Tires 1500D tires

[winndl]

I have a couple of questions about rear tires for my YM1500D.

The Firestone site says the "stock" 8.3-22 rice tires have an overall diameter of 37.5" and a rolling circumfernce of 113".

They also have a 9.5-22 tire that has an od of 39.2" and a rolling circum of 117".

1. Is the diameter and circum. too much difference for a 4 wheel drive?

2. Will the 9.5's fit my rims? (Firestone shows they should be put on a 8" width wheel, my Yanmar apparently has 7".)

 

[PAB_OH]

It depends on the front tires and the ratio. Ideally you want to have 3-5% "OVERDRIVE" on the front wheels. By increasing the RC of the rear you are eating into that amount, assuming it was correct from the factory.

Find out what the RC of the front tires are and divide the rear RC into that (probably between .65 & .72). Vary by too much and you'll be replacing some pretty expensive driveline components.

 

[winndl]

The RC for the front tires (from the web site) is 72", so if I understand you correctly,
72/113 = .637 would be stock 8.3-22 tires
72/117 = .615 would be with the 9.5-22 tires

Am I looking at this number correctly? Is the front a little less overdriven with the bigger tire on the rear? Or is this too much difference from original?

Its not that big of a deal to me either way. I just wanted to know because the 9.5's are available at my local Firestone dealer. He thinks he can order the 8.3's.

 

[PAB_OH]

I think you will be borderline. You are effectively reducing the size of the front tires which will result in less overdrive or lead on the front axle.

Here's a scenario to consider.

Let's assume that the original tires are set up to provide exactly 5% overdrive on the fronts when compared to the gearing between the front and rear axle (I think realistically this almost never happens).

.05*x+x = .637
x=.606

So if you change the rear wheels by making them slightly larger the NEW rear wheel will try to cover MORE ground per revolution while the fronts will be covering the same amount of ground. This causes the rear axle to "push" more. "Push" is a relative term though. Since we started with a built in lag in the rear or overdrive in the front (saying the same thing, just from different perspectives) we haven't reached a true "push" condition yet.

That point would be reached when the tires RC ratio drops below that of the axles (.606). Everything I've ever heard about tractor 4WD systems is that a 3-5% lead by the front axle is optimum. It provides needed traction while staying within the physical limits of the materials used in the driveline. This is different than on-road setups because it is assumed that the tractor is operating in loose material or mud. This allows the front wheels to safely slip without loading the driveline excessively. Passenger vehicles must maintian as close to a 1:1 ratio as possible since asphalt doesn't give so easily. If one axle leads or lags serious wear and breakage WILL OCCUR. This is also why you NEVER drive a tractor on hard surfaces while in 4WD.

So if we set the lower limit at 3% lead on the front we know we should have at minimum a .624 ratio. You're only at .615 and that is with no wear on the front tires. Don't forget, as the front tires wear they become smaller, effectively covering less ground per revolution and decreasing the lead of the front tires over the rear tires.

If it were me, I'd do it anyway. I'm willing to bet there is enough slop now in the driveline from normal wear that you won't notice anything. That is my personal opinion. If you're truly worried about it though you can run the rear tires on the low side and inflate the front tires toward the high side or get the 8.3x22's.

You have to remember this is all theory though and based on nominal numbers. I hope this helped though.

Good luck...

 

[winndl]

I can't thank you enough for this explanation.

This is exactly the logic I was looking for.

Considering the price for these new tires is less than the price of new rice tires, my front tires are brand new, my tractor seldom leaves my property and its all dirt and relatively flat so I don't use 4wd hardly at all, I think I'll take my chances.

Thanks again

 

[PAB_OH]

Glad I could help.

One more thing to consider and I forgot to mention. In really sloppy conditions (mud) I don't think the lead on the front axle is so important. I think others will argue with me about that and that is fine. But think about this. If I'm basically suspended in mud (tires are floating) it doesn't matter what ratio the tires have to each other (front vs. rear). All that matters is that BOTH axles ARE SPINNING. I'm going to get some help from that front axle rotating my tires. When it becomes important is on loose packed material especially going up a grade. You want the front tires to "pull" the tractor up the hill.

I think you'll be happy with the 8.3's especially if you seldom use 4WD.

Good luck.