FRONT SUSPENSION – AN
END TO BOUNCE?
Hard work in Holland for front suspension tractors, as the first comparison of its kind pits four makes
against each other on the field, road and test surface. Andrew Pearce brings home the results
ENGRAVED on the corporate heart of every major tractor maker is the slogan "Evolution, not revolution". Commercial sense says theres no point in making something you cant sell, particularly to conservative and sceptical farmers.
But progress certainly happens. Powershift transmissions are here to stay (try taking one away from an operator whos used to it) and electronics are going the same way, despite let-downs which have doubters cackling with "I told you so" glee. Whats the next big thing? Maybe front suspension, particularly as tractor road speeds edge upwards. Case, Deere, Deutz, Fendt and Valmet already have production systems, while Massey and Renault are rumoured to be close.
Its simple to list the likely benefits. An easier life for the operator and equipment, faster work over rough terrain and potentially more secure roadwork. But are they delivered in practice?
* Where, what and who
In July an international team descended on Lelystad, east of Amsterdam. Here in a vast flatness of polder the bottom of the sea turns into agriculture, with only lines and clumps of mature trees breaking a view to all horizons. Its a land of big fields and black soils high in clay; of arrow-straight roads which rise only to cross a canal or dyke.
On site were a Case CS120 (120hp), Deeres 6810 (125hp), a Deutz Agrotron 120 (120hp) and Fendts 140hp Favorit 514C Turboshift; Valmets entry hadnt been announced so couldnt be included. All but the Deere featured front suspension switchable from the cab, allowing immediate on/off comparisons. Deeres version carried a non-standard external lock-out valve.
Testing happened in two sections. For impressions of how the tractors feel to drive with and without front springing, four writers used each on winter barley stubble, on cobble road and on good quality tarmac – see panel (right) for details. To lay down a technical base, a team from the machinery section of Colognes Fachhochschule instrumented the tractors, drove them both over a man-made obstacle course and on the road, then analysed suspension behaviour. More on this in the Reading the Graphs section opposite.
Reports for each tractor start with suspension layout. Then come subjective driving impressions, a graph showing suspension performance compared to the group average, and a short discussion. To finish the test, we use general graphs to draw out several key points on front suspension effectiveness.
Its important to be very clear that all subjective comment comes from seat-of-the-pants feeling. Drivers are not machines, so are not necessarily alike in their interpretations. But all jottings on suspension behaviour are agreed between four testers and the science team.
• All tractors wore Michelin XM108 low-profile radials, 540/65 28s up front and 650/65 38s at the back. Pressures were set to 0.8bar (12psi) and 1.2bar (18psi) respectively. Both are higher than ideal for fieldwork given the weights and loads involved, but are necessary for the tractors 40-50kph top speeds.
• On top of this, the extra air reduces the tyres own suspension effect – a significant property of the Michelins and important when trying to isolate mechanical system behaviour. Tractors carried the front weight supplied as standard by each maker – 480kg for the Fendt, 730kg on the Deutz, 900kg on the Deere and 604kg on the Case.
• For fieldwork we used a four-furrow Opal 140 Vario and 3m Smarag tine/disc/ roller cultivator, both from Lemken. Stubble, heavily scored by sprayer and combine, was ploughed with the tramlines and cultivated at right-angles to them (the latter deliberately really tough on the driver and equipment).
• On the road we pulled a tandem axle Beco trailer (14.8t gross) and transported the plough, using the tractors hitch damping function (where available) for the latter. Two road surfaces featured; the good-quality tarmac that makes up the bulk of Flevolands country roads, and around 2km of uneven brick cobble along a dyke top.
WHOD BE AN ENGINEER?
Front suspension designers have a tricky balance to strike. The ideal set-up produces a good ride at all speeds, no matter whether its carrying only the tractors weight or something really heavy on the linkage or loader. Some makers switch off springing automatically according to weight and/or speed (which also gets round the problem of draft control with front-mounted tackle), others dont.
So far, all stick with pre-set pressure in the spring-providing nitrogen spheres. One way forward might be to vary gas pressure on-the-move according to load; another could be to use several spheres of different pressures and diaphragm areas, keeping more in circuit when load is light and switching to different combinations as weight to be carried changes.
A four-man team under Prof Dr-Ing Heiner Wesche gathered objective information on suspension behaviour. Three vertical acceleration sensors were fitted; one on the tractors front axle (one each side for the Case), one on the main frame under the nose, and the last up in the cab, under the drivers seat.
The tractor was then driven four times over a series of increasingly high wooden rumble strips at 10kph, 16kph and 30kph with suspension switched off and on. The job was repeated with a plough on the back. Then, after averaging each set of four results and charting the relative movements of axle, frame and cab, each suspensions bump absorption could be worked out.
To find real-road behaviour, instrumented tractors were driven at 40kph over a set section of cobble road with and without a plough and with and without hitch damping.
Fig 1 shows typical information coming from the sensors. The centre box represents the wooden rumble strips, which varied in height from 250mm to 750mm, seen from above. The red line is vertical movement of the tractors nose, the green line is vertical movement of the cab. Time in seconds runs along the bottom, acceleration in g is to the left.
On the following pages, individual tractor reports carry a single graph comparing that tractors behaviour with the group average. (Fig 2)
Bars show vertical accelerations for the nose and the cab, both with and without front suspension and the plough. Each coloured bar is the average of all values for that tractor on the test strip and on the road.
By averaging many values a tractors particular strength might be hidden (for example, one might be better at 10kph over the test strip than another) but a large sample allows for easier comparisons.
Making comparisons begs two obvious questions, particularly where differences in average values are small. The first is personal – how big does a change have to be before a driver can feel it? Its a key point, which the Cologne team couldnt quantify. Our subjective impression, though, is that small variations are noticeable.
The second question is statistical. How can you be sure that the difference between any two averages is real? Only by crunching the numbers to produce a yardstick – something that can be used to judge whether that difference actually exists or is just the result of random variation. Lumping each tractors values together to make a simple picture blocks such an analysis, but Prof Wesche reports that test results within each set of four runs were consistently consistent.
All the tractors shared very clear bad-bounce zones on road and field: 33-36kph on smooth tarmac with the trailer, 10-11kph on stubble with the cultivator. Either side of these speeds all travelled noticeably better.
Prof Wesche quickly cleared this up. As the tractors all weigh roughly the same (6.5t-7t) and were on identical tyres at common pressures, their natural resonant frequencies were similar. In the critical speed bands, this frequency comes together with the surfaces bump frequency (or some harmonic of it) and the two add, so the tractors leap hard. Slowing or speeding up moves the frequencies apart and the bounce dies away.
• Any operator knows the big part played by seat condition and setting.
• All tractors featured an air-sprung throne, so to minimise the "seat effect" during work we pumped each to maximum hardness and switched out any fore-aft option. Generation of pain in the devotion to duty?