I have had may of really old school people say that if you want a stable car at high speeds you need weight. "oh, I had a 55' chevy that we could run 100mph and it was solid as a rock" they say. 100 might have been fast 35 years ago. ( the math doesn't add up because the people in question had it used) . the latest was a kid I work with was talking about hitting 140 in his eclipse. he said he backed down due to fear. our resident old fart said his car was too light.

I have a problem with this. how in the hell does adding more weight make for a stable car? I thought stability came from airflow and downforce. low pressure areas and suspension travel.

I asked about handling and braking and the only resopnce I get was that Nascar cars are over 3000 lbs or some tripe like that.

Is that what the old guys did back in the day, get a bunch of fat chicks to go fast?

I am looking for a description on what is really true, and the different sides of the arguement, but most of all I want the truth.

 

Formula 1 cars weigh 1300lbs and go 200mph+ CART cars go 240+ and weigh 1500lbs. More weight does not nessesarily mean more stability, a lot of it, especially at very high speeds is indeed airflow and suspension setup. A heavy car will be more stable than a lighter car in general at high speed, but if you keep the air from getting under a car, and add downforce, the lighter car can become much more stable. Most cars from the factory are not designed for 150mph+, and they dont have low enough front air dams, thereby letting too much air under the car.

 

A heavier car can give the feeling of being more stable, just by it having greater inertia (or polar moment of inertia, depending on how the two cars are different..). Other factors can lie in the car's alignment; steering axis inclination, caster (mechanical trail, coming directly across in the steering system)

Aerodynamics are paramount at high speeds.. just from driving on the highway with a hand out the window, the effects of the aerodynamics can be felt, first hand (uh.. sorry =] ).

 

exactly. the reason stock first gens feel sorta unstable at high speed is because that stock front valance welcomes air under the car and it begins to "lift" a bit. A heavier car will resist this "lift" in the front, due to the more weight it has.

 

Don't forget Top Fuel dragsters.

6,000+ horsepower, 2,150 lbs. with driver, and 320+ mph in the quarter mile.

Front wing: ~1,800 pounds of downforce
Rear wing: ~6,200-6,500 pounds of downforce

 

Downforce and the power to overcome it is the best combo no matter how light the car is, as shown by the above facts from different types of cars. Often if an engine builder found a bit more power the team would just add more wing. Lacking in either dept is usually not competitive or downright scary.

 

downforce is like creating extra weight.

 

More or less, but adding more weight isn't an efficient method for increasing downforce.

Using spoilers, wings, or canards attached to the vehicle to divert airflow and push the car downward and keep it solidly planted on the pavement at high speed carries no penalty at lower speeds like adding static weight would.

And no matter how heavy a vehicle is, if its aerodynamics generate enough lift at a given speed, it turns into an airplane...

 

You don't need downforce to be stable, you just need zero lift.

 

Actually, you do need more downforce than required to simply negate lift in order to increase tire traction for cornering, braking, and accelerating at high speeds.

Top Fuel dragsters are generating far more downforce than their weight requires in order to put their power to the ground. A dragster launches at about 17% power and the adjustable clutch packs gradually apply more power during the course of the run. They never reach full clutch lock-up during the course of the run, because the full 6,000+ horsepower would simply anihilate their tires, even with over 3 tons of downforce on the rear of the car. In order to keep the tires planted, maximize contact patch, and therefore maintain traction with that kind of power, far more downforce is required than would be required to just generate "zero lift".

The same goes for CART cars. They generate enough downforce at 100 mph to keep the car suspended upside down if it maintained that speed. Downforce at that point equals the weight of the car and driver, far more than just "zero lift". At speeds up to 240 mph, they're generating far more downforce than is required to simply negate lift in order to keep the tires planted for cornering, braking, and accelerating, as mentioned above.

"Stable" is a relative term. If by "stable" you mean "not becoming airborne", then yes, zero lift is probably all that's required in a straight line. For maximum performance at speed, and especially for turning corners, you want more downforce than would be required simply to have zero lift.

 

Making the car heavy so that if feels stable at high speed is the old school way of doing it.

Its not thier fault they didnt have good aero back then....weight was all you had to combate the lift

Give him the examples of Cart and see how he explains that. nascar needs it cause thier cars suck

STEPHEN

 

I agree Jim; I was referring to driving in a straight line on the highway. If the car is not accelerating or turning it has no need for downforce, only zero lift.

I for one would like to have a hydraulicly actuated wing like what's on top of the WOO cars

 

No it is not. Down force is aerodynamic drag, not weight. It applies more downforce as the speed increases, not weight. This downforce does not make the car want to go straight off of a turn as extra weight would. Weight has inertia and downforce does not.
Sure you need more power to move through that drag at the same rate as a car that did not have any aero and weighed the same, but you can go through a corner a hell of a lot faster.

 

I have always wanted to come up with words that say that but couldn't. Thanks.

 

Agreed. Excellent post, RX-Heven

 

That's because weight is not equal to mass. Weight is an effect gravity has on mass. When you add weight, your tires will grip harder, but the extra mass (and thus, intertia) increases. Since your tires do not double in grip when you double in weight, increasing the weight by increasing mass is not the way to go

 

Also, remember stable and good handling are sort of opposites... A "stable" car will have tons of intertia and polar moment of inertia as mentioned above, and will be very unresponsive when you try to turn it.

 

Not true at all. I can make a paper airplane that flies perfectly straight.

 

Downforce is aerodynamic lift, not drag. It just happens to be lift applied in a negative vector. Your tradeoff between increasing downforce is always going to be some factor of increased drag, increasing the amount of thrust required to move the vehicle faster.

Aico is actually kind of right. In effect, downforce increases the amount of effective weight at the wheels. The car doesn't actually mass any more (which would have effects on inertia), but there's positive force pushing the vehicle down into the ground, more than just gravity. A tires ability to generate friction is proportional to the amount of force it is able to apply to the ground (up to a point, but that's all tied up in tire compounds and structures). That friction is what allows you to create the lateral forces to turn and accelerate the vehicle.

A vehicle, benefits tremedously from being as light as possible, in most performance requirements. However, F=M*A. A lighter vehicle requires less in the way of force to generate sideways or vertical acceleration, which can make the car unstable. Adding downforce to such a car allows the tires to generate enough grip to alleviate those issues.

 

I don't know what you think is stable or handles good, but I don't think I want to drive it. But if you mean a loose car is a fast car (through turns) then you could be right (Id love to drive it), but then again you dive into the ambiguous world of 'driver feel' and thats assuming they can even drive.

 

Which is still drag.

 

Then should autocrossers, who don't rely on downforce much, if at all add more weight to the car to generate more friction between the tires and ground to corner faster and be more stable?

p.s. I love being an argurable ***

 

You can argue that all night long, but you'd still be wrong. 2 separate forces acting in two totally separate vectors.

PaulC

 

Depends, do you want stability? I never thought the primary attribute of an auto-x car was it's stability.

More mass, more inertia, harder for outside forces to cause it to change direction, more stability, less agile and responsive.

PaulC

 

Speaking of aero dynamics at work, take a look at your local A-mod cars in auto-x for a big lesson in this.

PaulC