C5 engine undertray and water in footwells

[Northern_Mike]

I lost my under tray in early 2004. 100k miles and nearly 10 years hence I have had no problems.

I think you've already paid for at least a couple of them from the extra petrol you have been burning on the motorway

Made no difference at all to my fuel consumption according to the trip computer. That's 20 miles on country roads at between 40 and 60mph and 22 miles on the M4 every day.

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[Jeff]

C5 electronics lower the car when doing hign speeds on motorways thus increasing efficiency, irrespective of the undertray being there or not. Unless someone has stats regarding drag I doubt there is much (if any?) difference between having an undertray or not.

[qprdude]

That would be why F1 car designers just leave the underside of the car open then. Drag is always improved by a smooth aerodynamic surface. I don't really care one way or tother, but if a part is designed to be on a car, I like to have it there.

[Northern_Mike]

That would be why F1 car designers just leave the underside of the car open then.

Nope it wouldn't be.

F1 car bottoms are flat because the rules stipulate that they are. The designers would love to be able to design curves and air channels into them to increase downforce and various other aero effects.

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[Jeff]

F1 cars actually have a drag coefficiency worse than your average saloon car.

Saloon cars 0.3 to 0.50 and F1 0.70 to 1.1 depending on circuit set up. F1 cars have so much speed they need to create downforce to stop them taking off, hence more drag.

[osx]

C5 electronics lower the car when doing hign speeds on motorways thus increasing efficiency, irrespective of the undertray being there or not. Unless someone has stats regarding drag I doubt there is much (if any?) difference between having an undertray or not.

So if you have your windows open it doesn't make a difference?

[qprdude]

As much as I respect the opinions (and that's what they are) of the guys on the forum, I'll go with the designers as they are the experts, with ALL the data at their fingertips.


How Automakers Improve Aerodynamics While some shapes are inherently more aerodynamic than others, aerodynamicists and designers subtly shape every vehicle to reduce drag. "We look at all areas of the car that come in contact with the air. Upper surface shape, under floor, wheels and even cooling and engine bay," says Ian Anderton, aerodynamicist at the Jaguar Design Studio in Gaydon, England. Automakers fine-tune the way the air attaches to the vehicle's surface, and the way it leaves the rear end.

To improve Cd, designers may make the following changes:
•Round the edges of the front end
•Tune the grille and fascia openings
•Tune the wheel openings
•Place spats (small spoilers) in front of the tires to reduce turbulence
•Tune the size and shape of the outside mirrors and their attachment arms
•Reshape the water channel on the A-pillars
•Adjust the front fascia and air dam to reduce drag under the vehicle
•Add side skirts
•Tune the deck height, length and edge radius
•Install a rear spoiler
•Adjust the angle of the rear window
•Tuck up the exhaust system
•Use a diffuser to tune air coming off the underside
•Install "belly pans," underbody panels that cover components and smooth airflow

[qprdude]

That would be why F1 car designers just leave the underside of the car open then.

Nope it wouldn't be.

F1 car bottoms are flat because the rules stipulate that they are. The designers would love to be able to design curves and air channels into them to increase downforce and various other aero effects.

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No, that's the board bottom(skid plank) 10mm thick plank down the centreline to stop the car hitting the road surface, nothing to do with the shell under the car nosecone.

[Northern_Mike]

No, that's the floor itself. No shaping is allowed. F1 rules prohibit under-car shaping or venturis, and mandate a minimum ride height enforced by a relatively low-tech wear plank attached underneath the car. Underbody must be, by the rules, completely flat, except for plank and Reference plane. However, there is still scope to shape the diffuser area directly under and behind the rear axle line

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[qprdude]

I was under the impression that we were discussing the merits of the under tray. Which has absolutely no bearing on a mandatory plank along the centreline of a car.
As I have said, I will go with the designers (Jaguar in the case I copied).
If I ever buy a F1 car, I will consider carefully the merits of a wooden skid plate, until then I prefer to leave the car as it was designed, with the assumption that Citroen designers are more clued up on aerodynamics than I am, and wouldn't add weight to a car without benefit.

[osx]

http://www.autospeed.com/cms/A_2456/article.html" onclick="window.open(this.href);return false; with numbers to show the improvement in fuel consumption with an undertray.

[Jeff]

Interesting article osx, good to see the undertray decreases drag when only just behind the front wheels plus still allows some access to carry out routine maintenance getting at the sump, oil change etc.

[taffy]

id rather have an undertray...after seeing what the weather does to everything the whole car should have one!!

[Northern_Mike]

I was under the impression that we were discussing the merits of the under tray..

We were, then you brought in the F1 reference and incorrectly stated why F1 designers must use a flat floor - not just the centre plank (there to prove the car has been at the permissible ride height, nothing else). Don't get uppity with me because I corrected something you incorrectly stated.

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[qprdude]

Calm down mate. The bottom of an F1 car ain't flat. Under the nosecone is well shaped as is the rear end and under the side pods. The middle of the car is flat as per regs, but what I meant was that the car is not open (as one without an undertray!!) which was the original point. Anyhoo, it looks as if the proper scientific tests rather than opinions say that the undertray does make a difference. Not that I'm surprised.