Good evening All, Could someone please tell me the Correct tires for My Orig Wheels on my 79 155D?, The orig turfs are still on the back ,,Rottin!
, I want to know correct Diameters so I dont Strain the 4whl drive unit, Thankyou, I have some 12" car tires on the front now, Any help? Thanx Jr
Are your original rear turfs tires diamond or block tread?
According to some old factory literature I have. The factory stock turfs were 6-12 FD (full diamond) fronts and 8-16 FD rears. Most likely that's it - particularly if it has diamond tread. Though by now it could be anything.
The YM155D became the YM165D about 1980. I didn't think there were any changes to the tranny or Front/Rear drive ratio with the model change. The YM165D had the same diameter tires but one step wider. Doing that is bound to have changed the tire's rolling circumference and the F/R ratio. In fact, it did.
At about that time ('79 -'81) the YM155D/165D were also offered shod in a super wide turf tire on a wider rim than the diamond turf. That tire had a slight block instead of a diamond tread and was for mowing more delicate soils and steeper slopes. Those tires were 20.5 x 8.00-10 fronts coupled with 29 x 12.00-16 rears. That ratio wasn't as ideal as the first one, and was better for mowing than for loading. It could lead to binding of the front drive shaft, difficulty in shifting in/out of 4WD, tough steering, and mechanical problems. If that's what you have, you could improve it by measuring the mechanical ratio and adjusting rim width to make the tire ratio better than the stock tires. Or go back to full diamonds on the FD rim.
I agree with not wanting to strain the 4WD drive....although if you don't have a loader on it that isn't likely. Without the front end being loaded down, the 4WD system is stout enough to safely unload the differential torsional stress by skidding a front tire on any surface - even those real high traction surfaces like turf tires on dry pavement or ag tires in soft clay.
If you do have a loader with weight in the bucket then the machine shouldn't be used in 4WD anyway unless the tires can be allowed to slip. Naturally enough, we all do it anyway. But we do it knowing just how far we can push it before it tears things up.....and willing to pay if it happens.
It's real good practice to match the front and rear tire ratio to the internal front and rear gear ratios. If you pay attention to that it will be amazing how much nicer things work. You may or may not want to do this by matching the size printed on the side of the tire - doing that way sort of involves having faith in industry not changing much in some 30 years now. It is probably close to good enough. If it gets hard to shift in 4WD you'll know it wasn't close enough....
The way that Yanmar did it originally was to match the internal F/R gear ratio to the ratio of the ROLLING CIRCUMFERENCE of the tires they chose. Typically that spec is taken loaded with the tractor's weight but not with implements. It's about as good a number as you can find, and is available from the manufacturer's specs. Why do we not use tire diameter? Well, when loaded, the axle isn't in the center of the tire, so the diameter gets you into the ball park but not the game. The error works out to be large enough to matter. So you want true rolling circumference rather than diameter. Once you figure out these numbers you can match the F/R ratios perfectly and it will work fine of course. If you do that it will be very easy to shift into and out of 4WD, and the tranny's output shafts will never see a differential in their torsional stress. Do it that way if you are moving heavy loads in deep mud or are chained up on all four tires. For less aggressive work and best road steering I like for the front tires to "outrun" the rears by a few percent. One or two percent is plenty, I consider a 5% higher ratio Front to Back to be a maximum rather than a desirable goal. You can measure that internal gear ratio for yourself by jacking up one side and carefully rotating the tires and counting revolutions on one tire while the other tire makes a complete revolution. You can measure rolling circumference yourself by driving over chalk marks on the road or looking at lug prints in the dirt. Then buy tires to match both chosen ratio. Make any mismatch in the direction of allowing the front tires to outrun the rears. Keep it under 5%. That is always the best way. Second best is to use the factory specs.
Enjoy! rScotty
BTW, The Yanmar wheel bolt circle is standard for small rims just like most tractors and similar size machines. The same diameter rim can be ordered in almost any rim width (or offset). Tires typically have a specified mounting width, with adjustments in the rolling circumference possible by changing the rim width by inch increments.
