View Single Post
Old 05-12-2020, 06:34 PM   #12 (permalink)
Join Date: Nov 2017
Location: Australia
Posts: 1,615
Thanks: 92
Thanked 1,293 Times in 905 Posts
I am sorry, but this is your typical dump of misinformation, misunderstandings and plain outright wrong statements - as usual, dressed up in pseudo-scientific language.

Originally Posted by aerohead View Post
*Firstly, Hucho appears to downplay apprehensions about 'lift' in passenger cars as related to stability. Below 100 km/h he says it's not an issue, even in crosswind. Hucho does emphasize the importance of lift in high speed sports cars and racing cars.
Well, perhaps read a bit more widely and recently than Hucho, second edition?

You say you have my book. Well then, read pages 179 - 182. Note specifically the reference to SAE 1999-01-0651 that shows how even small amounts of lift adversely impact stability, and to SAE papers 2009-01-004 and SAE 2015-01-1537 that again show how even small amounts of lift (especially rear lift) adversely affect car handling and stability.

But reading and driving are different things. I drive a car that, because of its modifications, develops downforce. It is easily measurable at 100 km/h. The car is significantly improved in its driving behaviour - eg cornering speed. In addition, it actually rides better because the sprung/unsprung weight ratio has effectively changed.

I might have believed what you say before I did the research for my book, and experienced the benefits of reduced lift for myself.

Originally Posted by aerohead View Post
* I believe that the lift issues that you believe to be the major reason for rear lift would actually be attributed to separated flow over a horizontal portion of the aft-body, ahead of the transom. Squarebacks are incapable of generating rear lift. Proper fastbacks don't typically generate dramatic lift. Notchbacks are historically the worst offenders, and responsible for the commercial development of the rear spoiler, by Kamai, in 1982,beginning with a product for the BMW 2002.That spoiler merely extended the tail surface up into the inviscid flow, to provide re-attachment on the boot, while sequestering the low pressure of the greenhouse turbulence away from the 'base' of the car's transom, which would otherwise contaminate the entire wake, lower the base pressure, and increase pressure drag.
I am sorry, but this is just rubbish. (1) The lift being described is nothing to do with separated flow. (2) Squarebacks can create lift. As Barnard specifically says in his book, even a brick can develop lift. Fastbacks are notorious for lift. (3) Notchbacks are not historically the worse offenders, and of course as most people know, the first spoiler was fitted to the classic Porsche 911 in 1973 (page 195 of my book). The 911 was a high lift shape, just as you'd expect with that long upper body curve.

Originally Posted by aerohead View Post
*The Cayenne and I-Pace both will create this separation with, zero chance of re-attachment, as they violate the limits of the Mair/Buchheim departure slope angles, necessary to allow for re-attachment.The plan-view section of the roof exposed to the turbulence is at the lowest static pressure, compared to the suction peak at the windshield header,and this low, acting over the 'lever arm 'spanning the distance to the tail creates the 'moment' which lifts the tail. Simply extending the roof to the back of the vehicle cancels the moment, leaving a higher base pressure, and lower pressure drag. On the Jaguar, this is part of the reason for the Cd 0.29,vs the Tesla S 0.26. The Porsche Taycan is close to the 'template' and enjoys Cd 0.25.
I am sorry, but this is all just rubbish. Have you ever measured pressures on a car? Obviously not! There is attached flow on these cars (eg Cayenne) to the trailing edge of the roof - the roof is not exposed to turbulence. There are plenty of wind tunnel videos around of the latest Cayenne, showing just this. In fact, the idea that the flow separates on the roof of these cars is just laughable.

I can see now why some of the misinformation is spread here: it appears that you just make up theories, completely without evidence, and then happily apply them!
  Reply With Quote
The Following User Says Thank You to JulianEdgar For This Useful Post:
aerohead (05-13-2020)