16 October 1998

FRONT SUSPENSION: WORTH IT OR NOT?

EVERYTHING so far has concentrated on how the individual tractors perform. Looking wider its possible to draw some general conclusions about front suspension, and the effect on ride and road driveability of tyre pressure and hitch suspension.

To produce global graphs for movement at the nose and under the drivers seat (Figs 3 and 4), the science team averaged all measurements on all surfaces and speeds for all tractors. Other tests showed the effect on ride of tyre pressure (Fig 5) and hitch suspension (Fig 6).

Take a look at Fig 3. This shows average vertical accelerations (movement) at the nose, with and without a plough on the hitch. Movement without front suspension is taken as 100% (left). Its clear that running empty, front suspension cuts nose disturbance by 43%. But just carrying a plough cuts nose movement by 18%, and switching on front suspension only brings movement to the same level as before.

Whats happening? The ploughs weight is compressing the rear tyres, making them act more as springs. Video taken at the test strip backs this up; in slow motion, every tractors Michelin 108s (all the same size and at the same pressure) could be seen to pancake going over man-made ridges.

Fig 4 shows movement under the drivers seat, a place much closer to home. Disturbance without suspension is again taken as 100%. Comparing the two left-hand values shows that front suspension is much less effective at the seat than at the nose, which is no great surprise as the driver is sitting close to or over the unsprung rear axle. But look at the benefit gained by just adding a plough, (Fig 4, right). Its clear that carrying a heavy implement does much more for seat-area comfort than front suspension, though springing still adds something.

If the tyres are playing such a big part, what happens if you fiddle with pressures? Fig 5 comes from a separate set of results, and lays out the differences at the nose between front tyres at 0.8bar (12psi) and 0.6bar (9psi). The latter is Michelins fieldwork recommendation for XM 108s, given tractor size, weight and speed. Rear tyres stayed at 1.2bar (18psi).

Left-hand values are the same as in Fig 3 and reinforce the big effect of suspension on nose movement. But look what happens on dropping pressure by 3psi (25%) in those big front balloons: you gain a large chunk of the effect that would otherwise come from front suspension.

All but the Case featured hitch damping in transport. Fig 6 is an example of what happens when a tractor with hitch damping and test-standard tyre pressures was driven down the test strip at 27kph, or moderate transport speed. All units showed the same pattern.

Left-hand bars are average unsprung values at the nose and cab and are taken as 100%. Adding just suspension helps the nose more than the cab, while adding just hitch damping sees the nose getting a harder time. Using both systems together really calms the nose and, subjectively, makes the outfit feel much more stable to drive. If movement in the cab directly affects driver comfort (and with seat suspension off, it does), then the drivers life is improved equally by all three hitch/front springing permutations.

How come? Because the driver is effectively sitting on a see-saw, so remains largely unaffected as in turn, front suspension looks after the front end and damping calms the implements bounce. If your tractor has a hitch damping option it will help ride (hands up anyone who hadnt found that out for themselves). If it has front suspension as well, outfit stability in transport will be a lot better – probably because the front wheels can spend more of their time on the road, steering and gripping.

THELASTROUND-UP

So what can be drawn from these forests of figures and earnest impression-noting? Several points, some very obvious and some not.

&#8226 Front suspension does most of its work at the tractors nose. Sitting close to or over the rear axle, the operator still feels most of the bumps and all of the side-to-side shaking particularly in draft work, where the rear tyres carry less weight than in transport.

&#8226 Front suspension has little subjective effect in low-speed fieldwork. That is apart from on rough headlands, where turns can be both faster and kinder to the operator/equipment.

&#8226 Switching suspension in and out during ploughing or cultivation has no obvious effect on linkage draft control effectiveness.

&#8226 Hitch damping helps when carrying an implement on the linkage. And the further forward the driver sits, the closer he is to the front suspension/implement pivot point and the less pitching he feels.

&#8226 As fieldwork speed increases, front suspension boosts driver comfort. In harsh conditions an effective system will make the difference between continuing to work or not. In more reasonable going good suspension allows more consistent forward speeds and helps workrate.

&#8226 Front suspension increases transport stability, particularly with loads on the hitch or hook, and the faster you go the more it helps. Beyond moderate speeds, driver control may be better in emergency excursions from tarmac.

&#8226 Tractors have a natural bounce frequency which, in certain speed band(s), comes together with road surface undulations to make the unit hop. The problem may be complicated by out-of-round wheels or tyres. Front suspension kills or greatly reduces this speed-related bounce.

&#8226 So front suspension has a lot to offer on farms where road-work is common.

&#8226 Modern low-profile radial tyres, used at the makers suggested pressures, can deliver most (but not all) of front suspensions effect, especially when transferred weight is on the rear axle. Pressures higher than optimum sacrifice the comfort bonus, as well as losing grip and increasing compaction.

&#8226 A good seat, adjusted for the driver, is as important as front suspension.

&#8226 Front springing can never be more than a partial solution. But its a useful step towards the Holy Grail of full suspension.