tag:blogger.com,1999:blog-2399966929964838853.post4603041422400999232..comments2019-11-12T06:12:44.455-08:00Comments on The Flamboyant Introvert: Slow down. My philosophy for life also applies to the road.Bakari Kafelehttp://www.blogger.com/profile/06037720771479419105noreply@blogger.comBlogger2125tag:blogger.com,1999:blog-2399966929964838853.post-16921269112808993492014-09-12T15:17:37.683-07:002014-09-12T15:17:37.683-07:00HAhaha!
Alright, you caught me. I over simplified...HAhaha!<br />Alright, you caught me. I over simplified some of the details of the math and physics. But as you note in almost every one of your criticisms, the final point I am making is still valid.<br /><br />1) Drag = Cd * A * .5 * r * V^2<br />Cd - coefficent of air, A - cross sectional area, r - density, V - velocity.<br />Notice velocity is squared. IE Drag is parabolic relative to speed, or twice the speed = 4 times the drag<br /><br />2) I should have said "if you are going twice as fast, it takes 4 times the force to stop IN THE SAME DISTANCE" which is ultimately what matters.<br />Sure, you could stop a train with car brakes and tires *eventually*...<br /><br />3) Even in the case of SUV versus bike the SUV fairs better at low speeds versus high. At low speed, maybe a scratch. At high, new bumper, new windshield, maybe even a new hood. The same would hold true for a small car - low speed against bike is easier on car than high speed against bike. It is still true that speed plays the higher role in the amount of damage in a crash than does mass.<br /><br />4) I didn't mean to suggest stopping distance was the ONLY factor in crash statistics by vehicle type, just that it was one of them. The biggest factor seems to be differences in driver demographics. As I point out in http://biodieselhauling.blogspot.com/2013/08/refuting-big-car-safe-myth.html subcompacts are actually at both the top and bottom of the safety stats, because some very small cars are sports cars, and others are econoboxes. One is typically driven much faster and more recklessly than the other. The same goes for mini-vans versus SUVs.<br /><br />5) I wasn't comparing with 70mph, because 65 is the legal limit on urban interstates in most US states. Furthermore I strongly disagree that a 20 mile highway trip is "reasonable" for anything other than special occasions. Certainly not as a daily commute!<br />See http://lifehacker.com/5855550/the-true-cost-of-commuting-you-could-buy-a-house-priced-15900-more-for-each-mile-you-move-closer-to-work for why a 20+ mile commute is a terrible idea<br /><br />6) The CBR Shadow is a motorcycle. It gets much better than average mileage as it is. Motos don't weight much, but they have terrible aerodynamic profiles, so drafting has a much larger effect than for a typical car.<br /><br />Bakari Kafelehttps://www.blogger.com/profile/04002145755975841287noreply@blogger.comtag:blogger.com,1999:blog-2399966929964838853.post-63324034048412059442014-09-12T14:33:57.922-07:002014-09-12T14:33:57.922-07:00You have the right idea, but there are some flaws....You have the right idea, but there are some flaws.<br /><br />1.) "... at higher speeds air resistance increases exponentially** relative to speed." This statement is not correct. It does not increase exponentially (or parabolically) and it does not increase only at higher speeds. Resistance due to friction (be it wind or the engine turning) increases approximately linearly with speed. However, since you are interested in fuel efficiency as a function of distance travelled, reducing the resistance to lowest possible (by sitting still) obviously won't benefit you. As it turns out driving relatively slow at constant speed in the highest gear possible will give the best fuel efficiency. This is somewhere in the 35-55 MPH range (depending on the vehicle). One big loss in fuel efficiency is using the brakes. This is simply lost energy and as you mentioned, it is the kinetic energy (proportional to velocity squared) that is lost. Speeding from one light to the next in a city is fairly awful on the efficiency. It is also why hybrid cars work; they recover some of that energy during braking.<br /><br />2.) "... if you are going twice as fast, it will take four times as much force to stop." This statement is also not correct. A vehicle can be stopped with any opposing force. It is the time (and distance) in which it is done that is of importance. The limiting case in stopping force will be the friction of the tires. This is relatively constant for various speeds (but very different depending on road surface). However, you are right, the vehicle needs to dissipate all that kinetic energy. Thus the final relationship holds; the braking distance will be proportional to speed squared.<br /><br />3.) "... speed plays an overwhelmingly larger role in how bad a crash is than mass." This is true if and only if the crash is with something else very heavy, say a wall. However the crash will be dictated by a change in momentum (and transfer of energy). If an SUV hits a bicyclist at any speed, the SUV will fair very well compared the bicyclist. That's because the change in momentum is approximately equal for both, but the SUV is much heavier and thus will have very little energy change. Of course, that isn't to say that speed isn't the more important factor overall.<br /><br />4.) "[full-size SUVs (along with all other sizes of SUV, all sizes of truck, and even heavier full-size cars] are statistically more dangerous than mid-size cars" True, but if the only issue was stopping distance as dominated by speed, as you suggest, why are compact and subcompact cars at the top of the list and large vans at the bottom?<br /><br />5.) "3600 seconds / 65mph = 55 seconds." Since when is seconds divided by MPH equal to seconds? I think you mean 1/65 [hours/mile] * 3600 [sec/hour] ~= 55 sec/mile. To put a more precise formula to it, the factor time saved is equal to p/(1+p) = 1-v1/v2, where p is the factor increase in velocity, p = (v2/v1) - 1, and v1 and v2 are the speeds to be compared. As an example, lets take a reasonable 20 mile highway trip. The time decrease of driving is just over four and a half minutes (21 percent) at a common 70 MPH compared to the suggested 55 MPH. Now is a 17 minute trip that much better than a 22 minute trip, probably not. Let's say you take that trip twice a day for 5 days a week, 46 weeks of the year for 40 years. That's almost 60 days saved in time and sounds awesome. If we perceive your time to be valued at $10/hr, that's $14,000. Is that enough to justify the possibility of an accident, lost wages, fuel, etc. Statistically, probably not. Does it justify being a dick and possibly hurting someone else. Definitely No! <br /><br />6.) "Here is a real-life story..." Mythbusters did an episode on this ("Drafting For Money"). It sounds like that person was driving WAY to close (much less than the minimum 2 sec lead time) to be safe. Drafting is something left for the track.Anonymousnoreply@blogger.com