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Why one UK farmer thinks his tractor would probably roll before his trailer

With steep land threatening to unsettle grain trailers, one grower turned to a local engineering company for a stability solution.


Two seasons on, he believes the development is so good, his tractor would probably roll before the trailer. Geoff Ashcroft reports...

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Why one UK farmer thinks his tractor would probably roll before his trailer

Farm trailer evolution could have compromised stability for those working on rolling land, says Dorset grower John Martin.


“Farmers want wider tyres, lower ground pressure and more carrying capacity, but we have found that there comes a point where trailer stability overtakes those criteria,” says Mr Martin.


“Adding wider tyres has compromised chassis width to remain within a 2.55-metre overall width, and we have noticed this has had an impact on trailer stability on our farm.”


Not such an issue for those farming the flat lands of East Anglia, but in the rolling hills that surround the 320-hectare all-arable Deverel Farm, Milborne St Andrew, near Blandford Forum, a ‘twitchy bum’ has been part and parcel of grain carting for years.


“Steep slopes are challenging for us,” he says. “And when you are sat in the cab of a Hillmaster combine, lack of stability is far less noticeable than when you are pulling a grain trailer with your feet wedged in the window and knuckles going white while taking a load from the combine.”


Where most growers would run up and down a sloping field, Mr Martin’s focus on soil biology, incorporating residues and managing compaction, means fields are combined at 90-degrees to tramlines.

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His choice of 600/50 R22.5 flotation tyres on his 16-tonne and 18t AW trailers, while paying dividends with reducing compaction, have aggravated the stability problem.


Fortunately, the farm has only rolled a trailer once.


“One was too many,” he says.


“We had to find a solution for our grain trailers. Both are physically identical, apart from the height of the body. Where the 16-tonner has a 122cm tall body, the 18-tonner is another 30cm taller.”


He says that choosing super-wide tyres means the trailer’s leaf-springs need to be mounted closer together, to keep the trailer’s overall width legal.


“This narrowing of axle-to-chassis mounting points does affect the overall balance of the trailer,” he explains. “I could see the problem, but I could not see an easy solution.”


He contacted local engineering friend Richard Bower, of RDBS Engineering, and the pair embarked on some collective head scratching and number crunching.


Mr Bower says: “We used a fully-loaded grain trailer and pulled it across several banks so we could monitor movement, angles and how the trailer’s centre of gravity behaved as slopes became steeper.


“And what we found was the spring on the uphill side of the trailer was the instigator of a roll-over.

“Once the spring on the downhill side had compressed, and the gradient continued to steepen, the uphill spring would push against the weight and try to open. This would lift the higher side of the trailer and this movement was the catalyst to encourage the trailer’s body to flip.”

Leaf springs


The pair knew adding leaf springs to stiffen the ride would never help.


He says: “We needed to find a mechanism that would keep the body and chassis virtually parallel to each other, while still enabling suspension and load-sensing brakes to operate correctly.”

His answer was the development of the Hydrapowered trailer stability system, which behaves in much the same way as an anti-roll bar, but uses fluid dynamics rather than torsion bars.


It requires each axle to be equipped with a pair of hydraulic cylinders, mounted vertically, to connect each axle to the chassis. It complements the trailer’s existing suspension system, and sees the top of one cylinder piped to the bottom of the other, and vice-versa.


Mr Bower says the Hydrapowered stability system’s closed circuit operates with a low working pressure of 12 bar, which is applied to the system by a pump on installation. Though pressures are tweaked on installation, to allow axle movement in response to a trailer’s payload.


“Standard hydraulic cylinders simply will not work,” says Mr Bower.


“I have re-engineered the inside of each ram to afford a gradual restriction of stroke, as the trailer is filled.


“This retains movement and compliance when the trailer is empty, but progressively stiffens as weight is applied.”

Importantly, it affords enough movement between chassis and axles to correctly operate the trailer’s load-sensing brake valve and maintain braking efficiency regardless of payload carried.


“It is a system that needs no intervention from the operator, and is virtually maintenance-free,” he says.


“And being a low-pressure system, leaks and failures are greatly reduced, boosting longevity.”


In operation, as the trailer crosses a slope and weight shifts, the ram on the downhill side of the trailer is compressed. At this point, it transfers oil from the cylinderside of its ram, to the rod-side of its neighbour.


Doing so moves a greater volume of oil into the uphill ram, to pull the chassis onto the spring. This restricts body roll, and the rate of movement has been calculated to allow just one degree of body roll between body and axle, with a fully loaded trailer.

“The more weight that’s in the trailer, the more the rams get compressed, so a greater amount of roll restriction takes place,” he adds.


“We do need some roll to prevent the chassis from bending, as the whole assembly would simply become too rigid.”


He adds that the positioning of the two rams helps to restrict lateral movement of the axle, meaning tyres are required to absorb a little more when turning.


This could be viewed as a system that affords more tyre scrub, but two years into operating, the system on his 18t AW trailer, John Martin has noticed little wear on the trailer’s Michelin Cargo X Bib 600/50 R22.5 tyres.




Mr Martin says: “I run the tyres at 2.5 bar pressure, and the trailer treads very lightly indeed,” he says.


“But more importantly, the trailer is now much more planted and secure when working across slopes. And this gives my operators greater confidence too.


“It is not a totally fool-proof system of stopping roll-overs, but it does dramatically increase trailer stability. I am sure our tractor would now roll before the trailer.”


He has also noticed how much better the trailer rides too, as the Hydrapowered system stops one side of the trailer dropping or rising much more than the other.


"If one side of the trailer crosses a hole, for example, the wheel does not drop in, but is held up by the stability system,” he says.


“So the trailer is much less susceptible to dramatic changes in surface condition,”


He adds that the Hydrapowered system has not been touched for two years.




“It has proved to be a fit and forget system,” says Mr Martin. “And we tend to use the 18t trailer more than the 16-tonner on steeper land, simply because of the greater stability it offers.”


At a cost of about £1,000 per axle, plus fitting, it is not going to break the bank when you consider some top-spec trailers now cost upwards of £30,000.


“Those with tankers and silage trailers who find themselves travelling across sloping ground, would benefit from the increased stability this system offers,” he says.


So what is next for the Dorset arable farmer? “I would like a third axle to spread the weight of my loaded 18t trailer, and further reduce compaction,” he says.


“But then I would need two steering axles to help manoeuvrability. And to get two steering axles, I would have to use narrower 540 tyres to allow room for steering movement without needing a narrower, less stable chassis.


“And six 540 tyres are no more of a benefit than running four 600s.”


He reckons a non-steered liftaxle fitted with 600 tyres is the answer.


“The axle could lift and lower automatically based on the steering angle sensed between tractor and drawbar,” he says.


It looks like another visit to the RDBS Engineering workshop will soon be on the cards for this innovative grower.

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