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Thread: differences in rear tyre size on 4wd tractors

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    differences in rear tyre size on 4wd tractors

    Hi, I would like comments on whether changing a pair of rear tyre's with a rolling circumference of 5229 mm to a different make and size with a rolling circum. of 5344 mm is feasible. The tractor is 4wd and ideally i dont want to have to change the fronts as well. What tollerance is acceptable and would this wind up the transmission to an unacceptable level.

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    Re: differences in rear tyre size on 4wd tractors

    Couldn't you just give us the sizes in rim diameter,width,profile etc instead? Your first RC would suggest 18.4x38 but the second could be ???? depending on the profile.

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    Re: differences in rear tyre size on 4wd tractors

    as Agrimax said, to work it out you need to supply more info.

    The tyre guru's IIRC usually say that the front axle should have a lead of 4% max. 2% would be preferred. Can you supply sizes of tyres that are currently fitted and also there should be a plate on the front axle or the dropbox giving its gear ratio. With this info someone will be able to work it out, I used to get our Ex- Michelin- Ex Micheldever Tyres rep to do ours and unless your 110% sure the current fitment is correct, I would check it properly.

    We have found before that a nearly new tractor we looked at had been specced new by a dubious dealer to an adament customer with the widest tyres they could get even though it had a huge negative lead. If your not sure, check it.

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    Re: differences in rear tyre size on 4wd tractors

    If I'm not mistaken,the RCs that you have given would only be 4.5 inches of difference which seems very little,especially if you take wear into consideration.Can't imagine there being a big difference in diameter to change the RC that much but the actual tyre sizes would be better to go by.

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    Re: differences in rear tyre size on 4wd tractors

    Hi, in response the tyres are michelin omnibib 520/70 r38 (rc of 5229 mm) and dneproshina agrowork 30.5lr32 or 800/65 r32 in new money (rc of 5344mm). I had worked out the difference is 115mm which makes the ukranians 2.2% bigger in rc terms than the french. I have read that the 'lead' for a 4wd is typically 1-5% so im guessing that 2.2 % in tyre rc terms would be acceptable. Bit of a belt and braces man as after all, mistakes can be expensive and there's not much margin in anything to spend it in the workshop un-necessarily.

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    Re: differences in rear tyre size on 4wd tractors

    Quote Originally Posted by milk-king View Post
    Hi, in response the tyres are michelin omnibib 520/70 r38 (rc of 5229 mm) and dneproshina agrowork 30.5lr32 or 800/65 r32 in new money (rc of 5344mm). I had worked out the difference is 115mm which makes the ukranians 2.2% bigger in rc terms than the french. I have read that the 'lead' for a 4wd is typically 1-5% so im guessing that 2.2 % in tyre rc terms would be acceptable. Bit of a belt and braces man as after all, mistakes can be expensive and there's not much margin in anything to spend it in the workshop un-necessarily.
    Would it depend on how much lead the existing rears have vs the fronts though? If the lead was already 4% on the original set the new ones could be increasing it beyond a sensible margin?
    Stay in Northamptonshire - meadowviewcottages.co.uk

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    Re: differences in rear tyre size on 4wd tractors

    Quote Originally Posted by foxbox View Post
    Would it depend on how much lead the existing rears have vs the fronts though? If the lead was already 4% on the original set the new ones could be increasing it beyond a sensible margin?
    The fronts need to lead. Larger rears would reduce the lead

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    Re: differences in rear tyre size on 4wd tractors

    Quote Originally Posted by Maico490 View Post
    The fronts need to lead. Larger rears would reduce the lead
    Fair enough, not something I'm familiar with as you can tell!

    Does this mean then that if the current set up is close to the lower limit the new tyres could cause issues? I guess they'd wear in though
    Stay in Northamptonshire - meadowviewcottages.co.uk

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