The rear and front tires must maintain a proper rolling circumference ratio.
If the ratio between the
front and rear tires change, it can cause damage to the driveline or reduce the wear life of the tire. It is
acceptable to increase or decrease the overall diameter on the rear tires, but the front tires will also need
to be changed. Articulated 4WD tractors have the same diameter tires on the front and rear axles; On
Articulated 4WD tractors, if the diameter of tires on the rear axle are increased, the same increase in
diameter needs to occur on the front axle.
Rolling circumference can be defined as the distance a tire travels in one revolution. Due to
the increase in mechanical front wheel drive (MFWD) tractors in the marketplace, it is very
important to understand rolling circumference when trying to determine the proper tire setup for
your tractor.
4WD front tires must be properly matched to the rear tires (0 percent-5 percent overrun) to prevent
excessive front tire wear, unnecessary powertrain loads, and overall poor tractor performance.
In order to achieve maximum drawbar pull, maintain proper steerability, and reduce tire wear and fuel
consumption when custom-fitting 4WD front and rear tires, the following guidelines must be followed:
Transmission front/rear ratio for standard-width 5000 Series tractors is 1.3495. The inverse, used for
4WD matching calculations, equals 0.741. The 0.741 factor determines the front tire with 0 percent
overrun. A 0.778 factor, 0.741 x 1.05, determines the front tire with 5 percent overrun.
1) Determine the rear tire desired.
2) Obtain the rolling circumference (RC) of that tire from the tire manufacturer's handbook.
3) Calculate the rolling circumference of the smallest and largest compatible front tire by using the
following formula:
RC (Rear Tire) x 0.741 = RC (Front Tire - Smallest)
RC (Rear Tire) x 0.778 = RC (Front Tire - Largest)
4) Choose a satisfactory matching front tire by scanning the RC column of a tire manufacturer's
handbook. A tire is a satisfactory match if the RC falls between the smallest and largest RC's
determined above.
UNDERSTANDING ROLLING CIRCUMFERENCE Rolling circumference can be defined as the distance a tire
travels in one revolution. Due to the increase in mechanical front wheel drive (MFWD) tractors in the
marketplace, it is very important to understand rolling circumference when trying to determine the
proper tire setup for your tractor. Since both the front and the rear tire on MFWD tractors are engaged,
the front tire requires more revolutions to cover the same distance as the rear tire. It is necessary to
know the gear ratio of the tractor (available from the tractor manufacturer) in order to select the tires
that will adequately match the tractor’s front-to-rear gear ratio. The gear ratio on the typical MFWD
ranges from 1.2 to 1.5; therefore, the proper setup should allow for front tire over-speed up to 5%
(consult with the tractor manufacturer for the preferred lead percentage). When selecting a new size or
type of tire for your MFWD tractor, you must choose a front and rear tire combination that matches the
ratio provided by the original tires. To make this easier, the tire and tractor industries have created a
chart to better assist customers with tire selections for their tractors. (See chart on next page.) The first
column of the table refers to the RCI (rolling circumference index) group. All tires in a group will have a
similar rolling circumference and OD. The larger the RCI group number, the larger the tire diameter and
rolling circumference. The second column lists the rolling circumference in inches. The third column lists
the tire diameter (OD) for that RCI group of tires. The chart then displays columns of wider tire widths in
both inches and millimeters as you progress from left to right. The top part of the chart shows the width
of field rows, and in the columns below each row width, the chart shows the recommended tires for
that row width. To better understand the chart, take a look at the following example: A MFWD tractor is
set up for 30" rows with 480/80R46 (group 47) tires on the rear and 420/90R30 (group 42) tires on the
front. Most tractors are five-step tractors, meaning that there is a difference of 5 between the RCI group
number of the rear tire and that of the front tire.* To convert from 30" to 20" rows, go to the 20” row
column and select the 320/90R54 rear, and matching front, 320/85R38
The ratio between the number of revolutions in the front and rear axle is very
important in a mechanical four-wheel drive (this is determined by gears in the
gearbox). This ratio is referred to as the gear ratio and, depending on the
brand and type of tractor, is normally between 1.20 and 1.50. Dynamic
measurement of the gear ratio 1.
There is a close relationship between the dimensions of front and rear tyres on four-wheel drive tractors. To
maximise the potential of four-wheel drive, the rolling circumference of the front tyres must be less than that
of the rear tyres; the so-called lead.
Gear ratio, rolling circumference and lead
There is a close relationship between the dimensions of front and rear tyres on four-wheel drive tractors.
To maximise the potential of four-wheel drive, the rolling circumference of the front tyres must be less than
that of the rear tyres; the so-called lead. Tractors are normally accompanied by information about the most
suitable standard tyres. Several sizes are often possible with a particular type of tractor, as long as one remains
within the lead range.
A lead of less than 0% and in excess of +6% have a negative impact on the tractor’s driving performance and
steering (particularly in corners) and result in (excessive) wear in front and rear tyres and premature wear and
tear in the transmission.
The ratio between the number of revolutions in the front and rear axle is very important in a mechanical four-
wheel drive (this is determined by gears in the gearbox). This ratio is referred to as the gear ratio and,
depending on the brand and type of tractor, is normally between 1.20 and 1.50.
Dynamic measurement of the gear ratio
1. Activate front-wheel drive and differential
2. Mark the front and rear tyre at the bottom or on the contact surface
3. Let rear wheel spin 10x (forwards)
4. Count number of revolutions in front wheel
5. Calculation: divide the front wheel revolutions by 10
For example: 13.5/10 = 1.35 The gear ratio!
Different width sizes on the same rim: convert from inches to mm
In some situations, you will want to use tyres with a different width size on an existing rim. This is possible,
but can sometimes be difficult to convert tyre sizes from inches to mm. We are pleased to explain how this
works.
An inch is equivalent to 25.4 mm. This means you can now calculate how a tyre size in inches can be
converted into a tyre size in millimetres. This will not be an exact figure, but an estimate. For instance, a
12.4R20 tyres is 12.4 inches wide, which is the same as 315 mm. The tyre size that corresponds with this is the
320/85R20.
In this example, the number 320 is the width of the tyre in mm, 85 is the ratio between the height and width,
and R stands for radial construction. The number 20 stands for the diameter of the rim in inches, which, of
course, must be the same for both tyre sizes
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.
There is a close relationship between the measurements of the front and rear tyres on four-wheel drive
tractors. To get the most from the four wheel drive, the ground speed of the front tyres must be greater
than that of the rear tyres - the so-called front wheel lead. With tractors, it is normally stated which
tyres can be fitted as standard. A range of sizes is often possible with a certain tractor type, within the
borders of a correct front wheel lead. To optimally exploit the four wheel drive of a tractor, the value of
the front wheel lead should be between +1% and +5% (ground speed front tyres > ground speed rear
tyres). The permitted front wheel lead value is between 0% and +6%. A front wheel lead below 0% or
above +6% can have a negative influence on the tractor’s performance and steering (especially on
corners), cause excessive wear of the front and rear tyres and premature wear of the gears. The
relationship between the revolutions of the front and rear axles on a mechanical four wheel drive is
fixed (determined by the cogs of the gearbox). This relationship is the gear ratio, which is usually
between 1.20 and 1.50, depending on the brand and type of tractor. The exact mechanical gear ratio is
given in the manual of every tractor. The following method can also be used to determine the figure. In
addition, there is an explanation of how to determine the rolling circumference of a tyre and the
aforementioned front wheel lead. Determining the rolling circumference of a tyre Take the
measurements on a smooth and hard surface as follows:
1. Switch off the 4WD (the front and rear wheels have to turn independently).
2. Mark the sidewall of a front and rear tyre in the centre of the contact area between tyre and surface.
3. Mark the surface at the point where the centre of the contact area of both the front and rear tyres
meets that surface (i.e. the point of the marking on the tyres).
4. Drive the tractor in a straight line until the front wheel has made exactly 10 revolutions and mark the
surface where the middle of the contact area of the tyre meets the ground (i.e. the point where the
marking on the front tyre is after precisely 10 revolutions).
5. Repeat the process for the rear wheel (i.e. make and mark 10 revolutions).
6. Measure the distance between the markings of the start and finishing point of 10 revolutions of the
front and rear wheels.
7. Calculate the rolling circumference of the front and rear tyres as follows: Determining gear ratio,
rolling circumference and front wheel lead Distance after 10 revolutions of rear wheel Distance after 10
revolutions of front wheel Direction rolling circumference front (in mm) = distance covered by front
wheel / number of revolutions rolling circumference rear (in mm) = distance covered by rear wheel /
number of revolutions Determining the front wheel lead The following conditions are required to
measure the front wheel lead: The tractor must be fitted with the standard front ballast and the tyres
set to a nominal pressure. At least 60 metres of clear road is necessary. To determine the front wheel
lead: 1. Switch off the 4WD.
2. Mark the sidewall of a front and rear tyre at the contact point between tyre and ground. 3. Drive the
tractor for a distance of exactly 10 revolutions of the rear wheel and accurately count the number of
front wheel revolutions at the same time (i.e. including the part of the last, incomplete revolution). 4.
Repeat the measuring with the 4WD switched on. 5. Calculate the front wheel lead by dividing the
number of front wheel revolutions with the 4WD switched on by the number when switched off. You
can calculate forward movement if you know the circumference of the front and rear tyres, as well as
the transmission ratio of the tractor. Use the formula given below: front tyre rolling circumference x
gear ratio % front wheel lead = - 1 x 100% rear tyre rolling circumference Determining the gear ratio To
determine the gear ratio: 1. Switch on the 4WD. 2. Mark the sidewall of a front and rear tyre at the
contact point between tyre and ground. 3. Drive the tractor for a distance of exactly 10 revolutions of
the rear wheel and accurately count the number of front wheel revolutions at the same time (i.e.
including the part of the last, incomplete revolution). 4. Calculate the gear ratio by dividing the number
of front wheel revolutions by the number of rear wheel revolutions, e.g.: number of front wheel
revolutions gear ratio = number of rear wheel revolutio
