## Aerodynamic downforce v weight?

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DeltaVee
0
Joined: Sun Oct 11, 2015 1:36 pm

### Aerodynamic downforce v weight?

OK, here's a question that has puzzled me for a long time.
The spoilers on an F1 racing car produce down-force to hold the car on the track. They produce the most down-force when the car is on the straight, going at it's fastest, exactly the time when the car needs to be as light as possible! They are at their least effective when the car is in the slow corners and the car needs to be pushed down onto the track as hard as possible. Effectively, the spoilers give the car the worst in both circumstances.

Here's a thought. Why not make the car heavier and turn the spoilers the other way round so that they actually generate lift? That way, the weight of the car will hold it down in the slow corners, but on the fast straights the spoilers will generate lift so that the car is effectively lighter!?

OK, it would take a bit of "tweaking" to get the best compromise between weight and lift, but that's already true of spoilers generating down-force.

Any thoughts??

Alan Marlow

Belatti
85
Joined: Tue Jul 10, 2007 8:48 pm
Location: Argentina

### Re: Aerodynamic downforce v weight?

Hahahahahahahahahahahahahaha

Last edited by Steven on Sun Oct 11, 2015 8:19 pm, edited 1 time in total.
Reason: Merged posts
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bhall II
861
Joined: Thu Jun 19, 2014 7:15 pm

### Re: Aerodynamic downforce v weight?

Hovepeter
1
Joined: Fri Aug 30, 2013 1:10 pm

### Re: Aerodynamic downforce v weight?

first of all weight is a problem at the straight under the acceleration, but drag is too, so if you maked the car heavier, and still have the same wings producing drag it would defiantly just accelerate slower.

but the car will also be slower in corners, because the weight of the car is a bigger problem than the ekstra friction on the tyres can compensate for. all the ekstra weight wants to get the opposite way, when you drive into a corner, so you can't drive as fast without going of the track.

weight is one of the biggest enemies, if not the biggest one for a racecar, because its bad in corners, its bad when you accelarate and its bad when you are braking

Vyssion
139
Joined: Sun Jun 10, 2012 1:40 pm

### Re: Aerodynamic downforce v weight?

Essentially, the generation of downforce is the way that allows for the car to (in lamans terms) "be heavier than it actually is, without the bad stuff that comes with an increased weight".

The downforce and the normal (weight) force of the vehicle is the simplified mechanism by which the tyres gain their grip. And again, over-simplifying things for a purely aero discussion, the more grip you have, the faster you can theoretically corner at.
$F=(coefficient of friction)*N$ (Can't display an actual 'mu' symbol so will refer to it as C_f from here)

The "N" denotes normal force which is made up of the vehicles weight force and any downforce currently being produced.
$F=C_f(mg + F_d)$

If you combine this equation with the formula for centripetal acceleration (which could be adapted to fit a corner if a constant radius)
$F_l_a_t_e_r_a_l=(mv^2)/r$
and rearranged to give:
$v=sqrt((F_l_a_t_e_r_a_l*r)/m)$

Then you get the following formula:
$v=sqrt((Cf(mg+DF)*r)/m)$

This then shows that for a constant coefficient of friction, that velocity is proportional to $sqrt(g+(F_d/m))$ (meaning that an increase in the right hand side of the equation will increase the left hand side). If this is differentiated to get this in terms of time, it shows that time is proportional to $1/sqrt(g+(F_d/m))$ (or simply that it is inversly proportional).

The important term here is this $F_d/m$ which is often referred to as the "specific downforce" of the vehicle. So if we are to increase this term, by means of increasing downforce or reducing the cars mass, the theoretical maximum velocity we can corner at will increase and hence the time taken to travel the corner will decrease. Add in that since you can carry more speed through the corner, braking time is reduced in the lead up to it and the acceleration beyond it begins from a higher speed, and you can begin to see the benefits.

By increasing mass initially, this would have the effect of decreasing this term, which would be counter productive.

This argument here is purely from an aerodynamic standpoint (and extremely simplified!!!) though and doesn't even mention things like the weight transfer or suspension changes and other vehicle dynamic effects that would be required to handle the higher initial weight during a race. But hopefully it helps explain why it is the way it is!!
If you can't explain it simply, then you don't understand it well enough.
- Albert Einstein

#aerosaruman

DiogoBrand
68
Joined: Thu May 14, 2015 6:02 pm
Location: Brazil

### Re: Aerodynamic downforce v weight?

At least we can't blame him for not thinking outside the box.

By the way, very good post by Vyssion.

PlatinumZealot
328
Joined: Thu Jun 12, 2008 2:45 am

### Re: Aerodynamic downforce v weight?

DeltaVee wrote:OK, here's a question that has puzzled me for a long time.
The spoilers on an F1 racing car produce down-force to hold the car on the track. They produce the most down-force when the car is on the straight, going at it's fastest, exactly the time when the car needs to be as light as possible! They are at their least effective when the car is in the slow corners and the car needs to be pushed down onto the track as hard as possible. Effectively, the spoilers give the car the worst in both circumstances.

Here's a thought. Why not make the car heavier and turn the spoilers the other way round so that they actually generate lift? That way, the weight of the car will hold it down in the slow corners, but on the fast straights the spoilers will generate lift so that the car is effectively lighter!?

OK, it would take a bit of "tweaking" to get the best compromise between weight and lift, but that's already true of spoilers generating down-force.

Any thoughts??

Alan Marlow

Your reasoning has a few fallacies. You have to carefully separate your forces: Weight, Lift/downforce, drag and thrust, lateral grip. Then there is something that is not a force.. that is the mass of the car.. (this remains the same no matter the net acceleration of the car).
Down-force is not weight. Down-force is not drag (I see alot of seasoned members chat this crap to this day!). The thrust comes from the power put down to the ground (hey keep that wheel spin down).. then there is the lateral reaction force of the tyres that keeps the car on the path you want it to go (or hope it will go on!).

For a car to go fast you want to minimize the mass of it as much as you reasonably can. This mass is a resistance to acceleration. So a heavy car will accelerate slower. Downforce won't affect this too much, but Drag will though! A high downforce wing tends to be draggy... but if you can make it efficient you can get more downforce out of it or reduce the drag on it.

At the end of the straight you don't want the car to be too "light" (lacking downforce!) as you say it because the tyres make the most grip with downforce! and you need that grip to brake in time for the corner. haha.. A light (lacking mass!) car stops better too. See the confusion of how light can have two meanings? Mass is no way related to downforce! That is why you are puzzled.

When Colin Chapman famously said "Add lightness!" he wasn't telling the people to take off the wings he was telling them to reduce the mass of the car. lol
"The true champions are also great men. They are capable of making difficult decisions, of admitting their mistakes and of pushing harder than before when they get up from a fall."

- Ferrari chairman Sergio Marchionne

mep
34
Joined: Sat Oct 11, 2003 2:48 pm
Location: Germany

### Re: Aerodynamic downforce v weight?

Has anybody ever considered that this guy is just taking the piss of you guys?
I don’t believe that somebody who talks about the efficiency of a wing does not know the effect of inertia.

Vyssion
139
Joined: Sun Jun 10, 2012 1:40 pm

### Re: Aerodynamic downforce v weight?

DiogoBrand wrote:At least we can't blame him for not thinking outside the box.

By the way, very good post by Vyssion.
Haha, thanks mate!!
mep wrote:Has anybody ever considered that this guy is just taking the piss of you guys?
I don’t believe that somebody who talks about the efficiency of a wing does not know the effect of inertia.
He may be, but I'm sure that there are people out there somewhere who may wonder the same thing genuinely... but lets hope that they are few in number!!
If you can't explain it simply, then you don't understand it well enough.
- Albert Einstein

#aerosaruman

gambler
1
Joined: Sat Dec 12, 2009 6:29 pm

### Re: Aerodynamic downforce v weight?

I can see how one would misinterpret if windtunnel numbers for "down" were say 3500lbs and the car weighs 3500lbs, it would not be like the engine is pushing a 7000lb car. I dont understand the math formula, but I get what Platnum is saying.

Paul
21
Joined: Wed Feb 25, 2009 6:33 pm

### Re: Aerodynamic downforce v weight?

Although at some point the engine wouldn't mind losing the downforce, because rolling resistance.

bhall II
861
Joined: Thu Jun 19, 2014 7:15 pm

### Re: Aerodynamic downforce v weight?

Antonio Pizzonia ran an upside down rear wing flap at the Monza round of the 2002 International F3000 season.

I don't think it worked very well.

ChrisF1
28
Joined: Thu Feb 28, 2013 8:48 pm

### Re: Aerodynamic downforce v weight?

It must have worked reasonably well, he took second place, which was his best result of the season (Before being Disqualified due to the aforementioned wing )

stijnhesse@gmail.com
0
Joined: Tue Mar 13, 2018 1:13 pm

### Re: Aerodynamic downforce v weight?

Vyssion wrote:
Sun Oct 11, 2015 4:46 pm
Essentially, the generation of downforce is the way that allows for the car to (in lamans terms) "be heavier than it actually is, without the bad stuff that comes with an increased weight".

The downforce and the normal (weight) force of the vehicle is the simplified mechanism by which the tyres gain their grip. And again, over-simplifying things for a purely aero discussion, the more grip you have, the faster you can theoretically corner at.
$F=(coefficient of friction)*N$ (Can't display an actual 'mu' symbol so will refer to it as C_f from here)

The "N" denotes normal force which is made up of the vehicles weight force and any downforce currently being produced.
$F=C_f(mg + F_d)$

If you combine this equation with the formula for centripetal acceleration (which could be adapted to fit a corner if a constant radius)
$F_l_a_t_e_r_a_l=(mv^2)/r$
and rearranged to give:
$v=sqrt((F_l_a_t_e_r_a_l*r)/m)$

Then you get the following formula:
$v=sqrt((Cf(mg+DF)*r)/m)$

This then shows that for a constant coefficient of friction, that velocity is proportional to $sqrt(g+(F_d/m))$ (meaning that an increase in the right hand side of the equation will increase the left hand side). If this is differentiated to get this in terms of time, it shows that time is proportional to $1/sqrt(g+(F_d/m))$ (or simply that it is inversly proportional).

The important term here is this $F_d/m$ which is often referred to as the "specific downforce" of the vehicle. So if we are to increase this term, by means of increasing downforce or reducing the cars mass, the theoretical maximum velocity we can corner at will increase and hence the time taken to travel the corner will decrease. Add in that since you can carry more speed through the corner, braking time is reduced in the lead up to it and the acceleration beyond it begins from a higher speed, and you can begin to see the benefits.

By increasing mass initially, this would have the effect of decreasing this term, which would be counter productive.

This argument here is purely from an aerodynamic standpoint (and extremely simplified!!!) though and doesn't even mention things like the weight transfer or suspension changes and other vehicle dynamic effects that would be required to handle the higher initial weight during a race. But hopefully it helps explain why it is the way it is!!
Hi there I'm new to the forum and I just started working on the aerodynamics of a Formula Student car. But as a Mechanical Engineering student I'm not familiar with aerodynamics...yet! . As I tried to use the rewritten formula created by Vyssion our vehicle looked like it needed an enormous amount of downforce of 5000N at 17m/s(!!)

The rearranged equation: F.downforce = (v^2 * m)/(''mu'' * r) - m*g

v = 17m/s
m = 350 kg
''mu'' race tyre = 1.2
r = 8m
g = 9.81N/kg

But how is this possible, did I make a mistake? As Vyssion said, this is a simplified formula, but can it diffur that much from this result? And what would the full equation look like?

A lot of questions, but I hope somebody can help me out

Kind regards,
Stijn Hesse

Greg Locock
159
Joined: Fri Jun 29, 2012 11:48 pm

### Re: Aerodynamic downforce v weight?

pulling 3.6g at 17 m/s is a bit unreasonable.