A new chassis layout has resulted from a radical rethink for Boral, when specifying agitators. The constant quest for a lower tare has sent cement companies heading down the route of using lighter spec chassis, for instance, only to see cracked chassis as a result. Getting more concrete on board has often led to durability issues.
However, at the heart of the a fleet renewal project for Boral was a drive to improve safety even if it meant coming out at the same tare mass. The emphasis was on making a better mouse trap, not necessarily a lighter one.
A quick look at the typical agitator shows us where one of the issues lies. The agitator barrel is mounted on a subframe and this is bolted onto the top of the chassis rails. The whole unit sits proud of the chassis and well above the truck – it looks inherently unstable. Follow one around a roundabout and you can see the sway as the high centre of gravity works the rear suspension hard.
The other aspect of the design identified as affecting the rollover tendency was the relative flexibility built into the chassis and suspension. The 7mm-thick chassis rails do flex and the airbag suspension allows the whole vehicle to tip when turning.
“What we wanted was a better handling truck, to start with,” said Merv Rowlands, Boral Fleet Engineering Manager-Logistics. “The three areas of focus for us were the chassis stiffness, the roll characteristics of the suspension and the centre of gravity for the whole package.”
Starting with the chassis, on the Mack, it is made with 8mm steel, but some have been specified at 7mm thickness – this does introduce flexibility into the frame itself. By going up to 9.5mm, Boral reintroduced stiffness into the chassis. To further enhance stiffness, the cross members were also beefed up.
Looking at the suspension, Boral found a Volvo suspension which uses an inverted leaf steel rod set-up, which is available for fitment to Mack trucks. This particular five-leaf suspension has been approved by the road authorities as being road friendly. This latter aspect is an important consideration in the concrete game where payload is paramount.
Added to this, a roll bar was included at the rear of the chassis, further beefing up the stiffness. The chassis height, in relation to the axles, was lowered a further 50mm with the use of a lowering block in the chassis mounting.
One of the issues around rollovers is the feedback given to the driver by the suspension in the moments before a rollover. The airbag suspension was found to give the driver little idea the truck was about to tip – with the steel suspension the feel is much improved, from the driver’s point of view.
Another change was to go to 275/70 tyres. This brought the chassis height down to 68mm below the standard used in the past.
The next step was to get Cesco, who make the agitator barrels used by Boral, to design one without a subframe. The intention was to use the improved stiffness in the chassis as the subframe for the barrel.
“The beam fitted as a subframe on the barrels doesn’t actually add a lot to torsion,” said Merv. “I don’t like using U bolts, so what we’ve done is fitted chassis angles front and back onto the chassis rails, similar to those used to hold a trailer turntable on a prime mover.
“Then we got Cesco to design the pedestals so they sit on the chassis angles. Importantly, it puts the stress and the load where it should be. The chassis angle is designed to transfer weight onto the web of the frame rail rather than onto the top and bottom flange. On a normal mixer, that’s where the weight sits. It spreads the load out and it holds it down really, really firmly. There’s no U bolts, twisting and moving.
“Cesco could redesign the front and rear pedestal. This enabled them to drop the barrel down as low as possible. When you look at the overall height of the agitator it’s not a lot different. When the truck backs onto something like a pump it needs to be at the same height.”
In fact, the barrel has been angled up, going from 13.5 degrees to 16 degrees, increasing the rake of the whole thing. This has led to a number of changes to the chutes, but ending up with the concrete pouring out of the barrel at the same height as before. At the same time, the change in angle has brought the centre of gravity much lower.
One of the unforeseen advantages of the redesign is a reduction in the amount of spillage out of the back of the agitator. The steeper angle holds the concrete further from the lip of the barrel. Currently, Boral has to cut back loads with a high slump to avoid spillage en route to the job. With the new barrel position this is no longer necessary.
The whole new set-up ended up slightly lighter than the previous design, around 100kg. However, going to a Hardox barrel has seen the overall weight saving hit 330kg. This constitutes a considerable saving in a sector of the industry where tare weight is king.
“This new truck tared at 8.04 tonnes, a standard one is 8.4 tonnes,” said Merv. “That’s really important in concrete. There’s a lot of steel in those subframes and the front pedestal is much bigger on the old design.”