University of Missouri Extension

G1231, Reviewed October 1993

Are Radial Tractor Tires Cost Effective?

James C. Frisby
Department of Agricultural Engineering

When Pirelli Co. developed the first radial tractor tire in 1957, the company claimed it would develop 20 percent greater traction with less slippage at equal load than an equivalent bias-ply tire. (Forrest, et al. 1962) Subsequent research has confirmed that radial tires have the advantage in tractive efficiency. H. Erdal Ozkan, state extension specialist at Ohio State University, has reviewed all available research results related to field performance of tractors equipped with radial and bias ply tires.

Author Increase in tractive efficiency for radials
Seleznev and Kovalez (1968) 13 percent
Bohnert and Kenady (1975) 7 percent
Hoffman (1983) 8 to 9.5 percent
Hausz and Akins (1980) 7 to 13 percent

In most cases, the research was done with the same wheel slip for radial and bias tires. This may have kept the radial tires from reaching the predicted 20 percent advantage.

Drawbar pull tests were reported as follows:

Author Wheel slip Soil Advantage of radial
Forrest (1962) less than 15 percent Sand
Loam
Clay
8 percent
23 percent
21 percent
Thaden (1962) 16 percent   29 percent
Mumgaard and Rudakov (1975) 15 percent Firm 15.5 percent
Bohnert and Kenady (1975)   Tilled
Sod
14.6 percent
18.8 percent
Gee-Clough (1977) 20 percent   5-8 percent

Field capacity and fuel consumption were studied:

Author Increase in field capacity for radials Reduction in fuel consumption for radials
Seleznev and Kovalez (1986) 10.7 percent 13 percent
B.F. Goodrich Co. (1981) 12.3 percent 12.8 percent
Bohnert and Kenady (1975)   16.4 percent
Hauck and Kucera (1983) 2.1 percent 6.5 to 8.1 percent
Hausz and Akins (1980) 5 to 7 percent 6 to 10 percent

Conditions used by investigators were not constant, and results cannot be directly compared. Radial tires, however, do seem to have the advantage in every category except purchase price. Resistance to abrasive wear seems about the same for radial and bias tires, but radial tires have much longer tread life than bias tires. Radial tires will have service life 25 to 40 percent longer than equivalent bias tires (Ozkan, 1986).

Ownership cost usually includes depreciation, interest on investment, tax, shelter and insurance. For a comparison of tires, the difference in tax, shelter and insurance cost was thought to be negligible and was omitted. The capital recovery factor was used to estimate ownership cost because it accounts for both depreciation and return on investment.

To compute annual tire ownership cost; unit tire cost; fuel cost for radial and bias tires; labor cost for radial and bias tires; and total savings per hour and hours use required to break even, use the following equations:

Equations

Equation 1

TOC = TP(IR (1 + IR)n)
(1 + IR)n - 1
TOC = Tire ownership cost ($)
TP = Tire purchase price ($)
IR = Annual interest rate (decimal)
n = Average service life (years)

Equation 2

UTC = TOC
AU
UTC = Unit cost (cost per hour)
AU = Annual tire use (hours per year)

Fuel cost for radial and bias tires may be computed using Equations 3 and 4.

Equation 3

FB = GPH x UFC

FB = Fuel cost for bias tires (cost per hour)
GPH = Fuel consumption for bias tires (gallons per hour)
UFC = Unit fuel cost (cost per hour)

Equation 4

FR = FB (1 - RF)

FR = Fuel cost for radial tires (cost per hour)
RF = Reduction in fuel consumption for radial tires (decimal)

Labor cost for radial and bias tires may be estimated as follows:

Equation 5

LR= LB__
1 + IFC
LR = Labor cost for radial tires (cost per hour)
LB = Labor cost for bias tires (the assumed hourly wage rate)
IFC = Increase in field capacity for radial tires (decimal)

Total savings per hour and hours required to break even are computed as follows:

Equation 6

NSH = (FB x FR) + (LB - LR) - (UTCR - UTCB)

NSH = Net savings per hour (cost per hour) for radials
UTCR = Unit ownership cost for radial tires (cost per hour)
UTCB = Unit ownership cost for bias tires (cost per hour)

Equation 7

BE = TPR - TPB
NSH
BE = Tire use required for radial cost to break even with bias cost (hours)
TPR = Tire purchase price for radial tires ($)
TPB = Tire purchase price for bias tires ($)

Example calculations

Assumed input data:

Tractor maximum PTO power = 150 hp
Tractor annual use = 625 hours
125 hours stationary (tires not used)
500 hours mobile (tires used)
Diesel fuel cost = $1 per gallon
Cost for 2 radial tires = $1,500
Cost for 2 bias tires = $1,000
Expected bias tire life = 2,500 hours (5 years)
Labor cost = $5.20 per hour
Increase in field capacity for radial tires = 10 percent
Expected increase in life for radial tires = 30 percent
Expected fuel saving for radial tires = 6 percent
Average interest rate = 9 percent
Fuel efficiency for tractors with bias tires = 14 hp-hours per gallon

Annual tire ownership cost (using Equation 1)

TOCB = 1000 (0.09(1 + 0.09)5 )
(1 + .09)5 - 1
= $257.09 per year
TOCR = 1500 (0.09(1 + 0.09)5)
(1 + 0.09)5 - 1)
= $385.64 per year

Unit tire cost (using Equation 2)

UTCB = $257.09 per year
500 hours per year
= $0.51 per hour
UTCR = $385.64 per year
500 hours per year
= $0.77 per hour

Fuel cost (using Equations 3 and 4)

Fuel efficiency with bias tires (given) = 14 hp-hours per gallon

Fuel consumption = ____150 hp____
14 hp-hours per gallon
= 10.71 gallons per hour

FB = 10.71 gallons per hour x $1 per gallon = $10.71 per hour

FR = 10.71 (1 - 0.06) = $10.07 per hour

Labor cost (using Equation 5)

LB = $5.20 per hour

LR = __LB__
1 + IFC
= __5.20__
1 + 0.10
= $4.72 per hour

Total savings per hour (using Equation 6)

NSH = (10.71 - 10.07) + (5.20 - 4.72) + (0.77 + 0.51) = $0.86 per hour

Use required to break even (using Equation 7)

BE = 1500 - 1000
0.86
= 581.39 hours
_____581.39 hours_____
500 hours mobile use per year
= 1.16 years

References

 

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