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Moving rear spoiler - what's its purpose?
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Rettopian   
Mon Jul 29 2013, 04:00pm
Joined: Feb 13 2013
Member No: #1195
Location: Wiltshire
 Hi,

Can someone tell me what the primary purpose of the moving rear spoiler is? Is it there to reduce drag at speed or is it there to reduce lift at speed?

If its to reduce drag - does anyone know how effective it is in that role? (Any Cd/Cx figures anywhere?)

I ask because I went up to the midlands at the weekend with the cruise set at 75 after a slightly quicker burst that raised the spoiler to its higher position & the car returned about 3mpg more than on the return leg where I trundled along at 70 with the spoiler only raised to the first position.

Any ideas?
Lars of Sweden   
Mon Jul 29 2013, 04:28pm
Joined: Aug 17 2012
Member No: #991
Location: Tenhult
 I'm pretty sure that I have read that at lower speeds, partially up, it is to help with breaking, while at higher speeds it is to press the end firmer to the road, i.e. helping with road holding. How well this works in practice, I have no idea....
C6Dave   
Mon Jul 29 2013, 06:20pm

Joined: Oct 01 2009
Member No: #1
Location: Northumberland
Lars of Sweden wrote ...

while at higher speeds it is to press the end firmer to the road, i.e. helping with road holding. How well this works in practice, I have no idea....

The car also reduces the ride height at speed to improve the aerodynamics and roadholding
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dsharples   
Mon Jul 29 2013, 06:34pm
Joined: May 18 2010
Member No: #119
Location: Woodbridge
 I can only see it functioning as a DRS "Drag Reduction System" to offset drag caused by the curvature of the rear window/boot lid - and therefore aid fuel consumption.

I've certainly noticed the car to be more stable with it deployed in challenging weather conditions.

I can't see it generating "downforce" - as I can't really imagine that's an issue for a 1900kg car at 7Xmph - but happy for those of you who use autobahn's to correct me.
Rettopian   
Mon Jul 29 2013, 07:16pm
Joined: Feb 13 2013
Member No: #1195
Location: Wiltshire
 Dropping the ride height makes sense on the drag reduction front. What speed does this happen at?

So is the thinking that is acting a little like a Kamm tail?
Bishop   
Tue Jul 30 2013, 07:50am
Joined: Apr 16 2012
Member No: #868
Location: Harpenden
 I have always assumed that the higher position is to reduce lift. My assumption stems from memory of Audi TT's with a similarly shaped rump being reported as falling off roads when they were first launched without any form of spoiler due to undue lift and therefore instability at high speeds. That car quickly gained a somewhat ungainly looking large lip to resolve these issues. The second gen TT gained a c6-a-like dynamic spoiler, but I don't think it has the added sophistication of an intermediary position.
smihaialex   
Tue Jul 30 2013, 11:40am
Joined: Sep 18 2012
Member No: #1031
Location: Bucharest
 The analogy with the TT (as well as the Bugatti Veyron which also sports a similar but significantly larger rear deflector) is spot on.

It generates downforce at speeds higher than 130km/h necessary to keep the rear more firmly attached to the ground. You see, the car is 1,900 kg, in fact closer to 2,200 kg with driver, some luggage and a full tank of gas, but most of this weight is distributed towards the front. And, as we know, a car's ability to hold the road is given by the ability of the rear wheels to follow the front wheels - the quality of the rear's adherence (it's why you put the best tyres at the rear, not at the front). That's where the rear deflector comes in - at high speeds it makes up for the weight imbalance. It's really necessary, not a gimmick. It's also supposed to act as an air brake but I've never seen that in real life situations...

As for the 3mpg more, I think that has more to do with the speed than with the rear deflector - this car eats up less at 80 than it does at 70

Cheerios,
Sam
Rettopian   
Tue Jul 30 2013, 01:13pm
Joined: Feb 13 2013
Member No: #1195
Location: Wiltshire
 Thanks chaps.

Very interesting. And yes the TT did have a reputation of trying to reverse off the autobahns at speed.

Less fuel consumed at 80 than 70 eh? Super! Well I'm off to France shortly so 130kmh is about 80mph - so I will conduct a hightly objective test!
smihaialex   
Wed Jul 31 2013, 09:51pm
Joined: Sep 18 2012
Member No: #1031
Location: Bucharest
 Great!

When I'm in Germany my go-to speed is 150 km/h (~93 mph), in Italy and France it's 140 km/h (86 mph) - 10 km/h above the 130 km/h speed limit is usualy tolerated so you shouldn't get a speed ticket. Sometimes I set the cruise control to 139 km/h just to be sure

Funny story, while driving from Prague to Berlin, I was doing 150 km/h on the motorway, and a C5 3.0 HDi V6 passed by me like I was standing still - I recognized it as a 3.0 HDi because of the large twin exhausts. It must have been doing at least 180 Km/h. But the funny part is that it was also registered in Romania, in fact it was one of the press cars of Citroën Romania B xx VPC (voiture presse Citroën).

One thing that I noticed about our cars is that the speedometer is spot on - in most cars there's a difference in the speed reported by the speedometer in the dashboard and the actual speed reported via GPS for example, but in the Six it is exactly the same - when the speedometer says 141km/h, the GPS reports the same 141km/h, and because it's digital/electronic, it doesn't vary with speed - analog speedometers have a variable margin of error - they might repoort 60 km/h when you're doing 58 km/h, so a 2 km/h error, and say 120 km/h when you're doing 116 km/h, so a 4 km/h error

Cheerios,
Sam
 

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