Aerodynamic lift- A real problem

[Hermie]

Along with drag, anyone that drives on the highways and freeways should also be concerned about aerodynamic lift.

When air pressure builds up and generates lift, it reduces your tires' available traction, since it is taking the weight off of the road, but inertia still applies. This leads to longer braking distances, less control when swerving to avoid an accident, and the possibility of spinning out of control.

Since to some people it seems to be completely foreign, I thought they would like to know it's common knowledge to those in the real world:

http://www.camaro-untoldsecrets.com/...es/rpo_d80.htm

"The Corvair was found to wander excessively at highway speeds, dangerous nose lift was a real concern and for this reason aerodynamic studies were performed inside engineering."

"The front valance and rear deck spoiler were more than mere styling gimmicks. They actually made a measurable contribution to cornering and stability at highway speeds"

Over time, I'll be collecting data and articles related to the negative aspects of lift.

[Frank Lee]

Data: good.

We may actually get somewhere then.

[Christ]

While you're at it, collect some data about exactly how much positive lift is created by a series of vehicles at a series of speeds, then compare it to their weight.

And don't think for a second that anyone is going to be taking data seriously when it's in regard to a car built in the 1960's. We might take you seriously if you find reasonable data from cars that people still have readily available.

Oh - the Corvair had a serious problem with center of gravity and center of balance, as well. The aerodynamic lift was probably not much worse than ohter cars of the area, but it didn't have the weight up front to deal with it safely.

Another car that was a little squirrely at very high speeds was the Thunderbird of the same era. It had a boat nose that split the air just above the bumper, and the lower section of the nose acted more like a wing than a bumper valance. At 70+ MPH, even with the weight of a (390?) V8 in there, you had something much touchier than power steering, slightly south of safe.

Again, that was in the 60's. I've not seen a car designed like that since then, that I can recall. That tells me that OEM's have addressed the concern with regard to vehicle safety at normal highway speeds.

[Christ]

Very nice, Frank.

I also agree about adding a little weight to the trunk of the RR drivetrain vehicles (in the front, obviously). This is evidenced by the addition of weight increasing stability on streamlined vehicles, as well.

For reference, watch: The World's Fastest Indian, where Burt talks of "Speed wobble", and someone suggests adding weight to the nose to keep the rear stable.

Also, see Craig Vetter's Streamlined designs, at some point he was experimenting with lead/steel weights on the nose of a bike to increase it's "upward" stability, or reduce the tendency to sway axially on the tires.

EDIT: This post should be below Frank Lee's post... not sure what happened there.

[Frank Lee]

That's pretty much the deal. Aero wasn't much (any?) of a design factor in the '60s and yes for that old stuff there was such a thing as aero-induced misbehavior at highway speeds which was noticeable because it magnified the instabilities built in to many '60s suspension systems (especially tires which were nowhere near as stable as today's designs).

Corvairs are very light in the front. Most of the instability comes from having a Cg that's BEHIND the Cp (Center of Pressure); the analogy for that is to visualize firing an arrow backwards. Old RACING VW Beetles could become airborne due to lift... at 140+ mph. As a current owner of an old Beetle, an old Microbus, an old Corvair, and Tempos too, I can comment first hand on their handling characteristics. What's astonishing on the rear-engined vehicles is the simple addition of a little weight up front- evidently enough to get the Cg ahead of the Cp- boy they calm right down even in crosswinds. I believe that to be a far greater factor in stability than lift. BTW I've taken Vairs up to 100 mph and lived to tell about it- it wasn't that bad.

But I think we are talking about Tempos and similar stuff, likely from the '80s to now. The Cg/aero/lift/stability issues have been addressed by the factory boys, who really aren't as dumb as the guys at Modified Magazine or Autospeed would have you believe.

I've read that Tempos went through almost 1000 detail design changes in the wind tunnel for drag/lift tweaking.

FWD cars now are very stable. The Cg is well ahead of the Cp and not only that, even heavily loaded they don't get a rear weight bias unless REALLY loaded or overloaded. The Cg doesn't vary wildly from the empty to the loaded condition like it does in the old rear-engine cars- a Corvair is like two completely different cars when comparing trunk empty vs trunk full, while in a Tempo empty vs full is scarcely noticeable. Heck, in a Vair you can tell the handling difference between a full and empty gas tank (it's in the front). The only way a Tempo will be anywhere close to catching air is going off a ramp. If we say 75 mph is likely the top legal speed, the dreaded lift still only amounts to a tiny percentage of the weight of the car UNLIKE the examples frequently found in racing oriented aero articles. Increased speed rapidly increases aero effects.

If we could find the Cl for Tempos we could calculate a good estimate for the lbs of lift the cars experience at various speeds. My WAG is yes of course there's some lift but it's probably something like 20 lbs which for our non-racing purposes = nothing. Consider that the Tempo front wheels probably have 1000 lbs on em each.

Tempo/Topaz seem to suffer sagging rear springs as they age. I've read about that and seen it in other cars anyway- none of mine are or ever were like that. If the rear is dragging and the nose is in the air and it's going 75 mph I can see where things could start to get "floaty". But then a car in that condition probably also has worn out struts, worn out suspension bushings, worn out tires that are half-flat, 200 lbs of junk in the trunk, and a driver that possibly isn't in the top percentile. I wouldn't be surprised to see that whole mess in the ditch either. I have personally experienced the effects of worn-out suspension components and they are an order of magnitude more important for handling than aero at <75 mph. Many people like to disrespect Tempo suspension design but the part of the story they often leave out is they are complaining about a worn-out old hunk of crap. When Tempo suspensions are in good working order I find they have very good handling characteristics.

So yeah if you want to make yourself useful find the Cl values for Tempos... or even anything similar.

[JeepNmpg2]

Hey, while you are finding info - what about vehicles with higher ground clearence? I would assume that having more undercarriage exposed would be an issue with "floatiness" but all things being equal, does the exposure of axles, driveshafts, steering linkage, etc... make a higher vehicle more "spooky" at freeway speeds?

[Hermie]

Just remember, mass doesn't dissapear. It's still present, and inertia never goes away. Simple laws of physics. Aerodynamic lift reduces cornering traction because of these simple, easy-to-understand laws.

Consider that the Tempo front wheels probably have 1000 lbs on em each.

Probably closer to 700. I don't think the rear wheels have just 300 pounds each (which they would with your numbers)... That would make it EXTREMELY unbalanced.

[Daox]

That is true of any car that is braking. The front wheels do 70-80% of the braking. This is why manufacturers use biasing valves for the rear brakes so you don't lock them up. There is no way around the weight distribution under braking.

[MetroMPG]

Originally Posted by Hermie

Over time, I'll be collecting data and articles related to the negative aspects of lift.

Super. Hopefully it'll be more relevant than 40 to 50 year-old information about Corvairs.

The first generation Audi TT might be a good example of a modern car with a lift problem serious enough that the automaker took corrective action. And it would be instructive to note the speeds at which lift actually became a problem.

[Christ]

Regarding Jeeps and other high-clearance vehicles:

I'd have to say the Cg is more of a problem than aerodynamic lift. As hermie says, mass and inertia still apply... if your suspension doesn't counteract inertia properly during braking, turning, etc, you're going to topple.

Of course, there is no way to compensate for all possible situations, so the automakers tend to build for "worst case scenario" within reasonable specifications.

(That's to say that worst case scenario could really just be flying off a cliff or something, which they're not going to tune the suspension to compensate for your 4,000 foot drop, ya know?)