[U4IA187]

OK Im a newbie and couldnt find an existing link. I understand the aero components of the exige provide 100lb of downforce (although the old hand book has it as 100kgs) However, the actual construction of having the wing attached to a weak rear engine cover that sits on the rear clam and not stabilized to the main substructure makes me wonder how much of the down force is actually transmitted to the suspension and not just lost on a floaty engine cover. Does that make sense? Ive spent a fair amount of time in aviation and never had a functional wing that wasnt integral to the substructure. Ok dorkey question and double negatives, sorry. It still looks cool though.

 

[E.D.]

check for serious wings attached to substructure:
http://www.lotustalk.com/forums/f95/group-buy-reverie-1650mm-wing-68041/
http://www.lotustalk.com/forums/f92/new-wing-option-come-soon-apr-61257/

 

[apk919]

OK Im a newbie and couldnt find an existing link. I understand the aero components of the exige provide 100lb of downforce (although the old hand book has it as 100kgs) However, the actual construction of having the wing attached to a weak rear engine cover that sits on the rear clam and not stabilized to the main substructure makes me wonder how much of the down force is actually transmitted to the suspension and not just lost on a floaty engine cover. Does that make sense? Ive spent a fair amount of time in aviation and never had a functional wing that wasnt integral to the substructure. Ok dorkey question and double negatives, sorry. It still looks cool though.

Umm... no, it doesn't make sense. If 100 pounds of down force is generated by the rear wing, 100 pounds of additional force will be placed on the rear wheels... it doesn't matter how flimsy the rear engine cover is; as long as it doesn't break (or deflect so much that the wing's angle of attack is compromised), it will transmit the force.

EDIT: Thought experiment: remember the wheels aren't directly connected to the chassis of any car... there is a suspension (usually some type of spring and damper arrangement) separating the chassis (where aero structures are usually anchored) from the wheels. In equilibrium, all vertical force on the chassis (whether from the weight of the car, or through aerodynamic downforce) is transmitted to the wheels through the springs and dampers.

 

[VisualEchos]

The 100 mph downforce figures are as follows:

Elise:
Front: 8.6 lbs
Rear: 4.4 lbs

Exige:
Front: 42.5 lbs
Rear: 48.2 lbs

The downforce increase on the Exige is specifically because of the front air-splitter and rear spoiler, and increases as speed increases. The difference between the two can easily be felt at 75 mph entering a corner.

 

[U4IA187]

I understand that there s a conservation in energy with a total downforce being distributed to the rear of the car. However i wonder if it is as efficient since it has a kinematic component, ie a energy loss due to compliance in the rear engine cover. Some of the energy is eventually los to movement, pressure changes etc. Not all 100 lbs is distributed to the rear wheels.

 

[apk919]

Well, you do agree that all downforce is distributed to all four wheels, even if there's a dynamic component (energy storage in springs, other compliant structures)? Imagine the wing was mounted on a set of springs... the springs would compress, but all the force the wing generated would still be transmitted to the structure that the wing was attached to. The biggest problem would be that the wing's support was so compliant that the wing's position changed enough to reduce downforce (F1 teams have been accused of doing this intentionally to reduce both downforce and drag at high speeds).

Given the location of the wing's center of (negative) lift relative to the coil springs and dampers (which transmit forces from the chassis to the wheels) I think its safe to say that most of the wing's downforce it transmitted to the rear wheels.

 

[Monsi]

even with springs force on one side of the spring will be mirrored on the other...

they measure it at the wheels... so it really doesn't matter how it gets there

 

[U4IA187]

i dont think you can assume all downforce is transmitted solely to the wheels when some of the energy is lost to compliant things like springs etc. It takes force to cause deformation in the springs. the total amount of force is fixed and static, if a perentage is lost in compliance, then the rest would be transferred to the wheels. So no I dont agree it goes straight from lift to wheels.

 

[U4IA187]

This is getting too academic. All that really matters is concontrol in a corner.

 

[AustinP]

i dont think you can assume all downforce is transmitted solely to the wheels when some of the energy is lost to compliant things like springs etc. It takes force to cause deformation in the springs. the total amount of force is fixed and static, if a perentage is lost in compliance, then the rest would be transferred to the wheels. So no I dont agree it goes straight from lift to wheels.

Sorry, but your intuition is wrong. Energy is irrelevant, the only thing that matters is force. When you apply force to one end of a spring there will be an equal and opposite force at the other end of the spring, it doesn't matter how much the spring compresses. The energy consumed (or "work done") by compressing the spring equals force-times-distance, but it makes no difference to the force balance. Really.

 

[sleepe]

Can anyone postulate or guess as to how much downforce the Reverie 5-Element difusser and Reverie front spolier/w splitter extensions would add to a n unmodified Elise?

I want more downforce on my Elise, but am trying to accomplish this without the addition of a BIG [email protected]@ WING.
-robert

 

[_ace_]

i dont think you can assume all downforce is transmitted solely to the wheels when some of the energy is lost to compliant things like springs etc. It takes force to cause deformation in the springs. the total amount of force is fixed and static, if a perentage is lost in compliance, then the rest would be transferred to the wheels. So no I dont agree it goes straight from lift to wheels.

i'm an aerospace stress analyst, although a pretty junior one. i'll support that the loss you're describing is real--i did an analysis for one on a helicopter.

for the elise, i'd be more than willing to bet that the strain energy (or lost force) would be small enough to be neglected, ie order of 1% or less.

if it was designed reasonably (which i would assume), it won't vibrate significantly, and your spoiler will push the trunk and the whole thing will pretty much deflect down and then hold until the car slows down. the energy loss will be next to nothing.

now, if it's vibrating, or if the spoiler is bouncing up and down visibly with wind gusts, then the dynamic effects of having the spoiler intermittently lose it's downforce will probably make the driver angry before anything -- the rear would be twitchy.

 

[U4IA187]

Sorry, but your intuition is wrong. Energy is irrelevant, the only thing that matters is force. When you apply force to one end of a spring there will be an equal and opposite force at the other end of the spring, it doesn't matter how much the spring compresses. The energy consumed (or "work done") by compressing the spring equals force-times-distance, but it makes no difference to the force balance. Really.

not necessarily, this goes beyond high school physics.

 

[Bgwilly31]

The factory exige wing, visibly vibrates or bounces up and down at speeds 80mph+ or so.


However very interesting conversation.

Ive had this mod in my head for awhile. I love the look of the hatch mounted wing much better than clam mounted wing.

 

[apk919]

not necessarily, this goes beyond high school physics.

:huh: What part of "high school physics" do you think this goes beyond?

The factory exige wing, visibly vibrates or bounces up and down at speeds 80mph+ or so.


However very interesting conversation.

Ive had this mod in my head for awhile. I love the look of the hatch mounted wing much better than clam mounted wing.

True, but the vibration is not from the hatch, but from the fact that there's a large part of the wing that's actually in a "cantilever" configuration (a term I learned in HS, by the way).

I'm willing to bet that there would be a negligible difference in downforce generated by wing whether it was mounted to the hatch cover, or with a steel support directly into the chassis (as long as the actual position of the wing isn't altered).

 

[AustinP]

not necessarily, this goes beyond high school physics.

That's funny. Actually it only takes high school physics to understand that your statements were wrong. ace's statement is also wrong, but I suspect there is a grain of truth buried in his experience. If you include the dynamic motion of the wing (i.e. when the wing moves down, due to increased downforce on the wing compressing the various elastic elements), there are some minor dynamic elements (acceleration and deceleration of the wing mass) affecting the instantaneous force on the suspension, but due to the small weight of the wing it is so small as to be negligible. Vibration of the wing is a whole different matter. Vibration would indicate unstable air flow, which could have a large fluctuating effect on the downforce produced by the wing, but that's not what we were talking about.

 

[U4IA187]

That's funny. Actually it only takes high school physics to understand that your statements were wrong. ace's statement is also wrong, but I suspect there is a grain of truth buried in his experience. If you include the dynamic motion of the wing (i.e. when the wing moves down, due to increased downforce on the wing compressing the various elastic elements), there are some minor dynamic elements (acceleration and deceleration of the wing mass) affecting the instantaneous force on the suspension, but due to the small weight of the wing it is so small as to be negligible. Vibration of the wing is a whole different matter. Vibration would indicate unstable air flow, which could have a large fluctuating effect on the downforce produced by the wing, but that's not what we were talking about.

Interesting, a two dimentional answer for a three dimentional problem. Thats fine, although I doubt you are understanding the subtleness of the question. What you have said is right in terms of conservation of energy. BTW, force is energy in reference to a less than intelligent prior statement. That distrubution of energy isnt as simple as a spring and two arrows. Place it in a three dimentional world, for example bullets and kevlar, distrubute the force, and integrate the result. This conversation is getting sad. Everyone go have a beer. We havent gotten to discuss the shape of the wing that really insnt a wing or the lateral slip stream that decreases its supposed effectiveness. BTW, vibraton is not unstable airflow-- unstable airflow is properly considered laminar vs nonlaminar flow. We are working on different levels obviously. anic:

 

[apk919]

Interesting, a two dimentional answer for a three dimentional problem. Thats fine, although I doubt you are understanding the subtleness of the question. What you have said is right in terms of conservation of energy. BTW, <b>force is energy</b> in reference to a less than intelligent prior statement. That distrubution of energy isnt as simple as a spring and two arrows. Place it in a three dimentional world, for example bullets and kevlar, distrubute the force, and integrate the result. This conversation is getting sad. Everyone go have a beer. We havent gotten to discuss the shape of the wing that really insnt a wing or the lateral slip stream that decreases its supposed effectiveness. BTW, vibraton is not unstable airflow-- unstable airflow is properly considered laminar vs nonlaminar flow. We are working on different levels obviously. anic:

I hope you're not under the impression that force equals energy... if you are, you fail high school physics!

 

[AustinP]

Interesting, a two dimentional answer for a three dimentional problem. Thats fine, although I doubt you are understanding the subtleness of the question. What you have said is right in terms of conservation of energy. BTW, force is energy in reference to a less than intelligent prior statement. That distrubution of energy isnt as simple as a spring and two arrows. Place it in a three dimentional world, for example bullets and kevlar, distrubute the force, and integrate the result. This conversation is getting sad. Everyone go have a beer. We havent gotten to discuss the shape of the wing that really insnt a wing or the lateral slip stream that decreases its supposed effectiveness. BTW, vibraton is not unstable airflow-- unstable airflow is properly considered laminar vs nonlaminar flow. We are working on different levels obviously. anic:

OK, I think the problem here was in your poorly stated problem statement in your first post. If what you are worried about is that the angle of the wing could change due to the relative non-rigidity of the support (maybe that's what you mean by your reference to 3D), well yes, that's probably true, but based on observation it doesn't seem to change very much (track speeds of 130+ mph). And yes, by unstable flow I meant turbulent air flow, which can be one cause of vibration (but not only potential cause). I tried to start at where I perceived your "level" to be, based on your poorly worded question at the outset. Be careful what you assume about other posters, I happen to have a Ph.D. in M.E.

 

[apk919]

OK, I think the problem here was in your poorly stated problem statement in your first post. If what you are worried about is that the angle of the wing could change due to the relative non-rigidity of the support (maybe that's what you mean by your reference to 3D), well yes, that's probably true, but based on observation it doesn't seem to change very much (track speeds of 130+ mph). And yes, by unstable flow I meant turbulent air flow, which can be one cause of vibration (but not only potential cause). I tried to start at where I perceived your "level" to be, based on your poorly worded question at the outset. Be careful what you assume about other posters, I happen to have a Ph.D. in M.E.

+1

i dont think you can assume all downforce is transmitted solely to the wheels <b>when some of the energy is lost to compliant things like springs etc.</b> It takes force to cause deformation in the springs. the total amount of force is fixed and static, if a perentage is lost in compliance, then the rest would be transferred to the wheels. So no I dont agree it goes straight from lift to wheels.

The statement in bold is just plain wrong. Energy can be stored in a spring, but it's not lost (other than parasitic friction). Plus, it's force that we're concerned about, not stored energy.