Page 6 of 6 FirstFirst ... 456
Results 151 to 167 of 167

Thread: Ground pressure question

  1. #151
    giles1
    Guest

    Re: Ground pressure question

    Quote Originally Posted by homefarm View Post
    Clive did ask two questions
    1:- which tractor has the lowest ground pressure on the soil and 2:- will do least soil damage

    I have been trying to point out that the answer to question 1 is not necessarily the same as the answer to question two if you decide to use them in a field.
    Would a crawler analogy work to answer part 2?Same area of track in contact with soil,so same ground pressure,but one with shorter length and wider tracks,other with narrow long tracks.

  2. #152
    Feldspar
    Guest

    Re: Ground pressure question

    Quote Originally Posted by CATAG View Post
    Yes had studied that research before and you have related only one part of it relating to the specific test. (I dont tend to use rule of thumb with anything because it is not a fact) Under the part you already quote it also says:

    "The best way to determine contact pressure is to calculate the load in pounds per wheel and divide it by the area of the tire that touches the soil"

    Forgive the spelling of tyre above it is taken directly from the research which is american - however it all means the same - just remember also that the psi in america would be different to that of the UK because we have american pounds and uk pounds....

    This reinforces my earlier information - there is nothing relating to physics (relating back to the original question from Clive) he asked which tyre, same tractor, same manufacturer of tyre etc has the most contact area - he did not ask or worry about traffic paths or compaction ratios or soil smearing tendencies or grip etc etc - he asked about ground contact.

    Ground contact in psi = weight of machine in lbs
    _____________________

    area in inches sq of tyre contact

    Until you have measured the ground contact area on both tyres you cannot answer the question he asked. It could be that a longer footprint is exactly the same as a wider footprint but if the tyres on the tractor weigh a tonne each more than the "smaller tyres" then the psi will be higher because the machine weight will be higher.


    Richard
    Of course there will be an error associated with any measurement of the tyre contact area and so the calculated ground pressure will not be completely accurate. In your eyes does this calculation yield a fact or not? I don't think it is wise to always ignore rules of thumb. I think they have an important place. Certainly science would be hugely maimed if approximations were not admissible. The key is to always bear in mind the likely error, IMHO.

  3. #153
    CATAG
    Guest

    Re: Ground pressure question

    Last post - for tonight anyway.....

    So if you have a 50 tonne tractor on tyre size x (doesn't matter about the tyre but lets say they say rated pressure is 25psi)

    then we take a 20 tonne tractor on exactly the same tyres at exactly the same pressure

    which one has the highest Ground pressure in psi?

    OR

    try it another way round i drive my car into a field and it gets stuck the tyres are set at 40psi so i drop the pressures to 20 psi and it drives straight out of the field by making its tyres bulge did I make the car lighter - or did I spread the cars unchanged weight over more surface area to spread the weight - I increased the ground contact by lets say 10% or 20% max while dropping 50% of the tyres inflation pressure so where is the relationship between tyre pressure and ground pressure other than in the greater contact with the ground......

  4. #154
    Feldspar
    Guest

    Re: Ground pressure question

    Quote Originally Posted by CATAG View Post
    Last post - for tonight anyway.....

    So if you have a 50 tonne tractor on tyre size x (doesn't matter about the tyre but lets say they say rated pressure is 25psi)

    then we take a 20 tonne tractor on exactly the same tyres at exactly the same pressure

    which one has the highest Ground pressure in psi?
    I think I can be consistent with the above two posts I made and give the response that you think is correct - that is, the ground pressure will be greater in your first case. The reason is that the rule of thumb does not apply because in one of the two cases the inflation pressure will be a long way away from what is recommended.

    This example you have given is not very realistic as one would never, in a field situation, vary the weight of a tractor by this amount without changing the tyre pressures. What about a slight modification:

    So if you have a 10 tonne tractor with tyres that are inflated to the recommended pressure for the loads they support.

    Then say we add a 250kg weight to the tractor such that added weight does not alter the ratio of loads between the axles.


    Does the ground pressure change with the addition of the extra weight?


    I would say in this modified case that to a good approximation the ground pressure would not change.

  5. #155
    Feldspar
    Guest

    Re: Ground pressure question

    Quote Originally Posted by CATAG View Post

    try it another way round i drive my car into a field and it gets stuck the tyres are set at 40psi so i drop the pressures to 20 psi and it drives straight out of the field by making its tyres bulge did I make the car lighter - or did I spread the cars unchanged weight over more surface area to spread the weight - I increased the ground contact by lets say 10% or 20% max while dropping 50% of the tyres inflation pressure so where is the relationship between tyre pressure and ground pressure other than in the greater contact with the ground......
    I do not claim (after doing some reading and modifying my position) that halving the pressure would double the contact area but I do claim that there is a link between ground pressure and tyre pressure. This is an internally consistent statement if you note that "a link" does not necessarily imply equality between the two variables; it does, however, imply some degree of correlation.

  6. #156
    homefarm
    Guest

    Re: Ground pressure question

    While I understand Clives question was theoretical does it have any practical relevance regarding soil structure damage and choice of tyre.

    I do not think tyre choice/pressures can be chosen without soil type, job being undertaken and power available being considered first.

  7. #157
    agricontract
    Guest

    Re: Ground pressure question

    I think Clive needs to get a 724 Fendt on demo with 710s on it and swop the wheels on one side with the wheels off one side of his 939 (If it is on 900 that is )and go drilling at 8psi and see what it looks like you could do 20 acres with each tractor and watch the progress of the crop
    Drive at different speeds as well

  8. #158
    Ian01
    Guest

    Re: Ground pressure question

    Basically think of the bottom of a tyre as a diaphram. Apply more pressure to one side of the diaphram than the other and the diaphram will flex until the pressures are equal. No different to what happens in a Ramsey valve. If you have a tyre on the ground loaded with say 3 tonnes at 8psi, add another ton of weight and the pressure from the ground to the carcass will increase. Therefor the tyre will flex more until there is enough surface area and the pressures equal out. The tyre is not much more than a membrain transferring weight and load, via pressure within the tyre, to the ground.

  9. #159
    Colhonk
    Guest

    Re: Ground pressure question

    CLIVE must have been very bored, so he did what he likes doing, stirring you lot up. He has done a great job with his pointless question as only an Idiot would run a larger tyre at the same pressure as a narrower one for the same loading. (btw my 2peneth, will you sink further into the mud of your wheat field if you are stood on one foot or two feet????????)

  10. #160
    Clive
    Guest

    Re: Ground pressure question

    Quote Originally Posted by Colhonk View Post
    CLIVE must have been very bored, so he did what he likes doing, stirring you lot up. He has done a great job with his pointless question as only an Idiot would run a larger tyre at the same pressure as a narrower one for the same loading. (btw my 2peneth, will you sink further into the mud of your wheat field if you are stood on one foot or two feet????????)
    Rumbled

  11. #161
    Mr Bean
    Guest

    Re: Ground pressure question

    Quote Originally Posted by Colhonk View Post
    (btw my 2peneth, will you sink further into the mud of your wheat field if you are stood on one foot or two feet????????)
    Makes no difference in most fields this year, you would still be stuck.

  12. #162
    Right-arm fast
    Guest

    Re: Ground pressure question

    Good god, talk about over complicating something, no wonder we cant sort out a badger cull.

  13. #163
    v12cat
    Guest

    Re: Ground pressure question

    To me, the first paragraph of the document I posted is very clear.

    The tire contact area for any aircraft tire is calculated by dividing the single wheel load by the tire inflation pressure. If the load is expressed in pounds, and the tire pressure in pounds per square inch, then the area is in inches squared. The same thing works with kilograms and kg/cm2 - the result will then be in square centimeters.
    I can't add anything else to this thread really so I'll read with interest.

  14. #164
    OhDeere
    Guest

    Re: Ground pressure question

    Quote Originally Posted by Benwales View Post
    +1
    That's what your physics A level says!

    Right. At lower pressure the carcass will be softer, allowing for a bigger contact surface thus exerting less pressure ( pressure=force(weight)/area ). Same as comparing radial tires to cross-ply tires, the latter being much harder and inflexible and thus creating more surface pressure.

    In this case the longer duration of contact at lower pressure has a minimal effect on compaction.

    No-one really took up the issue slip. What slip does, is actually creating traction and therefore creating pull. The bars of the tire penetrate ground assisted by wheelslip and the actual forward pull (traction) is cased by the friction (sand) and cohesive (clay ect) forces inside the ground between the bars.

    More slip will be created by 900 tires, if there's no sufficient downwards force to make bars penetrate the ground. With any tires, it could be simply put, to a certain degree the more slip the more grip. Wheel slip is the necessary evil.

  15. #165
    SKIMMER
    Guest

    Re: Ground pressure question

    Has anyone noticed a significant crop improvement by using a certain tyre at a set pressure,If you can get below 10psi is that good enough,would you get your money back by speccing 600 instead of 460 or 900 instead 710 it does seem there are a lot of people not making the full advantage of the tyres they have.

  16. #166
    homefarm
    Guest

    Re: Ground pressure question

    Good god, talk about over complicating something, I dont even think we have started yet.

    How do different brands compare?

    How about tread depth and angle, dont see many 22degree ones today?

    How about self cleaning, does it happen and does it make a difference ?

  17. #167
    SKIMMER
    Guest

    Re: Ground pressure question

    Quote Originally Posted by homefarm View Post
    Good god, talk about over complicating something, I dont even think we have started yet.

    How do different brands compare?

    How about tread depth and angle, dont see many 22degree ones today?

    How about self cleaning, does it happen and does it make a difference ?
    What about spade lugs could be useful this year.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •