Aerodynamic lift- A real problem

[NeilBlanchard]

That is an incomplete list: creating lift and creating down force are accompanied by increased drag. If I am not mistaken, low drag also means that the vehicle is low lift and low down force.

[EngineeringStudent]

you're on the right track. even in an airfoil, which has relatively low drag, does create lift-induced drag:
Lift-induced drag

when you're designing anything, you need to look at all factors involved. you can't just focus on one thing and expect everything to be fine. gotta think outside the box!

here's a bit from the new audi a8 that was referenced elsewhere:
"Low lift coefficients at the front and rear axles ensure confident stability at highway speeds. "
2011 Audi A8 revealed amidst the stars at Design Miami 2009 — Autoblog

like before, it's pretty much a trade-off. without dramatically changing your car's shape, some mods that reduce overall drag might increase lift, so it all depends on what your goal is. but like the audi, its fully possible to have both.

[Frank Lee]

Audi politically has to make a big deal out of it as their name is associated with excessive lift. I think it's been shown to be a non-factor amongst post-'60s vehicles at U.S. legal speeds.

However if anyone comes up with better real data that would sure be nice to look at.

[Nerys]

On a heavily loaded vehicle SURE maybe. but on a lightly loaded vehicle I don't think it will make enough difference to offset the drag it adds. ie even down to 15 pounds my metro barely looks like the tires are low IE its very lightly loaded already. so the bearings are not working very hard and the tire patch is not being enlarged very much by the mass of the car IE not much drag to "remove" from rolling resistance.

[Christ]

I dont' think anyone ever had a question regarding the benefits of lift, negative or positive in nature. The question that commonly comes into play is that for all intents and purposes, any designed lift in a shape increases it's Cd.. which means that it takes more power to do the same work against wind resistance. Negative lift puts pressure on the tires which increases rolling resistance, and stresses the drivetrain more.

We're trying to be more efficient, not worry about whether or not the car, which is already designed to stay on the ground, is going to stay on the ground.

[Frank Lee]

Just for fun I plugged some numbers into a lift equation:

LIFT = (0.5) * Cl * DENSITY * SPEED * SPEED * AREA

I used a Tempo for the area figures and a Holden Commodore's front lift coefficient... just because I have nothing better.

Worked out to 19.6 lbs front end lift at 60 mph FWIW if I did it right which is remarkably close to my WAG of 20.

Grows to 78 lbs front at 120 mph. No worries, 2.3 a/t Tempo won't see 120 mph unless it's going off a cliff.

Recall, about 2000 lbs down force on the front end via gravity.

Yeah, 20/2000=1%.

I should add, how to define "front end" area? I used the total length/2. I suppose do the calc again for the rear, add em, and get total lift.

[Christ]

Originally Posted by Frank Lee

Just for fun I plugged some numbers into a lift equation:

LIFT = (0.5) * Cl * DENSITY * SPEED * SPEED * AREA

I used a Tempo for the area figures and a Holden Commodore's front lift coefficient... just because I have nothing better.

Worked out to 19.6 lbs front end lift at 60 mph FWIW.

HOLY **** SHE'S GONNA FLIP!!!

:chuckle:

[shovel]

Originally Posted by EngineeringStudent

so lift is a trade-off. you can either get less rolling resistance with lift, or more control with less/negative lift.

less rolling resistance is fine, but lift/downforce both come with drag penalties. the energy to push or pull on something (up or down) isn't free.

[Frank Lee]

As I previously alluded to, I think the misinterpretation of this article is the root of all this nonsense:

Automotive Aerodynamics - Sport Compact Car Magazine

and this is probably the passage that traumatized poor Squirmie so:

The result is lift, a curse to almost all production cars. For example, the 1995 BMW M3 has a lift coefficient (CL) of about .34, which means that a lifting force of approximately 500 pounds is generated by the body at 100 mph.

... which seems like an error of some sort to me because no other similar cars for which I've looked at the data are anywhere near close to that.

Were I to run the equation again with the BMW M3 l x w the lift numbers would be even smaller because the car is somewhat smaller.

I bet it was supposed to say 50 lbs.

[Christ]

Frank -

Sorry, just felt like posting jibberish.