September 2008, Work Truck - Feature
Avoiding the Pitfalls in Spec'ing Powertrains & Transmissions
By Joe Bohn
PTO Compatibility Important
One of the biggest traps a fleet can fall into is not ordering automatic transmissions with PTO compatibility.
For years, Allison Transmission, for example, provided PTO compatibility with all its medium-duty transmissions. More recently, Allison, as well as Ford Motor on lighter-duty models, also offer non-PTO compatible transmissions as lower-cost alternatives.
As a factory option, PTO compatibility generally costs a few hundred dollars. But the option can run into the thousands of dollars if installed retroactively, since aftermarket installation typically requires swapping out the entire transmission, says ARI’s Duford.
PTO compatibility is required for running any type of hydraulics, such as cranes or booms. Electrically operated cranes and booms can be used in lieu of PTOs on trucks up to 26,000 lbs. GVW for lighter loads or shorter boom operation. Electrics can provide crane capacities of 1,000, 1,500, or even 3,000 lbs.
But the key issue — even for lighter-duty applications — is the flexibility PTO compatibility provides.
A PTO-compatible transmission, spec’ed with hydraulic capabilities, can still be used for operating an electric crane or boom, if the fleet operator chooses that lower-cost alternative. However, without PTO capability, the transmission can’t operate a hydraulic boom, or a dump or lift truck operation.
PTO capability also improves trade-in/resale value.
Attend to Axle Ratios
According to Khosla, not paying enough attention to gear ratio selection can either undermine fuel economy or short-change a vehicle operator who needs extra pulling power at the lower end of the rpm range.
Typically, spec’ing axle ratios isn’t a problem for trucks up to 19,000 lbs. GVW because normally, few choices are offered (1-3 selections, depending on whether the vehicle has a manual or automatic transmission).
Selecting ratios for trucks in the 19,000-lb. to 26,000-lb. GVW range can be tricky and requires a performance analysis.
"Selecting the right ratio on a medium-duty truck is one of the easiest things to get tripped up on," notes Khosla.
One truck, for example, might require a 3.50:1 or 3.70:1 axle ratio, which provides the best fuel economy, if it’s a supervisor’s truck that typically runs lightly loaded.
But, the identical truck might require a 4.10 or 4.11:1 ratio if used in the field with a service body or crane application or operates off-road or in a mountainous area.
Khosla adds that some engine/transmission combinations offer eight or more axle ratios. The optimum selection can also come down to picking the "sweet spot" that best accommodates the truck in a variety of uses. WT