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 02-07-2000, 00:00 Post: 12595
JEFF



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 DIFFERENT TIRE SIZE

HI!I WAS WONDERING IF ANYBODY COULD HELP ME?I OWN THE 855 4WD FRONT TIRE SIZE IS 24X8.50-12 AND THE REAR IS 31X15.50-15 ON A 1995 BY THE WAY. MY BROTHER OWNES A 1993 855 4WD AND HIS TIRE SIZE IN THE FRONT IS 23X8.50-12 AND THE REAR TIRE SIZE IS 33X12.50-15.BOTH WERE BOUGHT USED. I WAS WONDERING IF THE DRIVETRAIN WAS GOING TO GET WRECKED BECAUSE OF THE TIRE SIZE DIFFERANCES.BY THE WAY THEY'RE ALL TURF TIRES.ANY IMPUT WOULD HELP! THANKS!!!! JEFF






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 02-07-2000, 00:00 Post: 12596
Don



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 DIFFERENT-TIRE-SIZE

Check your owners manual. If that is missing in action, a call to eather a JD dealer or a trip to a Central Tractor type store selling manuals should give you tire perameters.






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 02-07-2000, 00:00 Post: 12598
Roger L.



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 DIFFERENT-TIRE-SIZE

According to an owner's manual for the 855 that I have kicking around, your brother is running stock turf tires. No mention in there of any other sizes.....which doesn't mean that yours are wrong, just that I don't know. The first thing that you will notice when running a mis-match on the diameters vs the front and rear gear ratios is that is will be VERY difficult to get the lever out of 4wd mode. Sometimes you have to back up and rock the tractor to get it to let loose of all the shaft wind up. But lots of tractors are moderately hard to get out of 4wd because there is always a little bit of drive shaft wind up. With severe mismatches in snow you can feel it "jump" every once in a while...and I once mis-matched one so badly that the rear wheels were outrunning the fronts. In 4wd it was real tricky to steer it in a straight line. Turfs will usually slip before any damage is done if you are on dirt or snow or grass. Pavement might hurt it...but I wouldn't run 4wd on pavement anyway because there is always a little bit of mismatch.






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 02-08-2000, 00:00 Post: 12607
TomG

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 DIFFERENT-TIRE-SIZE

There was a lengthy discussion here or on tractorbynet last fall. The discussion contained ways of calculating tire circumferances and using axle ratios to determine distances turned by front and rear tires. There also was some information about typical allowable ratios between front and back travel.

If you have non-standard tires, there was enough information in the discussion to determine if they're outside of manufacture's recommendations.

I found it easier to refer to my owner's manual to make sure I had a standard set. If not, I would have probably asked my dealer.






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 02-08-2000, 00:00 Post: 12616
Murf

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An 'old timer' at my dealer says the easiest way to NOT screw up the calculation is to not do it. His method, although a little more work, is a simple & as accurate as it gets, especially if you have a loader. It goes like this, jack up the rear axle till both wheels clear the ground, place axle stands under the axle and jack up (or lift with loader) the front axle. Now stand something beside a rear wheel as a marker (I use the jack)and place a mark at "6 o'clock" on the tire, now do the same with the front tire on the same side. Now turn the rear wheel one revolution, the front should have turned exactly one also, if not the ratio is wrong. If the ratio is off, it doesn't matter by how much, it MUST be fixed.






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 02-08-2000, 00:00 Post: 12643
Roger L.



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Murf, don't you be buying anything mechanical from that 'ol timer. He missed this by a mile! I'd almost say that he is completely wrong....and I don't say that very often. But sort of like the stopped clock that is right twice a day, there is one - and only one circumstance where his advice is correct. But I can't think of any tractors that are built that way..... Smile...Can you?






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 02-09-2000, 00:00 Post: 12665
MikeC



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 DIFFERENT-TIRE-SIZE

If you've got a tractor with same size front and rear tires, then you can use that method to find out if you have a tractor with same size front and rear tires.






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 02-09-2000, 00:00 Post: 12666
MChalkley



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 DIFFERENT-TIRE-SIZE

There's always some sort of mechanical connection between the front and rear axles (assuming 4wd, of course). If the front and rear tires are the same, the connection is 1:1, or very close. When fronts are smaller, as is ususal with Compacts, you need to know what this mechanical ratio (let's call it MR) is before doing any tire fiddling. (I've done a lot of it! My L4310HST had 10-16.5 Titan fronts, 17.5L24 Firestone rears, now has 12-16.5 OTR fronts, 19.5R24 Radial Michelin XM27 rears.) Then, from the tire manaufacturer's data books, you find the Rolling Circumference (RC) for the tires in question. The front tire RC X the MR should preferably be between 1% and 3% more than the rear tire RC. Identical is ok. It must NEVER be less, or the dreaded 'power hop' can cause a very dangerous situation. Much more than 3% in either direction, and drivetrain damage may occur on hard surfaces. Typical MR's are around 1.5. If you just want to check what you've already got, you can also measure it physically by counting the number of turns of the front tires per 10 turns of the rear tires on a flat dirt surface with both 4wd engaged and not engaged. The number of times the fronts turn when 4wd is engaged should be between 1% and 3% more than when it isn't, for obvious reasons. I hope this helps.






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 02-10-2000, 00:00 Post: 12672
TomG

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 DIFFERENT-TIRE-SIZE

Mark's post contained really good information.

A point that might be missed in a quick read by a very basic person like myself. Tractors designed for different sized front and rear tires have different front and rear axle gear ratios. You have to know what the ratios are to determine an appropriate tire match.

Mark's gave a good method for determining if a tire match is OK. However, if the match isn't OK, then you have to do the calculation or use a standard tire set. Otherwise, you're just guessing.






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 02-10-2000, 00:00 Post: 12674
MChalkley



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TomG, that's the purpose of the mechanical ratio (MR) in the equation. If you know that, then you know how many times the tractor is going to force the front tires to turn for each turn of the rear tires, and that's the only thing that really matters. Except, of course, that the tires will physically fit without rubbing on something. (Ask me how I know...)






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Discussion Boards > Active Subjects > Messages as Posted > John Deere Review Forum

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Don 1 | JEFF 1 | MChalkley 4 | MikeC 1 | Murf 2 | Roger L. 3 | TomG 4 |




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