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Old 09-09-2020, 09:04 PM   #1 (permalink)
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Lift

As it happens, today Rob Palin (ex Tesla, Bentley and F1 aerodynamicist) has been in communication with me about lift (giving me feedback on that chapter in my book).

As always, what he has to say is fascinating. He states that one car company (that I am not going to name here) has these requirements:
  • Front coefficient of lift greater than rear coefficient of lift up to 15 degrees yaw
  • Actual forces need to be less than 10 per cent of each axle weight at top speed

He also emphasizes what I have been saying, in that steady state wind tunnel (and, he adds, short-run CFD) often doesn't capture the actual quick variations in lift. He writes:

In research I did back at MIRA, we saw easily 100% fluctuations in lift forces, on all manner of vehicles, with the associated spectrums varying wildly between vehicle shapes, and with individual modifications. A rear spoiler, for example, may have zero net effect on the mean lift, yet reduce the standard deviation of the forces away from that mean quite dramatically - and vice versa.

I just wish I'd had his feedback before I wrote my book, not afterwards!

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Old 09-09-2020, 10:28 PM   #2 (permalink)
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In the 1950s my cousins that lived on the Southern Oregon coast claimed the worst possible case is cresting a hill at high speed, into a 100mph headwind. Anecdotal, of course.
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Old 09-10-2020, 05:14 AM   #3 (permalink)
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It's really funny one. If you never drive over 80 km/h - 50 mph - I wouldn't worry about lift figures at all. Not at all.

But if you drive at 110 km/h - about 70 mph - then I think it becomes really important. Especially with, as you say, headwinds and humps. And even more so, with overtaking.

Between where I live (hamlet: population 150) and the nearest town (population: 1000) there is 10km (6m) of empty road where sometimes I drive very fast. It has a hump over a corner where the the car can get very light indeed. Then, having downforce is excellent.

But it's a bit like any area of car modification.
  • If you haven't experienced the difference that varying levels of acceleration throttle enrichment can make, you might assume: who cares?
  • If you haven't experienced the difference that varying levels of power steering assistance can make, you might assume: who cares?
  • If you haven't experienced the difference that varying levels of suspension damping can make, you might assume: who cares?

And so on.

Being able to change car behavior and then see how it feels is almost always a revelation.

It's exactly the same with lift/downforce.
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- Dr Wolf-Heinrich Hucho, the founder of modern vehicle aerodynamics
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Old 09-10-2020, 05:28 AM   #4 (permalink)
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Rob Palin's comments, and the SAE paper research I did for the book, start to show why lift/downforce is so important.

Having received his comments, I am tempted to start doing some high-speed logging of body pressures.

High-speed pressure logging would be quite cheap and fairly easy to do... but then I think - why would I bother? None of the car modification community (of any complexion) seems to be much interested in real aero data.

After all, even basics like simple averaging pressure measurements with an analog gauge apparently is like rocket science, beyond the reach of normal mortals. (Has even one person on this group done it? It costs under US$100 - for equipment that will last a life-time - and takes about 10 minutes. No, so much better to believe in the absolute BS being constantly spouted than find out the truth for yourself.)

It's all a bit like estimating the weight of an elephant by guessing its volume, its density, and the measurements taken from other animals. Like, you don't have scales to weigh the darn elephant? Oh I see, you can get them - but you can't be bothered.

After all - and get this - here even wool tuft testing is the stuff of mystery and fallacious interpretation! Fxxk me.

So I think: let someone else actually do some actual real on-road testing of high speed pressure variations. Just maybe you might find out new and really useful information. Hell, that would never do, would it...
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Old 09-10-2020, 01:17 PM   #5 (permalink)
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Quote:
Originally Posted by JulianEdgar View Post
After all, even basics like simple averaging pressure measurements with an analog gauge apparently is like rocket science, beyond the reach of normal mortals. (Has even one person on this group done it? It costs under US$100 - for equipment that will last a life-time - and takes about 10 minutes. No, so much better to believe in the absolute BS being constantly spouted than find out the truth for yourself.)
I have. And I just yesterday ordered the digital pressure gauge another poster suggested in a thread earlier this summer and two pitot tubes. (And your book--looking forward to reading it!).
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Old 09-10-2020, 02:34 PM   #6 (permalink)
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I'd like to install ride height sensors, but it's hard just to keep up with the car insurance payments. And pay down a Metro XFi.
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Old 09-10-2020, 05:49 PM   #7 (permalink)
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Quote:
Originally Posted by freebeard View Post
I'd like to install ride height sensors, but it's hard just to keep up with the car insurance payments. And pay down a Metro XFi.
It's actually pretty cheap to install ride height sensors. The major hurdle is in fact the time it takes to do it - it's not as easy as it first sounds.
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Old 09-10-2020, 06:25 PM   #8 (permalink)
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Quote:
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I have. And I just yesterday ordered the digital pressure gauge another poster suggested in a thread earlier this summer and two pitot tubes. (And your book--looking forward to reading it!).
What took you so long to buy it?!

All the expert feedback I have now received has identified only one mistake - the equation on changes in drag with top speed on Page 21. Dr Thomas Wolf of Porsche has a more accurate, and simpler, equation for this that he gave me.

Since the book was written, I have developed the throttle-stop method for testing drag changes, and now use the static port of a pitot tube as a pressure ref rather than a tank, so obviously neither of these techniques is in the book.
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- Dr Wolf-Heinrich Hucho, the founder of modern vehicle aerodynamics
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Old 09-11-2020, 12:56 PM   #9 (permalink)
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quick variations in lift

Quote:
Originally Posted by JulianEdgar View Post
As it happens, today Rob Palin (ex Tesla, Bentley and F1 aerodynamicist) has been in communication with me about lift (giving me feedback on that chapter in my book).

As always, what he has to say is fascinating. He states that one car company (that I am not going to name here) has these requirements:
  • Front coefficient of lift greater than rear coefficient of lift up to 15 degrees yaw
  • Actual forces need to be less than 10 per cent of each axle weight at top speed

He also emphasizes what I have been saying, in that steady state wind tunnel (and, he adds, short-run CFD) often doesn't capture the actual quick variations in lift. He writes:

In research I did back at MIRA, we saw easily 100% fluctuations in lift forces, on all manner of vehicles, with the associated spectrums varying wildly between vehicle shapes, and with individual modifications. A rear spoiler, for example, may have zero net effect on the mean lift, yet reduce the standard deviation of the forces away from that mean quite dramatically - and vice versa.

I just wish I'd had his feedback before I wrote my book, not afterwards!
Does that even matter if the vehicle's inertia, and suspension, dampen any actual physical excursions of the body?
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Old 09-11-2020, 01:04 PM   #10 (permalink)
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70-mph

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Originally Posted by JulianEdgar View Post
It's really funny one. If you never drive over 80 km/h - 50 mph - I wouldn't worry about lift figures at all. Not at all.

But if you drive at 110 km/h - about 70 mph - then I think it becomes really important. Especially with, as you say, headwinds and humps. And even more so, with overtaking.

Between where I live (hamlet: population 150) and the nearest town (population: 1000) there is 10km (6m) of empty road where sometimes I drive very fast. It has a hump over a corner where the the car can get very light indeed. Then, having downforce is excellent.

But it's a bit like any area of car modification.
  • If you haven't experienced the difference that varying levels of acceleration throttle enrichment can make, you might assume: who cares?
  • If you haven't experienced the difference that varying levels of power steering assistance can make, you might assume: who cares?
  • If you haven't experienced the difference that varying levels of suspension damping can make, you might assume: who cares?

And so on.

Being able to change car behavior and then see how it feels is almost always a revelation.

It's exactly the same with lift/downforce.
-----------------------------------------------------------------------------------
' Lift by itself is not a serious problem in the current passenger vehicle at highway speeds ( 70-mph ).' Kent B. Kelly, Harry J. Holcombe, General Motors Styling Staff, SAE Paper No. 649A, 1963, SAE Congress.

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