The same applies to all of the powertrain components. My rule of thumb is 95%, says Johnson. If I have a big block engine that is creating 1,800 ft.lbs. of torque, I would want to find a transmission that is rated for at least 100 ft.-lbs. more. If you are running right at the maximum, you are pushing it. Something will work fine when it is new, but when it gets a little wear and tear and age on it, and you get a little bit of driver abuse, that buffer protects it.
The High Cost of Over-specing
According to David McKenna, Mack Trucks, an over-specd truck results in misspent money, added unnecessary tare weight and a poor ride.
For example, if a customer over-specd the engine needed for an application, then he will most likely be heavy on the front axle, he explains. This is going to decrease the payload capacity of the truck because the heavier engine is eating up payload. So with each and every load, the customer could be giving up 400 to 500 lbs. of payload. That may not sound like much; but when tallied up over the course of a year, it has a substantial impact on the bottom line.
In todays economy, with the impacts of higher fuel prices, more expensive emissions equipment and an overall higher cost of doing business, the name of the game is to have the most efficient truck, not necessarily the biggest truck or the biggest engine, says Bryan Howard, Daimler Trucks. Our goal is to help the customer right-size or, in this case, right-spec the truck so that the business goals are met.
Weight and cost share a complex relationship. Generally speaking (not exclusively), the lighter you make the chassis, the higher the maintenance costs related to wear-outs and premature component failures, says McKenna. It really depends on the business model of each company. A lighter truck can carry more product and generate more revenue per trip, which is a good thing. However, this must be balanced with the added expense of maintenance and repair of lighter crossmembers, suspension components, brake drums and life cycles of smaller displacement engines and lower torque capacity transmissions.
There are ways to decrease tare weight and still maintain GVWR and durability of the truck, notes Howard, such as the use of an aluminum cab. The aluminum cab allows us to decrease the tare weight of the truck and still provide our customer base with cab durability performance.
Another example is through collaborative design with body manufacturers. For years, the chassis manufacturer and the body manufacturer each designed their piece of the puzzle somewhat in a vacuum. In some cases, you ended up with a vehicle that was way over-designed once you married chassis and body together due to the combined strength of the chassis frame and the body frame, Howard comments. We pursue opportunities with body manufacturers to collaboratively design and spec truck chassis with these things in mind. In some cases, we have been able to eliminate hundreds of pounds of weight by things such as removing frame liners where they are not necessary due to the strength the body frame adds to the entire vehicle.
Steer axle location is also significant. An axle-back chassis tends to have a shorter turning radius, since the wheelbase can be shorter for the same cab-to-axle dimension, says McKenna. Wheel and tire type and size also determine wheel cuts. Most axle-forward chassis are for use either on-road or in states that recognize Federal Bridge Laws, where overall length in combination with inner axle dimensions play into maximizing the legal vehicle Gross Combination Weight Rating (GCWR).
It is true that, at the end of the day, the overall wheelbase and axle spacing will be driven heavily by local regulations, agrees Howard. But there are two primary drivers of maneuverability. The first is wheel cut. There are no two ways about it the greater the wheel cut, the greater the maneuverability of the truck. If a customer knows that maneuverability is going to be important, then this should be an area of focus. Wheel cut is affected by wheel and tire choice, so keep that in mind.
He adds, With regards to set-back/set-forward axle position, a set-back position will increase the maneuverability of a truck... However, in some cases, it may make sense to spec a set-forward axle to achieve increased payload and a more desirable axle load distribution. You will lose some maneuverability because a set-forward axle position effectively increases wheelbase and generally has a slightly less available wheel cut. There is some give and take here and the decision is generally driven by determining which is more of a commercial advantage for the customer between maneuverability and payload.