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Southern Forest

Forest Service Wearch nc_,___ ---- ---- Note

Experiment Statlon

so-31 1

Februarv 1985

Load Deflection Relationships for Three Log-Skidder Tires

Jerry L. Koger, Eddie C. Burt and A. C. Bailey

SUMMARY

Load deflection curves and regression equations

are presented for three skidder tires at two inflation

pressures. The tires tested were a IO-ply 18.4-34, a

12-ply 24.532, and a 12-ply 3OSL-32.

INTRODUCTION

Recent cooperative studies (Burt et al. 1982, Iff et

al. 1982) by the U.S. Forest Service and the National

Tillage Machinery Laboratory (NTML) demonstrated

the need to develop load-deflection relationships for

tires used on articulated, four-wheel drive, rubbertired skidders. Load deflection curves were needed

to determine such factors as rolling radius, tractive

performance, and skidder stability. These curves can

also be used to help loggers determine the right

combination of tire size and inflation pressure for

different load conditions. Earlier studies by Biller

(1972) have shown that matching the proper tire size

for specific load conditions would reduce many of

the problems that loggers have with skidder tires.

The purpose of this paper is to summarize the load

of deflection results developed from these earlier

studies at the NTML.

MATERIALS AND METHODS

The single-wheel tire tester and other facilities of

the NTML were used to obtain the load deflection

relationships. The testing capabilities of the singlewheel tire tester have been described in detail by

Burt et al. (1980).

Three skidder tires (fig. 1) were selected to represent a range of sizes that are currently being

used on articulated, four-wheel drive, rubber-tired

skidders operating in southern forests. The three

skidder tires used in this study, supplied by Firestone Tire and Rubber Companyl, Akron, Ohio, were

a lo-ply 18.4-34, a 12-ply 24.5-32, and a 12-ply

30.5L-32.

The load deflection tests were run with the tires

at a stationary (non-rotating) condition. During each

test, the dynamic load was increased uniformly and

continuously from 0 to approximately 10,000 pounds

and then decreased uniformly and continuously

back down to 0 pounds. By obtaining tire deflection

measurements as a function of increasing and decreasing dynamic loads, an indication of tire hysteresis was obtained. Each tire was tested on a rigid

surface at inflation pressures of 15 and 25 psi.

ANALYSIS AND RESULTS

Tire deflection measurements were obtained at

approximately 30-pound intervals. For each test,

there were about 300 data points for tire deflection

as a function of increasing dynamic load, and about

300 data points for tire deflection as a function of

decreasing dynamic load. Since a graphical analysis of the data indicated only a small hysteresis,

and skidder tires under forest conditions are sub-

‘The use of trade, firm, or corporation names is for the

information and convenience of the reader. Such use does

not constitute an official evaluation, conclusion, recommendation, endorsement, or approval of any product or Service to

the exclusion of others that may be suitable.

Southern Forest Experiment StatIonIT-102lO U.S. Postal Services Bldg., 701 Loyola Avenue, New Orleans, La. 70113

Forest Service, U.S. Department of Agriculture.

Serving Alabama, Arkansas, Louisiana, Mlsslsslppl, E. Oklahoma, Tennessee, E. Texas, Puerto Rico, U.S. Virgin Islands

1
Figure l.-Skidder tires used in load deflection tests (18.4-34 on left, 30.5L-32 in center, 24.5-

32 on right).

jetted to sudden dynamic load changes, the tire

deflection data (increasing and decreasing dynamic

load) were combined for the regression analysis.

The general form of each regression equation

was: y = a + bx + cx2. In the model, y was tire

deflection, a was the vertical axis intercept, x was

dynamic load, and b and c were the regression

coefficients to be determined. Because the definition

of tire deflection is zero at zero dynamic load, the

constant a (vertical axis intercept) in the above

model was always zero. The developed regression

equations are shown in table 1, and their graphical

forms are shown in figures 2-4.

DISCUSSION

For the same dynamic load and inflation pressure,

a reduction in tire size was associated with an increase in tire deflection. As dynamic load increased,

the lower inflation pressure (15 psi) was associated

with a progressive increase in deflection for the

smallest (18.4-34) and largest (305L-32) tires. The

middle-sized tire (24.5-32) behaved differently from

the other two. It had a higher tire deflection at the

higher inflation pressure (25 psi) than at the lower

inflation pressure (15 psi) for dynamic loads up to

about 6,500 pounds. A possible explanation is the

difference in sidewall position at the various inflation

pressures. At lower inflation, the sidewalls could

have been supporting a significant amount of the

load in a manner analogous to a column. When the

tire was inflated to the higher pressure, the sidewall

could have been curved due to the increased pressure, thereby reducing the load-carrying capacity of

the sidewall. For loads up to 8,000 pounds at the

higher inflation pressure (25 psi), average tire deflection per thousand pounds was about 0.47 inches

for the 18.4-34, 0.38 inches for the 24.5-32, and 0.34

inches for the 30.5L-32.

LITERATURE CITED

Biller, Cleveland J. Are you overloading your skidder tires? Res. Paper NE-247 Upper Darby, PA:

U.S. Dept. of Agric. Forest Service, Northeastern

Forest Experiment Station; 1972. 7p.

Koger is a Research Engineer with the USDA Forest Service in Auburn, Alabama. Burt and Bailey are Agricultural Engineers at the

National Tillage Machinery Laboratory, USDA, SEA, ARS, Auburn, Alabama.

2
Burt, E. C.; Koger, J. L.; Taylor, J. H. and Bailey,

A.C. Performance of log-skidder tires. Winter

Meeting American Society of Agricultural Engineers; 1982 December 13-16; Chicago, IL. ASAE

Paper No. 82-1596. St. Joseph, MI, ASAE. 1982.

9P.

Burt, E. C.; Reaves, Carl A.; Bailey, A. C. and Pickering, W. D. A machine for testing tractor tires in

soil bins. Transactions of the ASAE. 23(3):546-

547, 552.

Iff, Ronald H.; Koger, Jerry L.; Burt, E. C., and Culver, Wade E. C-A-R-T-S: capacity analysis of

rubber-tired skidders. Winter Meeting American

Society of Agricultural Engineers; 1982 December

13-16; Chicago, IL. ASAE Paper No. 82-1594. St.

Joseph, MI, ASAE; 1982.19p.

Table 1 .-Load-deflection regression equations for the three skidder tires

Tire Inflation

size pressure Regression equation* w C.V.

18.4-34 15 psi Y=O.O00743(x)-0.0000000193(~)2 0.94 5.20

18.4-34 25 psi Y=0.000689(+0.0000000268(~)2 0.87 6.23

24.5-32 15 psi Y=0.000518(+0.0000000146(x)2

24.5-32 25 psi Y=0.000589(+0.0000000266(x)2

30.5L-32 15 psi ~=0.00051 a(+o.oooooooi 38(x)2

30.5L-32 2 5 psi Y=O.O00426()0-0.000000011 2(x)2

*Y = tire deflection, in inches.

x = dynamic load on a single tire, in pounds.

0.91 6.82

0.91 6.35

0.95 7.29

0.95 6.32

0 2000 4000 6000 8000 10000

LOAD (POUNDS)

Figure P.-Load deflection curves for the 18.4-34 skidder tire Figure 3.-Load-deflection curves for the 24.5-32 skidder tire

(dashed line segments indicate test loads beyond (dashed line segments indicate test loads beyond

manufacturer recommendations). manufacturer recommendations).

z 2.80

%!

2

= 2.10

5

0

0 2000 4000 6000 8000 I0000

LOAD (POUNDS)

3
25PSI

” I , /

0 2000 4000 6000 6000 lOdO

LOAD (POUNDS)

Figure 4.-Load-deflection curves for the 30.5L-32 skidder tire

(dashed line segments indicate test loads beyond

manufacturer recommendations).

4