Can I use a different size tire on my 4wd tractor?

Proper tire sizing/matching on front wheel assist tractors is a function of insuring that the front axle 'lead"  specification is kept within acceptable design and operational limits.  Front axle lead (or slippage) is necessary on front wheel assist tractors in order to realize the benefits of front axle tractive assistance.  Desired front axle lead is expressed as a positve increase in front axle speed over that of the rear axle, usually in terms of percentage (ideally +1%-+5% for most front wheel assist tractors).  Lead percentages outside of this range will lead to problems such as accelerated tire wear, difficult steering, loss of tractive assistance from the front axle as well as increased drive train wear with premature failures.  Negative lead (or slippage) will, from the outset, totally negate any potential advantages offered by front wheel assist. 

 

Determination of the front axle lead (slippage) for a given tractor is easily determined if one has access to the following information;

• Rolling circumference specifications are available from the tire manufacturer.  This is the only tire "specification"  that matters for this calculation.  Rim size is of no importance at this point.   

• Drive train ratio is available from the tractor manufacturer.

 

Calculation for determining front axle lead is as follows;

 

                                      Front Tire rolling circumference x Front/Rear drive train ration 

                  %Slippage=                      Rear Tire rolling circumference

 

Example;

Front tire size- 11.2x24 with rolling circumference of 129.6"

Rear tire size- 18.4-28 with rolling circumference of 175.9"

Tractor Front/Rear ratio= 1.4

 

                                          129.6 x 1.4                 181.44             

                                              175.9        =             175.9    =        1.031

                                                                                                   (1.031-1) x100 = percentage

                                                                                                    1.031-1 = .031

                                                                                                     .031 x 100 = +3.1% front lead (Good!)

 

Now, what will occur if we substitute a rear tire size of 18.4x30 having a rolling circumference of 183.5"?

 

                                          129.6 x 1.4                 181.44    

                                              183.5       =             183.5    =         0.989   

                                                                                                   (0.989-1) x100 = percentage

                                                                                                   .0989-1 = -.011

                                                                                                   -.011 x 100 = -1.1%  front lead (Notice that this is 

                                                                                                                                  negative lead meaning 

                                                                                                                                  the rear axle is over-

                                                                                                                                  running the front axle. 

 

This is obviously a bad choice of rear tire size for the given front size.

 

In order to use the 18.4x30 tires on this tractor, it would be required as well to install 12.4x24 front tires having a rolling circumference of 137.0".  This will result in a lead of +4.52% which is within the desired range of +1.0%-+5.0%.

 

If you have owned a 4wd tractor for any length of time, you have probably noticed that the front tires have a tendency to wear at a higher rate than the rear tires.   This is normal.  Keep in mind that, because of the design slippage of the front axle, when 4wd is engaged the front axle is, in effect, trying to "outrun" the rear axle.  Since the rear axle is supporting 60% of the total tractor weight, the front axle cannot drag the rear axle along with it at its slightly increased speed; therefore the front wheels must be content to slip (or spin).  Did you ever watch a teenager "peel out" at a red light?  This is exactly what is occurring in the front axle, only at much lower speeds with the associated tire wear being nearly imperceptible. 

Interesting to note is the fact that the rate of front tire wear decreases as the tread wears.  As the tread wears down, rolling circumference is reduced, bringing the front axle lead (slippage) percentage closer to 0%. 

This serves to illustrate why it is important to be judicial in the use of 4wd, avoiding usage on hard surfaces as much as possible. 

A related subject is the use of front loaders on front wheel assist tractors.  Ideally you want to maintain a load bearing ratio between the front and rear axles of 40%front/60%rear.   Asking the front axle to carry more than this share is setting yourself up for expensive repairs down the road.  It is important to use appropriate counter-balance on the rear of the tractor to offset the loads that are carried in the loader bucket.  Take note that front axle components are generally smaller and of lighter construction than the rear axle.  These front axle components were not designed to support 60%, 70% or in some extreme cases up to 90% of the total "load".
If your rear axle is scooting along, just barely contacting the ground now and then, this means the front axle is supporting near 100% of the total weight of the tractor, loader and bucket payload!  This is an almost iron-clad guarantee of front axle failure! 

Be cautious when changing tire sizes.  Even the same size from different manufacturers can have a significant impact on front axle leads.