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DIFFERENT TIRE SIZE

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JEFF
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2000-02-07          12595

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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Don
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2000-02-07          12596

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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Roger L.
Join Date: Jun 1999
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2000-02-07          12598

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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TomG
Join Date: Feb 2002
Posts: 5406 Upper Ottawa Valley
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2000-02-08          12607

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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Murf
Join Date: Dec 1999
Posts: 7249 Toronto Area, Ontario, Canada
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2000-02-08          12616

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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Roger L.
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2000-02-08          12643

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..... :-)...Can you? ....

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MikeC
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2000-02-09          12665

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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MChalkley
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2000-02-09          12666

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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TomG
Join Date: Feb 2002
Posts: 5406 Upper Ottawa Valley
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2000-02-10          12672

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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MChalkley
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2000-02-10          12674

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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Murf
Join Date: Dec 1999
Posts: 7249 Toronto Area, Ontario, Canada
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2000-02-10          12680

Roger, I re-read my message, obviously his method was better than my memory of it... They say the memory is the first thing to go, I don't remeber what they said goes next..... If I remember, I'll ask the 'ol guy' next time I'm there, it was a nifty trick & it worked when I tried it, I remember that much....LOL ....

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TomG
Join Date: Feb 2002
Posts: 5406 Upper Ottawa Valley
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2000-02-11          12692

OK, I never turn down invitations. Besides, your posts generally tell me things I need to know. So: How do you know? ....

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MChalkley
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2000-02-11          12695

TomG, it was mostly a figure of speech meaning "I found this out the hard way", but as you know, I'm always happy to tell what I know or, often, don't know but think I know. You know? When I switched to the 19.5R24s, they're 2" or 3" taller than the 17.5's, in addition to being, of course, 2" wider. I had carefully checked the width to make sure it wouldn't rub on anything. What I hadn't checked was the clearance to the backhoe. So the first time I mounted the backhoe, I couldn't drive off with it because the cleats of the tires were hooked over both sides of the stablizer mounts. So I had to make 1.5" spacers to go between the backhoe and the mount. To make the switch from 10-16.5 to 12-16.5 on the front, I had to order wheels that were wider and had more offset. Unfortunately they didn't have as much offset when I got them as I had asked for, so the inside of the rim would just barely rub against the tie rod when I turn hard. So I had to put a spacer on the stop to prevent that. I like the Michelin radials so much that I'll be switching the fronts to 11-16 XM27's like I've got on the rear as soon as the wheels come in. Those wheels are about the same width as the original wheels, so I can take the spacers back out then. So, like I said, I learned that you need to check for clearance problems in all directions with all equipment the hard way. But it keeps life interesting. And I wouldn't give up those Michelin's for anything. They give a much softer ride, have a huge traction advantage and will handle anything I can throw at them at only 9 PSI, which keeps them on top of a lot of goop that the original tires would just dig holes in. They're basically R4 tires with R1 tread. At least that's what they look like. Whatever they are, they work. ....

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TomG
Join Date: Feb 2002
Posts: 5406 Upper Ottawa Valley
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2000-02-12          12729

Uhhhhhhhhh, oh no. I get the picture. And you're still on speaking terms with your tractor? Yeh I know, with people one can duck and blame somebody else, but not with machinery.

I was thinking about going from turfs to ags on larger rims--still a standard tire set. Bet I would have the back hoe problem too. Part of the 3ph mount goes on the draw bar. The instructions say that the mount can't be further than 1" from the draw bar support. Spacers wouldn't work for me. If I go to ags, I should check the clearances real well. And, like I said, I usually read something I need to know in your posts. ....

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Roger L.
Join Date: Jun 1999
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2000-02-12          12737

Mark, that is really interesting on the tires and wheels. I hope to do something similar in fine-tuning my tires on mine...which naturally leads to a few questions. I know that mounted circumference varies with the width of the wheel, and that some tires can be mounted on wheel rims that are an inch wider or narrower than nominal. Does this apply to radials? Where are you getting your wheels? It seems that you have looked into the subject deeper than most.
....

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MChalkley
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2000-02-12          12739

Roger, I always seem to get in pretty deep. People are always telling me so. When I start describing something, they say "Boy, it sure is getting deep." Anyway, Titan is the OEM wheel provider for Kubota and, I think, JD. They don't sell direct, so you have to buy them through an afiliate company, OTR. If you want info on the specific salesman in the GA office I get them through, email me. I don't know if it's appropriate for me to post his name and number online without permission. One of the best things you could do is get your hands on the Michelin and Firestone tire data books. They're a gold mine of info. They give RC's, acceptable wheel widths, ballast capacities, etc. It's true that mounted circumference does vary with wheel width. But rolling circumference doesn't vary appreciably, according to the engineers at Michelin and Firestone that I've talked to, with bias ply tires and not at all with radials. I can't emphasize enough the benefits of radials vs. bias, if you start playing with tires. Incredible improvements in ride and traction. Radials seem to generally work better with narrower wheels than bias tires, for a given section width. Thus, the wheels for my original 17.5L24's were just barely on the narrow side, but perfectly acceptable, for the 19.5R24's I've got now. ....

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