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  1. #11
    New Member
    Join Date
    Feb 2006
    Posts
    18
    Location
    Davidsonville, MD
    Tractor
    Kubota B2400; Kubota B1750 with heated cab

    Default Re: can I replace front tires w/1/2" smaller diameter on Kubota 4wd/hydrostatic?

    This topic has been discussed in a couple of other threads. See
    http://www.tractorbynet.com/forums/k...-5x8-10-a.html
    http://www.tractorbynet.com/forums/k...tire-help.html

    I just went through the process of replacing my front 20.5x8-10s on by B1750 4x4 with a tire with a more aggressive tread than the turf style. I finally decided on the Carlisle All Trail II 22x9.50-10. Check out the links above for a full discussion.

  2. #12
    Platinum Member
    Join Date
    Nov 2008
    Posts
    715
    Location
    Brampton, On\lot Powassan, ON Canada
    Tractor
    Kubota B4200\MF 135\Kubota B3200

    Default Re: can I replace front tires w/1/2" smaller diameter on Kubota 4wd/hydrostatic?

    Adding to this, while in 4x4 every time you make a turn, those said gears in the Tranny get loaded up, thus the need to only use 4x4 on loose surfaces. In other words, even with perfectly matched tires, you will still get wear if you drive in 4x4 on hard surfaces.
    Unimogdave

  3. #13
    Veteran Member
    Join Date
    Jan 2010
    Posts
    1,662
    Location
    Piedmont, NC
    Tractor
    Kubota L4610 & BX2230, Farmall Super M, Super A

    Default Re: can I replace front tires w/1/2" smaller diameter on Kubota 4wd/hydrostatic?

    A couple of points here. First, on these 4wd (technically front wheel assist) tractors, the tractor and tire manufacturers usually recommend a front/rear tire combination that results in the front tires leading, or covering more distance than the rears, by 0 - 6 %. Second, you want to avoid a lag situation, where the fronts travel less than the rears. If the front tires lead, they help to pull the front of the tractor in turns; if they lag, the rears are pushing the front slightly. Perhaps more importantly, a lag situation can cause binding in the drivetrain and may result in increased wear on its components. The worst scenario is a lag configuration operating in 4wd on a hard surface, which can result in severe binding, even going in a straight line. Obviously, if the 4wd is not engaged, there is no lead/lag to consider because the non-driven front axle simply matches the rolling speed of the rear axle.

    To figure the lead/lag, you need to know the rolling circumference of each tire and the tractor's front/rear gearing, and then do some math.

    There is some tolerance, such as to allow for tire wear. For example, suppose you had brand-new front tires and bald rears. Your fronts would lead more than if both sets of tires were new. Conversely, if the fronts were bald and rears had full tread, it would have more lag.

    Since the tread depths on these tractors are quite a bit more than 1/2", the change in tire diameter the OP mentioned should not by itself cause an issue. Note that you should verify the manufacturers specified rolling circumference for the tire, not just the diameter. Depending on the gearing and tire rolling circumference, it might even give a better lead/lag than the OEM setup. And, you'd still want to estimate the lead/lag both with and without tire wear.

    I am by no means any expert on this, but this topic was extensively covered by some TBN pioneers more than 12 years ago, whose discussions I followed at the time, and I hope this represents those discussions correctly.

  4. #14
    Veteran Member
    Join Date
    Jan 2010
    Posts
    1,662
    Location
    Piedmont, NC
    Tractor
    Kubota L4610 & BX2230, Farmall Super M, Super A

    Default Re: can I replace front tires w/1/2" smaller diameter on Kubota 4wd/hydrostatic?

    Here's a link with some further explanation:

    Mechanical lead - Trelleborg Wheel Systems

  5. #15
    Elite Member
    Join Date
    Aug 2004
    Posts
    3,553
    Location
    North of Mtl,Que,Can (Ste Adele)
    Tractor
    MT180D

    Default Re: can I replace front tires w/1/2" smaller diameter on Kubota 4wd/hydrostatic?

    Grandad is correct, I also did a lot of reading on the issue as I needed to make changes up front due to trashed rims due to previous owner having used calcium ballast up front.
    I lucked out with garden tractor rears that fit stud pattern as well as rolling diameter and as a plus were wider for better floatation.
    In my case replacement rims were simply not available.

  6. #16
    Silver Member JD3520Turbo's Avatar
    Join Date
    Oct 2008
    Posts
    178
    Location
    Holt, MO
    Tractor
    2013 5083E Limited

    Default Re: can I replace front tires w/1/2" smaller diameter on Kubota 4wd/hydrostatic?

    I have always been told that 2.5% is the magic number when it comes to differences from 0 lead or lag.
    In other words you can have up to 2.5% lead and you are okay. I agree with the people above. You don't want to be in a lag situation if you can help it. That just binds things up and causes the tractor to fight itself. When in doubt error towards lead.
    What I have done with situations like this is to take the Overall Diameter (OD) of the rear tire and divide it by the front to get a ratio that is roughly the ratio of the gearbox. For example my little JD 3520 had a gear box that was a 1.623 to 1 ratio. Meaning that the front tires went 1.623 around to every one of the rear.
    I got that two ways. One was I divided the front and back tires to get the ratio. Then I figured there would have been a little lead roughly 2% built into it, which there was. I then called JD to see if I was close, and they confirmed that I was.
    In your case, I would try and see if I could find the actual ratio of the gear box and then get your ODs from your tires and see if that smaller one will work.

    Just for clarification. The measurement from the center of the axle to the floor is called Loaded Radius. and the measurement of one revolution of a tire at a given load and pressure is called Rolling Circumference.

    And as was stated previously, if you are only using your front wheel assist in wet conditions, and there will be ample slippage then you should be fine. Its just the binding in the turns or on dry pavement than could cause an issue.
    Past Rigs:2008 3520. June 2008 to July 2011
    2011 5083E Limited July 2011 to Feb 2013. Long Story
    2013 5083E Limited Feb 2013

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