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wasmeneh
Join Date: Jul 2003
Posts: 4 Wilmington, NC
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2003-04-14          53140

I am thinking about putting turfs on my Shibura SD 1843, 4wd. Currently it has 9.5X 24 rice tires on rear Diameter 39.75", Fronts 6 X 14 diameter 24 Ratio 24/39.75= .60377
New Bridgestone turfs are rear 39.8 diameter, front 24.2 (according to dealers spec sheet) 24.2/39.8 = .6084
The measurements on the rice tractor are as on the tractor.

Will I have a wind up problem or is .60377 close enough to .6084.
Ok experts
Please help.
Mike


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Peters
Join Date: Feb 2002
Posts: 3034 Northern AL
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2003-04-14          53144

I would think that is close enough remember you can adjust the running diameter slightly by reducing or increasing the air pressure in the tires. With the turf wider foot print you might want to run a little lower pressure anyways. ....

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TomG
Join Date: Feb 2002
Posts: 5406 Upper Ottawa Valley
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2003-04-15          53154

I agree with Peters. I did get curious about it and went through a numbers exercise to think it through. Assuming the tractor with existing tires produces a 5% lead on the front axle, I think that the new tires would increase the lead by about 1%. The trouble is that I don't know if I'm thinking about it right, but here's what I did--and please remember I'm looking for somebody to tell me it's wrong.

I reversed the diameter ratios and calculated circumferences so I could think in terms of how many time the fronts turn per one revolution of the rears. I got ratios 1.66 for the original tires and 1.645 for the new. The circumferences are 124.88" and 125.0" for the rear; and 75.4"
76.0" for the fronts.

A rear to front axle final drive ratio the same as the tire ratio (1:1.66) should produce 0% lead with the original tires. So, I reasoned what drive ratio would dive the original tires with a 5% front lead? I reduced the front tire circumference by 5% and calculated at new tire ratio for the original tires of 1.74. So, I tell myself that a drive ratio of 1:1.74 should produce a 5% lead with the original tires. When I apply that drive ratio to the new tires, the rears travel one revolution for 125.03" while the fronts travel 132.55" (76.03" x 1.74) for a lead of 6%. However, sometimes I don't think these things through very well first thing in the morning.


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Art White
Join Date: Jan 2000
Posts: 6898 Waterville New York
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2003-04-15          53156

I'm going to throw something at you that will blow your ideas. On a rice and cane tire as well as most ag tires, they do not run on the lugs that you are measureing from. When you get into tilled soil you work from the casing out. Think about the rice and cane tires themselves and the height of the lugs. I tried to match rice and cane rears to a larger front to offset the front's that were rice and cane and couldn't keep the u-joints from cooking the grease out in thirty minutes from replacement. See if you can mate the model of a comparable New Holland tractor and work off there specs for turf tires. I feel you will not be happy with what you have started. ....

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TomG
Join Date: Feb 2002
Posts: 5406 Upper Ottawa Valley
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2003-04-15          53157

Thanks Art! I did think of that and wondered where the diameters were measured from. Since I was more interested in whether I could reason from circumferences and final drive ratios I simplified. I'm familiar with tires speced in terms of rolling circumferences rather than diameters and wondered about that too. I guess the idea of lugs is somehow contained in this somewhat mysterious but standard concept of 'rolling circumference.' I'm still not certain if I reasoned correctly but now that I've got the thought juices flowing after my exercise I can go off to my morning meeting and I might even sound vaguely human. Your comment about cooking u-joints doesn't surprise me. Reasoning often avoids the practical. ....

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DRankin
Join Date: Jan 2000
Posts: 5116 Northern Nevada
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2003-04-15          53158

Wasmeneh, If I were in your shoes, (and I have been) I would do the lead/lag test on the Firestone web site. After that you will have three sets of numbers on which to base your decision.

1)Your current measured diameters
2)Your actual diameter ratios in 2wd, and
3)Your actual gear ratio lead/lag in 4wd.

I would do the test on a hard gravel road or similar surface so that you eliminate the problem Art spoke of with the ag tires sinking in to the soil.

Once you have the actual gear ratio you can find the right tire sizes and build in a small lead.

Arts advice to find a similar model is a good idea, might give you a clue also that you may need different size/width rims.

Also, before buying, get a thin flexible tape measure and measure the exact circumference, as that is the most accurate way.


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