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Please stay on the trail.

FAQ and threads for those just starting to hike the Colorado 14ers.
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Re: Please stay on the trail.

Postby Dex » Mon Oct 01, 2012 6:30 pm

Bean 2 wrote:
Bean wrote:
Dex wrote:So what you are telling us is that if the max pressure on the tire is 50 psi, then the max pressure it can a bike with 400lb rider can put on the ground is 50 psi?
Seriously?

Yes. The contact patch will increase in size. There may be slight pressure changes as the tire deforms but that will be insignificant. Assuming a square contact patch, all you'd need is 2" x 2" (per tire) at 50psi to support 400lb.


Dex, at the end of the day all of this physics talk is insignificant.


Only if you have something else to do ... like Hitler trying to win a war.
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"Please use the comments to demonstrate your own ignorance, unfamiliarity with empirical data and lack of respect for scientific knowledge. Be sure to create straw men and argue against things I have neither said nor implied. If you could repeat previously discredited memes or steer the conversation into irrelevant, off topic discussions, it would be appreciated. Lastly, kindly forgo all civility in your discourse . . . you are, after all, anonymous." Barry Ritholtz

Re: Please stay on the trail.

Postby Bean » Mon Oct 01, 2012 6:37 pm

Dex wrote:
Bean wrote:
Dex wrote:So what you are telling us is that if the max pressure on the tire is 50 psi, then the max pressure it can a bike with 400lb rider can put on the ground is 50 psi?
Seriously?

Yes. The contact patch will increase in size. There may be slight pressure changes as the tire deforms but that will be insignificant. Assuming a square contact patch, all you'd need is 2" x 2" (per tire) at 50psi to support 400lb.


No it won't, it is a tire - too much pressure and it will blow.

You are equating internal psi with external psi on the ground.

Where is Bean2 when you need him.

I just checked and my tires suggest 35-65psi (35 is too much, 65...lol) which means they could easily support our friend.

We're rapidly approaching an "airplane on a treadmill" level of discussion.
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Re: Please stay on the trail.

Postby Bean 2 » Mon Oct 01, 2012 6:56 pm

Bean wrote:
Bean 2 wrote:Dex, at the end of the day all of this physics talk is insignificant. The point is that walking downhill is for hiking boot wearing troglodytes (yes, that's right, troglodytes) and mountain biking/skiing is the way to go. If you are looking to build muscle (you can always use my calves for a good example of built muscle (my calves (which are gigantic FYI) can be seen in multiple threads and trip reports on this site)) you can always take up mountain biking and skiing, as they are most definitely NON-troglodyte methods of travel. With the aid of my physics textbook I can talk PSI and other such concepts all night but let's get to the bottom line here; and that is building muscle in it's finest form (my calves for example (which happen to be huge)) is not related to physics or trail design. Ergo this PSI conversation is more or less irrelevant...

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gdthomas wrote:

Bean, you're an idiot 2.

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Re: Please stay on the trail.

Postby mtnfiend » Mon Oct 01, 2012 7:20 pm

Well, all is right with the world.....it seems Bean is correct. It's not intuitive or obvious at first, but I had to put pen to paper to convince myself or loose my sanity (I mean honestly, Bean can't be right, can he??).

90 kg person+bike (~200 lbs). I weight about 165lbs and my bike is roughly 35lbs (heavy I know, but it's old and I'd rather spend my money on ski gear).

Since Newton was the pimp-daddy of math and calculus: F=ma; so 90kg*9.8m/s^2. = 882N.

P=F/A, rearrange to solve for area; A=F/P. I measured the pressure in my rear tire at 18 psi (124,105 Pa).

Solving for area; A = 882N/124,105N/m^2 = .0071 m^2 (~11 in^2).

I'll admit, when I first ran the calc, 11 square inches seemed absurd especially since the width of my tire is roughly 2" (meaning the contact length would need to be about 5.5 in). I know tires deform, but that much seems ridiculous, right??

I sat on my bike with as much weight on the back tire as possible (the calc above assumes 1 tire is carrying all the weight) and had my wife mark the contact length on the garage floor (slid a straight edge under the front and back of the rear tire until it touched the tire, then marked the location with a sharpie).

Measuring the distance between the 2 marks yielded 5.75 in. - means good enough for government work.

Now that we've got that solved, we can go back to ridiculing people for not staying on the trail........or bean can keep posting topless photos of himself (I'd wouldn't post that kind of stuff on the interwebz if I were you, but that's your deal). [-(
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Re: Please stay on the trail.

Postby HikerGuy » Mon Oct 01, 2012 7:24 pm

Dex wrote:
Bean wrote:
Dex wrote:So what you are telling us is that if the max pressure on the tire is 50 psi, then the max pressure it can a bike with 400lb rider can put on the ground is 50 psi?
Seriously?

Yes. The contact patch will increase in size. There may be slight pressure changes as the tire deforms but that will be insignificant. Assuming a square contact patch, all you'd need is 2" x 2" (per tire) at 50psi to support 400lb.


No it won't, it is a tire - too much pressure and it will blow.

You are equating internal psi with external psi on the ground.

Where is Bean2 when you need him.


Internal PSI = External PSI. Simple physics, opposite and equal forces. Bean is correct in the example he uses. The pressure in a tire does not change due to load.

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Re: Please stay on the trail.

Postby RckyMtnWildman » Mon Oct 01, 2012 7:35 pm

Please stay on the trail," should state: "please stay on one of them trails." Like June Carter tells Johnny Cash: "Y'all can't walk no line!" Doesn't mean the chosen path is wrong, it just deviates. I'm all for preservation, treading lightly, and keeping our mountains pristine; but if I see that beautiful summit lurking in the distance, the shortest path is a straight-line and that path will be taken as long as the obstacles concur.

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Re: Please stay on the trail.

Postby cheeseburglar » Mon Oct 01, 2012 7:39 pm

After 9 of us stormed Fortress Peak on Saturday, which is some obscure 13er hill somewhere in the wilderness, there should be a trail.
Please follow it.
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Re: Please stay on the trail.

Postby Dex » Mon Oct 01, 2012 8:14 pm

mtnfiend wrote:
Now that we've got that solved, we can go back to ridiculing people for not staying on the trail........or bean can keep posting topless photos of himself (I'd wouldn't post that kind of stuff on the interwebz if I were you, but that's your deal). [-(



The issue isn't about the PSI in the tire. It is about the PSI on the ground - bike tire vs a hiker's foot.

I know it is getting late but, let's try to focus.
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"Please use the comments to demonstrate your own ignorance, unfamiliarity with empirical data and lack of respect for scientific knowledge. Be sure to create straw men and argue against things I have neither said nor implied. If you could repeat previously discredited memes or steer the conversation into irrelevant, off topic discussions, it would be appreciated. Lastly, kindly forgo all civility in your discourse . . . you are, after all, anonymous." Barry Ritholtz

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Re: Please stay on the trail.

Postby ajkagy » Mon Oct 01, 2012 8:43 pm

Dex wrote:
mtnfiend wrote:
Now that we've got that solved, we can go back to ridiculing people for not staying on the trail........or bean can keep posting topless photos of himself (I'd wouldn't post that kind of stuff on the interwebz if I were you, but that's your deal). [-(



The issue isn't about the PSI in the tire. It is about the PSI on the ground - bike tire vs a hiker's foot.

I know it is getting late but, let's try to focus.


weight (bike + rider + gear) / surface area in inches touching ground = ground PSI

pretty simple. Sorry, but just because you have 20 or 30 PSI of air in the tire doesn't mean there is that much PSI touching the ground. The PSI in the tire is the outward pressure force of the air pushing against the rubber in the tire. Think of it this way...if I have a high pressure air tank with 5000 PSI of air in the tank laying on the ground, it's not putting 5000 PSI against the ground, lol
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Re: Please stay on the trail.

Postby cheeseburglar » Mon Oct 01, 2012 8:57 pm

I thought all hikers hate mountain bikers because of their scheme to make everyone leash their dogs.
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Re: Please stay on the trail.

Postby MUni Rider » Mon Oct 01, 2012 9:16 pm

mtnfiend wrote:Well, all is right with the world.....it seems Bean is correct. It's not intuitive or obvious at first, but I had to put pen to paper to convince myself or loose my sanity (I mean honestly, Bean can't be right, can he??).

90 kg person+bike (~200 lbs). I weight about 165lbs and my bike is roughly 35lbs (heavy I know, but it's old and I'd rather spend my money on ski gear).

Since Newton was the pimp-daddy of math and calculus: F=ma; so 90kg*9.8m/s^2. = 882N.

P=F/A, rearrange to solve for area; A=F/P. I measured the pressure in my rear tire at 18 psi (124,105 Pa).

Solving for area; A = 882N/124,105N/m^2 = .0071 m^2 (~11 in^2).

I'll admit, when I first ran the calc, 11 square inches seemed absurd especially since the width of my tire is roughly 2" (meaning the contact length would need to be about 5.5 in). I know tires deform, but that much seems ridiculous, right??

I sat on my bike with as much weight on the back tire as possible (the calc above assumes 1 tire is carrying all the weight) and had my wife mark the contact length on the garage floor (slid a straight edge under the front and back of the rear tire until it touched the tire, then marked the location with a sharpie).

Measuring the distance between the 2 marks yielded 5.75 in. - means good enough for government work.

Now that we've got that solved, we can go back to ridiculing people for not staying on the trail........or bean can keep posting topless photos of himself (I'd wouldn't post that kind of stuff on the interwebz if I were you, but that's your deal). [-(


I knew I remembered this whole argument somewhere else before. LOL!

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Re: Please stay on the trail.

Postby mtnfiend » Mon Oct 01, 2012 9:37 pm

I can't believe I'm doing this....

Dex wrote:The issue isn't about the PSI in the tire. It is about the PSI on the ground - bike tire vs a hiker's foot.


Yes, that was one of my immediate thoughts as well. I know the math I posted above is simplified, but please, please, post a mathematical solution that contradicts my above. Please!!

ajkagy wrote:weight (bike + rider + gear) / surface area in inches touching ground = ground PSI

pretty simple. Sorry, but just because you have 20 or 30 PSI of air in the tire doesn't mean there is that much PSI touching the ground. The PSI in the tire is the outward pressure force of the air pushing against the rubber in the tire. Think of it this way...if I have a high pressure air tank with 5000 PSI of air in the tank laying on the ground, it's not putting 5000 PSI against the ground, lol


I like your thought process ajkagy, and it is that simple. Although the high pressure gas cylinder is not a straight apples to apples comparison. A high pressure gas cylinder is a rigid structure, quite unlike a tire, and that is exactly why I set out to measure the contact area of my bike while it had a known force on it. To determine the pressure I exert on the ground, I have to know my force (not weight - although they are sometimes the same quantity) and the contact area. Perhaps the experiment was overly simplified and there was certainly some error in the measurements, but please put pen to paper and explain why the math above is incorrect. It would be a very strange coincidence that the measured pressure and contact area would be so close to the calculated values - I'm not talking about just being in the ball park here, I'm talking about being in the same section of the ball park!!

But actually isn't this discussion a little contrived?? We are talking about the pressure on dirt here. Does dirt get offended or hurt when it gets stepped on?? Are we engineering some sky scraper, road, or bridge??
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