INTRODUCTION

The main reasons for fitting traction aids to forest machinesare to increase the grip between machine wheels and theground surface and to spread load weight. The area of thewheel in contact with the ground is called the ‘footprint’area. If traction aids have a greater area in contact with theground than wheels alone the footprint size increases andground pressure exerted by the machine is reduced. Tractionaids can also increase machine stability, improve flotationand protect tyres (Figure 1). Inappropriate selection oftraction aids can have a negative impact on forest soil, water,the standing crop, and on machine and operator ergonomics.Appropriate traction aid selection for use with a machine ina given situation is therefore very important.

TYPES OF TRACTION AIDS

Tyres

Traction is created where a machine makes contact withthe ground; therefore the first consideration forincreasing machine traction is the tyre specification.Tyres are available in a diverse range of specificationsfrom a number of manufacturers. Externally, differenttread patterns impart different flotation, grip, loaddistribution and self-cleaning properties to the wheel;changing tyre specification can result in gains in traction.If further traction is required then ancillary traction aidsshould be considered.

Wheel chains

Wheel chains are a robust, easy to transport, quick to fitmeans of gaining additional traction (Table 1, Figure 2).Correct fit of chain to tyre is important to obtainmaximum benefit and avoid damage to the tyre throughslippage.

The incorporation of rings into tyre chains providesincreased grip although this may cause greater grounddisturbance and operator discomfort due to a rougherride. Special care is needed when fitting wheel chains tomachines with bogie wheels and those with articulatedsteering, as the clearance between the adjacent wheelsmay preclude the use of some chain types. Chains donot have the potential to significantly influence theflotation of the machine because, unlike bandtracks,there is no appreciable increase in the machinefootprint area.

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Traction Aids in Forestry

231 Corstorphine RoadEdinburghEH12 7AT

www.forestry.gov.uk

FCTN013

T E C H N I C A L N O T E

SUMMARY

This Technical Note discusses the benefits of traction aids in forestry. Generic types of traction aids and their effects onmachine travel are described. Guidance is provided on when and where traction aids should be used along with theircharacteristics under different operating conditions and their selection according to site. Advantages and disadvantages ofusing traction aids are listed and alternatives to achieving similar benefits through operational best practice that allowsefficient timber harvesting while safeguarding the condition of the site are also described.

A U G U S T 2 0 0 6B Y D U N C A N I R E L A N D O F F O R E S T R E S E A R C H

Figure 1

Forwarder converted to carry out ground preparation fitted withbandtracks on the front and rear bogies.

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Figure 2

Common features of wheel chains. Note that slack should beavoided; correctly tensioned chains will jingle slightly whendriving.

Table 1 Wheel chain characteristics.

Wheel chaindescription

Assembly of specialised chain links and/orrings fitted over the bare tyre to provideadditional traction and tyre protection.Relatively lightweight option forincreasing traction.

Effect on traction Friction between the wheel and groundsurface is increased and chains withwelded studs on the linkages aid gripdespite mud build-up. Chains create lessrolling resistance than tracks.

Effect on flotation Chains do not significantly aid flotationalthough preventing wheel slip canprolong the use of access routes.

Effect on grounddisturbance

Gaining extra traction through the use ofchains but the footprint area remainingunaltered means that a scouring effect canoccur on routes where wheel slip occurs.

Monotracks

A monotrack is similar to a bandtrack, but rather thanfitting to a pair of bogie wheels a monotrack attachesaround a single wheel. There is little experience andinformation on the use of monotracks (also called ‘wheeltracks’) as a traction aid in UK conditions and little isknown about their effects on traction, flotation andground disturbance (Table 2).

Bandtracks

Bandtracks increase machine traction and flotation. Avariety of specifications provide the machine with

Table 2 Monotrack characteristics.

Monotrackdescription

Traction aid made up of metal platesconnected by heavy duty linkages that fitover a single wheel.

Effect on traction Machine traction is increased. Seebandtrack characteristics (Table 3).

Effect on flotation Increased, although footprint area is not assubstantially increased as when usingbandtracks.

Effect on grounddisturbance

Little experience of use in UK conditionstherefore their effects on grounddisturbance are largely unknown.

Studs welded tothe chain linksprovideadditional grip.Links may besingle or double studded

D–shacklesecures chainaround thewheel

Chains areavailable invarious gaugeswhich influencesease of fittingand durability

Chaincomposed ofuniform links.Chains may alsoincorporatelarger ‘rings’ toincrease grip

different characteristics (Table 3, Figures 3, 4 and 5).Bandtracks add significantly to the overall weight of themachine as each track may weigh a tonne or more butbecause the machine’s footprint area is increased (Figure3) ground pressure can be reduced.

The footprint area of a forest machine is influenced bythe number of wheels, tyre pressure, tyre dimensions,tyre tread pattern and the use of traction aids thatincrease the area through which the weight of themachine is transferred to the ground.

A wheel is designed with a round profile to rollsmoothly, allowing efficient machine movement.Attaching bandtracks reduces the ability of machinewheels to roll freely, an effect known as rollingresistance. In recent years developments in track platedesign have attempted to overcome rolling resistance,while still maximising the benefits of tracks for improvedtraction and flotation.

If fitted correctly the track will be in intimate contactwith the wheel with no slippage between the tyre and thetrack. If slippage does occur it can result in tyre wear.Correctly fitted tracks will prolong the life of the tyre,protecting it from damage during use. Two principaltypes of bandtrack design are commonly available in theUK: those with a side link and those that feature sidepaws (see Figures 4 and 5). A number of bandtrackmanufacturers produce their own version of these tracktypes in a range of models to suit different machines,tyre specifications and operating conditions.

Side links and side paws

Bandtracks have traditionally been designed to curve roundthe wheel profile with the ends of each track plate partiallywrapping ‘around’ the tyre. The curved plates areconnected with forged links at the end of each plate, as

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Table 3 Bandtrack characteristics.

Bandtrackdescription

Metal plates connected by heavy-duty linkages that fit over a pair of bogie wheels. Varying designs and plate profilesgive different flotation, traction and steep ground working capabilities to the machine.

Effect on traction Machine traction is increased. Narrow track plates convey the greatest traction benefits, but offer minimal flotation.Lugs welded onto the outside of the track provide increased grip and prevent side slippage but prolonged travel alongforest roads has potential to erode the surface. Track plates with an ‘L’ or ‘C’ shaped cross-section help the machine togain extra purchase, although their aggressive scouring effect is only appropriate on stable and firm ground.

Effect on flotation Flotation is increased as the footprint area is greater than that of the wheels. The broader the bandtrack plates, thegreater the increase in flotation; some tracks are designed with broad plates and the smallest gap possible betweenplates to maximise the machine’s footprint area.

Effect on grounddisturbance

The degree of disturbance varies with track specification. Aggressive tracks designed to give the greatest tractionadvantage can disturb the soil significantly. This can be a particular problem when the machine is turning onto theforest road from the harvested site. Track plate profiles that are folded at the ends minimise this effect.

Footprintarea

Footprintarea

Footprint area

With tracks fitted the effective area through which the machine loadweight is transferred to the ground is increased. The tension of thebandtrack is not absolute. Therefore, due to flexing of the track, theincreased flotation achieved is not the total theoretically possible ifthe weight of the machine were evenly spread through the footprintarea of the bandtrack.

Figure 3

Comparison of machine footprint area for bogie wheels withand without bandtracks.

shown in Figure 4. Side link tracks are best suited tomachines that carry out minimal movement between siteson forest roads as the position of the track plate link endscan cause disturbance, but they do provide a lighter weightoption than side paw tracks and maintain good tractionand flotation.

Side link tracks are comparatively simple in their design andcheaper than tracks with side paws. They are well suited toa wide range of forest machinery. Recent designdevelopments include the production of tracks substantiallywider than the supporting tyre, allowing improvedflotation. These tracks have track plate links positionedcloser to the crown height at the side (outside radius) of thetyre and therefore include side paws to keep the track onthe tyre. Side paws allow a convenient way of extending thetrack plate to greater than wheel width while stillmaintaining firm contact with the sidewall. They alsosupport the track on the tyre when exposed to lateral forcesduring turning and travel on side slopes, and reduce groundscuffing by the track plate ends and rolling resistance.

Figure 4

Common features of bandtracks that include side links.

Track plate Anti-skid spike Side link Track plate end linkage

Track plate Track plate ends

Anti-skid spike Tyre grouser bar

Track curves around the wheel

Repeating track plate and link unit

With bare tyres the whole machine weight acts through a smallfootprint area.

contact with the tyre which prevents slippage. However,weight and cost increase as more plates are required tomake up each track. The greater distance between platesalso helps the track shed sticky mud on soils with highclay content.

Double wheels

Double wheels are used to gain extra flotation byincreasing the footprint area of the machine, particularlywith skidders; experience of using double wheels withforwarders is not widespread. The additional machinewidth has operational implications for the management ofthe work site.

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Plate spacing

Track plates should ideally follow a curved profile aroundthe wheel to reduce stress on the machine transmission.Track plates wider than the machine wheels have flotationbenefits, but stress is placed on the bandtrack and themachine transmission as the track effectively acts as a levermultiplying lateral force through the wheel or bogie. Anti-skid ‘spikes’ or ‘lugs’ give lateral grip to the track, helping toprevent lateral movement on side slopes; they can bepositioned at the side of the plate enabling them to providegrip while preventing them from standing proud of the plateat its centre. In this way the potential for damage is reducedwhen travelling on stone surfaced roads. With both side linktracks and side paw tracks the spacing between the trackplates influences the flotation and traction characteristics ofthe track. The space formed between adjacent plates isgoverned by the plate design and the link size. The longer theforged links between the plates the greater the interplatedistance and the more aggressive the track. As the gapbetween the track increases, flotation reduces (see Figure 6).

Shorter links equate to a greater track surface area,increasing the flotation of the track and increasing surface

Figure 5

Common features of bandtracks that include side paws.

Figure 6

Plate spacing increase with link size.

Track interplate distance increases

Track joining link size increases

USING TRACTION AIDS INFORESTRY

Principles of ground pressure

The ground pressure of a forest machine is influenced bythe footprint area (see Figure 3), the weight distribution andthe design of the parts in contact with the ground.

Mean ground pressure exerted by the machine can becalculated, assuming a moderate sinkage rate of 15%,using the following formula.

P = WR x B

where:P = mean static ground pressure (kg/cm2)W = wheel load: total weight/number of wheels

in contact with the groundR = unloaded tyre radius (cm)B = tyre width (cm)

Track plate Anti-skid spike

Sidepaw

Track plate end linkage

Track plate Track plate ends

Anti-skid spike Tyre grouser bar

Side paw

Track ‘sits’ over the wheel with the track paws in contact with the contour of the wheel

Repeating track plate

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Worked exampleFor an 8-wheeled machine with total weight of 18 000 kg (18 metric tonnes) and tyres of dimension 60 cm unloaded radius, 70 cm width, the mean groundpressure will be:

The following formula is useful in comparing change inmachine ground pressure with bandtracks fitted.

=W (kg)/no. of wheels

R(cm) x B(cm)18 000 kg/8

60 cm x 70 cm

=2250 kg4200 cm2

= 0.54 kg/cm2

P =W

B x (1.25 x R+L)

where:P = mean static ground pressure (kg/cm2)W = wheel load: total weight/number of points

of contact with the groundB = track width (cm)R = track wheel radius (cm)L = distance between track wheel centres (cm)

Ground disturbance

Wheel slippage is a major contributory factor to ruttingon access tracks. Increasing machine flotation andtraction though the use of traction aids can reduce wheelslip and wheel spin. The likelihood of soil damage varieswith soil type.

SELECTION OF TRACTIONAIDS ACCORDING TO SITE

As explained, the operational characteristics of traction aidsvary with their specification. Bandtracks offer widelydifferent operating advantages and these are often opposing,e.g. a narrow, widely spaced track plate will confer goodgrip but poor flotation, while a broad plate profile withnarrow spacing between plates has far better flotation butwith less effective grip. It is not possible to make specifictraction aid recommendations for a given machine since siteterrain and ground conditions vary considerably, offeringunique circumstances. Selection of appropriate traction aidsmust be made on a case-by-case basis; Table 4 offersguidance from a number of example site types.

ADVANTAGES ANDDISADVANTAGES OF TRACTION AIDS

Advantages and disadvantages of using traction aids areshown in Table 5. A decision on the most appropriateoption requires careful consideration of the manyadvantages and disadvantages. The best option willfrequently be a compromise, especially if variation existsin on-site operating conditions.

Figure 7

Diagrammatic explanation of formulae. Source: P. G. Mellgren, 1980: Terrain classification for Canadian forestry.

R x B

L

RR

BB

Footprint area = B ( 1.25 R + L )Footprint area = R x B

Assuming15% sinkage

B ( 1.25 R + L )

Wheel tyre Bandtrack

The calculations in Figure 7 assume a 15% sinkage rateand include a correction factor to account for interplategaps of 25% of the bandtrack area. The formula forcalculating ground pressure of a tracked machine includesa multiplication factor of 1.25, assumed to allow for thearea of the track between the wheel centres on the bogie.

Despite the theoretical reduction in static ground pressurethere may be less of a benefit achieved due to the manyvariables involved including the ground conditions,ground profile, slope and dynamic loading of the machine.

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Groundconditions

Tyres only Wheel chains Tracks

Dry, load-bearingground

Non-aggressive grip will result inadequate traction and minimise effectof vibration on operator comfort.Suitable

May provide some protection to tyresin areas where hard, stony conditionscould cause tyre damage throughabrasion.Consider

Likely to be unnecessary unlesstraction is a limiting factor, and mayresult in unnecessary disturbance,greater fuel use due to extra weightand increased vibration. Couldprolong the life of tyres in aggressive,abrasive conditions although unlikelythat this factor alone would justifytheir use.Unsuitable

Brown earth undermoderate climaticconditions

Traversable with standard tyrespecification.Suitable

Operate efficiently; may be used onone pair of wheels to provide extratraction when terrain demands. Consider

Generally excessive for normal use;isolated wet patches can be tackledby other means, such as reinforcingracks with brash.Unsuitable

Soil with a highclay content underwet conditions

Tyre tread may clog with soil,reducing traction. Open treadpatterns have greater ability to shedsoil. In wet conditions wheel slip islikely to lead to reduced traction.Unsuitable

Can have an aggressive scouring effecton wet soils; they increase the wheelgrip but not the footprint and tend to‘bite’ through the ground surface untilgrip is achieved.Consider

Tracks designed to maximise tractionact by aggressive gripping, likely tocut up wet soils. The potential toreduce ground pressure by increasingfootprint area offers the greatestbenefits for working wet sites.Consider

Moderate groundconditions withsteep slopes

On limiting slopes wheel slip canresult in tread pattern clogging withsoil and losing any traction benefits.This is worse on heavy, sticky claysoils.Unsuitable

Give additional grip to the machinebut to a lesser degree than usingtracks.Consider

Can impart excellent traction forworking safely up and down slopestowards the limit of the machine’soperating ability. Specification shouldbe suited to maximise grip ratherthan flotation, but caution should beapplied in order to prevent damageto forest roads.Suitable

Wet soft/boggyground

Unlikely to provide adequate tractionfor large-scale forest extractionmachinery unless specialisedoperational techniques are used suchas reinforcing racks (see Alternativeoperational methods section).Unsuitable

May improve grip and reduce wheelspin that can result in the machinebecoming bogged but no significantflotation gain.Unsuitable

Some tracks with wide, broad platesare specifically designed to provideincreased flotation as well as traction.Depending on severity may be usedwith good effect in combination withchains or with tracks on all wheels.Suitable

Snow with softground

Snow and soil accumulation in thetread of the tyres can result in a lossof grip. More aggressive tyre treadpatterns are likely to be equallyaffected, while potentially causingmore disturbance in the process.Unsuitable

Provide useful additional grip but dueto lower flotation than tracks maydisturb the ground and lead to erosion.Consider

Well suited, providing grip andflotation and preventing grounddisturbance especially during thaw.Some tracks are not recommended assnow build-up and compressionwithin the track can stress themachine bogies.Consider

Snow with hardfrozen ground(unlikely to beexperienced forprolonged periodsin the UK)

Snow and soil accumulation in thetread of the tyres can result in a lossof grip.Unsuitable

Provide excellent tractionimprovements in slippery conditions.Suitable

Effects of vibration may causeoperator discomfort similar to that forhard ground working. Addedflotation is likely to be of significantadvantage.Consider

Key: Suitable: most suitable option under givenconditions.

Consider: may offer specific benefits but maynot prove to be optimum.

Unsuitable: not recommended/no significantadvantage in using.

Table 4 Operational characteristics of traction aids under varying ground conditions.

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INTERACTION BETWEEN TYREAND TRACTION AID

Very aggressive tread patterns may prove incompatiblewith some traction aids as widely spaced lugs havereduced surface area for track plate grip and support.Traction aid and tyre compatibility should be checkedwith tyre suppliers prior to purchase.

To keep tyres in the best possible condition they must beused at the appropriate tyre pressure, stated by themanufacturer. This may vary with and without traction aids.Tyres should also be inspected regularly to establish whethertraction aids are causing wear. Correctly fitted aids shouldprolong the useful life of machine tyres with no wear.

When considering the use of wheel chains on machinebogie assemblies, care must be taken to ensure sufficientclearance between bogie wheels for them to turn freelywithout snagging.

New bandtracks must be checked regularly as, over time,they can adjust to increase in length as they bed in.Conversely it is important that tracks are not too tight onthe bogie assembly as this is liable to increase stress on bothaxle and hub bearings. Track tension can be varied byreplacing long links with short links or removing an entireplate or plates as required. As an indicator, the centre sag atthe top of the track when fitted at the correct tension shouldbe between 40 and 70 mm; specific recommendationsshould be checked with the manufacturer.

ALTERNATIVE OPERATIONALMETHODS TO INCREASETRACTION

Alternatives to using traction aids to increase flotationand/or traction include reducing the weight of themachine, substituting lighter equipment and the load (partloading), and strengthening the travelled routes.

Careful site organisation to minimise the use of low loadbearing ground over the felling site will also help to reducerutting. Technical Development Report 35/91 (Spencer,1991) and Technical Note 11: Protecting the environmentduring mechanised harvesting operations (Murgatroydand Saunders, 2005) provides detailed information onoperational methods to maximise traction and flotationwhen operating over soft ground.

CONCLUSIONS

• Recent developments by manufactures have resulted ina wide range of available traction aids with variedcharacteristics that provide traction and flotationadvantages under a diverse range of operatingconditions. It is important to appreciate that gainingthe best results in terms of traction and flotationduring forest harvesting operations is achievedthrough a combination of appropriate traction aidsand operating methods. Inappropriate use of tractionaids can be detrimental to the site.

Advantages Disadvantages

Increased ability to work on slopes coupled with increased safety. Can increase width of the machine and some aggressive characteristics cancause damage to standing trees and root systems.

Increased traction. On hard surfaces increased vibration may have negativeconsequences for operator ergonomics.

Increased flotation. Can damage forest roads if used inappropriately.

Prevent tyre wear and damage. Potential for site disturbance if used inappropriately.

Increased manoeuvrability of the machine due to traction gain. Poor fit can cause tyre wear and damage to the machine.

Reduced downtime as the machine will become bogged less often. Additional weight and rolling resistance may lead to higher fuelconsumption and reduced manoeuvrability.

Appropriate matching of traction aid to site can increase efficiency ofmovement and save fuel consumption.

Increased cost, although possible to offset this against increaseduseful life of tyres, for example.

Allow a single machine to operate over a wider range of operatingconditions.

Require time to fit and /or remove when moving between differentsite types.

Many different types and specifications to meet a variety of operatingconditions.

No one traction aid will be suited to every operating condition –compromise always has to be reached between traction, flotationaggressiveness, weight, ergonomics and efficiency.

Table 5 Advantages and disadvantages of traction aids.

• With increasing emphasis on silvicultural methodssuch as continuous cover forestry in the UK, thenecessity for regular thinning interventions will makerepeated mechanised access essential. To preserveforest access in a suitable condition, care must betaken to minimise the impact of mechanisedoperations such as timber extraction, through, amongother things, the appropriate selection of traction aids.

• Safety considerations are imperative, and traction aidrequirements should be included as part of the riskassessment process during forest operations.

ACKNOWLEDGEMENTS

I would like to thank the following for their support andadvice: Elspeth Macdonald and Steve Morgan (ForestResearch), Jeff Livingston (Forestry CommissionMechanical Engineering Services) and Douglas Clark (W.M. Clark & Son (Parkgate) Ltd).

FURTHER READING

FORESTRY COMMISSION (1998).Forests and soil conservation guidelines. ForestryCommission, Edinburgh.

MELLGREN, P.G. (1980).Terrain classification for Canadian forestry. ForestEngineering Research Institute of Canada (FERIC),Canadian Pulp and Paper Association, Montreal.

MOFFAT, A.J. (2006).Managing brash on conifer clearfell sites. ForestryCommission Practice Note 13. Forestry Commission,Edinburgh.

MORGAN, S. (2000).Review of tyres and traction aids. Forestry CommissionTechnical Development Internal Report. Forest Research,Ae, Dumfriesshire.

MURGATROYD, I. and SAUNDERS, C. J. (2005).Protecting the environment during mechanised harvestingoperations. Forestry Commission Technical Note 11.Forestry Commission, Edinburgh.

SPENCER, J.B. (1991).Soft ground harvesting review of methods to minimise sitedamage. Technical Development Report 35/91. ForestryCommission, Ae, Dumfriesshire.

SAUNDERS, C. J. and IRELAND, D. (2005).Extraction routes on sensitive sites. Forestry CommissionTechnical Note 10. Forestry Commission, Edinburgh.

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Enquiries relating to this publication should be addressed to:

Duncan IrelandForest ResearchTechnical DevelopmentRydal HouseColton RoadRugeleyStaffordshireWS15 3HF

T: 01889 586844F: 01889 585225E: duncan.ireland@forestry.gsi.gov.uk

www.forestresearch.gov.uk

© CROWN COPYRIGHTISSN 1470-6792ISBN 0-85538-700-9

FCTN

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B)/IH(JC

)/NM

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G06