It's true that mass has little to do with skidpad scores, in the normal case when the vertical force on the tires (=mg= the car's weight) is directly proportional to the lateral force on the tires due to centrifugal acceleration (=mv²/r). However, if we reduce the vertical force by some amount via aerodynamic lift, without changing the mass, they are no longer proportional. In that case, the maximum lateral acceleration "g's" the tires can withstand would necessarily decrease (approximately) proportionately. That's the direct opposite of race cars designed with downforce to get them around curves faster. Whatever the lift (or downforce) is, it doesn't result in a single gram "less mass to move down the Interstate." For travel in a straight line, your 200 pounds of lift would provide one minor benefit, reduced load on the tires, assuming other factors equal. (Reducing mass wouldn't otherwise save energy anyway, under the conditions you've assumed of "This is also not about getting over a hill," and "Acceleration is not what I was thinking off.")
Hi RedylC94: As posted, the length of airflow over the top of a Prius is longer than that underneath. This in turn would normally generate lift. If that lift has to be counteracted with a downforce producer - spoiler, wheel arches, grille opening design etc., that adds drag. If the base drag of an airfoil produce lift, that is less mass between the roadway and tires and should provide a slight increase in efficiency. That is if more drag was not required to produce the natural lift of the air foil shape. I have a sneaky suspicion there is something to this but cannot come up with a total pkg. as to why? Regarding mass, they should cancel out on both sides of the skid pad equation. I can bet the 16 MX-5 is closing in on 1G and it is the lightest sports car out there right now. It has to do with Cg, tires and suspension to maintain the tire to road surface horizontal while the body begins its roll to the outside of the turn. The speeds on a 300' diameter skid pad are to slow to create any measureable downforce too. Wayne
Maybe the underside of the prius is so smooth that the whole car is not producing as much lift as before (airspeed underneath has "sped up"). Now they have to make the top side a little more like a wing to keep everything "in balance". /"minger" in this physics forum seems to have some insight on the subject, but I'm not saying he would agree (or disagree) with what I"m suggesting https://www.physicsforums.com/threads/car-aerodynamics.456270/ /for maximum fuel efficiency you would want the very minimum of lift (positive or negative) being produced, . no?
Hi Carcus: Fun to read link! I would think the reduction of drag with a lift to downforce balance would be the best answer but with the shape of an aerodynamic car, there will be drag. What if you could trade some of that inherent drag for lift and achieve higher overall efficiency at speed? Wayne
Hi Wayne, My guess would be that they have to build in some downforce at highway speeds for stability. So I'm thinking ideally they would want lift neutral but at some point (50 mph and up?) they have to start building in some downforce... keeping in mind any lift produced (up or down) will be at the cost of induced drag. Total guess, .. I really don't know. /I remember (a long time ago) riding in a friends 67 'vette doing about 135 mph. The front end started floating like it was going to lift up off the road. Scared the crap out of me. //add ... I wouldn't go under the assumption that the whole car has a positive Angle Of Attack and is producing positive lift. The hood and windshield (at least on a "conventional" car -- not a 67 corvette) to me look more like they would be producing some down pressure (?) ///maybe this is a situation where you can maximize aerodynamics for efficiency OR you can maximize aerodynamics for stability --- but you can't do both
Wayne, you're forgetting the difference between mass and weight. Adding lift will reduce weight on the tires, but won't change the mass that's trying to go sideways. If you reduce mass, it mostly washes out. Less mass = less weight = less grip but there's also less mass to turn so it all works out about the same. If you're reducing weight by adding lift, without changing mass, the mass-to-grip ratio changes for the worse.
Umm ... Yes, that's correct, as I already said in different words. Mass, force, and weight are three different things, although related. (Weight being force due to gravity acting on a mass...) From high-school physics, we should remember that a vector in one direction does not increase or decrease a vector component in a perpendicular direction (although they can be added vectorially).
No, but ..... Weight on wheels will affect rolling resistance .....(perpendicular vectors) Lift will affect induced drag -- induced drag is drag due to lift ..... (perpendicular vectors)
Just saw this as well online and was about to post. Looks like the wait is now over! 60mpg EPA 125mpg Wayne?
Hi George: Toyota and I have been working on a little something with the 2016 Toyota Prius for almost 3 months now. I should not say much more but when I told them 100 mpg+ was a possibility and they may want me to lower that final result for the current plan, they looked at me like I was an alien. Wayne
Just received this in email from Toyota. It said to tune in at 9pm Pacific Time. https://livestream.com/Toyota/2016Prius
Apparently, they didn't reveal any specifics. Which is a letdown. The most noticeable aero feature I have noticed is the bottom edge of the rear bumper. It extends farther back than the rest of the car, and it seems to be forming a nearly complete Kamm back. Maybe there is a complete belly pan? We can only hope!
The livestream "reveal" was very brief. And I think most Prius fans were grateful: it was painfully splashy, hard to watch without cringing. The Priuschat team had an opportunity to look the car over, and there's a lotta threads over there, plus a near hour long video of their round table comparison of notes, a "what drives us" episode. Sentiment is mixed.
Sure, it looks a little goofy , but it shouldn't scare away the faithful. The unwashed masses.......... I dunno. We won't see any until January ( my wild-assed guess). I will watch that Priuschat video tonight.
