rScotty
Super Member
- Joined
- Apr 21, 2001
- Messages
- 9,551
- Location
- Rural mountains - Colorado
- Tractor
- Kubota M59, JD530, JD310SG. Restoring Yanmar YM165D
I can't comment on the traction part of your question. Traction would seem to depend too much on local conditions. Anyway it seems like you are mostly concerned about the front/rear differential ratio and how tire affects that ratio. I live in a deep snow area and ran chains on 4wd tractors for the past 30 years. Learned a lot.
First of all, I think you are on the right track. I'm assuming that you did some research before starting all this and already know that your mounted turfs with the chains on are about the same diameter as the industrials that you took off. In tractors, knowing the diameter doesn't get you wheel circumference directly like it does in HS math...but at least it gets you close.
What you really want to know is the actual rolling circumference of the tire. The rolling circumference is the measure of how the tire is "geared" to the ground. Unfortunately, rolling circumference varies with the type of surface as well as the inflation, but we can get very close.
You can do this with the original industrial tires in several ways. The easiest way I know is to mark the tread and measure between the marks it makes on the ground. An easy way with industrial tires is by putting a spot of paint on the tire tread and driving down the road. Do that for the fronts and the rears and measure the distance between the dots. This will give you your actual front to rear tire size ratio. In a perfect world, this would also be the ratio of the mechanical front versus rear axle gearing inside the tractor. But it isn't.
And now you need to find the factory front-to-rear internal gear ratio by taking the tractor apart and counting all the various teeth and gear diameters between the front wheel and the rear. UGH! Or you can do it more simply directly by jacking up one side of the tractor, put it into 4WD, mark both tires and then slowly (key is slowly) rotate the front wheel and count the number of rotations of the front wheel until the back tire makes one complete revolution.
If you compare that internal gear ratio to the actual rolling circumference you will find that they are not th same. Reason for this is that the original tires are always sized by the designers at the factory so that the front end of the tractor is slightly higher geared than the internal gearing would suggest. Somewhere between 5 and lightly under 10% is typical.... This provides for the very necessary positive steering control, but it also puts a torsional stress on axles and drive shafts when in 4wd mode. Thjx deliberate mismatch is the accepted but unfortunate compromise in any inexpensive 4Wd machine. The more expensive solution is to have another degree of variable coupling between the front and rear drive shafts. Most Ag and Homeowner tractors I know of do not have that more expensive sophistication - although you will see it on most industrial machines and full time 4wd cars. However, in any tractor having this deliberate mismatch you don't want to ever use 4wd in any high traction situation......Not on dry pavement and also not on snow with all 4 wheels chained. The torsional wind-up stress is additive as you drive and will eventually either cause a tire to slip on the surface or else something will break. BTW, you can often feel this wind-up stress as a reluctance of tractor to shift out of 4wd mode. It can feel like the 4wd shifter is bound up. What you are actually feeling is normal wind-up stress due to the front tires being higher geared relative to the rears.
After a bit of experience, the amount of reluctance to shift out of 4wd mode makes for a pretty good "cheap and dirty" way to measure just how close to right you have gotten the tire diameters. The harder it is to shift out of 4wd, the more attention you need to place on effects of varying traction.
I've played with this front/rear ratio thing a fair amount over the years on several machines.. If you have questions we can work on them.
enjoy! rScotty
