PowerTorque’s Technical Editor, Bob Woodward, spoke to industry about the effect that design changes to converter dollies have had on their transport operations.

Richard (driver) – CHS Broadbent:

“I’ve been driving B-double for about 20 years and more recently road trains (A-doubles), the A-doubles on bulk grain. Initial operations with A-doubles were with the conventional hinged drawbar.

“When I was allocated a newer combination with a rigid drawbar, my first impressions were chalk and cheese. I could not believe the difference, no pushing and shoving and no cab slap. The rigid drawbar tracks and feels more like a B-double.

“We haven’t identified any negatives and it’s a positive when splitting to tip at some sites where splitting and reversing onto the hopper is necessary. The other drivers have really embraced the concept. Dollies are being converted and currently there are six rigid drawbars in the fleet.

“The rigid drawbar converter dolly displays considerably less brake and tyre wear.”

Alcoa, Bridgestone, Hendrickson, Jost, MaxiTRANS and Wabco all acknowledge the ITC Dolly Project as partners.

Robert Smedley – Smedleys Engineers:

“From a technical perspective, the rigid drawbar design results in a noticeable increase in coupling vertical forces, however these vertical forces are within design limitations of the automatic style pin coupling. The data analysis and the visual observation of the converter dolly correlates well with the visible observations made during testing with the combination being more stable on the road, when cornering and major braking events. The rigid drawbar design undoubtedly offers better safety performance than does a hinged drawbar converter dolly.”

Smedleys Engineers completed dynamic behavior comparison of the rigid and hinged drawbar converter dollies in a 35-metre road train, where the conclusions were:

“Testing was conducted to compare the relative dynamic stability of a hinged drawbar dolly with a rigid drawbar dolly through analysis of three axes of accelerations and three axes of gyroscopic movement at the axle groups of the three trailers of an A-double type combination. Additionally, strains were measured at the tow coupling of both dollies to compare the resolution of forces through the two different configurations.”

The rigid dolly pitches less. It has less movable links in the system to softly absorb loads fed into the fifth wheel so it sees slightly more small longitudinal vibrations.
When the rigid drawbar dolly was used, the lead trailer exhibited more side-to-side movement. This appears from the data to have come from driver steering inputs. The driver stated he made the inputs due to a feeling of the trailers moving the prime mover around when the rigid dolly was fitted.
Once the side-to-side movement of the lead trailer was mathematically filtered out of the dolly and tag trailers, the rigid dolly appeared more stable and ‘calmed down’ the tag trailer more.
The rigid drawbar dolly had less very fast (small and too fast to see) snaking movements.
The hinged drawbar dolly pitches a lot very early on when braking, however as the brakes on the tag trailer axles balance out their contribution, it flattens off.
The rigid drawbar dolly barely pitches when braking.
The hinged drawbar dolly pitches a lot when accelerating as the drawbar hinge point is below the fifth wheel hinge point. This is related back to the “cab slap” reported by test drivers.
The rigid drawbar dolly does not pitch when accelerating or exhibit “cab slap”.
The tow coupling is more stressed by the rigid drawbar, but the stresses are not that high.
Following the test vehicle through long curves over bumps during those curves, the rigid dolly observably did not pitch over bumps while the hinged dolly did.

PBS combination assessments that include a converter dolly are often based on the assumption that the dolly has a rigid drawbar. Not all are the same!

So, what’s next? There is an increasing number of tri axle converter dollies in operation. Some goose-neck designs incorporate a rigid drawbar. However, the traditional are all hinged and incorporate multiple ride height valves.

Seems time for another industry project: To develop a general use rigid drawbar triaxle air suspension converter dolly design.