Setup question: ride height

[Giovaneveterano]

Hi folks. I'm a bit confused regarding the correct way to set up the ride height of an aero-type car (GT, open wheels, Proto...)

The lower the better?
I generally keep the same space from the tarmac as I change suspensions stiffness to get the same camber/aero balance.
Some tips?

 

[Neilski]

@RasmusP I think Neil doesn't read the messages on telegram...

Mmm, yeah certainly not all of them

 

[Mr Deap]

Will have a go, ta.
Is that physically realistic though? My (admittedly weak) understanding of car aerodynamics led me to believe that lowering a car (keeping rake the same) would always reduce the air pressure under it and therefore increase downforce.

It's the opposite. You need low pressure created under the car to get downforce by using the car movement.

If no air under it or almost none, there's not enough air under the car to attract the high pressure to the low pressure area to become neutral. If the car is set too low the whole low pressure area goes behind the car instead, thus you get more drag instead of downforce. It's like having the angle of attack too high creating a stall.

Lift to Drag Ratio:
https://www.grc.nasa.gov/www/k-12/airplane/ldrat.html

Aero Dynamic force:
https://www.grc.nasa.gov/www/k-12/airplane/presar.html

Basically drag/downforce/lift are the same force, if you remove weight in the equation... it make people explain it all weird & make it complicated like nobody business. I prefer to see it as difference in variation in pressure & center of pressure that create that dynamic force as it is easier to understand.

 

[Giovaneveterano]

I asked a question.
I have dozens of hours of awesome engineering stuff to read.
Gosh, I love this community

 

[Neilski]

Lift to Drag Ratio:
https://www.grc.nasa.gov/www/k-12/airplane/ldrat.html

Aero Dynamic force:
https://www.grc.nasa.gov/www/k-12/airplane/presar.html

Yes, I'm familiar with quite a bit of that stuff already. The treatments in most places though (like the NASA links) are purely 2D, and I was intrigued to see the relevance of the 3D stuff in one of the car-related posts above. (The 3rd dimension in this instance is the finite width of the car.)

 

[dud]

It's the opposite. You need low pressure created under the car to get downforce by using the car movement.

If no air under it or almost none, there's not enough air under the car to attract the high pressure to the low pressure area to become neutral. If the car is set too low the whole low pressure area goes behind the car instead, thus you get more drag instead of downforce. It's like having the angle of attack too high creating a stall.

Lift to Drag Ratio:
https://www.grc.nasa.gov/www/k-12/airplane/ldrat.html

Aero Dynamic force:
https://www.grc.nasa.gov/www/k-12/airplane/presar.html

Basically drag/downforce/lift are the same force, if you remove weight in the equation... it make people explain it all weird & make it complicated like nobody business. I prefer to see it as difference in variation in pressure & center of pressure that create that dynamic force as it is easier to understand.

Hmmm, interesting point about comparing to the angle of attack. I am still not entirely convinced that having a certain minimum amount of air entering the car bottom does create downforce in the forward areas. Out of curiosity, how does a different in height between front and rear of the car play into this theory? Do you require the rear (before the diffuser area) to be a bit higher?

In any case, the diffusers create a lot of drag. Even though they are not in the airstream over the car. That is why the Ferrari 488 has a DRS system that "shortcuts" most of the diffuser at a certain speed (it is in AC, you can see it in F5 mode from behind).

I asked a question.
I have dozens of hours of awesome engineering stuff to read.
Gosh, I love this community

Me, too.

 

[bigbawmcgraw]

Out of curiosity, how does a different in height between front and rear of the car play into this theory? Do you require the rear (before the diffuser area) to be a bit higher?

As i understand it, the rear needs to be higher because the rear wing pushes the rear end down at speed. The idea is to have it as level as possible when moving. This is where suspension stiffness comes into play too.

 

[dud]

As i understand it, the rear needs to be higher because the rear wing pushes the rear end down at speed. The idea is to have it as level as possible when moving. This is where suspension stiffness comes into play too.

But wouldn't a contiguously widening space under the body create more underpressure?

Looking at AC data, while I am unable to retrieve the lift coefficients from Python (*), I have a suspension height display and that indicates that the angle of the underbody doesn't really change for common cars with strong aero of some kind. That includes cars with and without big wings. For example 991 GT3RS and 991 R both go down pretty linearly and level with speed. I find that to be a bit odd.


(*) http://www.racedepartment.com/threads/is-anybody-able-to-get-acsys-cs-aero-from-python.146011/
Any ideas there? Anybody able to get the Aero values in Python?

 

[mclarenf1papa]

As i understand it, the rear needs to be higher because the rear wing pushes the rear end down at speed. The idea is to have it as level as possible when moving. This is where suspension stiffness comes into play too.

On race cars, typically not. Usually optimum downforce is achieved at some rake angle, so the goal is to be at that angle at cornering speeds. Drag is usually lowest at zero rake (or in any case at an angle lower than when optimum downforce is achieved), which is why rear springs are often softer than fronts on LMP cars (rake at medium speed, no rake at high speed).

Both the front and rear will be pushed down by downforce; engineers adjust the springs to achieve the desired effects on ride height.

 

[Mr Deap]

Hmmm, interesting point about comparing to the angle of attack. I am still not entirely convinced that having a certain minimum amount of air entering the car bottom does create downforce in the forward areas. Out of curiosity, how does a different in height between front and rear of the car play into this theory? Do you require the rear (before the diffuser area) to be a bit higher?

In any case, the diffusers create a lot of drag. Even though they are not in the airstream over the car. That is why the Ferrari 488 has a DRS system that "shortcuts" most of the diffuser at a certain speed (it is in AC, you can see it in F5 mode from behind).

The high pressure make most of the downforce for the front of the angle while the low pressure pull under the rear leading to an equal term when the air is traveling the object in question. When there's too much angle & not enough air flowing under, it create a low pressure area above the back end of the angle which create a lift & drag at the back.

Diffuser reduce drag. Instead of getting that drag force(or suction point or low pressure) pulling the car without a diffuser, you get force(or suction point or low pressure) pulling the car more downward. Drag/downforce/lift are the same force, if you remove weight in the equation.

About the 488 GTB diffuser, it probably has to do in how the upper holes channeling at speed in combination to the angle of the diffuser which result less drag. The diffuser on the 488 is too short to promote downforce.

EDIT: I believe it's mostly there to reduce drag more than anything else.


​

 

[Glaurung]

On race cars, typically not. Usually optimum downforce is achieved at some rake angle, so the goal is to be at that angle at cornering speeds. Drag is usually lowest at zero rake (or in any case at an angle lower than when optimum downforce is achieved), which is why rear springs are often softer than fronts on LMP cars (rake at medium speed, no rake at high speed).

Both the front and rear will be pushed down by downforce; engineers adjust the springs to achieve the desired effects on ride height.

The magic bullet of the active suspensions

 

[bigbawmcgraw]

But wouldn't a contiguously widening space under the body create more underpressure?

Looking at AC data, while I am unable to retrieve the lift coefficients from Python (*), I have a suspension height display and that indicates that the angle of the underbody doesn't really change for common cars with strong aero of some kind. That includes cars with and without big wings. For example 991 GT3RS and 991 R both go down pretty linearly and level with speed. I find that to be a bit odd.


(*) http://www.racedepartment.com/threads/is-anybody-able-to-get-acsys-cs-aero-from-python.146011/
Any ideas there? Anybody able to get the Aero values in Python?

Yep, with race cars the downforce would be more linear. I was using an F1 car (I should have made that clear) as an example where the rear wing exerts more pressure than the front wings.