Increasing downforce while reducing drag?

[Christ]

Originally Posted by MadisonMPG

"The TT has 148 pounds of lift at the rear axle at 125 mph without the spoiler, but only a 53-pound lift when the spoiler is fitted."

Twice the speed, four times the drag/lift/down force. So you will be doing 60 or so with a Tempo on the highway. (hopefully) Your car is not shaped like the TT and will not generate that much lift. But assuming it does, at 60mph you will have 36ish pounds of lift. Which probably makes your 1,000 pound rear end weigh 964 pounds. Not exactly something to be concerned about. Is your car even rear wheel drive?

Front wheel drive, 2.3L 4 cylinder. (3.0 V6 option). The won't go much faster than 70-ish in stock config without a bunch of *****ing and whining.

[lunarhighway]

the first thing to do if you want to make a car go faster safely is to check the suspention and the wheel alignment. also check all the rubber brushings and pieces where various suspenting compontents attach. if the rubber mounts of an anti roll bar for example are worn the car will feel much more "loose" and unstable especially during fast manouvres.

here's an intresting article on ride and handling with a little science on how humans precieve this as well

Automobile Ride, Handling, and Suspension

[MechEngVT]

Originally Posted by MadisonMPG

Is your car even rear wheel drive?

Is the Audi TT? The Volkswagen New Beetle? Both are FWD, but both have suffered from high speed instability due to aerodynamic lift at the rear.

In high-speed highway driving (and no, 80 mph doesn't count...I'm thinking Autobahn counts) rear downforce improves stability regardless of drive configuration. A stable vehicle is one that is slightly biased toward understeer. Having a front engine or front weight bias when combined with rear lift will make a vehicle more prone to oversteer. A little bit of oversteer at 125 mph during a lane change can be fatal. Oversteer is always unstable (when not throttle-induced). At high speeds during emergency maneuvers it can happen fast enough that no human driver can correct the steering quickly enough to catch it without going into oscillations. This is why every vehicle in mass production has varying degrees of tendency to understeer.

2000mc and lunarhighway are right; check rear for toe-in and replace old bushings, ball joints, and bearings. Any slop in bushings/joints can cause dynamic toe-out at the non-driven axle as the vehicle is pulled forward but the loose suspension lags behind. The opposite (dynamic toe-in) will happen during driving at the driven axle, but during braking a sloppy driven axle will transition to dynamic toe-out and make the car feel unstable during hard braking.

[Christ]

Originally Posted by MechEngVT

Is the Audi TT? The Volkswagen New Beetle? Both are FWD, but both have suffered from high speed instability due to aerodynamic lift at the rear.

In high-speed highway driving (and no, 80 mph doesn't count...I'm thinking Autobahn counts) rear downforce improves stability regardless of drive configuration. A stable vehicle is one that is slightly biased toward understeer. Having a front engine or front weight bias when combined with rear lift will make a vehicle more prone to oversteer. A little bit of oversteer at 125 mph during a lane change can be fatal. Oversteer is always unstable (when not throttle-induced). At high speeds during emergency maneuvers it can happen fast enough that no human driver can correct the steering quickly enough to catch it without going into oscillations. This is why every vehicle in mass production has varying degrees of tendency to understeer.

2000mc and lunarhighway are right; check rear for toe-in and replace old bushings, ball joints, and bearings. Any slop in bushings/joints can cause dynamic toe-out at the non-driven axle as the vehicle is pulled forward but the loose suspension lags behind. The opposite (dynamic toe-in) will happen during driving at the driven axle, but during braking a sloppy driven axle will transition to dynamic toe-out and make the car feel unstable during hard braking.

You sure about that? I might be wrong, but I'm fairly certain that dynamic toe settings move outward as you speed up.

[MechEngVT]

Originally Posted by Christ

You sure about that? I might be wrong, but I'm fairly certain that dynamic toe settings move outward as you speed up.

I thought about it again and still think I was correct (as you are) that toe trends to toe-out on the non-driven axle while the vehicle is in motion.

On the driven wheels...I guess it depends on what is "loose" and the setup in question. For a FWD car if your tie rod ends or rack bearings are loose or shot I think the drive traction would tend to toe inward. If your steering linkage is in tip-top shape but your lower a-arm inner bushings were shot you may toe outward under acceleration (forward and inward motion of knuckles/ball joints/kingpin axis with rigid steering would toe outward).

Of course this all assumes that you're running a typical tire/wheel offset that places the tread center outboard of the kingpin axis' intersection of the ground. Running extremely high backspacing wheels could reverse the tendencies for the driven axle.

For RWD live axle toe is irrelevant. For IRS/RWD it would be almost completely dependent on the exact linkage and how toe is set if it is even adjustable. For non-adjustable toe RWD/IRS (does such a thing exist?) I still think toe-in would dominate under drive torque as the tires push the suspension forward to provide the motive force on the sprung mass of the body.

[Christ]

Of course, I was assuming that all suspension parts are at OEM spec there. It's too difficult to accurately say what happens when things get worn, because no two worn parts will perform exactly the same, so it becomes a matter of speculation as to exactly what's going to happen in that case.

If you refer to physics, there is a motive force rotating the tires, but friction is attempting to stop that motion (equal and opposite blah blah blah). Under cruise conditions, the front tires (ideally) should be at 0 toe for less friction during cruising, and less power/fuel consumption. In order to achieve this, slack has to be "built in" to the suspension system (see: rubber bushings) so that the road surfaces frictional stress on the tires approximates a 0 toe angle. IIRC, most OEM settings call for toe IN to compensate for this, and adding stiffer/solid suspension bushings means you have to compensate for the lack of motive capability of the suspension now, so you should be closer to 0 toe from the get-go.

Is this incorrect? It's how I've always understood, but I'm not a suspension engineer by any means.

[Bicycle Bob]

Depending on the scrub radius, which is occasionally negative with FWD cars, toe has to be set to avoid toe-out during braking or acceleration. The more rigid you can get things, the closer you can run to zero toe at cruising. Camber matters for this, too, if you are using any.

[Christ]

Originally Posted by Bicycle Bob

Depending on the scrub radius, which is occasionally negative with FWD cars, toe has to be set to avoid toe-out during braking or acceleration. The more rigid you can get things, the closer you can run to zero toe at cruising. Camber matters for this, too, if you are using any.

This is the term I was describing above, I think.

[MechEngVT]

Yes that's it...scrub radius is the distance between the "tread center...and kingpin axis intersection with the ground." Couldn't remember myself either.

I wouldn't say I am a suspension engineer, but I did engineer a suspension on an off-road production vehicle a few years ago and had benefit of experience from an engineer and technician that designed a similar setup previously and a former tire engineer.

Bicycle Bob's right...it's more important to avoid toe out under braking as this is unstable. Driving down the road you should still be toe-in to compensate for compliance transition during braking. Toe-in during forward movement is comfortable for most as it assists the vehicle's steering finding center. There's less overall tire scrub as you approach 0 total toe so you want to shoot for the minimum level of static toe-in that prevents squirrelly handling under panic stops.

I was driving one of the off-road cars with the suspension I designed on it after a mechanic replaced a bunch of suspension parts must have neglected to set alignment. I thought I could see a visual toe-out (on 25" OD mud tires). I drove off to try it and it pulled to one side. I slammed on the brakes at an intersection on the test track and it shot to the other side of the road, putting me half off the road just short of the stop sign. Toe was out over 1/2".

[MetroMPG]

Buy & read Hucho's book if you're interested in aerodynamics. In it you'll find answers to all your questions, and as a bonus you get to avoid all us rude folks who care more about drag reduction than lift reduction.

Lowering the ride height will typically also reduce drag for a modern passenger car. I recall a chart in Hucho that relates ride height to lift as well, but I've lent my book to Ben so can't look it up. But I suspect it's the positive effect you're after.