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It’s All About the Gears

By Craig Weeks

Flashback: early 1980s, 1965 to 1973 muscle cars ruled the roads, and every high school gear head scrambled to save $500 for the rusty Mustang, Camaro, or Challenger with the “For Sale” sign in the window. It wasn’t about what it was but rather what it could be. At least here on the West Coast, it was all about the quarter mile—nobody cared about going around in circles. Blown big block, dual quads, dual-point distributor, four speeds, and don’t forget the 4.10 positraction rear end. You were really the big man on campus if you went to 4.56.

I know, it’s cool, but this is supposed to be about fire trucks, so get to the point. Well, let’s first understand what the gear ratio represents. We are specifically talking about the drive axle gearing. Using a 4.10 gear ratio as the example, this correctly would be printed as 4.10:1 representing 4.1 turns of the driveshaft to 1 turn of the wheel. So for the sake of argument, let’s say that this vehicle does 50 miles per hour (mph) at 2,000 engine rpm. Although there are many other factors affecting road speed, including transmission gear and tire size, let’s focus on the drive axle.

This is where it gets a little confusing. If I want to go faster at the same 2,000 rpm or if I want better fuel economy, I want a higher gear ratio, which would actually be a smaller number such as 3.73:1. But if I don’t care so much about top speed but want a quicker acceleration or to haul a heavy load, I would typically opt for a lower gear ratio, which is actually a larger number such as 4.56:1. And whatever the maximum engine rpm is combined with the drive axle gear ratio plus tire size and transmission gear ratio, that will be the maximum powered road speed—period.

Unfortunately, and based on my experience, the emergency apparatus industry in general does not embrace the full potential of the drive axle gear ratio. The primary focus seems to be on maximum road speed, and with the typical diesel engine governed to a maximum 2,100 rpm, options are quite limited. Here is example verbiage from a typical apparatus specification; “The drive axle gear ratio shall provide for a top speed of 68 mph at governed engine rpm.” This is known as being gear-bound.

My question is: why would you want to put the undue stress on the single most expensive component on the apparatus by running it at maximum speed while on the highway? The typical answer is, “It gives us the best all-around performance”. Really? What makes you so sure?

This is the muscle car mindset and it is wrong! But, it wasn’t always wrong. The early diesel engines used by the industry built their torque and horsepower closer to the same rpm—usually around 1,700-1,800—and mechanical governors could not be adjusted on the fly once the desired road speed was achieved. But, today’s modern electronic diesel engines with their torque curves in the 1,100- to 1,400-rpm range spool up too quickly and past their optimal torque curve when coupled with too low of a gear ratio, thus actually losing performance and efficiency. And, maximum road speed can be controlled electronically with a simple adjustment from a laptop. Don’t believe it? Ask for a performance report from the transmission manufacturer for this gear-bound gear ratio and then ask for two more consecutive gear ratios higher. Unless you care about startability on an 80 percent grade, look at your typical daily operating ranges, such as 0- to 30-mph performance and road speed on a six percent grade.

Here is what I am looking for when specifying new apparatus—let’s say a nontanker less than 50,000 GVWR with an NFPA-compliant, electronically governed, 68-mph speed limit. I prefer my governed 2,100-rpm engines to run no higher than 1,800 rpm at highway speed, so my starting point is the gear ratio that provides 68 mph at 1,750 rpm—fairly simple to find with a potential speed calculator available on the Internet. Then I request a performance report, available from most OEMs or a department’s transmission representative, along with one gear ratio above and one below. These reports can be quite large and detailed, but I focus on and compare specific operating ranges. If the higher gear ratio, for example, shows an improvement in those ranges, I will request one more gear ratio report higher until I see a performance decline. This is what I call the performance sweet spot, which is reflected in the specification.

Granted, the rear axle gear ratio is only one component of the drivetrain. But in my opinion, it is the most taken for granted and neglected. Stop the madness by trying to compensate performance with bigger engines and more horsepower, which will be discussed at a later date, and match all components to work in harmony. Let me ask this: do you experience diesel particulate filter (DPF) or excessive regeneration issues? If so, the engine may not be working in its prime operating range to perform passive regeneration because of an incorrectly specified gear ratio. Agree with me or not, you can’t argue with results.

CRAIG WEEKS is division chief/fleet manager for the Los Angeles County (CA) Fire Department and president of Specialty Fleet Consulting.

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