Ford Sierra XR4i Bi-Wing Spoiler?

[freebeard]

The Probe is all horizontal surface. The Escort RS Cosworth has those fat arms that act like trim tabs vortex generators if not rudders.

[aerohead]

One journalist had nothing but good things to say about Sierra's very stable high-speed characteristics. He drove at up to 120-mph, and in the rain.

[Snax]

Originally Posted by aerohead

One journalist had nothing but good things to say about Sierra's very stable high-speed characteristics. He drove at up to 120-mph, and in the rain.

I think this is an issue of context. In terms of other similar class cars, it probably did compare favorably, but up against other more sporty models it was still lacking.

It's odd what works for stability and what does not. As a teenager, my parent's F-250 felt great at 115 MPH. Later on, the second generation MR2 I had started feeling like a complete handful at that speed with a Cd of 0.28. It had the low spoiler, and now I wonder what one without it is like.

(And at 51, I have no idea what the top speed of any of my current cars would be. )

[aerohead]

Originally Posted by Snax

I think this is an issue of context. In terms of other similar class cars, it probably did compare favorably, but up against other more sporty models it was still lacking.

It's odd what works for stability and what does not. As a teenager, my parent's F-250 felt great at 115 MPH. Later on, the second generation MR2 I had started feeling like a complete handful at that speed with a Cd of 0.28. It had the low spoiler, and now I wonder what one without it is like.

(And at 51, I have no idea what the top speed of any of my current cars would be. )

112-mph was set as the benchmark for the fatalities experienced in the Audi TT.
The pre-production 1963 Corvette Stingray was reported to have 'uncontrollable lift at 140-mph.'
The nose of the reproduction Cobra Daytona Coupes are said to get light and wander around at 125-mph.
Seems like a mixed-bag out there.
It's probably a good thing that the MR2 had the spoiler.

[JulianEdgar]

Stability is a bit more complex than is being described.

1) If rear lift is higher than front lift, the car will be more unstable - so it's not just the absolute values but also the ratio of lift values. Recent papers from Porsche describe this very well.

2) Lateral stability depends largely on the lateral locations of the centre of pressure versus the centre of gravity, with a rearwards centre of pressure required for stability. (ie more side profile area towards the back of the car than the front.)

Re the Sierra, I haven't explored it in detail but I understand that they added separation edges to the sides of the rear glass to improve stability, so perhaps there was an oscillatory flow at those locations.

[j-c-c]

Originally Posted by JulianEdgar

Stability is a bit more complex than is being described.

1) If rear lift is higher than front lift, the car will be more unstable - so it's not just the absolute values but also the ratio of lift values. Recent papers from Porsche describe this very well.

2) Lateral stability depends largely on the lateral locations of the centre of pressure versus the centre of gravity, with a rearwards centre of pressure required for stability. (ie more side profile area towards the back of the car than the front.)

Re the Sierra, I haven't explored it in detail but I understand that they added separation edges to the sides of the rear glass to improve stability, so perhaps there was an oscillatory flow at those locations.

The adjoining issue with F/R ratios is additionally, there is seldom an aero system in place that the ratio remains constant over the useful speed band. Meaning one has to compromise and try to remain within the band where the stability is acceptable, to state the obvious,

[JulianEdgar]

Originally Posted by j-c-c

The adjoining issue with F/R ratios is additionally, there is seldom an aero system in place that the ratio remains constant over the useful speed band. Meaning one has to compromise and try to remain within the band where the stability is acceptable, to state the obvious,

Only if the CLf and CLr change dramatically with speed - and as far as I know with road cars, they don't.

[JacobLeSann]

Would love to see a video on the aerodynamics of this double wing, Julian.

[botsapper]

Frank Stephenson, designer of iconic car designs; the Mini, Maserati MC12, Ferrari F430 & FXX and the McLarens P1, 675LT, 570S & 720S, was also the Cosworth designer. He originally drew on an aerodynamically 'Fokker DR-1 Tri-wing' spoiler design on the race car and for the road car. The Ford bean counters deleted his third wing, it cost 5 deutschmarks too much!

https://www.youtube.com/watch?v=kFTepo8MzHY&t=258s

Wheeler Dealers' RS Cosworth episode, Ant Anstead fabricated the third wing! @4:40

https://www.youtube.com/watch?v=JzXlmjUNgUA