Please stay on the trail.

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

Postby madbuck » Tue Oct 02, 2012 1:31 pm

Vids wrote:Here is an example to help things along (using round numbers for simplicity's sake):
....


The only way the internal tire psi affects the psi exerted on the ground is by changing the contact area (less psi = bigger contact area).


But the support of a properly inflated tire is precisely the dominant mechanism for supporting the weight of the bike and rider (with a much lesser, but nonzero, influence of the tire's rigidity. Consider a tire closer to a balloon (almost no rigidity) and a steel tank (very much), to use examples earlier in this thread; also consider how a 20lb bike sinks to the ground on a flat tire, to get an idea of the degree of support from the sidewalls alone).

A cushion of air in a tire -- if sufficient pressure exists to keep the rim off the ground -- is responsible for nearly all of the support of the weight above it.
A tire is a container for holding sufficient air to support the weight above it. (Bridgestone).
The volume of the tire remains nearly constant (sinks lower and stretches fatter); the mass of the air in the tire remains constant; the pressure remains nearly constant.

The issue with your example is that the numbers are both simplifed and made up :-D
But if those were real weight and contact patch sizes, the pressure in the tire to yield those results would be close to 11psi.

See mtnfiend's actual experiment earlier in the thread. It was a great match of empirical observation to theory -- and he began the experiment being dubious of the theory!

In very low psi examples, when the pressure isn't enough to hold up the entire weight, gravitational forces are sufficient to overcome all of the air pressure in the tires, and any remaining unsupported weight is supported by the structure of the bike. Maybe this is the disconnect between people saying "internal psi != external psi" -- because it's an example when not all of the support is given by the air pressure.

Perhaps it helps to think of an inflated tire as a crude scale: the more weight pushing down on it, the larger the contact patch is, up until the internal pressure of the tire is overcome. In that case, the analogy would be a fat person standing on a spring-loaded scale, and bottoming out the scale! All we could conclude is that the weight was at least heavy enough to overcome the support of the air (or spring).
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TallGrass
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Re: Please stay on the trail.

Postby TallGrass » Tue Oct 02, 2012 1:34 pm

w = mg; weight equals mass times acceleration of gravity
f = ma; force equals mass times acceleration
Gravity is often treated like a constant (9.8m/s^2, 32.2'/s^2), but it varies by altitude, crust (oceanic, continental), as well as ground composition (e.g. if standing above a large ore deposit) hence the need for different equations, especially for the Mars rovers.
2.2lbs does not equal 1kg as one is force the other mass. In space it would basically be 0lb but still 1kg. Heard of the 14er weight loss program? Unfortunately you get it all back by the trailhead!

"Outside PSI = inside PSI" doesn't apply if the vessel also exerts an inward force. E.g., inflated to 3atm, a wheel's tire/tube/rim only needs to exert an inward force of 2atm which coupled with the 1atm outside yields equilibrium.

"Tire contact patch(es) [area] X tire PSI [pneumatic] = rider+bike weight" may be close in some circumstances but clearly is non-determinant as evidenced by a bike with both tires in a rounded muddy rut will have a contact area factors larger, and one with both tires rolling over angle iron factors less, but both will have the same weight on the ground and PSI in the tires. If it is fuzzy with bicycles, it sure isn't with 700lb of rider+motorcycle+gear (my 14er approach vehicle :-D ).


And who's to say the elephant isn't wearing floral pumps or a nice patent leather stilettos? (cue Pampilion)
Let's just hope they wear something more appropriate on a 14er trail. :)

MUni Rider wrote:Now what? :-k
Image


Jove help you if you go barreling down Elbert on that and blow a lace!
With right on front and left on rear, it could induce some interesting crabbing tendencies if it pro- or supinates!
Not sure if I'll do more 14ers. The trip reports are too tiring. :wink:
"A few hours' mountain climbing make of a rogue and a saint two fairly equal creatures.
Tiredness is the shortest path to equality and fraternity - and sleep finally adds to them liberty."
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tlongpine
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Re: Please stay on the trail.

Postby tlongpine » Tue Oct 02, 2012 2:00 pm

Now that this matter has been resolved...

What ground force (expressed as PSI) is required to damage tundra?
I am unable to walk away from the mountain without climbing it. An unclimbed mountain tugs at my consciousness with the eternal weight of time itself. Until I've pressed my face into it's alpine winds, hugged it's ancient granite walls, and put it's weathered summit beneath my heal I'm unable to resist it's attraction.Knowing nature gives the mountain more time than she gives us adds urgency to the obsession. As has been said before; the mountain doesn't care.

It can wait forever. I cannot.
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Greenhouseguy
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Re: Please stay on the trail.

Postby Greenhouseguy » Tue Oct 02, 2012 2:26 pm

tlongpine wrote:Now that this matter has been resolved...

What ground force (expressed as PSI) is required to damage tundra?


Wet tundra, dry tundra, frozen tundra, or snow-covered tundra? It makes a difference.
"May your boulder be your blessing." - Aron Ralston
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Re: Please stay on the trail.

Postby Vids » Tue Oct 02, 2012 3:02 pm

madbuck wrote:But the support of a properly inflated tire is precisely the dominant mechanism for supporting the weight of the bike and rider (with a much lesser, but nonzero, influence of the tire's rigidity.


Agreed, but I would say the psi in the tire (while under loading from the bike and rider) is a pretty close value to the force exerted on the ground. It had been stated above as a "simple law of physics" that internal psi always equals force on the ground, that's not entirely accurate.

Here is a quote from a study by Penn State (about tractor tires which are much stiffer, but you get my point):

"Contact pressure is the pressure that is exerted by a tire or track on the soil surface, expressed in pounds per square inch (psi). Reducing contact pressures will cause less topsoil compaction (Figure 5). In completely flexible tires, surface contact pressure is similar to tire pressure. With most farm tires, surface contact pressure is about 1 to 2 psi higher than tire pressure due to stiffness in the tire." The entire study can be read here: http://pubs.cas.psu.edu/freepubs/pdfs/uc186.pdf
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Re: Please stay on the trail.

Postby Vids » Tue Oct 02, 2012 3:05 pm

Greenhouseguy wrote:
tlongpine wrote:Now that this matter has been resolved...

What ground force (expressed as PSI) is required to damage tundra?


Wet tundra, dry tundra, frozen tundra, or snow-covered tundra? It makes a difference.


This one is even more complicated......not even going to begin to contemplate this. :lol: Depends on velocity of tire, friction factor between ground and tire, root structure of tundra grasses, moisture content of soil, etc.......
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jdorje
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Re: Please stay on the trail.

Postby jdorje » Tue Oct 02, 2012 4:24 pm

crossfitter wrote:The internal pressure really won't change very much. It will change a little as the tire deforms, sure, but that deformation is a small fraction of the total volume of the tire. In your balloon example, what really changes is the contact area between the balloon and plate. In the 0 psi case, the entire load is carried by the frame and the ground pressure is extremely high due to the thin edges and rigidity of the rims.


Makes sense. I think that's a function of the tire's rigidity though. If you put a plate on a balloon the balloon will deform a lot and the pressure will rise (leading to it popping). Tires don't deform nearly so much so the volume of air is not compacted.

ajkagy wrote:weather and water effect poorly designed and built trails more than any hiker/biker could ever effect it. All it takes is 1 big deluge to do some pretty serious damage.


I think that pretty much sums things up.
-Jason Dorje Short
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BillMiddlebrook
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Re: Please stay on the trail.

Postby BillMiddlebrook » Tue Oct 02, 2012 4:36 pm

True. Additionally, terrain and natural materials on our peaks don't always provide for "bomber" trail construction.
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jdorje
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Re: Please stay on the trail.

Postby jdorje » Tue Oct 02, 2012 4:49 pm

I have no idea what you could possibly be talking about. They should just get a backhoe and a bulldozer up there and stop the tundra damage once and for all.
-Jason Dorje Short
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BillMiddlebrook
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Re: Please stay on the trail.

Postby BillMiddlebrook » Tue Oct 02, 2012 5:01 pm

lol

At least a friggin' rope line!
"I have made it through the things others would surely die just watching"
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jdorje
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Re: Please stay on the trail.

Postby jdorje » Tue Oct 02, 2012 5:05 pm

Do you need 3000' of gain on your escalator ascent?
-Jason Dorje Short
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BillMiddlebrook
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Re: Please stay on the trail.

Postby BillMiddlebrook » Tue Oct 02, 2012 5:07 pm

And who's paying for the power? We'll have to revisit the TH collection boxes topic.
"I have made it through the things others would surely die just watching"
- Megadeth

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