Farming Ahead October 2012 No. 249 www.kondiningroup.com.au50

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| Workshop Tyre TracTors

M aking head or tail of tyre sizes can be frustrating, dare we suggest deflating, at the best

of times.While it is pretty easy to identify the

rim size, there seems to be myriad other cryptic codes on modern tractor tyres which all have meanings, but are not understood by most of us.

When shelling out the big bucks for a new set of triples on your articulated tractor, it pays to know exactly what all that sidewall terminology means on your 12 new tyres.

It also helps to have a bit more of a clue when fitting alternate tyre sizes to tractors and implements.

Tyre-size designations were once a simple matter, with the rim size and the width designated in inches.

But tyre sizes are now largely specified in metric, which confusingly also includes the rim size measured in inches.

But as you’ll read below, this is all fairly straightforward — once you are armed with the right information.

ContaCt ▶ Josh Giumelli name.name@company.com.au

With a mixture of imperial and metric measurements, ply ratings and service descriptions, it is little wonder tractor tyre sizes can be a little confusing. This month, Kondinin Group engineer Josh Giumelli attempts to make sense of tractor tyre terminology

Tricky tractor tyre terminology

www.kondiningroup.com.au No. 249 October 2012 Farming Ahead 51

Imperial sizesTraditionally, tyre sizes were specified in two imperial dimensions, and these tyres are still available today in these size designations.The sizes are relatively easy to interpret.The 13.6–28 pictured suits a 28 inch rim, and has a section width of 13.6 inches. The dash between the two numbers indicates a cross-ply or diagonal-ply tyre.An R designates a radial tyre (eg 16.9R34). An L between the section width and the rim size would indicate a tyre with a reduced aspect ratio.

Metric sizesNow metric sizing is becoming more common, such as the 900/60R32 shown above. Metric tyre sizes allow manufacturers to better control the diameter-to-width ratio and rolling circumference, which is crucial when matching different size front and rear tyres for front-wheel assist tractors. As with the imperial size tyres, the above tyre fits a 32 inch rim. The nominal section width is 900mm, and 60 is the aspect ratio. The aspect ratio is a percentage, and gives the tyre section height based on the section width (eg 0.60 x 900 = 540mm for this tyre).

service descriptionThe service description is an important rating, as it specifies the maximum load and speed at which the tyre can operate under normal conditions. In this case, 168 refers to the maximum load, or load index (5600kg, see Table 1). This means each axle can carry 5600kg for single tyres. For a dual-wheel configuration, the load index is reduced by 12 per cent, so it would be 5600 x 2 x (1- 0.12) = 9856kg.A8 is the speed rating for the tyre, which corresponds to 40km/h (see Table 2).

tread designationThe tyre tread designation describes the type of tread and intended usage for the tyre, and is typically R1, R2 or R1-W. R1 is a standard lug tyre used for general purpose, dryland farming, and has the shallowest lugs, and the closest spacing between lugs. R2 tread is used in wet, flooded conditions and has approximately twice the lug height of R1 tyres. They also have the widest spacings between lugs to help shed mud. These tyres tend to vibrate the most during road transport, and will have less tractive efficiency on drier surfaces than R1 or R1-W tyres.R1-W is a sort of cross between R1 and R2, having deeper lugs than R1 but wider and shorter than R2. They are most suited to traction in wet and sticky conditions.

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Farming Ahead October 2012 No. 249 www.kondiningroup.com.au52

Table 1 Tractor tyre load ratingsload rating kg load rating kg

135 2180 163 4875

136 2240 164 5000

137 2300 165 5150

138 2360 166 5300

139 2430 167 5450

140 2500 168 5600

141 2575 169 5800

142 2650 170 6000

143 2725 171 6150

144 2800 172 6300

145 2900 173 6500

146 3000 174 6700

147 3075 175 6900

148 3150 176 7100

149 3250 177 7300

150 3350 178 7500

151 3450 179 7750

152 3550 180 8000

153 3650 181 8250

154 3750 182 8500

155 3875 183 8750

156 4000 184 9000

157 4125 185 9250

158 4250 186 9500

159 4375 187 9750

160 4500 188 10000

161 4625 189 10300

162 4750 190 10600Source: Kondinin Group

ply ratingsManufacturers of diagonal (cross) ply tyres use a ply-rating system to indicate tyre strength, often in conjunction with the service description. The ply rating is not the number of layers of fabric in the tyre carcase, but rather a general indication of strength. Metric radial tyres generally use only the service description to indicate strength, whereas imperial-sized tyres can use either ply rating, service description or a star system. The star rating system consists of one to three stars, with three being the strongest.

Converting between metric and imperial sizesIt is fairly easy to convert sizes between metric and imperial designations, but be careful and avoid ‘mixing and matching’ metric and imperials on the same machine. This is especially important for FWA tractors which must have accurately matched front and rear tyres. A mismatch would result in a change to the lead (amount the front wheels turn in relation to the rear wheels), and possible severe transmission damage. Always use manufacturer-recommended tyres in such cases.The tyre pictured shows both rating systems, with 30.5 the approximate imperial size corresponding to 800mm. Table 3 lists metric sizes corresponding to imperial tyre sizes.

| Workshop Tyre TracTors

www.kondiningroup.com.au No. 249 October 2012 Farming Ahead 53

Tyre TracTors Workshop |

Table 3 Imperial and metric tyre equivalentsStandard size Standard size Wide size

Inch metric metric

7.50–16 260/70R16

280/70R16

7.50–18 260/70R18

280/70R18

10.5–18 275/80R18 280/70R18

7.50–20 260/70R20

280/70R20

9.5–20 275/80R20 300/70R20

10.5–20 275/80R20 320/70R20

11.2R20 280/85R20 320/70R20

12.4R20 320/85R20 360/70R20

12.5R20 340/75R20 375/70R20

335/80R20

340/80R20

13.6R20 360/70R20

14.5–20 375/75R20 405/70–20

380/75R20

9.5R24 250/85R24 300/70R24

11.2R24 280/85R24 320/70R24

12.4R24 320/85R24 360/70R24

13.6R24 340/85R24 380/70R24

14.9R24 380/85R24 420/70R24

16.9R24 420/85R24 480/70R24

16.9R26 480/70R26

23.1R26 620/75R26

28.1R26 750/65R26

9.5R28 25085R28

11.2R28 280/85R28 320/70R28

12.4R28 320/85R28 360/70R28

13.6R28 340/85R28 380/70R28

14.9R28 380/85R28 420/70R28

16.9R28 420/85R28 480/70R28

14.9R30 380/85R30 420/70R30

16.9R30 420/85R30 480/70R30

18.4R30 460/85R30 520/70R30

8.3R32 210/95R32

9.5R32 230/95R32

11.2R32 270/95R32

12.4R32 320/85R32

30.5LR32 800/65R32

12.4R34 320/85R34

Standard size Standard size Wide size

Inch metric metric

16.9R34 420/85R34 480/70R34

500/70R34

18.4R34 460/85R34 520/70R34

20.8R34

8.3R36 210/95R36

9.5R36 230/95R36

11.2R36 270/95R36

12.4R36 320/85R36

13.6R36 340/85R36

11.2R38 270/95R38

12.4R38

13.6R38 340/85R38

14.9R38 380/85R38

16.9R38 420/85R38 480/70R38

18.4R38 460/85R38 520/70R38

20.8R38 520/85R38 580/70R38

620/70R38

18.4R42 480/80R42

460/85R42

20.8R42 520/85R42 580/70R42

620/70R42

14.9R46 380/85R46

420/80R46

380/90R46

18.4R46 460/85R46

480/80R36

20.8R46 520/85R46 620/70R46

18.4R50 480/80R50Source: Kondinin Group

Table 2 Speed ratings for tractor tyresRating a1 a2 a3 a4 a5 a6 a7 a8 b D F J

speed (km/h) 5 10 15 20 25 30 35 40 50 65 80 100

Source: Kondinin Group