United States 

Department of 

Agriculture Skidder Load Capacity 

Forest Service and Fuel Consumption 

Northeastern Forest HP-4 1 C Program Experiment Station 

Research Paper NE-537 

1983 

Ross A. Phillips 
The Author 

Ross A. Phillips is research 

mechanical engineer in the Forest 

Engineering Research Project, 

Northeastern Forest Experiment 

Station, Wlorgantown, West Virginia. 

He has a B.S. degree in Agriculture 

and a Bachelor of Agricultural 

Engineering degree from Ohio 

State University, an M.S. in 

Agricultural Engineering from the 

University of Missouri, and an MSE 

specializing in transportation from 

West Virginia University. He began 

his career with the USDA Forest 

Service in 1965. 

Manuscript received for 

publication 5 July 1983 

Abstract 

This program gives the log 

weight that the skidder can move 

and gives fuel consumption either in 

liters or gallons per turn. Slope of 

the skid trail, skidder weight, and 

skid distance must be entered into 

the program. 
Loggers work for profit and 

therefore seek improved efficiency 

to increase profits. Increasing 

costs constantly face logging 

operators. Efficiently loading a 

skidder will help reduce costs per 

unit of wood skidded, and we have 

developed a program for d in this. !i' The Hewlett-Packard-41C handheld calculator program can be used 

as a tool to quickly compare loads 

and fuel used by wheeled skidders 

under different skidding conditions. 

Although a great number of 

variables influence skidding, only 

the major ones were used to 

simplify this hand-held calculator 

program and to give the operator 

items of input that can be readily 

determined. Iff and others (1982) 

incorporated 39 variables pertaining 

to skidder capacity in a Fortran 

program to obtain the skidder load. 

The program presented here will 

not compare to the highly 

sophisticated program. The tradeoffs gained are simplicity of use 

and the low-priced, highly portable 

equipment that can be used almost 

anywhere. 

The computer program 

described in this publication is 

available on request with the 

understanding that the U.S. 

Department of Agriculture cannot 

assure its accuracy, completeness, 

reliability, or suitability for any 

other purpose than that reported. 

The recipient may not assert any 

proprietary rights thereto nor 

represent it to anyone as other 

than a Governm ent-produced 

computer program. For 

information, please write: 

Engineering Research, 

Northeastern Forest Experiment 

l'~he use of trade, firm, or 

corporation names in this 

publication is for the information 

and convenience of the reader. 

Such use does not constitute an 

official endorsement or approval 

by the U.S. Department of 

Agriculture or the Forest Service 

of any product or service to the 

exclusion of others that may be 

suitable. 

Station, 180 Canfield Street, 

Morgantown, WV 26505. If you 

want to obtain the program, send 

three blank Hewlet t-Packard 

magnetic cards. Features of the 

HP-41C are presented in Appendix 

A. A program list is provided in 

Appendix B. 

Program Characteristics; 

The program gives not only a 

load that the skidder will pull but 

also the fuel per turn. Input items 

for load are skidder weight and 

slope for the loaded skidder. 

Distance of ski& is added to 

calculate fuel used per turn. When 

slope and skidder weight are 

entered into the program, the 

maximum log weight for the load 

appears. Values of traction, rolling 

resistance, and sliding friction used 

in the program limit the maximum 

weight of the load. The selected 

values for these variables should 

give an accurate answer in the 

majority of situations; however, 

extra soft soil or overly rocky 

conditions could readily change the 

practical value of the maximum 

load. 

Prompts 

After starting the program, the 

first prompt llSLOPE?" appears. 

Percentage of slope for the 

average skidding distance should be 

entered as a whole number. Use 

positive slope for uphill skidding 

and negative slope for downhill 

skidding. If vast variations in slope 

exist over the skidding distance, 

various sections of the skidding 

distance could be treated 

independently and then corn bined. 

Press the R/S (runfstart) key after 

entering slope and the next prompt 

"SKIDDER WT?" appears. 

Skidder weight may be entered 

either in kilograms or in pounds. 

Readout for log weight of the load 

will be in the same units as entered 

for skidder weight. If 10 percent 

has been entered for slope and 

12,000 kilograms or pounds entered 

for skidder weight, the calculator 

screen will show "LC WT = 5,6lOff 

in the same units used for skidder 

weight, giving the maximum log 

weight for the load after pressing 

the R/S key. Press the R/S key 

again and the prompt "LC WT?" 

appears. A value may be entered 

for load and the R/S key pressed or 

the R/S key may be pressed with 

no entry. With no entry, the 

maximum value of the load will be 

used for further calculations. By 

making no load entry and pressing 

the R/S key with the previous 

entries, the screen shows "FORCE = 10,071" in the same units used for 

skidder weight. This force value is 

the force exerted by the skidder to 

the soil to move the skidder and the 

load. 

Pressing the R/S key produces 

"SKID DIST?ll on the calculator 

screen. This asks for skidding 

distance for a section of skidroad 

with a relatively constant slope. If 

kilograms have been used for the 

skidder weight, the skid distance 

must be entered in meters. If 

pounds have been used for the 

skidder weight, the skid distance 

must be entered in feet. 

Press the R/S key and the 

screen shows VIETRIC?ll. If 

kilograms and meters have been 

used for entries, press number "1" 

to indicate yes. If pounds and feet 

have been used for entries, press 

"0" to indicate no. This permits 

either system of measure to be used 

with the same program, If a skid 

distance of 1,000 feet has been 

entered and "0" pressed for metric, 

pressing the R/S key will give an 

answer of ''GAL = 0.355,11 which 

would be the gallons of fuel used 

for one round trip. If a skid 

distance of 1,000 meters has been 

entered and "1" pressed for metric, 

the R/S key produces "LITERS = 

9.715," which would be the liters of 

fuel used for one round trip. The 

round trip and machine size are 

both much greater for kilograms 

and meters than they are for pounds 

and feet when the same numbers 

are used. 
Input 

Best values available should be 

used for input. Of course, 

compatible units must be used, 

either metric or English. The 

skidder weight given by the 

manufacturer generally would have 

greater accuracy than the other 

inputs. Because the program is for 

practical use, it does not require 

extreme precision. 

Skidding dis tance changes with 

almost every load, thus an accurate 

estimate may be sufficient. If map 

measurements or ground 

measurements are available, an 

average of the maximum and 

minimum distance will give a 

reasonable value if the difference 

of maximum and minimum is not 

too great. Where slope changes 

control a section of skidding 

distance, a ground measurement 

would be most accurate. Slope 

would best be determined with an 

Abney level, or with a more modern 

device. 

Readout 

Some answers appear before 

making all inputs. This shows in the 

load which has a readout for 

maximum load followed by a 

prompt asking for the load value. 

Because maximum load generally 

cannot be obtained with normal 

operating conditions, this sequence 

is necessary and a reasonable load 

may be inserted. Also, the 

maximum load does not change for 

downhill or nega.tive slopes. 

Fuel consumption per turn 

includes bringing in the load and 

making the return trip. Fuel 

coilsumption should be considered 

comparative since this program is 

greatly simplified from one that 

considers all of the variables. When 

a logger works in conditions similar 

to one where this program has been 

used, the accuracy of fuel output 

could be verified, or if necessary, a 

correction factor could be applied, 

This situation would permit using 

fuel output as an absolute value. 

In any event, a comparison of costs 

can be made bet ween full and 

partial loading. 

Loading a skidder probably 

affects efficiency more than any 

other variable in logging. This 

program along with production 

records will demonstrate to the 

user what additional costs occur 

from under loading skidders. 

Consistent loading to skidder 

capacity is impossible, unless logs 

are cut to the right size for 

specific conditions, Loads 

approaching the maximum can be 

achieved with experience. 

Downhill slopes permit 

enormous loads to be pulled. 

However, moving a load 

considerably larger than the 

skidder creates a hazard. The 

program contains a loop for 

negative or downhill slopes that 

gives a load that can be pulled on 

level ground. This maximum load 

for negative slopes is realistic for 

two reasons: (1) this gives a safesize load, and (2) most downhill 

skidding has at least one level 

location on the skid trail or landing 

and the load could not be 

efficiently changed for this 

location. 

The criteria set up to 

determine the maximum load were: 

(1) sliding friction of 85 percent, 

(2) rolling resistance of 20 percent, 

and (3) tractive coefficient of 48 

percent. Sliding friction of 85 

percent is near that measured by 

Falk and Peters (1979) for west 

coast cable thinning. They reported 

86 percent for an average. The 

value used also is in the lower range 

of tests run by Hassan and 

Gustafson (1981) in North Carolina. 

The rolling resistance of 20 

~ercent is suggested by Phillips 

(1981) for skidders on skidroads. 

Most of the skidding would be done 

along a skid trail, The tractive 

force of the skidder can range to 

near 85 percent (Iff and others 

1982), but slippage would be 

excessive for any practical use. 

Nebraska tractor tests (Nebraska 

Board of Tractor Test Engineers 

1980) for farm tractors stop when 

slippage reaches 20 percent. Burt 

and others (1982) indicate a range 

of 45 to 50 percent traction on a 

relatively firm skid road with 20 

percent slip. Thus, 48 percent was 

selected for the program. 

Conditions vary, and loads may 

be greater than those indicated as 

maximum. The calculated values 

should be realistic for most 

skidding conditions. Where the soil 

is very firm and traction excellent, 

tipping of the skidder would be the 

limiting factor on loading. Loads 

calculated by the program will not 

tip a skidder in any reasonable 

situation. 

For rnulas 

The program was written from 

derived formulas using slope, 

rolling resistance, sliding friction, 

and internal friction of the skidder. 

The assumptions were made that 

one-half of the log weight was 

supported on the skidder, and that 

the skidder and load were moving 

on a uniform slope (Fig. 1). 

Internal friction of the skidder is 

used for fuel consumption. 

Twenty-five percent of the skidder 

weight was used for internal 

friction. This percentage agrees 

with a low range of both Matthes 
Figure 1.-Foree diagram for loaded skidder. 

and others (1982) for skidders and 

Chew (1980) for futuristic trucks. 

Values 

Slope = 0 

Rolling resistance = 0.20 

Internal friction = 0.25 

Tractive coefficient = 0.48 

Sliding friction = 0.85 

Skidder weight = SW 

One-half log weight = L 

Gravity component normal 

to slope = cos 0 

Gravity component 

along slope = sin 0 

Tractive force = resistive force 

1. 0-48 SW cos 0 + 0.48 L cos 0 

= 0.2 SW cos 0 + 0.2 L cos 0 

+SW sin 0 + 2 L sin 0 + 0.85 

L cos 0 

transpose 

2. 0.48 L cos 0 - 0.2 L cos 0 

- 2 L sin 0 - 0.85 L cos 0 

= 0.2 SW cos 0 + SW sin 0 

- 0.48 SW eos 8 

combine and change signs 

3. 2 L sin 0 + 0.57 L cos 0 

= 0.28 SW cos 0 - SW sin 0 

solve for 2 L = log weight 

4. 2L = SW 0.28 cos 0 - sin 0 

0.285 cos 0 + sin 0 

Return to the basic concept of 

the skidder pulling a log on a 

uniform slope. The force exerted 

along the slope is F. 

5. F = 0.85 L cos 0 + L sin 0 

+ 0.2 Leos 0 + Lsin 0 

+ 0.2 SW cos 0 + SW sin 0 

com bine 

1 05 6. F = -+-(2L) cos 0 

+ 0.2 SW cos 0 + (SW + 2L) sin 0 

Internal friction of 0.25 SW is 

added to the force for determining 

energy: 

1.05 '7. F = - (2L) cos 0 + 0.2 SW 2 

cos 0 + (SW + 2L) sin 8 

+ 0.25 SW 

Energy consumed bringing the 

load in is force times distance, and 

for the return trip, 0 is changed to 

-0 and a load of 0 is used. Energy 

per turn is E. 

8. E = F X D + (0.2 SW eos (-0) 

+ SW sin (-0) + 0.25 SW)D 

E may be either in kilogrammeters or in foot-pounds. The 

conversion factor from kilogrammeters to kilowatt-hours is 

1,980,000. The conversion factor 

from foot-pounds to horsepowerhours is 367,100. 

Use 4 kilowatt-hours per liter 

of fuel oil (Chew 1980, Nebraska 

Board of Tractor Test Engineers 

1980) for a round number near 

average to convert energy to 

quantity of fuel. This value 

converts to 20.3 horsepower-hours 

per gallon of fuel oil. 

9. Fuel = E/1,980,000/4 liters 

turn. 

10. Fuel = E/367,100/20.3 gallons 

per turn. 
These for in ulas incorporated 

into the flow chart (Fig. 2) result 

in the program listed in Appendix 

B* 

0 START * INPUT SLOPE 

t 

INPUT SKIDDER WEIGHT 

e INPUT LOG WEIGHT 

-3 OUTPUT FORCE %L 

( INPUT SKIDDING DIST. I 

t 

OUTPUT GAL, /TURN 

1 c OUTPUT LITERS / TURN 

Figure 2.-Flow chart. 
Literature Cited 

Burt, Eddie 6,; Koger, J. L,; Taylor, J. Matthes, Kenneth R.; Watson, William 

H,; Bailey, A. C. Performme of F. ; Bailey, A. C. Performance of 

log skidder tires. Paper presented 1% &cider tires. Paper presented 

at Winter Meeting of American at the Winter Meeting of American 

Society of Agricultural Engineers. Society of Agricultural Engineers. 

Pap. No. 82-1596. MI: St. Joseph; Pap. No. 82-1596. MI: St. Joseph; 

American Society of Agricultural American Society of Agricultural 

Engineers; 1982. 11 p. Engineers; 1982. 11 p, 

Chew, Norman B. 'h~ekdes@l: A 

look to the future. Automot. Eng. 

88(11): 39-41; 1980. 

Falk, Gary D.; Peters, Penn A. A 

preliminary analysis of the lateral 

yarding forces in a cable thinning. 

Proceedings, IUFRO Mountain 

Logging Symposium; 1979 

December. Seattle, WA: Univ. of 

Wash.; 1979; 75-79. 

Hassan, Awatif E.; Gustafson, M. Lee. 

Factors affecting skidding forces. 

Paper presented at the Winter 

Meeting of American Society of 

Agaicultural Engineers. Pap. No. 

81-1586. MI: St. Joseph; 

American Society of Agricultural 

Engineers; 1981. 26 p. 

Iff, Ronald H.; Koger, Jerry L.; Burt, 

Eddie C.; Culver, E. Wade. 

C-A-R-TS= Capacity analysis of 

rubber-tired skidders. Paper 

presented at the Winter Meeting of 

American Society of Agricultural 

Engineers. Pap. No. 82-1594. MI: 

American Society of Agricultural 

Engineers; 1982. 19 p. 

Nebraska Board of Tractor Test 

Engineers. Nebraska Tractor Test 

Data 1980. Agric. Eng. Yearb. 

1980: 536-545; 1980. 

Phillips, Ross A. Theoretical energy 

consumption of timber harvesting 

equipment. Paper presented at 

Winter Meeting of American 

Society of Agricultural Engineers. 

Pap. No, 81-1600. MI: St. Joseph; 

American Society of Agricultural 

Engineers; 1981. 23 p. 
Appendix A 

Calculator Description 

The hand-held Hewlett-Packard 

41C contains programmable 

capabilities and a reasonably 

extensive storage. The calculator 

was designed for easy entrance of 

program storage. The program 

may be entered by direct 

programming, which is relatively 

simple, or through magnetic cards. 

Magnetic cards offer quick 

recovery of the program after 

some interruption such as using 

another program. The calculatorts 

continuous memory holds the 

program as long as the storage 

space is not used for something 

else and even when the calculator 

is turned off. The reader will also 

transfer a program to magnetic 

cards. 

Visual prompting for data shows 

on the visual display of the 

calculator so the program can 

readily be used at logging sites or 

any other place where a quick 

answer would be desirable. 

The calculator signals for low 

batteries when BAT appears in the 

lower left side of the viewscreen. 

This signal indicates the need for a 

battery change. Old batteries may 

cause erratic functions of a 

program, even before the 

calculator indicates a need for 

battery change. 
Appendix 

Skidder Load Capacity and 

Fuel Consumption HP-41C 

Program List 

01 LBL "FUEL" 

02 FIX 0 

03 ttSLOPE?tf 

04 PROMPT 

05 100 

06 / 

07 ATAN 

08 ST0 86 

09 COS 

10 ST0 82 

11 RCL 86 

12 SIN 

13 ST0 81 

14 "SKIDDER WT?" 

15 PROMPT 

16 ST0 03 

17 RCL 02 

18 * 

19 .28 

20 * 

21 RCL 03 

22 RCL (ill 

23 * 

24 - 25 RCL 01 

26 RCL 82 

27 .285 

28 * 

29 + 

30 1 

31 ST0 94 

32 RCL 83 

33 .9825 

34 * 

35 RCL 04 

36 XOY 

37 x< =Y? 

38 GTO 89 

39 GTO 10 

40 LBL 10 

41 RCL 84 

42 "LG WT =It 

43 ARCL X 

44 PROMPT 

45 ST0 85 

46 "LG WT?" 

47 PROMPT 

48 ST0 05 

49 GTO 88 

50 LBL 09 

51 "LG WT =" 

52 ARCL X 

53 PROMPT 

54 ST0 05 

55 "LG WT?" 

56 PROMPT 

57 ST0 05 

58 LBL 08 

59 1.05 

60 2 

61 1 

62 * 

63 RCL 82 

64 * 

65 RCL 03 

66 .2 

67 * 

68 RCL 82 

69 * 

70 + 

71 RCL 03 

72 RCL 85 

73 + 

74 RCL 81 

75 * 

76 + 

77 RCL 83 

78 '25 

79 * 

80 + 

81 ITFORCE =" 

82 ARCL X 

83 PROMPT 

84 ST0 85 

85 "SKID DIST.?" 

86 PROMPT 

87 ST0 07 

88 * 

89 ST0 09 

90 RCL 86 

91 CHS 

92 ST0 06 

93 COS 

94 RCL 03 

95 * 

96 '2 

97 * 

98 ST0 88 

99 RCL 86 

100 SIN 

101 RCL td3 

102 * 

103 RCL (if3 

104 -25 

105 * 

106 + 

107 RCL 88 

108 + 

109 RCL a7 

110 * 

111 RCL @9 

112 1- 

113 STO @l 

114 FIX 3 

115 "METRIC?'l 

116 PROMPT 

117 X=@? 

118 GTO B6 

119 GTO @l 

120 LBL 96 

121 RCE 81 

122 198W86 

123 1 

124 2g.3 

125 / 

126 'GAL ='* 

127 ARCL X 

128 PROMPT 

129 GTO 65 

130 LBL 87 

131 RCL 81 

132 367186 

133 1 

134 4 

135 / 

136 "LITERS =" 

137 ARCL X. 

138 PROMPT 

139 GTO 85 

140 LBL @5 

141 "ENDff 

142 PROklPT 

143 "RTN" 

144 STOP 

145 END 
* U.S. GOVERNMENT PRINTING OFFICE: 1983-705-029: 27 

8 

Phillips, Ross A. Skid& laad capacity and fuel eonsumption HP41C 

program. Res. Pap. NE-537. Broomall, PA: U.S. Department of 

Agriculture, Forest Service, Northeastern Forest Experiment Station; 

1983. 7 p. 

Program gives log weight that the skidder can move and gives fuel 

consumption either in liters or gallons per turn. Slope of the skid 

trail, skidder weight, and skid distance must be entered into the 

program. 

ODC 389 

Keywords: Skidder, load capacity, logging, computer programs, cost 

estimating, fuel estimating, skidding forces and distances. 
Headquaders of the Northeastern Forest Experiment Station are in 

Broomaill, Pa. Field laboratories are maintained at: 

husetts, in cooperation with the Universiry of 

Mwachuetts. 

@ Berea, Kerrtucky, in cooperation with Berea College. 

@ Burlineon, Vemont, in cooperation with the University of 

Vemont, 

@ klawaw, OM@, 

@ Durhm, New Hmphire, in cooperation with the University of 

New Hmw%m. 

@ Hmden, Conn~ticart, h ccooperation with Yale University. 

@ Mosgmtown, West Virghia, in cooperation with West Virginia 

Univemity , MorganCown. 

@ Orono, Mahe, in cooperation with the University of Maine, 

Brono. 

@ Pawns, West Virginia. 

@ Brineetoan, West Virginia. 

@ Syracw, New York, in cooperation with the State University of 

New York College of Environmental Sciences and Forestry at 

Sy lsaeuse University, Syracuse. 

@ University Park, Pennsylvania, in cooperation with the 

Pennsylvania State University. 

@ Warren, Pennsylvania.