Actual Ground Pressure? Wordy and rambling post warning!- Mtbr.com
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  1. #1
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    Actual Ground Pressure? Wordy and rambling post warning!

    I'm looking for a rough measurement of the contact patch size of a range of fat bike tires at say 15, 10, and 5 psi to figure out what the range of ground pressures is. Anyone willing to measure?

    My motivation is comparative in nature as I want to relate the bikes impact in reference to an engineering study.

    (In other words I just want to see what ground pressure is compared to various snow cats and muskeg vehicles in an engineering text at my local library. I know it's going to be a much higher ground pressure, although likely lower than most off-road vehicles.)

    And the why comes down to filling the gap of anticipation waiting to buy a new bike and contemplating riding it around my remote property which is surrounded by muskeg.

    I'm guessing that fat tires are roughly equal to a hiking boot in contact patch and I routinely hike the muskeg in question without sinking through the thatch so I think it will work in the summer, let alone winter.

    Any insight?

  2. #2
    A Surly Maverick
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    Hi, welcome

    Must be honest and say I had no idea what Muskeg was .... http://en.wikipedia.org/wiki/Muskeg

    But I do now !

    I suspect Velobike may be well qualified to answer this as he has an healthy obsession with trying to ride the bogs of Northern Scotland

    My experience is mainly sand riding and normal often wet trails.

    Fat Bikes excel in these circumstances due to the big tyre print and the ability to run really low pressures allowing an even bigger contact patch.

    I can ride over stuff that I sink upto my boot tops in if I try to walk !

    Hope that helps

    Dr FG.
    Life IS a Beach and then you Corrode :)

  3. #3
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    Contact patch will be very close to weight supported by tire / PSI

    So, 100 pounds of weight on a tire with 10 PSI yields a contact patch of 10 square inches.
    100 pounds of weight on a tire with 5 PSI yields a contact patch of 20 square inches.

    Experimentally, you could get more exact measurements by taking the imprint of a tire on a soft surface supporting a know weight.

    I think peak pressure under a hiking boot pushing off is probably several times that for a fatbike tire.

  4. #4
    Nouveau Retrogrouch SuperModerator
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    Quote Originally Posted by Pitchfire View Post
    I'm looking for a rough measurement of the contact patch size of a range of fat bike tires at say 15, 10, and 5 psi to figure out what the range of ground pressures is. Anyone willing to measure?

    My motivation is comparative in nature as I want to relate the bikes impact in reference to an engineering study.

    (In other words I just want to see what ground pressure is compared to various snow cats and muskeg vehicles in an engineering text at my local library. I know it's going to be a much higher ground pressure, although likely lower than most off-road vehicles.)

    And the why comes down to filling the gap of anticipation waiting to buy a new bike and contemplating riding it around my remote property which is surrounded by muskeg.

    I'm guessing that fat tires are roughly equal to a hiking boot in contact patch and I routinely hike the muskeg in question without sinking through the thatch so I think it will work in the summer, let alone winter.

    Any insight?
    It the simplest level, the inflation pressure tells you how much weight is supported. 15psi in the tire supports 15 pounds of load per square inch of ground contact. The load being carried determines the size of the contact patch. So if the load is 150 pounds on the tire inflated to 15psi the contact patch is 10sq/in (regardless of the tire size). High volume tires just let you use lower pressures before the tire bottoms out.
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  5. #5
    Nouveau Retrogrouch SuperModerator
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    Quote Originally Posted by Pitchfire View Post
    I'm looking for a rough measurement of the contact patch size of a range of fat bike tires at say 15, 10, and 5 psi to figure out what the range of ground pressures is. Anyone willing to measure?

    My motivation is comparative in nature as I want to relate the bikes impact in reference to an engineering study.

    (In other words I just want to see what ground pressure is compared to various snow cats and muskeg vehicles in an engineering text at my local library. I know it's going to be a much higher ground pressure, although likely lower than most off-road vehicles.)

    And the why comes down to filling the gap of anticipation waiting to buy a new bike and contemplating riding it around my remote property which is surrounded by muskeg.

    I'm guessing that fat tires are roughly equal to a hiking boot in contact patch and I routinely hike the muskeg in question without sinking through the thatch so I think it will work in the summer, let alone winter.

    Any insight?
    Should also note that a pneumatic tire acts differently than the (more) rigid track of a snowcat (also think snowshoe). The rigid structure distributes the weight over a given area. Increase the surface area of the structure and you decrease the ground pressure for a given load.
    mtbtires.com
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  6. #6
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    and reverse-engineering what shiggy said, if the density of the ground you're riding on requires a weight distribution area greater than your current contact patch in order to prevent sinking, then each step up in size makes a big difference in reducing the weight's effect in deforming the ground beneath it.

    consider this: the pictures of fatbikes on snow and the depth the tread sinks into the snow more closely resembles the depth a snowshoe would've sunken into the snow.
    hiking boots would've gone right through until the snow packed enough to support the person.
    of course this is all guesswork considering I can't remember anyone taking a snowshoe pic and a fatbike pic taken in the same area on the same day, but I'm sure there are pics of inglorious dismounts up to the knee beside a floating fatbike track in the snow.

    you mentioned the anticipation gap, aiming at the "current" bikes (3.7" tires roughly) or waiting for the 4.5's?
    I gotta say, having been stuck in musky swamp (not proper muskeg by a long shot) I'd go as big as I could if I were you.


    oh, and welcome mister first post!
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  7. #7
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    Never mind the figures.

    A simple test is to find a muddy stretch, then walk through it, ride an ordinary mtb through it, and then ride a fat bike through. Fatbike wins.
    As little bike as possible, as silent as possible.
    Latitude: 57º36' Highlands, Scotland

  8. #8
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    Footprints from a guy running on the beach today, my tyre marks, 8psi, 80mm rims. No way you would have ridden a regular bike on that, far too soft.

    Fatbike wins

  9. #9
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    awesome.
    standard foot and fatbike tread on same terrain, same day.

    'ere ya go!!
    getcha some FATbike or sink into the muskeg and be lost forever ever.
    If steel is real then aluminium is supercallafragiliniun!

  10. #10
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    Just looked up the definition of muskeg. Sounds like the sort of stuff we get in most of the Highlands of Scotland. Places where most mtb riders don't consider going.

    These are the sorts of places where I like taking my bike.









    As you can see, the lack of a trail is no deterrent.

    The reality is that even the 3.7" Larry tyres are too narrow for most of this going - I don't like leaving grooves in any terrain - so there is a fair bit of hike-a-bike involved. But it is still much better than using an ordinary mtb. In fact any bits on this sort of stuff that could be ridden with an mtb could probably be ridden with a cyclocross bike, and that's what I was doing previously because if you have to carry a bike, a CX one is lighter.

    I am eagerly awaiting the new Big Fat Larry tyres at 4.5" because that will allow a larger footprint, and I'll be straight off to the bogs to try them out as soon as I can get a pair.

    I think 6" tyres are what we really need

    This raises a question for Coastkid - what pressures are used on the tyres for greenkeeping equipment? Some of those fat wheeled trailers etc are probably equal to a Pug and rider in weight.
    As little bike as possible, as silent as possible.
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  11. #11
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    So… muskeg is sort of like peat bogs? How come they’re not brewing up single malt in Alaska?
    Don’t frail and blow if you’re going to Braille and Flow.

  12. #12
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    Thanks for all the responses!

    Muskeg is just another name for peat bog I believe. We don't dig up any bog people routinely, but same deal. In fact if we burned our trees, we'd be Scotland (same climate, same latitude). If I remember correctly Scotland used to have forests like Southeast, Alaska. Our water table is within an inch or two of ground level in most places and often quite deep to get down to anything solid. On our land solid ground varies from our one Volkswagon sized granite boulder on the surface, to over 8.5 feet down (never hit bottom there) then you hit bedrock or clay. It's tough going walking on floating thatch that may or may not hold together!

    I think my initial guess was rather pessimistic. Different thing by a great margin than the contact patch of a standard mt.bike.

    The 4.5's sound heavenly!

  13. #13
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    So after reading and considering the responses here I have to agree with the above on hardpack. But relative to muskeg and snow I believe that another factor comes into play that I would like to be able to quantify, that of "flotation".

    If my 200 lbs. of bike and rider are carried at 5 psi then my tire pressure on the road is approximately 40lbs/sq", but on snow or in boggy soils that isn't going to be enough and the tires will sink in and provide a greater contact area until "relative flotation" occurs. So if my tires sink in 3-4" then the contact patch doubles at least lowering the actual ground pressure well below the actual tire pressure.

    So now I have to find a decent way to quantify flotation. Of course to do so I will have to come up with a stability factor for what the snow at a given condition (or muskeg) will support. Comparing it to my boots helps give a frame of reference.

    I'm only posting this in hopes it will help someone down the road understand ground pressure, as my internet searches have been full of crap with a few bright spots (this being one, wiki another).

    So to review, tire psi (above structural load of the tires, to rule out the weight being carried by the sidewalls like a run-flat tire), so tire psi roughly equals hard surface ground pressure, but in soft soils and ground conditions a tire will sink until if bogs down or "floats" (which may well be more accurately referred to as "planing," something easier to understand here in S.E. AK in my skiff). Then whatever the soils will support will occur (or bogging will occur) by an increase in the surface area of the tire as it sinks which consequently lowers the ground pressure as measured by the surface area of the tire in contact with the soil, divided by the weight of the bike and rider, to give the actual ground pressure which is very often going to be below the tire pressure of a fat bike tire.

    I think that's right...

    The Bombradier Muskeg (tracked vehicle) has a loaded track ground pressure of 2psi. Other similar tracked vehicles (basically snow cats for summer) range from .5psi (Sure-go) to 2.5psi (Foremost 6T). I don't have a figure for a tracked Argo, but bet the Sure-go is close or better.
    Lacking a cone penetrometer I guess I'll just have to give it a whirl.
    Last edited by Pitchfire; 07-14-2011 at 04:24 PM.

  14. #14
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    Quote Originally Posted by Pitchfire View Post
    S
    If my 200 lbs. of bike and rider are carried at 5 psi then my tire pressure on the road is approximately 40lbs/sq", but on snow or in boggy soils that isn't going to be enough and the tires will sink in and provide a greater contact area until "relative flotation" occurs. So if my tires sink in 3-4" then the contact patch doubles at least lowering the actual ground pressure well below the actual tire pressure.
    In your first calulation example, output units should be square inches (not lbs/sq). That area then gets distributed among both tires, assuming you are not riding a unicycle or doing wheelies. Probably one of the big advantages of traveling with wheels on soft surface is that the pressure remains relativly constant as you move. I know I have been on snow conditions that would support my weight if standing still but then I would punch through as soon as I started walking (shifting all weight toward even smaller supporting area)

    Definitly intersting to think of each ground surface type as having a PSI value and that float is simply the relationship between your tire pressure being counter-acted upon by the available ground pressure.

  15. #15
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    Quote Originally Posted by Pitchfire View Post
    ...The Bombradier Muskeg (tracked vehicle) has a loaded track ground pressure of 2psi. Other similar tracked vehicles (basically snow cats for summer) range from .5psi (Sure-go) to 2.5psi (Foremost 6T). I don't have a figure for a tracked Argo, but bet the Sure-go is close or better.
    Lacking a cone penetrometer I guess I'll just have to give it a whirl.
    I use a thumb penetrometer. Shove it in the ground, then shove it into the tyre to gauge the differential and the chances of riding on that surface.

    It is a highly scientific method because it's digital.

    Those pressures you have quoted are useful because it's a guide to what we really need to be able to run our tyres at. I'm pretty sure I'm right about needing 6", but I eagerly await the BFL anyway.
    As little bike as possible, as silent as possible.
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  16. #16
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    Quote Originally Posted by GrayJay View Post
    In your first calulation example, output units should be square inches (not lbs/sq). That area then gets distributed among both tires, assuming you are not riding a unicycle or doing wheelies. Probably one of the big advantages of traveling with wheels on soft surface is that the pressure remains relativly constant as you move. I know I have been on snow conditions that would support my weight if standing still but then I would punch through as soon as I started walking (shifting all weight toward even smaller supporting area)

    Definitly intersting to think of each ground surface type as having a PSI value and that float is simply the relationship between your tire pressure being counter-acted upon by the available ground pressure.
    Agreed. His example has 40 sq/in of tire contact patch, not 40psi of ground pressure.
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  17. #17
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    Yeah, I missed that.

    Your thumb penetrometer isn't too far off the mark. The Swedish Forest Operations Institute put out a snow hardness guideline:

    A gloved fist can penetrate very soft snow (0-500g/cm2), an open gloved hand can penetrate soft snow (same range given for very soft snow), an extended gloved fore-finger can penetrate medium hard snow (500-2500g/cm2), a pencil can penetrate hard snow (2500-5500g/cm2), and a knife can penetrate very hard snow (>5500g/cm2).

    Obviously you can just ride it and see, but it's good to be able to quantify even what you cannot ride and to see what you would need to be able to ride it and whether the trade-offs are worth implementing.

    With all the talk about the new 4.5" tire and corresponding Moonlander, it leads me to ask at what point the trade-offs stop being worth it most of the time. Do I want to get a '11 Mukluk, or the new version, or do I want to go with the Moonlander? I don't yet know.

    I could strap various inner-tubes to a wide enough rim without a tire and find out pretty quick what a minimal weight tire could offer in terms of float over snow and sand (wouldn't be durable, no hard turns but would prove the concept).

    I guess everyone has to decide where diminishing returns begin for their riding tastes. Could always get a water trike...

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