Transmissions 101, Part 1: The basics - why so many gears?
Given what we do for a living, diesel powered engines are vastly superior to the gasoline alternative in almost every way. Every way but one: width of useful power band.

Marty Mullican
Technical Editor

the Eaton-Fuller FSO-8406A

A standard in many D Units

Automotive gas engines produce useful, economical power over a wide RPM range, usually about 1600 RPM (from about 1400 - 3000 RPM). Above or below that power band, the engine becomes either very inefficient, or can't produce significant useful power. Heavy-duty diesel engines on the other hand only produce useful, economical power in a narrow band of approximately 800 RPM (from about 1200 - 2000 RPM). Again, above or below that power band, the engine again becomes either very inefficient or can not produce useful power.

Because a diesel's power band is only one-half the width (in terms of RPM) of an automotive gasoline engine, we need to couple that power to the wheels in a way that keeps the engine operating in the RPM range where it works best. And because that range is so narrow (only 800 RPM), it takes more gear ratios to keep diesel engines running in their optimal power band as the wheels turn from 0 to 65-plus MPH.

So then, why don't diesel powered D-Units have as many gears as our Class 8 18-wheeled big brothers (often running 9, 10, 13, even 18 forward speeds)? Let me ask you this question: "Do you think your D-Unit is underpowered?"

From a four-wheeler's perspective probably so, but consider this; a typical D-Unit is significantly overpowered compared to our Class 8 18-wheeled brothers. By as much as 35%!

A typical 18-wheel refer or flat bed hauling coiled steel will be running right at gross weight (80,000 pounds). Add to that about twice the aerodynamic drag and rolling resistance of twelve more tires when compared to a D-Unit. Given the average horsepower of a modern Class-8 over the road tractor, 470 HP, you can calculate they are moving their loads with about 0.006 HP per pound.

A typical D-Unit on the other hand, with a 260 HP engine running at an average of 32,000 pounds (which is very high for an average given our deadhead percentages), yields a much more powerful 0.008 HP per pound. And all that power is pushing a vehicle with half the wind resistance and one-third the rolling resistance (only 6 vs. 18 tires on the road). On a HP per pound basis, D-Units on average are 25-35% "more powerful" than an 18-wheeled semi! Wow!

This is the reason why 18-wheelers need 9, 10, 13, and even 18 speed transmissions. Because of their power to weight ratios, they need these additional gears to keep running right in the middle of their engine's optimal power band. Because of our greater/better power to weight ratio, we can operate on the fringes of our power band, maintain reasonable performance and efficiency and get away with only 5, 6 or 7 speeds.

"But why do I hear about so many D-Unit operators having transmission problems?"

Unfortunately for the most part, the transmissions normally offered by dealerships on D-Unit sized Class 7 trucks are fully synchronized "city truck" gear boxes, designed for delivery non-professional "day drivers" to operate. This is done to shave a few dollars off the monthly payment. An excellent example here would be the Eaton-Fuller FSO-8406A, a.k.a. "Synchro-6," a standard in many

D-Units (see illustration below).

These transmissions have been engineered around durability standards based on an average of driving 30,000 miles per year, mating 190-250 HP engines producing 600-800 foot-pounds of torque to GVWs of 15-25,000 pounds. They do a fantastic job when operated within these design parameters, but start to wear significantly and break down when exposed to 90,000 miles a year of sustained high-torque operation in an overdriven top-gear at Interstate speeds.

A modern, 90-110,000 mile a year D-Unit goes well beyond what these transmissions were designed by their manufacturers to do. Think about it. Running 90% of the time at 65-plus MPH in top gear, our torque/power transfer (from engine to drive axle) runs significantly higher than any city truck would experience. Add to that overdrive ratios in top gear turning at Interstate speeds and it's easy to see why long haul Over The Road applications generate twice the heat, twice the wear, and twice the mechanical stress (torque) on a transmission than any other application.

What we need for the long haul, if a D-Unit O-O plans to keep a new truck for 400-500,000 miles, then trade, is a transmission that will handle our weight and speed, stay out of the shop and on the Load Board. A transmission that was designed from the very start with the specific intent of running 150,000 miles a year of high torque operation, spending 80-90% of its time in top gear at Interstate speeds. We need nothing less than legitimate Class 8 Over the Road strength and durability.

Mechanical Stress - Torque and Speed:

The two factors that stress transmissions the most are torque and sustained high speed (and speed means heat).

First let's look at torque: The most fuel-efficient way to operate an OTR truck is the hardest on transmissions. For maximum efficiency, a truck should be geared (the combination of engine RPM, MPH and drive axle ratio) to spend most of its time in top gear, just 250-300 RPM above the last shift point. Here, the engine is producing about 90% of maximum torque. When confronted with a grade to climb, still in top gear as the engine RPM lugs-down toward the downshift, it reaches 100% torque just prior to the shift point. Yes, our transmissions spend near 95% of their service life (running in the top two gears) with our engines producing near maximum torque! No "city truck" gearbox was designed to endure this level of mechanical stress to the tune of 90,000-plus miles per year!

Sustained high speed (heat): This one is easy. What percent of your mileage (not time) is spent at Interstate speeds versus running 30 MPH through town? What a difference this is when compared to the city delivery truck's mission! When your transmission is spinning fast, it's producing heat. City truck gearboxes just were not designed for hour-after-hour of highway speeds. Just to show you how serious this is, most "city truck" transmissions carry less than 20-pints of lubricant. A Class 8 transmission, about 30-40 pints. There aren't any more moving parts, this extra lubricant is used to dissipate heat.

The Class 8 transmissions I am talking about have been designed precisely for the type of service we need, and often carry five year, 750,000 warranties.

"But just how do you shift these multi-gear monsters, anyway (although I have a pretty good idea, I have always wanted to know for sure but have been too embarrassed to ask)?" That will be our topic in my next column.

Until next time, drive safely!

Marty