Runners: Why are you too good for the sidewalk?

[silverpig]

Actually, it's not. The concept is the same, but the amount of shock absorbed is vastly different.

I ordered a pressure sensor to add numbers to this discussion. I already know it's noticeably different because I can feel it and I did several calculations, but some people just won't believe something without a graph. I'll get the PCB back in a week or two and then I'll update this thread with the results.

That's exactly my point. The amount of shock absorption is so drastically different.

People have already done this study and in a lab.

 

[silverpig]

/facepalm

/doublefacepalm I can do that too...

 

[MrDudeMan]

That's exactly my point. The amount of shock absorption is so drastically different.

People have already done this study and in a lab.

Nothing in the equation changes in the actual situation other than the ground. Your contrived examples are useless due to that. Concrete reflects more energy into your body.

Studies always include injury rates. I've seen very few with raw data and none that specifically details force per step over two surfaces using a single runner. I know - sample sizes, statistics, blah blah - but if you take the same person and show actual data while transitioning from one surface to another, you should see it in the numbers. I'll probably throw in an accelerometer because my hunch is that you'll see a difference using that data as well.

Lastly, all the data in the world can't compete with experience. It was also 'proved' in the 1920s using calculus that a human could never run a mile under 4:00. Math isn't always right because you often have to assume too much. I'm well versed in the art of equation-bullshit being an engineer.

 

[Capt Caveman]

/doublefacepalm I can do that too...

Stay ignorant...

 

[Jeff7]

Nothing in the equation changes in the actual situation other than the ground. Your contrived examples are useless due to that. Concrete reflects more energy into your body.
...

Elastic vs inelastic collision sort of thing?


How much more can asphalt really deform, versus concrete?

 

[silverpig]

Stay ignorant...

You too. This is easy.

Link to a paper that says it makes a difference. I have quoted two lab trials that say it isn't significantly different and offered a physical explanation of why. You have repeatedly posted drivel, e-gestures, and posts of little substance.

 

[silverpig]

Nothing in the equation changes in the actual situation other than the ground. Your contrived examples are useless due to that. Concrete reflects more energy into your body.

Studies always include injury rates. I've seen very few with raw data and none that specifically details force per step over two surfaces using a single runner. I know - sample sizes, statistics, blah blah - but if you take the same person and show actual data while transitioning from one surface to another, you should see it in the numbers. I'll probably throw in an accelerometer because my hunch is that you'll see a difference using that data as well.

Lastly, all the data in the world can't compete with experience. It was also 'proved' in the 1920s using calculus that a human could never run a mile under 4:00. Math isn't always right because you often have to assume too much. I'm well versed in the art of equation-bullshit being an engineer.

The two papers on this thread probably do.

The concrete may reflect more energy, but that is absorbed by the shoe. That's the point of the shoe...

 

[MrDudeMan]

The two papers on this thread probably do.

The concrete may reflect more energy, but that is absorbed by the shoe. That's the point of the shoe...

I agree that's the point of the shoe. The human body is pretty sensitive, though, so I think that's why this debate keeps happening.

I should have said concrete reflects more energy per unit of time. Asphalt returns to it's original shape slower, which means less dE/dt. The hardness of concrete may be trivially more than asphalt, but the impulse should be significantly higher.

The weight of the runner is probably a first order component. Up to 3.5x the weight of the runner is exerted on the foot while running. Assuming proper form (landing on the forefoot with approximately 6 sq. in. of surface area):

• 120 lb person exerts up to 70 psi on the ground
• 200 lb person exerts up to 116 psi on the ground

To put that in perspective, a 4000 lb car (assuming even weight distribution) puts 1000 lbs of force on the ground under each tire. The pressure, however, is much lower because car tires have roughly 35 sq. in. of contact area. The end result is 28 psi. Even if you shift more weight to the front tires to account for the engine and transmission, the end result still doesn't come close to a 120 lb person. We know cars deform the road, so obviously humans deform it more.

The frequency of the reflection is important and I haven't been able to find any data on that. Shoes probably aren't very good at filtering high frequency components, so concrete should impart more energy into your body because it springs back into it's original shape faster. Maybe a single step makes no difference, but what about 10,000? I believe I should be able to see this with a series of accelerometers. Integrating over a 5 mile run should provide some good information.

 

[Matthiasa]

snip

We know cars deform the road, so obviously humans deform it more.

snip

It does not work like that...

 

[MrDudeMan]

It does not work like that...

Sure it does. I just asked my brother in law who is a civil engineer to explain it to me. The way cars deform asphalt is by applying pressure to it. The initial compression of a road is directly related to the weight of a vehicle and the number of iterations. Later the compression of the substrate becomes less significant, but still not insignificant, compared to upheaval of the surrounding material due to displacement. It, in fact, works exactly as I said. Cars aren't magical in their ability to deform the road. They drive over the same spots with much more repetition than runners step in the same spots.

 

[Matthiasa]

Yes... but you stated that a human would deform it more which is simply not true.

 

[MrDudeMan]

Yes... but you stated that a human would deform it more which is simply not true.

I'm not going to argue about semantics any more than this: Cars cause more permanent deformation because of increased repetition. A human, or anything that exerts increased pressure on the substrate, causes more deformation per impact, but it's wear balanced much more evenly.

 

[SpatiallyAware]

I am literally lol'ing at you Olympic class athletes who must run in traffic due to the differential between asphalt and concrete deformation under a 150lb load.

 

[episodic]

You could be in the south where they don't build sidewalks.

 

[MrDudeMan]

I am literally lol'ing at you Olympic class athletes who must run in traffic due to the differential between asphalt and concrete deformation under a 150lb load.

I haven't seen anyone claim, suggest, or imply that. No need to troll.

Personally, I only run on the road in neighborhoods. If I leave the neighborhood, I use the sidewalk.

 

[feralkid]

Seriously? I can't imagine asphalt having any more give than concrete, unless you weigh 5000 lbs.

Try parking your bike with the kick-stand on a hot day, watch it sink slowly into the asphalt.

 

[Capt Caveman]

I am literally lol'ing at you Olympic class athletes who must run in traffic due to the differential between asphalt and concrete deformation under a 150lb load.

Retard, yes you are.

 

[Capt Caveman]

The two papers on this thread probably do.

The concrete may reflect more energy, but that is absorbed by the shoe. That's the point of the shoe...

LOL. And the shoe is going to absorb the full impact no matter what the surface? LOL.

 

[MrDudeMan]

I had a longer conversation with my bro-in-law to try to get more information.

The modulus of elasticity determines how much deformation occurs in a material with a given load in the elastic region. The elastic region is defined as the range of loads starting from 0 to the point where the load becomes high enough to cause permanent deformation. With that said, the modulus of elasticity of asphalt in compression (which is the state it's in while you're running on it) is 0.2 ksi. 1 ksi = 1000 lbs per sq. in. The modulus of elasticity of concrete is 3 ksi. That means a load that causes a deformation of x on concrete will cause a deformation of 15x on asphalt.

Some math:

I previously calculated that a 200 lb human could put as much as 100 psi (rounded down for a whole number) on his foot at the moment of impact. A shoe probably reduces that by a factor of 4 (I failed to mention this earlier) because of the distributed load due to the sole of the shoe. So, a 200 lb person could put 25 psi of pressure on an asphalt road. That's 12.5% of the modulus of elasticity, meaning if the asphalt is 2" thick, it will compress 0.25" very briefly. Concrete would compress by 0.016". Even if the pressures are off by a factor of 3, asphalt would still compress noticeably more than concrete. My shoe compresses about 0.4", so an asphalt road compressing by even 0.05" would be noticeable (12.5%).

Those are peak numbers. Reduce the pressure exerted by the same person by 50%, which is more realistic for long distance running (force per step = body weight * 1.75 (instead of 3.5 like I used earlier)). Now the asphalt compression is roughly 6%. Changing the thickness of the asphalt, compression of the shoe, weight of the runner, and several other things will obviously affect the results. It may turn out to be less than 1% in some cases, so I doubt a person in that situation would be able to feel the difference. However, it could end up being close to 10% in other situations and I strongly believe a person can feel that. Note: this is all at 25C. The modulus of elasticity decreases as temperature increases, so on a hot day the asphalt will be weaker while concrete stays essentially constant at all survivable ambient temperatures.

As it turns out, my previous comments about asphalt returning the same amount of energy were incorrect. It should definitely return less as it absorbs much more than concrete. I read some of that on my own, but I trust this new information much more as it came from someone who I trust is an expert in this field.

 

[WelshBloke]

I had a longer conversation with my bro-in-law to try to get more information.

The modulus of elasticity determines how much deformation occurs in a material with a given load in the elastic region. The elastic region is defined as the range of loads starting from 0 to the point where the load becomes high enough to cause permanent deformation. With that said, the modulus of elasticity of asphalt in compression (which is the state it's in while you're running on it) is 0.2 ksi. 1 ksi = 1000 lbs per sq. in. The modulus of elasticity of concrete is 3 ksi. That means a load that causes a deformation of x on concrete will cause a deformation of 15x on asphalt.

Some math:

I previously calculated that a 200 lb human could put as much as 100 psi (rounded down for a whole number) on his foot at the moment of impact. A shoe probably reduces that by a factor of 4 (I failed to mention this earlier) because of the distributed load due to the sole of the shoe. So, a 200 lb person could put 25 psi of pressure on an asphalt road. That's 12.5% of the modulus of elasticity, meaning if the asphalt is 2" thick, it will compress 0.25" very briefly. Concrete would compress by 0.016". Even if the pressures are off by a factor of 3, asphalt would still compress noticeably more than concrete. My shoe compresses about 0.4", so an asphalt road compressing by even 0.05" would be noticeable (12.5%).

Those are peak numbers. Reduce the pressure exerted by the same person by 50%, which is more realistic for long distance running (force per step = body weight * 1.75 (instead of 3.5 like I used earlier)). Now the asphalt compression is roughly 6%. Changing the thickness of the asphalt, compression of the shoe, weight of the runner, and several other things will obviously affect the results. It may turn out to be less than 1% in some cases, so I doubt a person in that situation would be able to feel the difference. However, it could end up being close to 10% in other situations and I strongly believe a person can feel that. Note: this is all at 25C. The modulus of elasticity decreases as temperature increases, so on a hot day the asphalt will be weaker while concrete stays essentially constant at all survivable ambient temperatures.

As it turns out, my previous comments about asphalt returning the same amount of energy were incorrect. It should definitely return less as it absorbs much more than concrete. I read some of that on my own, but I trust this new information much more as it came from someone who I trust is an expert in this field.

I'm not sure asphalt compresses by a 1/4 of an inch however briefly.

 

[MrDudeMan]

I'm not sure asphalt compresses by a 1/4 of an inch however briefly.

It doesn't matter how you feel about it. Numbers are numbers. 0.2 ksi = 200 psi, so if you apply a pressure of 10 psi it will deform by 1/20th of the original thickness. I have no idea how long it stays that way and neither did my brother in law because he doesn't deal with transient loads. Maybe it only happens for 10ms or maybe it lasts for 100ms. I don't know either way. I also don't know how thick a typical road is. Maybe it's only 1" thick, which would reduce all of the results I calculated by half.

Also, my next statement (the one you didn't bold, but was still relevant) covered this case. If there's a lot less pressure than I calculated because I missed something major, as much as a factor of 3, the numbers still show a significant difference between asphalt and concrete. Regardless of what the pressure is, there's a 15x difference between asphalt and concrete. Even if it compresses by 0.02", concrete would compress by 0.0013". That ratio is going to be the same no matter what the rest of the equation looks like.

 

[WelshBloke]

It doesn't matter how you feel about it. Numbers are numbers. 0.2 ksi = 200 psi, so if you apply a pressure of 10 psi it will deform by 1/20th of the original thickness. I have no idea how long it stays that way and neither did my brother in law because he doesn't deal with transient loads. Maybe it only happens for 10ms or maybe it lasts for 100ms. I don't know either way. I also don't know how thick a typical road is. Maybe it's only 1" thick, which would reduce all of the results I calculated by half.

Also, my next statement (the one you didn't bold, but was still relevant) covered this case. If there's a lot less pressure than I calculated because I missed something major, as much as a factor of 3, the numbers still show a significant difference between asphalt and concrete. Regardless of what the pressure is, there's a 15x difference between asphalt and concrete. Even if it compresses by 0.02", concrete would compress by 0.0013". That ratio is going to be the same no matter what the rest of the equation looks like.

Numbers are indeed numbers. It doesn't mean they are correct in describing what happens however.

 

[MrDudeMan]

Numbers are indeed numbers. It doesn't mean they are correct in describing what happens however.

That doesn't make any sense. This is a pretty straight forward application of science and math. Impart energy into a material that behaves in a way that can be calculated and then calculate the results. Every engineering discipline does this. If you think there's something magical going on that can't be described with normal math - math that's good enough to build roads - then please explain.

 

[WelshBloke]

That doesn't make any sense. This is a pretty straight forward application of science and math. Impart energy into a material that behaves in a way that can be calculated and then calculate the results. Every engineering discipline does this. If you think there's something magical going on that can't be described with normal math - math that's good enough to build roads - then please explain.

Ok.

Go outside and poke at a bit of asphalt.

Seriously. Go outside and do it.

...

...

Ok done it?

You're saying it compresses by a quarter of an inch when you run on it. Did you press really hard with your thumb? How far did it compress?

Now go and find a bit of broken, loose asphalt. Stress it a few times. Is it snapping back into shape really quickly?

 

[MrDudeMan]

Ok.

Go outside and poke at a bit of asphalt.

Seriously. Go outside and do it.

...

...

Ok done it?

You're saying it compresses by a quarter of an inch when you run on it. Did you press really hard with your thumb? How far did it compress?

Now go and find a bit of broken, loose asphalt. Stress it a few times. Is it snapping back into shape really quickly?

You're getting hung up on 0.25" of compression in a very specific circumstance. That was a peak compression from a 200lb person running, which could result in more than 700lbs of force acting on the asphalt. If you can't understand how this is different than your contrived situation, then there's really not much else to say. I knew before I submitted that message that someone would get hung up on that number, but I figured repeated explanations would help clear it up. I guess not.

Now, onto your experiment. I was VERY easily able to feel movement under my thumb when I pressed it into the road. Concrete has exactly zero give when using the same pressure from my thumb. I also found a sizable piece of asphalt to compress between my fingers. Video.

No, it isn't popping back into shape very quickly, but I'm also putting very different load on it with my finger. I can't emulate running with my thumb. However, I can very easily show how compressible asphalt is compared to concrete with relatively little force.