Tires Mismatched rear tires (wear)

[svcguy]

Curious as to anyone out there who has serviced tractors that have run mismatched rear tires. The reason I ask is that the TN75F I purchased has an almost new tire on one side and an older worn out tire on the other. Both are 380/70R20 rear tires (orchard model) but one side obviously was replaced, the other not. I have ordered a new tire to replace the original, but realize they should be ideally replaced at the same time.

I am sure a dirt tractor would fair better than one that spends time on the pavement, but just not sure how much differential stress this causes. It has to be hard on the diff though. I also realize there is no way to assess the wear without opening up the case (winter project) but am curious if anyone has experience with this particular problem or has seen the results.

 

[sandman2234]

Mark a line across each tire and drive it one revolution and then measure the difference in the travel amount of each tire. If it isn't much, adjust tire pressure to see if it can easily be equaled. You might need to use the rollcage as a guide to measure the difference in roll.
I wouldn't think treadwear would amount to enough to justify buying new tires, but I have never measured the difference. Let us know what you determine.
David from jax

 

[svcguy]

The tires are too far gone from each other to make up for it with air pressure. The one is like new, ribs about 1 inch tall. The other tread has been worn down to less than a 1/2 an inch. That means the theoretical rolling circumference (diameter of 40 / 39 inches * 3.14) would have to differ from each other by about 3 inches. It seems that would put quite a strain on a differential. While the differential itself is designed to speed up or slow down side to side when a vehicle turns, two different circumference tires would cause it to bind. The only way the binding could be relieved (I think) is by wheel slip, something that would be pretty challenging on pavement with ballasted tires. I just don't have the background to know how differentials would hold up under that type of binding. Now all of this is figuring that the binding effect could only be alleviated by tire slip, but I am not sure how else it could be relieved.

 

[Eagleview]

An open diff center will allow for the mismatch , the sun and planet gears rotate as they would in a turn to make this up . An LSD diff center WILL have to slip which causes wear . A diff lock engaged will see some large stresses on the whole axle . It is as your thinking better to have closely matched and inflated tyres .

 

[jimmysisson]

Not sure why you call it binding. Every time you round a curve in your car the inner wheel turns fewer revolutions than the outer wheel per revolution of the driveshaft. Same if one tire is smaller, on auto or tractor. The spider gears will be running all the time, not just on corners. They're more or less made to run all the time anyway. If you've ordered a new tire anyway, just run on the old one til you can change it, no harm done.
Jim

 

[Eagleview]

The "spider" gears are stationary whilst the vehicle is going in a straight line if the tyres are the same size and at the same pressure ie. both axles are rotating at the same speed .

A Limited Slip Diff will be damaged by different sized tyres as there is a mismatch in axle speed with unequal tyre sizes .

A diff lock and axles can be damaged if it is engaged with different sized tyres .

Massive drive line damage can be done on hard ground if four wheel drive is engaged with mismatched tyres especially doing loader work . The wind up cannot be dissipated because of the traction of the tyres .

 

[svcguy]

Thanks for the info guys. I think I was misunderstanding how the diff works. I realize a differential allows for one side speeding up around corners, but I thought the side spiders worked against two other thrust type spiders in a set of four. Didn't realize an open center diff was okay. My TN75 doesn't have a limited slip diff, so if it doesn't cause binding, I guess it's a moot point - I had thought that it would eventually cause binding with mismatched rolling circumferences. Recognize using the diff lock wouldn't be healthy with mismatched tires, but don't really use it. The new tire will balance out the two for traction, so I don't mind spending the money. I am ordering another set of larger rims from Unverferth so that I can run larger tires on it, have sized a set that will have just less than 1% lead slip and still fit on the orchard model.

 

[LBrown59]

Thanks for the info guys. I think I was misunderstanding how the diff works. I realize a differential allows for one side speeding up around corners, but I thought the side spiders worked against two other thrust type spiders in a set of four. Didn't realize an open center diff was okay. My TN75 doesn't have a limited slip diff, so if it doesn't cause binding, I guess it's a moot point - I had thought that it would eventually cause binding with mismatched rolling circumferences. Recognize using the diff lock wouldn't be healthy with mismatched tires, but don't really use it. The new tire will balance out the two for traction, so I don't mind spending the money. I am ordering another set of larger rims from Unverferth so that I can run larger tires on it, have sized a set that will have just less than 1% lead slip and still fit on the orchard model.

Won't doing this miss match of fron't and rear tires cause cause binding on a 4 wheel drive between the front and rear wheels ?

 

[svcguy]

No, as long as the tires are sized correctly based on your front axle gear ratio and rolling circumference of both the front and back tires, you can change tires. That's why a tractor allows for a couple of possible wheel sets with 4WD based on the type of work they will be used for. But you are right in that you can't just go and change them, they have to be sized properly or it will damage the tractor. That is what I am refering to when I refer to less than 1% positive slip. In my case, the problem is front tire clearance of both the engine and the loader frame. The TN75F is a bit of a challenge because of minimal clearance issues.