Low ground pressure, unbeatable traction and greater productivity potential. That sums up the attraction of high horsepower tracked tractors to arable farmers who run them for cultivation and drilling in preference to a wheeled unit.
Inevitably there are compromises – as there are with all machines. But enough growers have come to live with those compromises to see rubber-tracked tractors become an established part of the arable farming scene.
Whether that position is secure remains to be seen, however. Wheeled tractor power outputs are well into tracked tractor territory now and continued improvements in radial tyre technology are eroding the traction belt’s fundamental advantages.
For sheer grip on firm, dry soil, a rubber belt takes a lot of beating. Its advantage comes from the large contact area created by stretching a flexible rubber belt between front and rear wheels. Or else by using a track assembly instead of a wheel assembly on both axles.
A 360hp Challenger MT755D sitting on 635mm (25in) belts has a static contact area of 3.1sq m, according to Agco calculations.
A top-spec radial tyre, such as Michelin’s AxioBib, installed on a tractor with IF 800/70R38 rears and IF 650/75R30 fronts will provide around 1.5sq m for high draft loads requiring lots of traction, or 1.8sq m on an equal-wheel tractor.
The larger contact area of a tracked machine makes a significant difference in a high draft load situation, says Challenger, such as primary cultivations or when hauling a large seed drill on heavy soils and is easier to set up for optimum performance and efficiency.
That is simply because other than piling on the ballast there is nothing else to adjust; with tyres, achieving the optimum set-up requires inflation pressures to be adjusted to suit the weight and draft loads involved, which demands a bit more effort and nous on the part of the operator.
Both traction devices achieve maximum operating efficiency with a degree of slippage in this situation. For tracks, that figure is between 3% and 5% and for tyres it is between 8% and 12%.
Tracklayer enthusiasts point to such figures as an indication that a belted machine will cover more acres for a given amount of fuel but tyre supporters counter that a wheeled machine running at the same working speed will achieve the same output and without necessarily consuming additional diesel.
It’s more a question of whether a wheeled machine can match its tracked equivalent in especially demanding circumstances.
Advances in tyre performance and operator understanding of how to get the best out of high-performance tyres has made that more feasible.
It’s in situations where traction is compromised by the condition of the soil that tracks tend to come unstuck.
On a wet cultivated soil with low shear resistance or on wet stubbles with a slimy surface, tracked machines have little or no leeway for maintaining secure traction and positive steering control.
The operator of a tractor on tyres may have to accept disturbingly high levels of wheelslip to keep working but should retain more effective steering control and, up to a point, has a good chance of getting the job done.
The question of relative power efficiency is a complex one. On the face of it, a tracked tractor running with 2-5% slip should be making better use of its power than one operating at 15% when one in eight revolutions provides no work.
But wheel supporters highlight rolling resistance – the effort needed to propel the vehicle – and the power needed to not only bend a robust and tightly bound belt around drive and idler wheels (a figure of 7hp per “bend” is suggested) but also to turn a twin track machine.
With both tyres and tracks, however, the effort needed to move the vehicle increases significantly the more it sinks into soft cultivated or ploughed soil, so the ability to work “on top” while generating high levels of traction is another factor.
Either way, it is important to recognise that tracks and tyres need sufficient weight and appropriate weight distribution to perform at their best traction-wise.
Both need plenty of mass up front to balance the effects of draft load as that is the only way to ensure that tracks pull effectively along their entire length and front tyres make their full contribution in addition to the rears.
For top-work applications especially, the ability of a big tractor to tread lightly becomes an important factor.
Wheeled tractor operators can take advantage of their machine’s greater flexibility in this respect by shedding the pounds and adjusting tyre pressures to suit.
Strip a Fendt 936 Vario of all its ballast and the operating weight drops to around 10.8t; a Case IH Magnum 315 of similar power weighs in at a trim 9.4t.
These figures contrast with the Challenger MT755D’s base weight of 14t imposed largely by the heavy-duty undercarriage.
Although the difference between the mass of the two types of machine is not enough to wipe out the Challenger’s absolute ground pressure advantage provided by the larger surface contact area of the traction belts, it certainly closes the gap significantly.
And the all-up weight of a vehicle does have an impact on the structure at depth of soils prone to compaction.
A wheeled tractor operator working a trailed implement with relatively modest draft load for top-work has scope to not only remove weight but also use a more favourable tyre inflation pressure to increase the length of the contact area than is advisable when full-bore tractive effort is required.
Any ground pressure advantage a twin-track machine may have is certainly wiped out on headlands when working with a heavy mounted implement – heave the tool into the air for a turn and a bigger proportion of the outfit’s weight is now concentrated on the rear half or less of the tracks.
In the same circumstances, big tyres deform and present a bigger contact area to minimise the impact of the weight transfer effect.
Track supporters parry this argument by highlighting the impact any measure of trafficking has on root development and plant growth.
A wheeled tractor on the tyre configuration described earlier impacts around 25% more of the cultivated area than the 635mm (25in) track belts as a result of their greater width.
A twin-track crawler can pretty much turn within its own length – although such a manoeuvre is not particularly desirable whether on loose soil or abrasive concrete.
Being able to steer using a wheel to alter the speed of the tracks relative to one another is certainly a huge improvement over the lever-operated clutches and brakes of old. But it does irritate operators that in the field, a “skid steering” tracklayer leaves ridges of soil in its wake on headlands.
Four-track tractors – whether purpose built like the Case IH Quadtrac or created by fitting the Soucy or Westtrack systems to a wheel tractor – do not suffer this problem and nor do fully wheeled machines, of course.
The effect can be mitigated with twin-track machines to some extent by taking a sweeping turn through a wider headland and by using high-definition auto steering guidance to work in a skipped-bout pattern.
Development of the rubber tracked Challenger and subsequently of John Deere’s tracked machines made “crawlers” a feasible option once more because of their ability to be driven on the road.
It’s a particularly welcome attribute for operators obliged to use the highways and byways to get their primary cultivation tractors from the home base to distant fields.
In contrast to wheeled tractors equipped with duals – or even very large singles on the biggest tractors – a tracked machine no more than 3m wide needs no escort and is less of a hazard to fellow road users.
The wheeled tractor’s ace card is its versatility. A tracked power unit can be used to good effect on a grain chaser before post-harvest cultivation and drilling gets under way in earnest but that is really its only role other than tilling and drilling.
The same is true of a big four-wheel drive, of course, and on economic grounds should apply to a heavyweight tractor of conventional layout.
But growers are getting more used to giving their 250-350hp tractors alternative chores such as grain, straw, seed and fertiliser haulage, for which wheels are clearly better suited than tracks.
That is one of the strongest arguments for the conversion specialists – fit tracks for the situations that suit them best but retain the use of wheels for road-going versatility.