What are the benefits of lowering a vehicle for racing

Question

Lowering a vehicle affects many aspects related to suspension geometry and aerodynamics. I am trying to understand these phenomena better. Note that although my question sounds like an opinion, it's not really my intent. I just wanted to describe how I understand things so that people can point me to the flaws in my understanding and wrong assumptions.

We read in many places on the internet that one of the main reasons for lowering a car is to reduce its center of gravity with respect to the ground, thereby reducing the body roll. This sounds like a myth to me, because as you lower your vehicle, you are affecting the suspension geometry, lowering the roll center (RC) more than the center of gravity (CG). As the force that results in body roll must be translated to torque by multiplying the distance between the RC and the CG, when you increase this distance you increase body roll. In this sense, you are counterproductive by lowering the vehicle and actually increasing the body roll in an attempt to decrease it. (I am purposefully neglecting the stiffer springs used to make a vehicle lower simply because it's not a good justification for lowering: you could have used stiffer springs that maintain the same ride height, or perhaps better yet, a stiffer anti-sway bar).

My invalidation of the lower CG argument relies on the assumption that your roll center is not corrected by adapting the suspension geometry. But even assuming you made corrections with say control arm spacers, you would still have been better off simply raising the RC and not lowering the car. There is still no point in lowering the vehicle. The torque that produces the body roll is independent of the vehicle height w.r.t the ground and fully dependent on the distance between the RC and the CG.

https://www.youtube.com/watch?v=Jgdpdtk3LSw

So again my question is why lowering race cars?

Another argument we see is aerodynamics. When thinking about aerodynamics I think of downforce. Downforce is generated by a higher pressure up top than down below. I cannot understand how lowering a vehicle is beneficial in this sense either. Assuming that your car has no angle (0 degrees between the underside plane and the road), and assuming that you don't have a rear diffuser, then you are certainly admitting a smaller amount of air through the front by decreasing the area facing the air on the front, but you are also reducing the volume underneath the car by the same factor ∆h (where ∆h is the amount by which the car was lowered). Following this reasoning, I don't see how the pressure underneath the vehicle could differ from the one above as a result of the lowering (it should still be atmospheric pressure in both places).

If you are using a rear diffuser, then I can see how reducing the volume underneath could help make the pressure even lower at the diffuser, thereby creating a downforce in the rear (since the increase in volume at the diffuser remains the same, while the total volume beneath the car decreases, so the proportion by which the diffuser lowers the air pressure by the Bernoulli principle is greater, and the downforce resulting from the difference of pressure between the top and bottom is increased).

One aerodynamic reason I CAN understand for lowering race cars is the reduction in the amount of air hitting the cross-section of the tires and wheel fenders, reducing the drag. But is this the only one or am I misunderstanding many things here?

Answer 1

I think the easiest way to realize your initial argument of why lower the center of gravity, can easily be shown as misguided by this one simple statement. If what you are suggesting is accurate, why not create race vehicles with the Center of Gravity (CG) 10 feet in the air? The simple reason is, your side to side stability would be gone. As you lower the CG, stability is improved.

Something to realize is, the CG and Roll Center (RC), while having a relationship, are two different things. By changing one, you do not necessarily change the other.

Two quotes found in the book, Tune to Win by Carroll Smith (pgs 29-30):

Center of Gravity:

THE CENTER OF GRAVITY of any body is defined as that point about which, if the body were suspended from it, all parts of the body would be in equilibrium - i.e. without tendency to rotate. It is the three dimensional balance point of the race car. All accelerative forces acting on a body can be considered to act through the center of gravity of that body. We want our race car's cg to be just as low as we can get it.

Roll Center:

THE ROLL CENTER of a suspension system is that point, in the transverse plane of the axles, about which the sprung mass of that end of the vehicle will roll under the influence of centrifugal force. It is sort of a geometric balance point. It is also the point through which the lateral forces transmitted from the tire's contact patches act upon the chassis.

The RC is measured by the geometric angles of the suspension, of which can change during vehicle operations, because the geometric angles of the suspension change during use. The CG will not change through operation of the vehicle (putting aside little things like movement of driver and fluids in the car, which would have some effect, but realistically very little).

I'm not sure what you mean by:

My invalidation of the lower CG argument relies on the assumption that your roll center is not corrected by adapting the suspension geometry.

I say that because the RC is completely adjusted and corrected by suspension geometry. How suspension geometry is setup dictates the RC.

This source states:

When cornering, centrifugal force is applied to the car’s CG, which tends to push the car to the outside of a corner. This causes the CG to rotate around the RC. Since the RC is below the CG, cornering force causes the car to rotate AWAY from the force. Hence, the car rolls to the OUTSIDE of the corner.

and continues with:

When the RC is far away from CG (lower RC), when the car corners the CG has more leverage on the RC, so the car will roll more.

When the RC is closer to CG (higher RC), when the car corners the CG has less leverage on the RC, so the car will roll less.

If the RC was right on top of the CG, when the car corners the CG has no leverage on the RC, so the car would not roll at all.

With this in mind, your main goal should be to have the CG as low as possible to make it easier to place the RC as high as possible, while still allowing the geometry not to be crazy.

On your question of aerodynamics and why. You already touched on the why, that being, the less air under the vehicle, the better. The more air you have going under the car, the more lift occurs on the vehicle. To combat this, engineers utilize three basic things: front splitter; under vehicle aero panels; rear diffuser.

The front splitter cuts the oncoming air. The portion of the air which is above the splitter is forced up, out, and around the vehicle. There is still some air which goes under the vehicle, but without the splitter, as the vehicle goes faster, more and more air is pushed under the car. More air means more pressure. More pressure means more lift. More lift means less stability at speed.

The under vehicle aero panels give what air does go under the car, less things to hit (i.e. smoother travel from front to rear of the car).

The rear diffuser is as you suggested as it helps, through the venturi effect, to suck the what air is under the car and throw it out behind the vehicle faster. This creates a vacuum effect under the vehicle, which pulls the car closer to the road. (Note: By "vacuum", I'm suggesting something less than ambient air pressure.) Think of the amount of surface space under the vehicle. Now, introduce a vacuum which is pulling down on all that surface. Say if you could get 1 lb per square inch of space, it could easily equate into hundreds of lbs of downforce.

By lowering the car, the amount of air which could possibly go under the car is diminished. The splitter provides the initial cut, but lowering the car means there's less air which could go under the car, causing less drag underneath allowing the car to slice through easier.