4 Wheel Drive and Tire Ratios

[npalen]

How about jacking up ONE side of the tractor just enough that the front and rear tires are just free to rotate with 4WD or FWA engaged. Will probably require a jack at front and rear.

Then, with a helper, run a rope, twine, tie strap or similar between the front and rear tires and wrapped around the outside of the tires. Mark the rope just behind the front tire also mark the floor at this point. Rotate the REAR tire while keeping the rope held in original position on the circumference of both tires.

Stop when the mark on the rope gets near the rear tire then measure the distance traveled. Then measure the slack in the rope and divide this by the the distance traveled to give and indication of percentage of front wheel lead.

The idea here is that the gear ratio should cause the front tire to "travel" faster than the rear causing the rope to go slack.

Disclaimer: I've not done this so it's subject to broad criticism, constructive or otherwise.

 

[zzvyb6]

You can't compare drive ratios with the tires off the ground. There they are ROUND. On the ground, they are deflected by the weights on them. Because the earth is FLAT, there needs to be slippage within the tread to allow the ROUND tire carcass to have a flat spot.

All my 4wd JD tractors have tire paired size charts in them for Ag, Turf and the Industrial constructions.

Additional notes (I was a Vehicle Dynamics and Tire Engineer for almost 40 years), Cars have a viscous differential that manages front to rear drive torque distribution. Trucks can have a viscous coupling AND a lockup selector needed when the going gets tough.

Keep in mind that tractor tires are usually operated off of pavement and for dirt, they need a significant mount of slippage to generate traction. (Go check out the tractor pull videos). Naturally, the amount of slippage at optimum traction is dependent on tread design, carcass stiffness and of course SIZE. Yet SIZE is based on the weight the axle will usually carry. Tractor tires also have softer tread compounds so that the front to rear slip averaging works better. That's why you don't get 50,000 miles out of them.

 

[npalen]

So how would you recommend determining whether the relationship between the front and rear tires falls within the +5% lead?

 

[rScotty]

So how would you recommend determining whether the relationship between the front and rear tires falls within the +5% lead?

First of all, npalen I've got to congratulate you for that crazy complicated method you proposed in message #11. That shows some real original thought and ought to work to compare the front to rear overdriven ratio based on unloaded circumference ....although the shear complexity makes it impossible to actually do it.... which is probably a good thing as xxvyb6 points out the answer you would get it isn't the same thing as loaded rolling circumference in the real world of tractor work. But congrats anyway. And BTW, a variation on your method will work for determining the internal Front to Rear gear ratio as long as there is no viscous coupling.

Getting back to your questions, here's how to do it a simpler way using wet paint spots on the tires driving on a dry slightly dusty asphalt road. All the numbers are averages of my own measurements between the paint dots we laid down on the road using our old 33hp Yanmar with OEM R1 Ag tires.

In 2WD I measured the Rolling Circumference (RC) of the front tire at 90.25", and the RC of the rear at 146". Front to rear ratio is then: 90.25/146.00 = .618

In 4WD I measured the Rolling Circumference (RC) of the front tire at 88.00", and the RC of the rear at 149". Front to rear ratio is then: 88.00/149.00 = .603

Now take a moment and look at those numbers. In 4WD, the REAR RC was longer - indicating that the rears were being dragged by the front tires. And also in 4WD we see that the FRONT RCs became slightly less, which on the road surface I used probably means that the fronts were slipping a bit as they slightly dragged the rears.
All this slight tire drag and slippage is typical of an overdriven ratio where steering control requires that the fronts turn a little faster than the rears.

How much faster? Well you can compare the ratios as a percentage of change from 2wd to 4wd by first subtracting one ratio from the other, and then dividing that answer by the original 2wd ratio, and finally multiplying by 100 to change the answer into a percentage. For the numbers above this is:

((2WD ratio - 4WD ratio) / 2WD ratio) times 100 = the overdriven percentage

((0.618 - 0.603) / 0.603) x 100 = 2.49% overdriven

And there's the answer. That Yanmar YM336D was about 2.5% overdriven as it came from the factory with Ag tires. I wanted to match that when changing to turfs.

Enjoy,
rScotty

 

[zzvyb6]

So how would you recommend determining whether the relationship between the front and rear tires falls within the +5% lead?

I don't worry about F/R drive torque error on dirt, grass or gravel. It's a big deal on concrete or asphalt. And, air pressure is about the only variable available to you to change it if your axle loads are set (loader, loaded tires, sprayer tanks, full fuel tank.

You can check it using a coast down technique if you have a manual transmission. Start with a very high front tire pressure, Run the tractor on a level surface in a gear to give you a fast walking speed. Push in the clutch and time the seconds to come to a complete stop. Lower the pressure 2 psi and do it again. If you can draw a graph on the back of an envelope, plot the pressure on the x axis and the coast down time on the y axis. After a few runs, you should notice that the curve climbs a bit, reaches a peak and then starts heading down again. The pressure at the highest coast-down time has given you the lowest front rolling resistance which has a strong drive torque component. When the drive torques are balanced (i.e. no strain in the front drive-line) you have the optimum drive ratio (theoretically). Your results will depend on the load case you choose (i'd pick the heaviest one because that's where the tire wear and the drive-line strain would be highest).

For a hydro unit, there is no clutch to disengage, so I would run the test with the same procedure, but write down the speed you attain when coasting down a decline in the road. Same strategy: The lowest rolling resistance gives you the highest speed steady state speed for the exact same throttle setting.

If I were working, we would put strain gauges on the front drive shaft and measure the minimum front drive torque and front tire pressure. But this ought to get you very close. You do need to find the peak of the time or speed curve, though. If you can't, then you might have a tire size mismatch that's causing a problem. Worn tread, wider that standard rim width, or just plain wrong front or rear tire size is the cause.

 

[zzvyb6]

Here is an example of the procedure. Note that if you start with a pressure too low, it can take a while to get the optimum settings. Start with high pressure. You can bleed down the tires quickly without having to run back to the compressor.

 

[npalen]

I've not heard of a viscous coupling on a SCUT or CUT. The only thing close that I can think of is the automatic clutch on some of the New Holland's.
And, yes, I do understand the difference between rolling circumference and the actual mechanical gear ratio between the front and rear drives. And, yes, variations of the two wheels off the ground method can also yield the mechanical ratio. Your "slow-down" method of matching tires is interesting, had not heard of that but makes sense.

Edit: Link to previous discussion of this subject if anyone might be interested:

http://www.tractorbynet.com/forums/tires/371468-tires-why-rolling-circumference-loaded.html

 

[rScotty]

I've not heard of a viscous coupling on a SCUT or CUT. The only thing close that I can think of is the automatic clutch on some of the New Holland's.
And, yes, I do understand the difference between rolling circumference and the actual mechanical gear ratio between the front and rear drives. And, yes, variations of the two wheels off the ground method can also yield the mechanical ratio. Your "slow-down" method of matching tires is interesting, had not heard of that but makes sense.

Edit: Link to previous discussion of this subject if anyone might be interested:

http://www.tractorbynet.com/forums/tires/371468-tires-why-rolling-circumference-loaded.html

I've not heard of a SCUT or CUT with a viscous coupling between front and rear either. Or for that matter any that are using a center differential on the driveshaft between the front and rear gearing. But all those things are common in offroad SUVs now, so we will probably see them in tractors someday. When that finally happens, I'm hoping that matching RC and mech. gear ratio won't be necessary. Yes, I know that you understand the various ratio matching methods, but not everyone understands it or why it needs to be considered.
Thanks for your comments,
rScotty

 

[PILOON]

LOL, forget all about ratios (front should lead by 5%), just drive only on wet grass or snow and never on pavement.
If you are doing snow chores a good set of chains on the rears will save the front end.

I had a 3/4 ton 4 x 4 plow truck with matched tires on all 4's and the instant I hit asphalt I fractured a U joint, dang truck cost me one axle per year, usually the rt front, but then all a cost of plowing business. (but also then plow truck purchase was all of $3500 so I can't complain) also it netted me over $5K per season as a 'pastime'. I later (after 6 yrs) sold it for $4k .
Not counting inflation I think I came out ahead, LOL.

 

[Redneck in training]

On a related note, does it cause undue stress on a drive train to leave the tractor in 4wd when using it on unpaved or uncompacted surfaces for extend periods of time? For instance when pasture mowing, some areas when turning on a slight grade or dip in the ground I need 4wd because the R4 tires will simply spin on the grass but the rest of the area is flat. Or is the best policy (what I call the redneck 4wd policy) to engage the 4wd after I'm stuck and hope it gets me out?

You should ask different question. When is it safe to be in 2WD? Example: You transport load of wet dirt in the FEL bucket going mild slope. You need to stop the tractor but it will skid rather long distance before it stops. Try that with 4WD. It will stop on the spot. Do not try it even with empty loader on wet grass and steeper slope. The tractor, while in 2WD, will skid to the bottom of the slope.
I had a leaky sight glass on the the HST so I decided to park the tractor nose down on about 25 deg slope to pop the glass out and replace it. Just after I drove over the top edge the tractor started to skid at increasing speed despite me pressing the brake, it made right turn on the bottom, balanced on two wheels for a second and settled back on all four. Since then I leave my tractor 90% of the time in 4WD and put it to 2WD only when bush hogging without a FEL bucket.