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etmax
User Rank
Gold
Re: Nice one
etmax   8/2/2013 11:54:18 PM
NO RATINGS
@Burghermeister I'm sorry I should have explained my concern a bit better. If BMW were to do everything just right in terms of the steering geometry and so on, it wouldn't change the fundamental fact that an SUV is like a poorly designed sail for side wind and the lighter the vehicle is with respect to the surface area it represents, the easier forward motion can be destabilised particularly when the wind is gusty and the forces pushing on the front of the vehicle will alternate stronger and weaker with respect to the force at the rear of the vehicle, and this will alternate as you pass various fronts. To put my concern in a nutshell and a worst/best case comparision, consider a brick sitting on the ground in strong wind then consider a cardboard box of the same exterior dimensions but at only 1% of the weight (perhaps less?). One will fly away in relatively light wind, the other will stay put unless it's a cyclone. This is all driven by exposed surface area to weight for a given geometric form "SUV" in this case.

Charles Murray
User Rank
Blogger
Re: Nice one
Charles Murray   8/2/2013 7:15:10 PM
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That's an excellent question, etmax -- one that applies to virtually every vehicle with a carbon fiber body. Unfortunately, it's not one that I'm likely to get an answer to. I checked BMW's 24-page press release and it doesn't even give the drag coefficient (as far as I can tell), let alone the wind-surface-to-weight-ratio. Sorry.

Elizabeth M
User Rank
Blogger
Re: city car
Elizabeth M   8/2/2013 8:27:09 AM
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That is a great license plate for a Prius driver, Nadine! Nice to see even the eco-minded types have a sense of humor (as I assume it is meant to be sarcastic, right? :)) On the earth day note, yes, it doesn't take much for any U.S. holiday, even one meant to celebrate nature, can become a reason to buy and sell things. :(

Burghermeister
User Rank
Iron
Re: Nice one
Burghermeister   8/2/2013 6:53:21 AM
NO RATINGS
I actually oversimplified the explanation in my previous post by leaving out the fact that air is a compressible medium, and its net effects change with velocity, surface area, roughness, etc....

This is helps account for that fact in addition to the net effects I previously described.

http://www.ara.bme.hu/oktatas/letolt/Vehicleaerodyn/Vehicleaerodyn.pdf

 

Burghermeister

Burghermeister
User Rank
Iron
Re: Nice one
Burghermeister   8/2/2013 6:00:17 AM
NO RATINGS
The question of why you get so much feedback in the wheel...a sideways tug with side windload is not only a function of the weight, but also a function of the suspension setup and the location of aerodynamic center of gravity (ACG) relative to the mass/physical (CG) location.  This is not magic, it is all related to applied phyics. 

If you are an engineering person or have taken physics, remember the free body diagrams.  There 3 mutually perpendicular axes of the vehicle to consider a vertical axis (yaw axis), a longitudinal axis (roll axis), and cross body axis (pitch axis) which all originate from the vehicle CG in 3D.

Remember the concept of forces applied at a distance of leverage creating a torque moment?  Think now in terms of vehicle response in a rotational mode around the yaw axis located at the CG, these are the torque moments that will generate steering feedback. The net torque moment will be fed back through the steering mechanism for the driver to experience. 

If the ACG is aligned with the CG, there is no net rotational torque moment to create feedback into the steering mechanism, and there will be merely sideload roll torque around the imaginary vehicle axis running parallel to the wheels..  The location of roll torque moment relative to vehicle CG around that logitudinal roll axis....in combination with spring/bushing stiffnes will dictate the amount of roll response of the vehicle to side winds. 

BMW certainly should have designed the suspension properly in consideration of the above, and the Center of Gravity (CG) of the vehicle should be very low with batteries and motor assembly sitting low on the vehicle. 

 

Driving the final vehicle will tell you how they did at dealing with the vehicle dynamics/physics. 

 

Burghermeister

etmax
User Rank
Gold
Re: Nice one
etmax   8/1/2013 9:35:16 PM
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I have to ask an engineering question here, if it has so much carbon fibre that it has 90km range with a 22kWhr battery, then what does its wind surface to weight ration look like? What I'm getting at is I drive a 1.5t 3.0l powered petrol car (which does get quite good economy on the highway) and when I'm driving at 100kph and I get a 100kph wind gust from the side I feel a real tug on the steering. In my wife's 900kg petrol car I feel the tug a little stronger. What happens in this car that looks like it might be the size of a medium SUV and maybe weighs only 70% of its traditional sister when that happens? Just a thought.

Charles Murray
User Rank
Blogger
Re: city car
Charles Murray   8/1/2013 7:14:59 PM
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Yes, Bunter, taxi cab fleets want that combination of price, utility and mileage that you describe. That's why so many cabs are now Priuses.

Charles Murray
User Rank
Blogger
Re: city car
Charles Murray   8/1/2013 7:13:24 PM
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That's a great license plate, Nadine. Apparently, Prius owners have a good sense of humor. "Curb Your Enthusiasm" did a show poking fun at Prius owners, even though Larry David himself is a Prius owner and an avowed environmental activist.

http://usatoday30.usatoday.com/life/people/2006-05-04-larry-david_x.htm

Bunter
User Rank
Platinum
Re: city car
Bunter   8/1/2013 1:27:46 PM
NO RATINGS
Hi Nadine,

It has always amused me that they chose Lenin's birthday for earthday.

That soemone would put that on their own Prius is hilarious.  Some folks act like all Prius owners are eco-nazis but most of them just seem to want an affordable mid-size car that gets great mpg.  And that is why, IMO, it dominates the market-the combination of price, utility and mileage is the best.

However my VFR800 get similar mpg and puts a grin on my face every day.  My "Eco-toy".

Smirk.

TTFN

Dennis

NadineJ
User Rank
Platinum
Re: city car
NadineJ   8/1/2013 1:10:19 PM
NO RATINGS
You're right.  The Eco-toy market is dominant.  I listened to a panel discussion with many who were instrumental in getting the first Earth Day up and running.  They lamented the fact that the holiday is just another opportunity for consumerism.

On a lighter note--The funniest thing I've ever seen on a Prius was a license plate that read "PIOUS". 

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