The sales engineer’s tool – before computers
Before computers did all the calculations for us, engineers relied on slide rules to do the maths necessary for truck design. Truck sales engineers wanted something simpler and quicker to allow them to specify the right powertrain for specific applications, so Cummins came up with the Truck Performance Calculator.
Back in the 1970s, electronic computers were a pipe dream and the fledgling desktop calculator was as good as mechanised calculations got. Design engineers were skilled at slide rule use, but the guys trying to specify truck powertrains in front of customers usually lacked that degree of mathematical aptitude.
In Columbus, Indiana, at the Cummins Engine Company Inc offices the engineering team designed a specific, easy-to-use slide rule that could fit into a sales engineer’s brief case and be used, with confidence, in front of a truck buyer.
The Cummins Truck Performance Calculator came in a vinyl push, with a printed instruction sheet, but the operating sequence was repeated in text on the Calculator as well, so when the instruction sheet was lost, it didn’t really matter.
The rule took the customer through a series of steps that calculated, in turn: rolling resistance horsepower; grade resistance horsepower; air resistance horsepower and finally, total horsepower required.
The inputs were done by sliding the rule until the appropriate gross weight, desired road speed, road surface type, truck frontal area, transmission and axle ratios, and tyre size were progressively entered.
Aligning the sliding section with the fixed part was made easier by a sliding perspex ‘gunsight’.
The next step was achieved by turning the Calculator over and reading off geared speed and grade startability.
At the end of a five-minute operation the sales engineer could give the potential customer the horsepower he’d need for a specific application, along with the appropriate transmission and drive axle ratios.
In our previous working lives, we at Historic Vehicles were very familiar with the Cummins Truck Performance Calculator and used it on an almost daily basis. Our surviving example is a treasured possession.
Nowadays, every truck maker has computerised programs that show the same information instantaneously.