uploaded 7/26/2000
Difference Between Weight and Downforce
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You may be wondering why, if aerodynamic downforce can increase
cornering speed, does a lighter car corner faster? Why is vertical
load provided by aerodynamics different than vertical load provided
by weight? Those are good questions! We’ll start answering
them by talking about friction.
Friction
Friction supplies the resistance to sliding that we use everyday.
Without friction we couldn’t walk or crawl and nails wouldn’t
hold.
Consider a block of some material, a cube about an inch on
a side. The block is lying on a surface, say a table. The block
has some weight because of gravity. We’ll call that weight
Fv, the vertical force, because it acts straight down. If you
push on the block with a force (F) parallel to the table, you
can make it slide on the surface. If you push hard, it will slide
right off the table. If you push against it very lightly, it
won’t move.
Intuitively, you know a block of wood will slide with a lighter
push than a hunk of rubber, and the wood block will also be easier
to move than a block of lead. Why is that?
This force needed to overcome friction is bigger if the block
weighs more, but it also depends on the properties of the surfaces
in contact—their coefficient of friction. The equation that
describes this is:
Ff = Cf x Fv. That reads: Friction force equals the
Coefficient of friction times the vertical Force.
You can see from this equation that the friction force is
larger when Cf is larger. Cf is why you have to push a rubber
block harder than a wood block. Rubber sliding on anything has
a higher Cf than wood sliding on that same material.
Fv, the vertical force, is why it takes more force to slide
a lead block than a wood one. The lead is heavier and so Fv is
bigger than with a wood block of the same size.
How about some numbers? Rubber has a relatively large friction
coefficient when tested on most surfaces. Let’s say it’s
0.8. If the rubber block weighs 1 pound, then the vertical force
is 1 pound, and it’s going to take 0.8 pounds of force to
push the block at a steady speed. That came from this calculation:
0.8 (Cf) times 1 pound (Fv) = 0.8 pound (Ff).
Let’s add some vertical force to the block. We could
just place a piece of lead on top of the wood block but that
wouldn’t be very interesting. Instead, let’s put an
upside-down wing on top of the block, blow some air over the
wing, and produce a downforce of 9 pounds. Now, Fv is 10 pounds
(1 pound of weight and 9 pounds of aero force), and it takes
8 pounds to move the block. That came from the calculation: 0.8
(Cf) times 10 pounds (Fv) = 8 pound (Ff).
You’re right, there’s some aero drag force too but,
since we’re in control here, we can rotate the wing and
blow the air at right angles to the path of the block. Then,
drag forces don’t act in the same direction as the friction
forces and don’t affect our numbers. Anyway, we’ve
got downforce in excess of the weight of the block and gained
a lot of friction force without adding weight to the block. What
about the weight of the wing? OK, we made the wing out of Unobtanium
which has no weight.
These calculations show we can add downforce to a racecar
with a wing and get more friction force from the tires, but it
doesn’t answer the part of the question about why a lighter
car can corner faster than a heavier car. We’ll answer that
in the next installment of this series.
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