.

As I said, marketing will grab the lowest number seen during a program, but other information sources may contradict this.

[JakobusVdL]

I've read that the components can make quite a difference to the car's Cd.
So the larger engine will have larger air demand, and more cooling requirements for the engin and gearbox.
Likely the more powerful model will also have larger brakes which require more cooling.