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Old 05-26-2021, 06:08 AM   #21 (permalink)
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It's only friction with the ground that provides ALL of a car's propulsive, braking and cornering forces. You can easily experience what a car that is only just in contact with the ground would be like by driving on a surface with almost no friction (like on ice, or aquaplaning with all four wheels on water). Basically, impossible to control.
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So, to get full ground contact again, the body needs to drop down what is the suspension lift. Lets say 30 cm. It takes 0,25 s to drop 30 cm by free fall. So, from the instant you "disable the lift" it still takes 0,25 s till your brakes or steering begin to work. Not worth the risk.

I think both of you are misunderstanding my proposal, I am not suggesting floating cars, or turning cars into flying planes or hovering ekranoplans, I am suggesting partially lifting cars.

Such that the force pushing down on the wheels is reduced to say 50% of static force. The wheels still provide the drive and steering (though maybe assisted by control surfaces).

We just have wings that take some of the weight off the tyres. (thread title changed to reduce confusion)

Assuming 50% weight reduction would still give 50% of the grip and that is excluding other control surfaces. Brakes would be airbrake assisted that also increase downforce like an SLR or Huayra.

Modern cars on a skidpan manage 1g lateral grip, that would mean that with 50% of the weight removed (excluding control surfaces) that 0.5g of lateral grip is instantly achievable, pushing up to 1g if lift is cancelled and maybe 1.5g with downforce.

Even in "lift mode" that would still give 70% of the cornering speed for the same radius.

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Originally Posted by seifrob View Post
say your car is lifted "just to touch". So, to get full ground contact again, the body needs to drop down what is the suspension lift. Lets say 30 cm. It takes 0,25 s to drop 30 cm by free fall. So, from the instant you "disable the lift" it still takes 0,25 s till your brakes or steering begin to work.
Car is lifted so weight is reduced, that is maybe a few cm of travel, I have never seen a 30cm free fall suspension but maybe that is for huge cars. Steering works at all speeds if only 50% of the weight is off the wheels.

And if we assume 2.5cm of suspension lift that would take virtually no time for full grip to be achieved, just through gravity.

Freebeard, I think lifting the rear wheel is clever but not really what I am talking about here, because that would be quite susceptible to the pit manoeuvre, or potholes, or crosswinds


Last edited by AeroMcAeroFace; 05-26-2021 at 06:25 AM.. Reason: addition of freebeard comment.
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Old 05-26-2021, 06:26 AM   #22 (permalink)
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Originally Posted by AeroMcAeroFace View Post
I think both of you are misunderstanding my proposal, I am not suggesting floating cars, or turning cars into flying planes or hovering ekranoplans, I am suggesting lifting cars.

Such that the force pushing down on the wheels is reduced to say 50% of static force. The wheels still provide the drive and steering (though maybe assisted by control surfaces).

We just have wings that take some of the weight off the tyres.

Assuming 50% weight reduction would still give 50% of the grip and that is excluding other control surfaces. Brakes would be airbrake assisted that also increase downforce like an SLR or Huayra.

Modern cars on a skidpan manage 1g lateral grip, that would mean that with 50% of the weight removed (excluding control surfaces) that 0.5g of lateral grip is instantly achievable, pushing up to 1g if lift is cancelled and maybe 1.5g with downforce.

Even in "lift mode" that would still give 70% of the cornering speed for the same radius. And if we assume 2.5cm of suspension lift that would take virtually no time for full grip to be achieved.
Maybe go and drive some cars that have a lot of aerodynamic lift? It's not a good experience at highway speeds, even with 10-15 per cent lift.

In fact, one car I have seen data for (14 per cent rear lift) had control problems on a public road. My mind boggles at how hard it would be at 50 per cent lift!

As I said, driving on ice, aquaplaning (or in my country, 'floating' the tyres over dirt road corrugations) all emulate the effect of having a high aero lift car.

Tyre grip doesn't go down linearly with reduced load:

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Old 05-26-2021, 06:47 AM   #23 (permalink)
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The reason that downforce works so well in improving handling is that the downwards push on the tyre increases (so increasing grip) but the mass of the car doesn't change. Therefore we have more grip without having an increased mass to accelerate (forwards, backwards or sideways).

Now contrast that with lift. Now we have less downwards push on the tyres, so decreasing grip (and, as the diagram above shows, sometimes disproportionately decreasing grip) but we have the same mass to accelerate (forwards, backwards or sideways).

Furthermore, the current research evidence (see my book pages 180 - 182) shows that lift, even small amounts of aerodynamic lift, has a major effect on car stability. (We're talking a couple of kg having a measurable affect.) That may be because the aerodynamic forces are oscillatory, and therefore may resonate with unsprung weight and/or suspension natural frequencies. Either way, all the evidence is that any aero lift reduces stability, handling and braking.

I therefore don't think that makes sense in cars on public roads, even with sensing systems and electronic over-rides. But it's easy enough to prove for yourself. Just put some high lift wings on your car and assess control and stability versus improved fuel mileage.

Going the other way (ie from lift to downforce) with my road car was an enormous learning experience, one I notice each time I drive my wife's Mercedes that has quite a lot of rear lift (maybe 10 per cent?).
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Old 05-26-2021, 07:30 AM   #24 (permalink)
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Maybe go and drive some cars that have a lot of aerodynamic lift? It's not a good experience at highway speeds, even with 10-15 per cent lift.

In fact, one car I have seen data for (14 per cent rear lift) had control problems on a public road. My mind boggles at how hard it would be at 50 per cent lift!
Presumably you are talking about the mk1 audi TT, I don't believe you when you say that lift is the sole cause of the issue here, that is a simplification. The issue in that case was lift but also suspension issues.

The real issue is unequal lift and/or centre of pressures ahead of the centre of mass. Planes do not suddenly crash when they get to 50% of their weight on their tyres.

When you added those side fins on your insight, you got extra rear lift but because the centre of pressure moved rearwards the stability improved?

If unequal loading was the sole cause of the problem we wouldn't have cars with weight distributions up to 70/30.

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But it's easy enough to prove for yourself. Just put some high lift wings on your car and assess control and stability versus improved fuel mileage.
But it isn't, because my cars aren't designed with aerodynamic stability in mind, they are aerodynamically unstable and rely on the tyres to stop the car from going sideways. This is a thought experiment about a car that is aerodynamically stable.

I have driven cars with front end lift, and I know it isn't pleasant, but then I have flown planes and gliders with much more lift and they have no directional stability issues. As I mentioned earlier the real issue is directional stability, rather than specifically lift.

Drop a plane and it will point in the correct direction, drop a car and it will tumble. Why? Aerodynamic directional stability.

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Either way, all the evidence is that any aero lift reduces stability, handling and braking
On typical road cars maybe, but a plane becomes more stable as more lift occurs, due to more corrective aerodynamic pressure. Handling, again on a typical car, but this imaginary car can cut the lift, and has control surfaces so that evidence doesn't apply. Braking, air brakes would be employed, lift cutting and added downforce would be employed.

I am not saying that it would be in any way easy to do, or legal to do, but I still see no reason why a car specifically designed for it, such as in a university challenge like a solar challenge, wouldn't work.

Tyre grip is not linear no, but I was approximating it to linear because the linear region is where most car tyres are. However, the graph you provided suggests that halving the force on the tyres leads to only a 25% reduction in grip.

Last edited by AeroMcAeroFace; 05-26-2021 at 07:56 AM.. Reason: added tyre part
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Old 05-26-2021, 09:26 AM   #25 (permalink)
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Less down force, even if only gravity, reduces traction. Traction is the only thing that allows you to steer a road vehicle.

Look at airplanes or helicopters landing, does it look elegant and highly accurate? Nope
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Old 05-26-2021, 09:47 AM   #26 (permalink)
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Don't normally agree with Julian, but his last two posts kinda hit the issue spot on.

Lots of just plain wrong data in this post mostly about how aircraft fly. And yes, I am a pilot, PSEL, taildragger with couple hundred hours in type, who builds weird toy aircraft that has consumed a forest of balsawood trees learning what does not work then learning theory of what does.

landing aircraft on icy runways rely mostly on inertia to stay on the runway and don't use the high-speed turn offs because human brains operate about 3/8 of a second in the past. They overrun the runway end if they can't use the brakes to stop in addition to all the reverse thrust. Stopping tons of something going +150mph in less that 2miles dissipates lots of energy.
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Old 05-26-2021, 10:02 AM   #27 (permalink)
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Who said anything about ice? Not me, the comparison by Julian was that lifting a bit of the car was like driving on ice. A car that generates no lift, as was proposed when the brakes are applied, has no difference to an ordinary car, other than the benefit of air brakes. I cannot see how that is in any way worse.

"Lots of just plain wrong data in this post mostly about how aircraft fly." Please let me know what is wrong.

Ground effect vehicles are very different to planes.

Edit:

To make things clear for everyone picking up on issues already addressed.
  • lift is only employed when steering angle is very small
  • lift is only employed when brakes aren't applied
  • lift can be cancelled out at any point
  • The car is never fully off the ground
  • Ride height controls the amount of lift
  • Lift is only employed in a straight line at cruise
  • The wheels still drive the car, because they never leave the ground

Going round corners is unaffected, because of steering angle sensors that cut lift. Braking is unaffected because of airbrakes and brake sensors that cut lift. All of these are already addressed.

The debate is about cruise.

Last edited by AeroMcAeroFace; 05-26-2021 at 10:15 AM..
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Old 05-26-2021, 10:17 AM   #28 (permalink)
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Quote:
Originally Posted by AeroMcAeroFace View Post
I think both of you are misunderstanding my proposal, I am not suggesting floating cars, or turning cars into flying planes or hovering ekranoplans, I am suggesting partially lifting cars.

Such that the force pushing down on the wheels is reduced to say 50% of static force. The wheels still provide the drive and steering (though maybe assisted by control surfaces).

We just have wings that take some of the weight off the tyres. (thread title changed to reduce confusion)

Assuming 50% weight reduction would still give 50% of the grip and that is excluding other control surfaces. Brakes would be airbrake assisted that also increase downforce like an SLR or Huayra.

Modern cars on a skidpan manage 1g lateral grip, that would mean that with 50% of the weight removed (excluding control surfaces) that 0.5g of lateral grip is instantly achievable, pushing up to 1g if lift is cancelled and maybe 1.5g with downforce.

Even in "lift mode" that would still give 70% of the cornering speed for the same radius.



Car is lifted so weight is reduced, that is maybe a few cm of travel, I have never seen a 30cm free fall suspension but maybe that is for huge cars. Steering works at all speeds if only 50% of the weight is off the wheels.

And if we assume 2.5cm of suspension lift that would take virtually no time for full grip to be achieved, just through gravity.

Freebeard, I think lifting the rear wheel is clever but not really what I am talking about here, because that would be quite susceptible to the pit manoeuvre, or potholes, or crosswinds
Here's a proposal:
Choose one vehicle that we have all, necessary data from which to do a full, table-top analysis.
We can run some numbers, within the context of that specific vehicle and see where it takes us.
CAR and DRIVER does the most comprehensive road tests, available to the lay person. I'd recommend choosing from their stable.
That will put us all on the same page.
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Old 05-26-2021, 10:25 AM   #29 (permalink)
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Well, firstly, airborne aircraft modify inertia, they don't have anything else to react on except compressable air. You can extend a literal barndoor, but the reaction to that is still relatively slow even if you can fling a highly reinforced large door open really fast. Watch the tailfeathers of a commercial airliner on a gusty crosswind landing, they are moving lots for not much attitude change.

Julians comment was flying on a road with reduced downforce was similair to driving on an icy surface. I concur having driven a oval race on a frozen river, or doing intentionally stupid things on an ice covered parking lot. If you reduce your ability to alter direction, then your learned ability to fix that situation becomes useless until you relearn your required response. Being 3/8 of a second in the past in a semi flying car, when that deer shows up, you have this really big problem really fast to fix with possibly learned erroneous solutions in a slowly responding vehicle
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Old 05-26-2021, 11:09 AM   #30 (permalink)
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Quote:
Originally Posted by AeroMcAeroFace View Post
To make things clear for everyone picking up on issues already addressed.
  • lift is only employed when steering angle is very small
  • lift is only employed when brakes aren't applied
  • lift can be cancelled out at any point
  • The car is never fully off the ground
  • Ride height controls the amount of lift
  • Lift is only employed in a straight line at cruise
  • The wheels still drive the car, because they never leave the ground
The debate is about cruise.
The debate was about a car equipped with lift-generating device to eliminate rolling resistance losses.

So, for strictly academic debate yes, it is possible to equip a car with such a device, that will create lift under specified conditions, and stop creating lift as needed and thus eliminate rolling resistance losses.
BUT
in practice:
- you are eliminating rolling resistance losses, but adding aerodynamic lossess (control surfaces, spoilers), acceleration losses (increased mass), internal power consumption losses (add-on ECU, sensors, actuators).

No time/desire to make even back-of-the envelope calculations, but i guess net effect would be negligible. I would put my foot down, even if you were my student and proposed it as a thesis/project.

All the safety hazards are relevant too, in the final, you are about to put real, living people in - reagrdless if it is on track or street.

Would YOU sit in a car that has deliberate half-second delay imposed in steering and brakes?

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