Tips for Ag Tire Size Conversions

When should a grower consider tire or wheel conversion on their machinery? While there may not be a “one size fits all,” there are some options to consider when selecting the right size and type of tire.

Different operations will require different sets of tires for the same equipment, says Norberto Herbener, OE Applications Engineer for Trelleborg Wheel Systems. An example of this is with sprayers, where during crop applications “skinny” tires are used to reduce crop damage, and large flotation tires are used for broadband application without a crop standing.

Rolling Circumference Index

A Rolling Circumference Index (RCI) chart is used to match front and rear tires to each other within a given gear ratio, says David Graden, Global Account Manager for Michelin Ag.

When and how to use RCI chart

Ken Brodbeck, VP of Technology, Precision Inflation Systems, provides the following MFWD (Mechanical Front Wheel Drive) Row Crop Tractor Tire Conversion example.

Let’s use the typical MFWD tractor with 480/80R50 rears and 420/85R34 fronts. This is defined as a “5 Step Tractor.” The tires need to have a 5 Step difference on the RCI Chart. RCI 48 – 43 = 5 Step difference between front and rear tires. Going to a smaller diameter front and rear will work as well by selecting any tire from RCI 47 rear – 42 = 5 Steps.

Alternate section widths (mm) range

The chart below has popular 5 Step MFWD tractor RCI 48 480/80R50 rears and RCI 43 420/85R34 fronts, showing alternate wider and narrower rear and front sizes highlighted below.

Narrower section widths (mm) range

If you are looking for narrower options, simply move to the left of the RCI 48 480/80R50 for rear tire options, then move to the left of RCI 43 420/85R34 for front tire options.

Wider section widths (mm) range

If you are looking for wider options, simply move to the right of the RCI 48 480/80R50 for rear tire options, then move to the right of RCI 43 420/85R34 for front tire options.

Alternate setups and important checks

Remember, tires are considered part of the tractor transmission. While most MFWD tractors have “5 Steps” between the front and rear tires, there are some 4 and 6 Step tractors out there. Always contact the tractor manufacturer to be sure steps are correct and alternate specific sizes will work.

On select large John Deere tractors, for example, you can look at the frame for Step Indicator:

In the case of a 4 Step Tractor, you simply select a RCI 48 Rear and matching RCI 44 Front. RCI 48 – 44 = 4 Step difference between front and rear tires.

Wheel Recommendations for IF VF Technology Tires

When moving from a Standard Tire to an IF/VF Tire, manufacturers recommend a wider wheel to maximize the benefit these tires offer for weight carrying capacity and lower air pressures, says Dave Paulk, manager of field technical services for BKT USA.

Because of the increased air pressures and load carrying capacities of IF/VF tires, make sure wheels are built to handle the higher pressures and loads these tires will carry.

Editor’s Note: This content originally appeared in AG Tire Talk to provide answers that farm equipment dealers have about farm tire technology. This series features a trending question followed by an abridged version of the answers. For the complete answers, check out agtiretalk.com.