4 Wheel Drive and Tire Ratios

[rScotty]

On 4 Wheel Drive and Tire Ratios, there has been a lot written about the advantages in matching the tire size with the internal Front/Rear gear ratios - especially for 4wd. It turns out that this Front to Rear ratio is easy to measure.

It makes sense that in 2wd, Front to Rear tire size ratio doesn't much matter. But when shifting to 4wd, matching the F/R tire rolling circumference to the tractor's internal F/R drive train gear ratio does a lot to reduce the stress on the drive train and also makes steering easier as well.

You can check this effect on any tractor by simply measuring the distance that the front and rear tires travel in one revolution and comparing the measurements in 2wd and then again in 4wd.

All that is required is a flat piece of ground or road, some way to put a mark on the tire tread that will transfer to the ground, and a tape measure. I have marked tire treads with a blob of wet paint on a dry road, and a different time by using a short lag bolt into the tread for sand and snow.

In 2wd, measuring the distance between the marks the tires leave on the ground will tell us the rolling circumference (RC) of the tires. This RC may vary from the manufacturing spec, but in 2wd without any tire slippage what we are measuring is the true rolling circumference for that tire on that tractor. We are also automatically accounting for factors like tread wear, inflation, and load.

Repeating the same measurement in 4wd on high and low traction surfaces and comparing with the 2wd numbers we got gives good insight as to what is actually happening when we shift into 4wd.

We know that a measured difference from the 2wd numbers has got to be the result of tire slippage. And we know that normally tractors are set up so that the front tires rotate about up to 5% more than the rears in order to maintain steering control. So we have some idea of what we are expecting to measure. But in spite of this I found that the first time I checked things the measurements didn't quite come out as I expected. That required some thinking.

It turns out that when we measure in 4wd the measurements are no longer simply showing independent rolling circumference for each tire. In 4wd the front and rears are coupled together, so the measurements reflect a combination of how the tractor is loaded, how much traction the tires are getting, and also the internal F/R gear ratio of the drive train. On some surfaces you may find that the front tires are dragging the rears, and on other surfaces that the rears are push-sliding the fronts. Or maybe a bit of both. It is worth thinking about for awhile; this is handy information for protecting the drive train.

BTW, you'll hear the overdriven ratio between 2wd and 4wd expressed as a percentage difference. It is often called the Front-to-Rear Overdriven Ratio, and a commonly heard figure is that you want to be 5% or less overdriven. These measurements will give you that % ratio for your tractor by comparing the 2wd F/R ratio with the 4wd F/R ratio.

Bottom line is that there is no perfect ratio. On slippery surfaces the drivetrain can protect itself pretty much regardless of any ratio difference. But when traction increases, so does drive train stress.
Good Luck,
rScotty

 

[mcfarmall]

That's an interesting analysis of the relationship between the front and rear wheels. It would be nice if manufacturers would list the Front-to-Rear Overdriven Ratio in the tractor specifications but alas, most average SCUT & CUT buyers wouldn't know what to do with the information and dealer salesmen would have even less of a clue what it meant or how to explain it to a customer.

Another handy bit of information would be a tire chart provided by the manufacturer of compatible "mix & match" sizes for those of us who would be interested in running R1's in the rear and R4's in the front or any other combination of R1, R4 & turf tires suitable for our applications.

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?

 

[Texasmark]

That's an interesting analysis of the relationship between the front and rear wheels. It would be nice if manufacturers would list the Front-to-Rear Overdriven Ratio in the tractor specifications but alas, most average SCUT & CUT buyers wouldn't know what to do with the information and dealer salesmen would have even less of a clue what it meant or how to explain it to a customer.

Another handy bit of information would be a tire chart provided by the manufacturer of compatible "mix & match" sizes for those of us who would be interested in running R1's in the rear and R4's in the front or any other combination of R1, R4 & turf tires suitable for our applications.

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?

I have seen some tire charts, when looking for a match to 12x16.5 rears, that show both numbers: The actual circumference and the actual rolling circumference at rated pressure and load. This helps to determine what you are talking about.

 

[LouNY]

It's all good information to have when considering new tires. One major item not mentioned is the loaded radius of the tires which is what determines the rolling circumference, the loaded radius will change drastically depending on the axle loading of your tractor. A tractor with no loader and only a 3 pt implement will increase the front tire load and radius while decreasing the rear tire radius while the load is being carried, the greater the load the greater the change. A tractor with a front end loader will show considerable change depending on the load in the loader, with the loader empty if you perform rScotty's test and then fill the loader with a heavy load and repeat you would find a considerable difference in the rolling circumference because the loaded radius changes with load now shorter on the front and taller on the rear. Your front end lead/lag will always change with load, so when you want it will decide the pressure you need in your tires which also changes the loaded radius. There is no end to tinkering with these thoughts :stirthepot::banghead:and :drink:

 

[rScotty]

That's an interesting analysis of the relationship between the front and rear wheels. It would be nice if manufacturers would list the Front-to-Rear Overdriven Ratio in the tractor specifications but alas, most average SCUT & CUT buyers wouldn't know what to do with the information and dealer salesmen would have even less of a clue what it meant or how to explain it to a customer.

Another handy bit of information would be a tire chart provided by the manufacturer of compatible "mix & match" sizes for those of us who would be interested in running R1's in the rear and R4's in the front or any other combination of R1, R4 & turf tires suitable for our applications.

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?

I believe that it is OK to leave the tractor in 4wd anytime a tire is so easy to spin - like when using R4s on grass. In fact, R4 industrials and R3 turf tires seem to spin almost too easy in the field. That's probably all to the good for protecting the drive train.

As you say, the manufacturer's Overdriven Ratio can be hard to find in the tractor specifications. But if you measure the front to rear rolling circumference (RC) on a slippery surface in 4wd - as described in the first post of this thread - and then divide the REAR RC by the FRONT RC it will give you a number very close to the manufacturer's internal gear ratio. It's close enough. The only time I checked it, the measured RC numbers were only off of the manufacturer's OverDriven Ratio by a tenth of one percent.
rScotty

 

[pmsmechanic]

Lift the tractor off of the ground and mark both the front and the rear tires. In 4wd turn the rear tires one full revolution. The front tire mark should be slightly past one revolution. If you measure the circumference of the front tire and then the difference you should be able to calculate the % lead of the front axle.

This is more of a question rather than a statement.

 

[rScotty]

Lift the tractor off of the ground and mark both the front and the rear tires. In 4wd turn the rear tires one full revolution. The front tire mark should be slightly past one revolution. If you measure the circumference of the front tire and then the difference you should be able to calculate the % lead of the front axle.

This is more of a question rather than a statement.

It's a good question. That's the right concept....and the method that you are describing works for a 4wd car or truck or for anything where the front and rear tires are identical, inflated the same, and assuming that the loading of the front axle is the same as the rear. The problem is that none of these conditions hold true on a our tractors, so we have to use a different methods.
thanks,
rScotty

 

[pmsmechanic]

It's a good question. That's the right concept....and the method that you are describing works for a 4wd car or truck or for anything where the front and rear tires are identical, inflated the same, and assuming that the loading of the front axle is the same as the rear. The problem is that none of these conditions hold true on a our tractors, so we have to use a different methods.
thanks,
rScotty

I'm not 100% sure that your statement is correct. Either I didn't explain myself correctly or you didn't read what I wrote.

It might take more than one revolution of the rear tires for the marks to be close and because the front tires are smaller they would travel more than one revolution. At some point in a set number of revolutions the marks on the tires should be close as both wheels are traveling the same distance. The front tires should rotate a little more because of the built in lead.

The loading and inflation of the tires is irrelevant as the tractor is off of it's wheels. The point I'm trying to make is this would be a rough way of calculating the lead built into the drive train.

 

[CincyFlyer]

There already exists a tire size classification system just for this purpose; I mentioned it sometime earlier in the year when I was considering turfs for my tractor. But basically you'd get a set that was the same number of steps apart as your existing set: if you had a B and a D, and you wanted to put A size on the front, you'd need to put C size in the rear.
I've since given up on turf tires, since in very large sizes they have very large prices (plus you need the wheels upon which they are mounted.)

 

[rScotty]

I'm not 100% sure that your statement is correct. Either I didn't explain myself correctly or you didn't read what I wrote.

It might take more than one revolution of the rear tires for the marks to be close and because the front tires are smaller they would travel more than one revolution. At some point in a set number of revolutions the marks on the tires should be close as both wheels are traveling the same distance. The front tires should rotate a little more because of the built in lead.

The loading and inflation of the tires is irrelevant as the tractor is off of it's wheels. The point I'm trying to make is this would be a rough way of calculating the lead built into the drive train.

And my point is that there is no "lead built into the drivetrain". The lead only appears about when you compare the difference in the internal drivetrain ratio with the difference in the rolling circumference ratio of the rear and front tires.
You need both the gear ratio and the tire ratios to calculate the lead, because it is the difference between them is the lead - often called the overdriven ratio.
good on ya,
rScotty