There you have it, the analysis from John MacAdam. -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of Christopher Goss Sent: Wednesday, August 28, 2002 1:28 AM To: Hanks, Jeffrey S; 'picqc'; Mike Prizy Cc: Kurt Bray; [EMAIL PROTECTED] Subject: RE: t-and-f: Concrete VS Asphalt (was: Why on the street?)
Ouch! Lots of bad science here, Jeff. Let's take today's issues one by one... 1) Hardness The proper comparison of running surfaces is indeed hardness and the golf ball test WILL provide an accurate relative comparison between surfaces of two hardnesses. When you hear engineers or scientists talking about the hardness of a material, they are usually speaking of one of several standardized measurements performed on the material. The most common measurements are Brinell, Rockwell, Vickers, and Shore. The selected measurement technique depends upon the material, as each method is better than the others over different ranges of hardness. The equipment used for these measurements is expensive and not very portable. A portable tool used to estimate material hardness is interestingly a ball dropped within a tube. The height of the bounce (measured by graduated marks on the side of the tube) is calibrated to the estimate of the material hardness. Pretend you are a billiards ball. Would you rather hit another ball (which is hard) or the table's rail (which is not nearly as hard)? The same thing applies to your feet. Lots more about hardness may be found at http://www.calce.umd.edu/general/Facilities/Hardness_ad_.htm . 2) Concrete versus asphalt Concrete has a narrow range of hardness whether it is old or new, cold or hot. Asphalt, however, has a much wider range of hardness that is indeed affected by the age, temperature, manufactured density, and composition of the material. I suspect this range is what has generated some of the differences in opinion regarding the hardness comparisons between concrete and asphalt. Fresh asphalt is not nearly as hard as concrete and is therefore a better running surface than concrete. This is because fresh asphalt does indeed have internal air pockets, as well as soft tar that can flex under load. (It has nothing to do with the microscopic interaction between the surface of the asphalt and the bottom of one's shoes. There is no "nestling" between shoes and the running surface.) Over time, the asphalt will compact, increasing the hardness as the tiny air pockets disappear and the tar simply hardens. The amount of gravel used in different asphalt mixtures will also affect the hardness. So, running on old asphalt with a high gravel concentration on a cold day may feel closer to concrete. Still not sure about asphalt and concrete? Go give both of them a whack with a large hammer or sledge. You'll feel the difference. 3) Temperature Remember science class when you turned water into ice by lowering the temperature? Same thing happens to asphalt and especially dirt -- anything with some liquid content that can experience a phase change through a lowering of the material temperature. Sure, asphalt can approach the hardness of concrete at very cold temperatures, but due to its relatively high moisture content, dirt can also become very hard. Cross country teams in International Falls can vouch for this one. So, Jeff is correct that the surfaces will be closer in hardness as the temperature decreases. 4) Shoes and hardness Someone mentioned earlier that the rubber in running shoes eliminated the issue of the hardness of the running surface. A cushioned running shoe will distribute the energy transmitted to the running surface over a longer period of time. This reduces the maximum force imparted to the surface. It's a bit like bending your knees to soften the impact when you jump off of tall box (like in plyometrics). Even with cushioned shoes, there is a force that is reflected back into the foot. This force is dependent upon the coefficient of restitution between the two materials. For our case, we can consider this relatively equivalent to the hardness. Shoes will soften the impact, but a runner wearing the same pair of shoes will feel a greater impact as he moves from a softer surface to a harder one. 5) Falling down In most cases, the severity of falling or stumbling injuries is not an issue of hardness, but roughness. Concrete is generally rougher than asphalt -- at least the sidewalk variety. Dragging skin across a rougher surface naturally produces greater abrasions. Of course if an elbow, knee, or head contacts either surface at an angle close to perpendicular, hardness is once again the issue regarding severity (see items 1 & 2). So, what did we learn today -- other than the fact that all of my classes will run late if I someday get a chance to quit the corporate engineering racket and pursue a career as a science teacher? We learned that running on asphalt is softer than running on concrete, but that the difference between the two depends upon the composition, density, age and temperature of the asphalt. Oh, and we also learned that there are a lot of people on this list (present company certainly included) that know how to speculate on peak age, nutrition, running surfaces, etc., but not nearly as many seem to know (or are willing to post) cold hard facts, results, splits, race reports, and statistics on track & field. ;) [EMAIL PROTECTED] -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Hanks, Jeffrey S Sent: Tuesday, August 27, 2002 3:32 PM To: 'picqc'; Mike Prizy Cc: Kurt Bray; [EMAIL PROTECTED] Subject: RE: t-and-f: Concrete VS Asphalt (was: Why on the street?) The experiments you guys are talking about don't really relate to the situation here. You can't make the argument that if you fall on concrete or asphalt it's going to hurt just as much. Nor can you say that because the amount of bounce you get off a golf ball reflects the hardness of a substance. There are a lot of other factors that come into play here. >From my understanding of the materials, asphalt is a much more porous substance (more microscopic holes in it) than is concrete. The absorption that you get from running on the surfaces has a lot to do with the rubberized material at the bottom of your shoes nestling into this porous holes. By doing so, they create cushioning that a surface with no holes in it doesn't provide. There is also a certain "give" factor that you have to consider. That is the amount of "give" that a substance has and like some other posters have stated, this has a lot to do with the outside temperature. The heavier an object (golf balls compared to a human's pound per square inch is very small) the more profound the pressure put on one particular spot and consequently the more potential give is the result. Asphalt gives more than does concrete, although I believe as temperatures approach freezing, this difference is not as pronounced. -Jeff -----Original Message----- From: picqc [mailto:[EMAIL PROTECTED]] Sent: Tuesday, August 27, 2002 13:08 To: Mike Prizy Cc: Kurt Bray; [EMAIL PROTECTED] Subject: Re: t-and-f: Concrete VS Asphalt (was: Why on the street?) I don't buy it. Fall down on concrete and fall down on asphalt and tell me which on hurts the most. You're wearing shoes that absorb shock and you have insoles that do the same. Drop the golf ball on a piece of foam that is on a layer of air(gel, water, whatever) and see how high the ball bounces. In Boulder you'd take your life into your hands running on some of the sidewalks because they were so uneven, broken, and torn up. In other places the crown of the road was so high it would throw your back out if you ran on it for very long. In other places the traffic was so crazy it was life threatening.So, you take the lesser of the evils and go for it. JL Mike Prizy wrote: > I just did the golf ball drop test and used my mail carrier - who just > delivered my September 2002 TFN - as the judge. He said it was > difficult to tell, but that the ball seemed to bounce higher from the > sidewalk, though I think the rough asphalt surface made the golf ball > take an angular path, appearing to bounce not quite as high. > > However, that is with two hard objects. I think the difference in > variables with a runner - shoe density, shoe wear, form, mechanics, > speed, etc. - will make the differences insignificant. > > Grass will make you last. > > Kurt Bray wrote: > > > Mike says: > > > > >The only asphalt I ever saw that was significantly softer than > > >concrete was on country roads when I ran in college. The only other > > >asphalt I ever saw that was softer than set > > >concrete was the asphalt > > >just before the steamroller went over it. > > > > Here's a simple experiment you can try. I did it myself a few > > minutes ago to confirm what I'd heard. Find a flat area that has > > both asphalt and concrete - say a parking lot with a sidewalk next > > to it. Stand on the concrete and drop a golf ball held out at > > shoulder height. It will rebound about up to your waist. Now step > > over and do the same on the asphalt, and you will see that bounces > > only up to about a little over your knees. > > Conclusion: there IS a noticeable and measurable difference in the hardness > > of the two surfaces. > > > > Add this difference up over and over again though hundreds of > > thousands of footfalls, and it could make a real difference in > > injury risk. > > > > I certainly agree with those who say that dirt is even better (the > > ball hardly bounces at all on dirt), and I do the majority of my own > > running on dirt for that very reason. But if you are running in an > > urban setting and you have only the choice between asphalt and > > concrete, take the asphalt. > > > > Kurt Bray > > > > _________________________________________________________________ > > Chat with friends online, try MSN Messenger: > > http://messenger.msn.com
