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Are Radial Tractor Tires Cost Effective?

James C. Frisby
Department of Agricultural Engineering, University of Missouri-Columbia
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.
AuthorIncrease 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:
AuthorWheel slipSoilAdvantage of radial
Forrest (1962)less than 15 percentSand
Loam
Clay
8 percent
23 percent
21 percent
Thaden (1962)16 percent29 percent
Mumgaard and Rudakov (1975)15 percentFirm15.5 percent
Bohnert and Kenady (1975)Tilled
Sod
14.6 percent
18.8 percent
Gee-Clough (1977)20 percent5-8 percent

Field capacity and fuel consumption were studied:
AuthorIncrease in field capacity for radialsReduction in fuel consumption for radials
Seleznev and Kovalez (1986)10.7 percent13 percent
B.F. Goodrich Co. (1981)12.3 percent12.8 percent
Bohnert and Kenady (1975)16.4 percent
Hauck and Kucera (1983)2.1 percent6.5 to 8.1 percent
Hausz and Akins (1980)5 to 7 percent6 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:

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 ($/hour)
AU = Annual tire use (hours/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 ($/hour)
GPH = Fuel consumption for bias tires (gallons/hour)
UFC = Unit fuel cost ($/hour)

Equation 4.

FR = FB (1 - RF)
FR = Fuel cost for radial tires ($/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 ($/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 ($/hour) for radials
UTCR = Unit ownership cost for radial tires ($/hour)
UTCB = Unit ownership cost for bias tires ($/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/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/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/gallon

Annual tire ownership cost (using Equation 1):

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

Unit tire cost (using Equation 2):

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

Fuel cost (using Equations 3 and 4):

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

Fuel consumption =____150 hp____
14 hp-hours/gallon
= 10.71 gallons/hour
FB = 10.71 gallons/hour x $1/gallon = $10.71/hour
FR = 10.71 (1 - 0.06) = $10.07/hour

Labor cost (using Equation 5):

LB = $5.20/hour

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

Total savings per hour (using Equation 6):

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

Use required to break even (using Equation 7):

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

References

B.F. Goodrich. 1981. Power saver radials — save eight ways and then some! B.F. Goodrich Tire Division, 500 S. Main St., Akron, Ohio.

Bohnert, L.F. and T.D. Kenady. 1975. A comparative analysis of radial and bias R-1 drive tractor tires. SAE Paper No. 751185.

Forrest, P.J., I.F. Reed and G.V. Constantakis. 1962. Tractive characteristics of radial-ply tires. Transactions of the ASAE 5(2):108-115.

Gee-Clough, D.M. McAllister and D.W. Evernden. 1977. "Tractive performance of tractor drive tires: A comparison of radial and cross-ply carcass construction." Journal of Agricultural Engineering Research 22:385-395.

Hauck, D.D. and H.L. Kucera. 1983. Radial tractor tires. Cooperative Extension Service, North Dakota State University, Fargo, North Dakota.

Hausz, F.C. and H. Akins. 1980. Optimizing tire/vehicle relationships for best field performance. SAE Paper No. 801021.

Hoffman, R. 1983. "Return of the traction tests." Farm Journal 107(11):18-21.

Mumgaard, M. and N. Rudakov. 1975. Radial versus bias-ply tractor tire performance. University of Nebraska Tractor Testing, Lincoln, Nebraska.

Ozkan, H.E. and A. Yahya. 1986. Radial tires — are they economical? Paper No. 86-1025. American Society of Agricultural Engineers. St. Joseph, Michigan.

Seleznev, I.I. and N.M. Kovalez. 1968. "Assessment of the operating characteristics of radial-ply tractor tires." Soviet Rubber Technology 28(3):1969-1970.

Thaden, T.J. 1962. "Operating characteristics of radial-ply tractor tires." Transactions of the ASAE 5(2):109-110.

Copyright 1999 University of Missouri
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