Is my mechanic stubborn ? Rim change:rolling ratio everything or binary also counts ?
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    Is my mechanic stubborn ? Rim change:rolling ratio everything or binary also counts ?

    Hello guys. I'm considering changing the rims on a 2850DT (european open station version) to lower it. The goal is so that I can use it on a fig orchard beneath the trees. I searched around and it seems that rolling ratio is what matters, but my mechanic consulted with someone that has the original service manuals and says that I shouldn't change the rear tires bellow 16.9R30. I told him that I searched a lot and that practically everywhere is mentioned that rolling ratio is what matters but he insists that I shouldn't do it because of the binary in the front axis differential, which he says is the reason the technical manual mentions 16.9R30 as minimum. But I'm thinking to myself, to achieve the max binary (I think he means the high torque on the gears when pulling) I would have to be loading the tractor with a lot of weight, but I'm not gonna use it for tilling anyway, otherwise I would destroy and tree's roots! Just need to do some mulching under the trees to avoid too much weeds and stuff. Heck for what I'm gonna use it I don't even need the 4wd engaged, in which case it doesn't matter.. It's a risk if I forget and engage the 4wd, but it shouldn't be a probablem if not used for a long time.

    But I want to be sure! Can anyone share their experiences ? I know in the US you guys use tractors mostly for extensive agriculture so probably only a few orchard farmers in here...
    Cheers
    Thanks
    Last edited by JgRa; 08-20-2018 at 11:40 PM.

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    mjncad's Avatar
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    My 2-cents for what it's worth.

    Are you currently using the smallest tire combination that Deere offered from the factory? That specification should be in the owner's manual, shop manual, and usually in the sales brochure.

    If you had a 2WD tractor or NEVER used 4WD drive; then it shouldn't matter if you went with a smaller tire and rim setup for more clearance in your orchard. BUT if you DO use mechanical 4WD and the ratios aren't matched within specifications; you have a higher chance of wrecking some expensive mechanical components.

    Think of it like this; you have a 4x4 pickup and the transfer case output front and rear is the same RPM; but your front axle has 3.73 gears and your rear axle has 4.11 gears. At any speed, one axle will be turning faster than the other one; yet they get the same input RPM's. Bad Juju is going to happen because of the mismatch.
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    JgRa
    I think tire sizes rear/front won't matter if you don't engage MFWD & if you intend to engage MFWD need to be in correct ratio frt to rear but in IMHO a smaller rear tire would offer less traction for deep tillage due to smaller footprint of tire(less soil contact area of tire).
    Last edited by Tx Jim; 08-21-2018 at 07:03 AM.
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    What are your current tires sizes front and rear?

    If you change both front and rear by the same percentage (use rolling radius) all should be OK in 2wd or 4wd. I.E. if you reduce the rear rolling radius by 6% also reduce the front rolling radius by 6%

    JD used five different rear axle gear ratios for different tires sizes. I would NOT want to change them. They only used ONE front axle gear set. Since they only used one front axle gear set, they used ten different gear ratios to power the front drive shaft. If you could not find a smaller front tire to obtain the needed size change percentage, you might be able to have the drive shaft gear ratio changed. You want the front tires to Lead/Pull 1-5% more than the rear tires.

    If you are willing to forgo 4wd, removing the front drive shaft to prevent damage would be a good idea.
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    I owuld think that if you keep the ratio between the front and rear the same, it shouldn't matter how small the new tires and rims are.

    For example, if you have a 1:2 ratio, and you use a 40" rear tire, you can use a 20" front. Same if you want to use 20" rears and 10" front, or 5" rears and 2.5" fronts.
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    Thanks for all the answers, current tire sizes are 18.4R34 and 13.6R24. I don't have the owners manual or anything and couldn't find any. I thought one had to change the gearbox to alter the drive shaft gear ratio, how is that done ? That's good news. No, no way I want to get rid of 4WD, it's night and day difference on heavy tilling So I would have to change tires according to the job, smaller rear ones for mulching on orchard and bigger (normal) ones for other tilling stuff. I would have to be extra careful not to engage 4WD by accident if I only change the rear tires. Thinking about it, front tires as big as the rear ones would look weird on a classic JD... But it's probably the best practical option.
    Thanks.
    Last edited by JgRa; 08-21-2018 at 01:07 PM.
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    Quote Originally Posted by johnH123 View Post
    I owuld think that if you keep the ratio between the front and rear the same, it shouldn't matter how small the new tires and rims are.

    For example, if you have a 1:2 ratio, and you use a 40" rear tire, you can use a 20" front. Same if you want to use 20" rears and 10" front, or 5" rears and 2.5" fronts.
    Close. Unless jacked up, tires are not perfectly circular (because they're under load and they deform) and different diameter tires don't deform proportionately.

    What you really need to look at is the rolling radius specification of the tire. Double that and the resulting number is your effective diameter. It will be less then the unloaded diameter of the wheel/tire combo.

    Al
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    Quote Originally Posted by AlKozak View Post
    Close. Unless jacked up, tires are not perfectly circular (because they're under load and they deform) and different diameter tires don't deform proportionately.

    What you really need to look at is the rolling radius specification of the tire. Double that and the resulting number is your effective diameter. It will be less then the unloaded diameter of the wheel/tire combo.

    Al
    Also known as static load radius.
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    Quote Originally Posted by JgRa View Post
    Thanks for all the answers, current tire sizes are 18.4R34 and 13.6R24. I don't have the owners manual or anything and couldn't find any. I thought one had to change the gearbox to alter the drive shaft gear ratio, how is that done ? That's good news. No, no way I want to get rid of 4WD, it's night and day difference on heavy tilling So I would have to change tires according to the job, smaller rear ones for mulching on orchard and bigger (normal) ones for other tilling stuff. I would have to be extra careful not to engage 4WD by accident if I only change the rear tires. Thinking about it, front tires as big as the rear ones would look weird on a classic JD... But it's probably the best practical option.
    Thanks.
    Same sizes as my 6415. What size rears do you want to run????
    Some other things to consider. At least with my tractor if the wiring fails/gets pulled off, default is 4wd. I.E. no power to the 4wd solenoid, it's in 4wd. When I use the brakes, it automatically engages 4wd for more stopping power.

    I have a Operators manual for the JD 6110-6510 and it has three pages that tell how to determine which transmission gear set is needed. you should be able to find it online at manuals.deere.com
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    My 2cents,,,Given everything mentioned, I would look into finding something that could do the task. Leave your current tractor as it is.
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