Weighing up the odds 28 August 2014

Inadvertent overloading is a perennial worry. Steve Banner talks to equipment manufacturers and specialists about solutions, costs, technologies and actions

Executing a chassis conversion can be a bit like playing three-dimensional chess. The overall permitted length must not be breached; turning circle restrictions must be obeyed; and the extent to which the rear end swings out must be kept within bounds. Furthermore, the truck must be designed to minimise any danger of an inadvertent gross or axle overload and, if the work is conducted pre-registration, then everything must comply with EC Whole Vehicle Type Approval. These days that includes staying within the electronic stability programme envelope, unless you want a hefty bill for having the vehicle tested before it can be registered.

Addressing axle overloading is almost the easy bit, despite the threat of prosecution if axle weights are exceeded. Trucks are often constructed with a theoretical risk of axle overload, but in practice it doesn't happen, because of the nature of their work. As Brent Carmichael, engineering operations manager at Britcom International (which handles 150—200 conversions annually, including lengthening and shortening wheelbases, fitting mid-lift and tag axles and turning tractors into rigids), says: "We've constructed chassis for removal vans that would suffer a drive-axle overload if they were evenly loaded to their maximum gross." But on removal work, that situation is highly unlikely. "We warn the customer nevertheless," he states, adding that you can never eliminate driver error.

But what about others on, say, multi-drop work, where vehicles unloaded from the rear experience weight transfer that might lead to the front axle being overburdened (the classic diminishing load problem)? To combat this, the driver would have to stop and move the load backwards: not always practical if there is no ready access to a forklift or a pallet truck, or if the load is not palletised.

A difficulty is knowing whether an axle is overloaded or still has capacity to spare. Fear of overloading and the impact that would have on a transport firm's OCRS ( operator compliance risk score) if they were caught is prompting some to send trucks out under-loaded with all the costs and inefficiencies that implies, according to Derek Hack, sales manager at Axle Weight Technology (better-known as Axtec).

"That was the situation at Hanson Building Products, which uses drawbars to deliver bricks and blocks to builders' merchants," explains Hack. "We installed dynamic drive-over axle weighbridges at a number of their sites and the efficiency improvements mean they've paid for themselves in two or three months." Also giving gross weights and said to be capable of ±0.25% accuracy, such weighbridges cost anywhere from £12,000 to £35,000 depending on the ground works needed, although Hack says it's typically £16,000—18,000.

A pair of portable axle weigh pads is another option: one person can easily carry both unaided. Accuracy is from ±50kg to ±100kg, says Hack, depending on the size of the truck. They cost just under £6,000 a set and should only be used on level ground swept clear of stones. "We do not, however, recommend their use with anything heavier than an 18-tonner," warns Hack.

Not everybody has access to a weighbridge or weigh pads. Hence the proliferation of onboard weighing systems that display loading on individual axles and the gross weight on a screen in the driver's cab. The data can be stored in the unit's memory and the device connected to a tracker so that the depot traffic office is instantly alerted to any overloading incident.

Onboard weighing equipment specialist Barry Napper warns that such equipment may not be quite as accurate as operators think – at least so far as individual axles are concerned. "Load cells can help, but all they actually do is weigh the load in the body and tell you how much is at the front and the back," he observes. In other words, you get load distribution, not the precise weight on each, and any attempt to improve on that can be frustrated by anything from tight bushes to worn springs.

"You can install an onboard weighing system that relies on spring deflection to get the axle weights," he agrees. "But while that's fine for light vehicles, unfortunately it won't stay in calibration for any length of time, particularly on, say, eight-wheeler that are in and out of quarries."

With prices starting at £1,500 for a two-axle chassis, Axtec offers onboard axle and gross weighers that use a mini load cell measuring spring deflection on axles with steel suspension or a pressure transducer for those equipped with air suspension. "We've written software so that operators can adjust our equipment to take wear and tear into account," advises Hack. However he does not claim pinpoint-accuracy. "What we offer works to ±2.5% on a two-axle rigid, increasing to ±5% on a three- or four-axle rigid," he says.

Bowmonk offers a similar system designed to tell the driver of an artic the vehicle's gross weight, how much is on the trailer bogie and how much on the tractor unit's fifth wheel – and if permitted weights are being exceeded. It taps into the air suspension to obtain the data and there is an audible overload alarm on the in-cab display.

Costing £1,475 and with an installation time of four to five hours, it can be linked to in-vehicle telematics so that the data can be transmitted to home base. It can also be linked by Bluetooth to any Android mobile phone, too. Claimed accuracy is ±0.5%.

Yet while such systems may not work to the nearest kilo, and are not at present available for trailers, they can indicate an overload problem to the driver. And that should prompt him or her to visit the nearest dynamic weighbridge.

Much the same is true of any such system – including Wabco's TEBS E trailer braking system. "Every EBS [electronic braking system] has a pneumatic or electrical connection to the air-suspension bellows," explains field service engineer Andy Chapman. "The data from this is used by the ECU [electronic control unit] to calculate the appropriate braking for a given axle or bogie load and, as a spin-off, we are also able to give the operator axle load information."

This can be done in three ways, he continues. "Data transmitted to the towing vehicle via the CAN ISO 11992 network can be viewed by the driver on a dashboard display, if the truck is suitably enabled," he explains. "The second and more common option is to show it on the Wabco Smartboard, attached to the trailer, while the third option is to make it available to the traffic office via a telematic link."

But again, what it indicates is the degree to which the bogie is overloaded. It does not reveal the loading on individual axles. On a drawbar trailer, however, an extra sensor is included, which allows the value to be split to show both front and rear axle total loads.
Because airbags and other components differ from trailer to trailer, Wabco makes no guarantees as to precision. "It's within a couple of per cent either way though, and the Smartboard offers the ability to calibrate the internal pressure sensor to make the data more accurate," contends Chapman. And a Smartboard will only cost you £150.

Author
Steve Banner

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