# Lift/downforce calculation help

#1

Hi

How can I calculate the actual lift forces of a car from the lift coefficient I have obtained from simscale?

Thanks

#2

You can actually calculate the forces on SimScale. you can add a Result Control Item (in simulation setup).
https://www.simscale.com/docs/content/simulation/resultControl/OF_forcesMoments.html?highlight=forces
This will give you force output plots and you can also copy the values from the Log as shown below:

If you have the coefficients only, then you have to convert it manually based on the formulation. See link below.

Best,
Ali

Front Wing target force values
I have done homework 1 several times but not yet received any simulation results?
Mesh error
#3

My problem is I would like to know the actual forces in lbs or kg. The force plots do not tell you this.

The link you pointed to only deals with a lifting body like an airfoil. The car body is more of a problem because
some of its surfaces are neutral some create lift and some downforce.
I was woundering if there was a way to work it out from the lift coefficient?

Best,
Ged

#4

Hi @gcanfield, I see what you are saying… I don’t think lift coefficient is what your looking for, it is not that useful for cars only really aerofoil profiles. Obviously it can be calculated for anything but I wouldn’t have said its accurate because you require a surface area to calculate lift from it. But what @Ali_Arafat was saying is you can select the surfaces you want to find the lift of, and calculate the force generated by the pressure around the surfaces.

This will be in newtons (kg.m/s^2) for x,y and z axis, which is what your after (probably z pressure forces depending on orientation). you can calculate this for the entire car by selecting all faces or selecting just faces of interest (such as rear wing).

I think this is the best way of doing it.

Does this help? if not if you provide a link to your project we can work through it.

Kind regards,
Darren Lynch

#5

Hi Darren

Thank you that makes sense and would make my life easier if the units are in newtons but I was under the impression that the units used in forces and moments were not of any particular unit of measurement? See previous posts below:

Hi,
I know I can measure down force of a car using Result control / forces and moments but I have two questions.

1. Are the results in a specific unit of measurement or are they just a numerical value for comparison?
2. Is it possible to measure the amount of down force on each of the four wheels individually ?

pfernandezPower User
27d

Hi Ged,

In incompressible flow the Navier Stokes are divided by the density so you end up with kinematic units (kinematic pressure, kinematic viscosity, etc.). If I’m not wrong, even though you can define a density, OpenFOAM will default its value to 1 when in incompressible. I usually stick to coefficients and then calculate the forces using the well known equations.

Regarding your second question, there’s no direct way to do that in OpenFOAM that I’m aware of. You can use bins to get the distribution of forces and then, with a static analysis, calculate the forces exerted on the front and rear wheels as in this project

https://www.simscale.com/projects/pfernandez/aerodynamic_analysis_of_the_tsc-03e/4

#6

Hi @gcanfield, this is true for incompressible however I believe this is why you define a density in the forces and moment result control, so if for example, you calculate downforce generated from your car at ground level you define the air density to be approximately 1.225 kg/m^3 and this will produce the answer you seek.

I think to calculate a lift Coefficient you need to define a profile width and then using the coefficient, along with surface area, density, and velocity you can calculate lift. this works well for wings and profiles but not so well on entire car. Although you could use it on aerofoils on the car design. However, this still wouldn’t be my preference.

However, I will run a simulation this evening just to ensure that I’m correct.

1. Are the results in a specific unit of measurement or are they just a numerical value for comparison?
Confirming with a simulation this evening I believe the unit is Newtons
2. Is it possible to measure the amount of down force on each of the four wheels individually ?
Yes, create four forces and moments controls and assign them to all faces for each tyre individually.

@Ali_Arafat am I correct with units? it may be handy to include units on the plot and documentation?

Kind regards,
Darren Lynch

#7

Hi @1318980

Thanks for your time with the forces experiment . I also tried a couple of simulations and agree the forces at that density do seem to be in newtons. I also created four forces and moments controls and assigned them to all faces for each wheel individually but this does not work. The results you get are just for the wheels and nothing connected to them so takes no account of the car shape. any other ideas?

Kind regards
Ged

#8

Hi @gcanfield, sorry so you want to find the force on each wheel due to the downforce of the car?

if so couldn’t you couldn’t you use pressure moments from an arbitrary centre and use the distance in the x,y and z from the centre to the wheels to calculate the force? so you could select all faces of your car except the wheels.

This would be my thinking, am I understanding your problem correctly? If not maybe you could elaborate more on what your project is and what you hope to achieve? are you finding forces on the suspension or something or just how much down force to see how much traction the car has? Sounds very interesting.

Kind regards,
Darren Lynch

#9

Hi @1318980
Yes that is correct downforce of the car. In actual fact what im working on is racing sidecars. I need to know the downforce on each wheel for balance purposes. The problem is sidecars have only 3 wheels all on different tracks which is making it even more complicated. any suggetions ?

kind regards
Ged

#10

Yes, ok @gcanfield. I think what I said about the moments should work. make a cad model with the origin somewhere in the middle of the geometry. Use the Coordinates where the wheel meets the ground as distances in x,y and z to get forces onto the road from the wheel and set your centre of rotation to be the origin. You might as well select all faces for this.

I think this link helps:
http://www.ce.memphis.edu/2131/PDFsF12/Moments%20in%203D.pdf

This might seem complicated at first but Matlab can make quick work of this if you are familiar with it.

Solving as a static problem we can double check the calculation using sum of forces due to moments in each direction across all wheels should equal the total force in each direction (also calculated by SimScale).

This is the way I would do it, I can’t think of any other way of doing this.

Kind regards,
Darren lynch

Drag Coefficient result?
#11

Thanks Darren I will have a go

Regards
Ged