Far North central Iowa, Northwood | Here;s somthing I found on my computer, comes from Deere.
18 November 2003
Tractor Talk Bulletin
Number:
Publish Date
9020 SERIESPOWERS OF RIGHT CHOICEPART 4 FOOTPRINTS IN THE SOIL, TRACKS VS. TIRES
One of the most commonly asked questions about track tractors is: Which has more flotation capability, a track or a tire? This bulletin explores track and tire sizes to see how they compare in the soil relative to footprint size and calculated ground pressure. Youll find, with the latest range of tire sizes and combinations, your dealership has a lot more to offer in choices for flotation capabilities on wheel vs. track tractors.
Theoretical Calculations
Lets take a look at two example tractors. A 9420T Track Tractor equipped with 30-in. tracks has a track footprint total of 6660 sq. in. If the tractor is ballasted for best flotation or light loads at 40,000 lb., the resulting average static ground pressure exerted is 6.0 lb. per sq. in.
of soil [40,000 / 6,660 = 6.0]. The same tractor ballasted up to 50,000 lb. would now exert an average 7.5 psi [50,000 / 6,660 = 7.5].
If a 9420 4WD is equipped with dual 710/70R42 radial tires, it has a flat plate tire footprint area totaling 5040 sq. in. If that tractor is as light as it can be (approximately 35,000 lb.), the resulting average static ground pressure exerted is
6.9 lb. per sq. in. [35,000 / 5,040 = 6.9].
But if that 9420 4WD was ballasted to 105 lb. per horsepower or 45,000 lb., then the average resulting pressure would be 8.9 psi [45,000 / 5,040 = 8.9].
Note:These are theoretical calculations based on the area of the footprints as if they were standing on hard ground or concrete.
Calculations in Soil Conditions
The same two tractors placed in soil conditions where they are both pressing into the soil surface will yield different results. The tire footprint in the soil becomes a little wider and much longer. 3-in. penetration rates are given in some of the tire manufacturers data listings, so for this example well use that figure.
The 710/70R42 duals on the 9420 4WD total 10,480 sq.
in. of surface contact with 3-in. of tire penetration. Based on a ballasted tractor weight of 45,000 lb., the new calculated static ground pressure would be 4.3 lb. per sq. in.
If the 9420T Track Tractor would penetrate the same 3-in., the track contact area would not increase as much as the tire resulting in only 8206 sq. in. If the track tractor was also ballasted to 45,000 lb., now the theoretical ground pressure average would be 5.5 psi.
Formula
The formula used below calculates theoretical ground pressure for a tractor:
Obtain tractor weight by weighing your tractor or calculating weight based on order codes, options, and ballast. Typical 4WD tractors weigh between 30,000 and 45,000 lb. Typical large track tractors weigh between 39,000 and 50,000 lb.
Obtain the area of tire or tracks from the following chart and use those two factors (weight and
area) to calculate PSI.
____________________ / ____________ = _______________ Enter Tractor Weight (lb.): Divided by: Enter area
(sq.in.): Equals: Ground Pressure (psi)
Litho in U.S.A. Page 2 of 2
Tire and Track Footprint Area Comparison Chart (4WD & Large Track Tractors)
Notes:
1) Tire flat-plate and 3-in. penetration area figures are from Firestoneag.com/tire data.
Track areas are calculated based on John Deere 9000T and 9020T Series undercarriage drive wheel and idler wheel dimensions and spacing.
Areas shown are calculated
total sq. in. for one tractor:
2) NA = Not applicable tire/track combination for that particular size of 4WD or large track tractor.
Additional Points
Based on the chart above, there are several combinations of tires that can be used to get footprint areas equivalent to a track unit. A properly equipped 4WD has the capability of achieving good flotation while putting power to the ground like a track tractor. The following are additional points to think about when comparing tires to tracks in the soil:
A quick/easy way to estimate ground pressure exerted under a radial tire is to add 1 or 2 psi to the internal air pressure of the tire. A radial tire set at 6 psi air pressure will exert approximately 7 to 8 lb. per sq. in. on the soil surface, regardless of weight.
When a radial tire experiences an increased load due to drawbar load and weight transfer, it will squat down on the soil, increasing (especially length) in footprint size. The net result is the larger footprint area spreads out the extra weight at about the same rate per sq. in.
When a track unit experiences an increased load and transfers weight, it will lean back on the soil, trying to lift up the front end. The net result for the track is the rear portion presses down with more force and actually increases the lb. per sq. in. load on the soil.
Bottom Line
So the answer to the question, Does a track or a tire tractor have more flotation capability? is YES. If equipped right with tire/track size and ballast, either one can be capable of handling flotation needs. Thank you for offering your customers the Power of Right Choice with John Deere wheel and track tractors.
Solid. Stable. Still John Deere.
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Tracks vs. Wheels
