Weight vs downforce, Stability and grip
 

In the why don't cars have aerofoils thread that degraded from the debate I wanted, I was arguing that downforce and weight are essentially the same.
Julian claimed something like a few kg increase in downforce dramatically changed the way a car handled.

To me all downforce creates is extra vertical force on the tyres with no increase in mass to accelerate around bends.

If a few kg of downforce, (a few % of the cars weight) dramatically changes the handling, why does the same effect not occur when putting fuel in the car or something in the trunk/boot?

(I haven't read the thread you mention)

The only reason I can think is that you don't have to accelerate the mass. And I mean accelerate in a new direction, like turning a corner changes the direction of your momentum. Which pushes the car sideways a little bit whereas the downforce is a straight down and no mass/inertia in the given direction.

Putting fuel in the gas tank does affect handling. 1/2 empty tanks are worse still particularly doing Gymkhana where fuel starvation and over rich chew up precious seconds not to also mention having weight transfer to the outside during tight fast turns.

If you don't notice the effect, you're not close enough to the edge and aren't competitive even in beginners class.

Back in the day, I ran an 1156 lightbulb on a stainless CB radio whip antenna and broke the bulb contacting the pavement on a really tight fast turn.

Don't, it descended into rubbish. I did find the quote that I was thinking about though(further down)

That is all I can think of, but a few kg?

I notice my car gets slightly more understeery with lower fuel, but that is a 50kg difference, but I am not driving close to the edge on public roads.

Try sharp fast cornering with a below 1/2 tank of fuel, like a fast left hand (you're UK, correct?) Or even a roundabout on a rainy day. Try wet parking lots at midnight. It will get your attention.

Try the same experiment with:

• A Model A Ford or Porsche 356 with the gas tank in the cowl.
• A Ford Pinto with the gas tank behind the rear axle.

Any difference?

You answered your question already, that downforce creates more friction for the tires without increasing the amount of weight that has to be accelerated around a corner.

I wonder what moon races would look like? Suspension would be everything as the low G combined with bumps would cause loss of traction. The vehicle could be shaped like the broad side of a barn and it wouldn't cause drag.

My fuel tank is in the front, so I can understand why it would have less grip and why application of throttle which would lift the nose would cause more understeer.

Interesting, for my car it's the opposite.
I need to have a spare wheel in my trunk or my car doesn't drive properly, but my gas level doesn't change anything.
However my car is rather unconventional, the gas tank is almost perfectly in the center of gravity and the trunk is at the front axle.

Anyways, aerodynamic downforce =/= adding weight.
If you add weight, you have more force pushing down your tires but also more inertia to overcome, so it cancels out.
With aerodynamic downfoce you have more force pushing the tire down, but no additional inertia.
This allows the car to brake harder or to take corners faster.

This is probably not best suited to the aerodynamics section, but it is here now.

But it obviously doesn't cancel out, because if it did then removing the spare wheel would make no difference. The inertia is reduced, there is less grip, both proportionally by the same amount because the tyre grip can be assumed to be linear.

I suppose the question is really, why does a small change in weight/downforce have a big effect on some cars, yet a negligible change in others?
And why doesn't the grip loss balance, less weight at the front, less grip at the front. The only explanation I can think of is weight transfer, cg height, moment of inertia, or maybe tyre pressures too high for the new weight.

To be clear, I don't believe that the sole cause of increased stability in Julian's case was a few kg of downforce, I believe that it is mostly to do with the centres of pressure moving further back, but I think he would disagree.