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Thread: Ground pressure question

  1. #1
    Clive
    Guest

    Ground pressure question

    Can't get my head around this despite my a level in physics (it was a long time ago !)

    assume you have 2 tractors that are identical and importantly the same weight

    tractor a is on 900 wide tyres and tractor b is on 710's

    both run a tyre pressure of 8psi - question is which tractor has the lowest ground pressure on the soil and will do least soil damage ?

    are they both the same as they run the same psi in their tyres ? or as I suspect tractor a with its wider tyres treads lighter (p= f x a )

    .........or does tractor a just spread the same force over more soil and as a result does more damage ??

    sure it's blindingly obvious but staff room discussion over a cup of tea just got quite heated with different opinions !

  2. #2
    foxbox
    Guest

    Re: Ground pressure question

    I would go with larger footprint equals less ground pressure for a given area, the weight has to be spread out somewhere unless you remove gravity from the equation. You could put your tractor on bike tyres at 8 psi and they would exert much more pressure for a given area than tractor tyres of any size but where you draw the line at "damage" is where the question really lies.

    And I think it needs to stop raining so you can worry about more important things!!

  3. #3
    svc
    Guest

    Re: Ground pressure question

    All forces have an equal and opposite the smaller the area the deeper the rut to support the same weight

  4. #4
    Timbo_1975
    Guest

    Re: Ground pressure question

    Surely b) on the 900's will exert less pressure per square inch / cm / whatever than a) due to the larger contact patch.

  5. #5
    Steevo
    Guest

    Re: Ground pressure question

    Less ground pressure by having a wider tyre, which in turn means less ground damage per cm2 of soil, though in total a greater area damaged.


    Narrow tyre:
    Less ground damaged, but the damage is much worse.

    Wide tyre:
    More ground damaged, but the damage is not as severe.


    As foxbox says, the question is at what ground pressure does ground become "damaged".

  6. #6
    scrambler
    Guest

    Re: Ground pressure question

    They will both have the same footprint and therefore ground pressure. The 710's footprint will be longer and the 900's will be wider. I think the 900's would do more damage as they are covering more ground. The fact is though that the 900's would be able to run at a lower air pressure and carry the same weight.

  7. #7
    foxbox
    Guest

    Re: Ground pressure question

    Quote Originally Posted by Steevo View Post
    Less ground pressure by having a wider tyre, which in turn means less ground damage per cm2 of soil, though in total a greater area damaged.


    Narrow tyre:
    Less ground damaged, but the damage is much worse.

    Wide tyre:
    More ground damaged, but the damage is not as severe.


    As foxbox says, the question is at what ground pressure does ground become "damaged".
    I've often wondered as threads on here appear regarding which are the heaviest sets of rolls at what stage rolling becomes "bad" and whether these wider tyres actually get down to similar sorts of ground pressure?

    If so, and bearing in mind that most "damage" or compaction is made on a first pass with a tyre with considerably less increase in damage the 2nd time, does CTF really produce the benefits being talked about?

  8. #8
    foxbox
    Guest

    Re: Ground pressure question

    Quote Originally Posted by scrambler View Post
    They will both have the same footprint and therefore ground pressure. The 710's footprint will be longer and the 900's will be wider. I think the 900's would do more damage as they are covering more ground. The fact is though that the 900's would be able to run at a lower air pressure and carry the same weight.
    Why would the 710's have a longer footprint for the same radius tyre at the same pressure? I'm not familiar with tyres beyond the fact that my tractor has got some so I'm intrigued by this. Keeping in mind that a 900 is around 28% wider than a 710 the 710 would need to be quite a bit longer to compensate?

  9. #9
    scrambler
    Guest

    Re: Ground pressure question

    Quote Originally Posted by foxbox View Post
    Why would the 710's have a longer footprint for the same radius tyre at the same pressure? I'm not familiar with tyres beyond the fact that my tractor has got some so I'm intrigued by this. Keeping in mind that a 900 is around 28% wider than a 710 the 710 would need to be quite a bit longer to compensate?
    If you look at a tyre from the side you will see that as you reduce the air pressure the bottom of the tyre gets flatter and flatter and more tread is in contact with the ground lengthways. The normal rule of thumb for field work is to have three lugs touching the ground when parked on a hard surface (it would be difficult to judge it on soil).
    If the 900's and the 700's had the same length of footprint, the wider tyre would have less air pressure and therefore ground pressure if everything else was equal.
    There are many people who do not make the most of there very expensive tyres.

  10. #10
    Tom H
    Guest

    Re: Ground pressure question

    I would personally go for the narrow one. as the reason it can run the same is that it will be a new type of tyre (xeobib,axiobib?) and the 900 will be something a little older? I would guess a trelli? The axio and xeo's will be one pressure for a set weight too, so you can go from field to down the road flat out and not worry about changing pressure/ knackering the tyre. Much better

    my 2p's worth

  11. #11
    Cab-over Pete
    Guest

    Re: Ground pressure question

    Quote Originally Posted by scrambler View Post
    They will both have the same footprint and therefore ground pressure. The 710's footprint will be longer and the 900's will be wider. I think the 900's would do more damage as they are covering more ground. The fact is though that the 900's would be able to run at a lower air pressure and carry the same weight.
    Clive originally stated that the two machines were the same weight and the two sets of tyres running at the same pressure.
    I would say that for the same radius/diameter and pressure, the 900 would do less damage to the soil. The 710 will have no longer footprint if the radius is the same, surely? Faffing about with pressures will change things greatly. I think Clive was talking about exact same make of tyre, just different sizes.

    It does bring up a point though. Always get the maximum diameter tyre you can fit on the machine. Height is good, especially where width is a restriction.

  12. #12
    Clive
    Guest

    Re: Ground pressure question

    the bit that is confusing is in my example both tractors are running at 8psi

    so the ground pressure is 8psi on both of them no ??

    ie they both exert the same force to the ground ?

    I know in reality the bigger tyre would most likely be capable of running a lower pressure than the narrow one but in my example they are running the same

    as Pete says assume all other things are equal like diameter etc

  13. #13
    willvf
    Guest

    Re: Ground pressure question

    Assuming that both tyres have the same carcass strength then if the air pressure is equal for each tyre the ground pressure will also be equal for each tyre. The 710 tyre will however have a longer footprint than the 900 tyre so the "damage" from the 710 will be concentrated on a smaller area of soil (as the soil will carry the weight for longer with the narrow tyre). The length of footprint takes a bit of understanding but it is basically because the weight on a tyre is primarily carried by the air in the tyre and the ground pressure cannot exceed the air pressure (if it did then where does the pressure come from - other than the carcass strength of the tyre which is typically 2psi and similar between all radials!) therefore if the same given weight is carried by different tyres at the same pressure, then the surface area (ground) in contact with a tyre must be equal, if the contact area cannot get wider then its only alternative is to become longer.

    That was my understanding anyway!

    I recall seeing research from the last century which showed that most damage is done in the 1st pass over soil with each successive pass adding less damage than the previous one (i.e. travelling of the same piece of ground 5 times did not do 5 times more damage than one pass). On the basis this is correct, a narrower tyre that can operate at the same pressure as any wider tyre would do less damage overall (as the total area of ground affected must be lower).

  14. #14
    scrambler
    Guest

    Re: Ground pressure question

    Quote Originally Posted by Cab-over Pete View Post
    Clive originally stated that the two machines were the same weight and the two sets of tyres running at the same pressure.
    I would say that for the same radius/diameter and pressure, the 900 would do less damage to the soil. The 710 will have no longer footprint if the radius is the same, surely? Faffing about with pressures will change things greatly. I think Clive was talking about exact same make of tyre, just different sizes.

    It does bring up a point though. Always get the maximum diameter tyre you can fit on the machine. Height is good, especially where width is a restriction.
    In Clive's example, the 710's will have a longer footprint. The more weight you add the longer the footprint will get in order to keep the ground pressure the same.

  15. #15
    JD Andrew
    Guest

    Re: Ground pressure question

    Pressure = Force / Area

    So if force (weight of tractor) is the same, but the area (tyre touching the ground) is different then ground pressure will be different.

    8psi is the pressure inside the tyre, not the ground pressure. The pressure would alter the rolling radius and sidewall flex, and therefore length and width of the footprint.

  16. #16
    Clive
    Guest

    Re: Ground pressure question

    Quote Originally Posted by Tom H View Post
    I would personally go for the narrow one. as the reason it can run the same is that it will be a new type of tyre (xeobib,axiobib?) and the 900 will be something a little older? I would guess a trelli? The axio and xeo's will be one pressure for a set weight too, so you can go from field to down the road flat out and not worry about changing pressure/ knackering the tyre. Much better

    my 2p's worth
    assume wide or narrow both are the same brand / technology etc

  17. #17
    Clive
    Guest

    Re: Ground pressure question

    Quote Originally Posted by JD Andrew View Post
    Pressure = Force / Area

    So if force (weight of tractor) is the same, but the area (tyre touching the ground) is different then ground pressure will be different.

    8psi is the pressure inside the tyre, not the ground pressure. The pressure would alter the rolling radius and sidewall flex, and therefore length and width of the footprint.
    so wider floats better regardless of inflation pressure ?

    this is what I thought but it all got confused in our staff room debate !

  18. #18
    homefarm
    Guest

    Re: Ground pressure question

    I am not sure the pure physics of the size and pressure of the tyre can give a definite answer to the question of damage.
    Soil type, wet/dryness, type and speed of operation will all affect which tyre is better regarding damage.

    As I described in the CTF thread speed does make a big difference and never seems to be mentioned by experts.

    Can easily be seen with a cambridge roller the difference between 2k and 20k is chalk and cheese.

  19. #19
    back'o'beyond
    Guest

    Re: Ground pressure question

    I'm with scrambler. The contact area will be the same for both tires, just a different shape. The 900 would be wider and have say 2.5 lugs in contact and 710 narrower with say 3 lugs in contact. The 710 will be in contact for longer down the wheel way and so will push down for longer at a given spot in the wheel way.
    Each tire will deflect until the the 8 psi ground pressure is reached. Add more weight and the tires will deflect more to spread the weight over a greater area until the 8psi ground pressure is reached again. In essence the 710 will look flatter than the 900 at any given weight.

  20. #20
    Richard
    Guest

    Re: Ground pressure question

    If the tyre covered 100 acres footprint at 8psi it cant put the same ground pressure over that area like a small tyre can it?.. where would all the weight come from?

  21. #21
    JD Andrew
    Guest

    Re: Ground pressure question

    Quote Originally Posted by Clive View Post
    so wider floats better regardless of inflation pressure ?

    this is what I thought but it all got confused in our staff room debate !
    A tyre exerts a 'cone' of downwards pressure on the soil. If the weight on the tyre is kept the same, but the tyre is made wider, then the cone will be wider at the top, but shallower (as the force is the same). So a wider tyre may consolidate more at the top, but do less damage deep down.

  22. #22
    davidroberts30
    Guest

    Re: Ground pressure question

    Quote Originally Posted by JD Andrew View Post
    Pressure = Force / Area

    So if force (weight of tractor) is the same, but the area (tyre touching the ground) is different then ground pressure will be different.

    8psi is the pressure inside the tyre, not the ground pressure. The pressure would alter the rolling radius and sidewall flex, and therefore length and width of the footprint.
    yes
    to know the ground pressure you need to measure the contact area of the tyre and weigh the tractor

  23. #23
    welshfarmer
    Guest

    Re: Ground pressure question

    The hours I've spent pondering this!

  24. #24
    Krampeman
    Guest

    Re: Ground pressure question

    Wider tyres were developed for flotation in wet conditions initially but over the years it's been argued that it's more ground pressure.

    In reality I believe the less the tyre touches the soil better off you are so a 710 tyre will touch less soil than a 900.

    If you have a field that is 100m long and you run a single 900 wheel up it, basically it will compress 90 sqm.

    If you run a single 710 tyre up it then you will compress 71 sqm.

    So by choosing a taller 710 tyre like you have on the 939 you reduce the compressed soil area but extend the footprint for traction.

    So basically keep the 939 wheels and make them fit the 724.

  25. #25
    davidroberts30
    Guest

    Re: Ground pressure question

    Quote Originally Posted by Krampeman View Post
    Wider tyres were developed for flotation in wet conditions initially but over the years it's been argued that it's more ground pressure.

    In reality I believe the less the tyre touches the soil better off you are so a 710 tyre will touch less soil than a 900.

    If you have a field that is 100m long and you run a single 900 wheel up it, basically it will compress 90 sqm.

    If you run a single 710 tyre up it then you will compress 71 sqm.

    So by choosing a taller 710 tyre like you have on the 939 you reduce the compressed soil area but extend the footprint for traction.

    So basically keep the 939 wheels and make them fit the 724.
    yes but your 90sqm will have (for arguments sake) 5 cm depth of compaction
    and the 71 sqm will have 8cm depth of compaction

  26. #26
    Fran Loake
    Guest

    Re: Ground pressure question

    Quote Originally Posted by foxbox View Post
    If so, and bearing in mind that most "damage" or compaction is made on a first pass with a tyre with considerably less increase in damage the 2nd time, does CTF really produce the benefits being talked about?
    Would that not actually support the CTF arguement though? With several passes on a limited area in CTF rather than single unregulated 'first' passes over a larger area.
    Fran

  27. #27
    Ian01
    Guest

    Re: Ground pressure question

    Quote Originally Posted by willvf View Post
    Assuming that both tyres have the same carcass strength then if the air pressure is equal for each tyre the ground pressure will also be equal for each tyre. The 710 tyre will however have a longer footprint than the 900 tyre so the "damage" from the 710 will be concentrated on a smaller area of soil (as the soil will carry the weight for longer with the narrow tyre). The length of footprint takes a bit of understanding but it is basically because the weight on a tyre is primarily carried by the air in the tyre and the ground pressure cannot exceed the air pressure (if it did then where does the pressure come from - other than the carcass strength of the tyre which is typically 2psi and similar between all radials!) therefore if the same given weight is carried by different tyres at the same pressure, then the surface area (ground) in contact with a tyre must be equal, if the contact area cannot get wider then its only alternative is to become longer.

    That was my understanding anyway!

    I recall seeing research from the last century which showed that most damage is done in the 1st pass over soil with each successive pass adding less damage than the previous one (i.e. travelling of the same piece of ground 5 times did not do 5 times more damage than one pass). On the basis this is correct, a narrower tyre that can operate at the same pressure as any wider tyre would do less damage overall (as the total area of ground affected must be lower).





    I think your spot on

  28. #28
    munitor
    Guest

    Re: Ground pressure question

    The pressure in the tyre is the pressure exerted to the soil. So for the same tractor the 710 and the 900 will have the same contact area the 900's contact area will obviously be wider while the 710's will be longer.

    If both tyres have ultra flex technology and they are on the same diameter rim and had the same rolling radius the 900 would be able to go at a lower pressure than the 710 for any give weight.... I think

  29. #29
    Krampeman
    Guest

    Re: Ground pressure question

    Quote Originally Posted by davidroberts30 View Post
    yes but your 90sqm will have (for arguments sake) 5 cm depth of compaction
    and the 71 sqm will have 8cm depth of compaction
    No it won't because it's a taller longer footprint tyre!

    That's the whole point Michelin etc developed the very tall 710 tyres because a 710 can do everything from haul, drill, plough, bedtill etc if and it's a big IF it's has the footprint to match which has to be longer.

  30. #30
    Right-arm fast
    Guest

    Re: Ground pressure question

    In clives question i think the narrower tyre would exert more ground pressure with the same weight and inflation pressure, because it carries the same load spread over a smaller footprint. I think the footprint would still be smaller on the 710 than the 900 with the same pressure and weight.
    I am certain that the air pressure with in the tyre does not represent the pressure exerted on the ground, although the tyre's inflation pressure affects the ground pressure by altering the tyre's shape under load.

    the ground pressure would be calculated using force (Newtons- kgs x 9.81) divided by area (metres2).

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