Maintenance Q&A
Vibration, downspeeding and thermostats

By Paul Abelson, Senior technical consultant

Since many of you have written with essentially the same question, allow me to summarize them all into one:

I have a (year, make, model) with extra-heavy-duty brakes for shorter stopping distances. After my last brake job, I developed a vibration in my steer axle when I brake. What can I do about it?

This is a common problem brought about by the increased mass of newer, heavier drums. Corrosion may also affect it. It was discussed at the recent TMC meeting. Also, Meritor issued a technical bulletin on steer axle braking vibration.

As a result of being larger (16 1/2 inches vs 15 inches), drums and brake components are all heavier to take the increased stopping torque. Brakes work by creating friction that changes the kinetic energy of the truck’s motion into heat energy. The quicker you slow or stop, the more heat is generated. The heat is absorbed into the brake drums and then dissipated to air passing over the drums. With more heat, you need more mass to absorb it, and more area to release it.

More mass (weight) and larger size makes the balance of the total wheel end assembly (everything outboard of the spindle) a contributor to the situation. According to the Meritor technical bulletin, the three main components of the total indicator reading (TIR) – the measure of runout determined by a dial indicator – should not exceed 0.015 inches. Those components are drum runout or out-of-roundness, hub-to drum mating surfaces, and wheel bearing end play. According to TMC RP618, end play should be between 0.001 of an inch and 0.005 of an inch – although in recent years systems to achieve measurable and repeatable pre-load have been announced by suppliers Dr. Preload and Rather Engineering. But I digress.

Since you are having vibration, we can assume that TIR is greater than 0.015 of an inch, although it is a good idea to measure as each measurement taken individually can reveal problems in other areas. You should find the source of the runout. Disassemble both wheel ends and repair or replace any components in need.

Measure brake drum diameters. Both drums on any axle should be within 0.025 of an inch of each other. Using a caliper, make sure wear on the drum’s friction surface is under 0.08 of an inch. This allows for wear during the current service interval.

Verify that both drums weigh the same when new and that the original drum weight is adequate to handle the heat generated in the current application.

Remember, brake fade is caused by excessive heat, causing resins in the brake shoes to gasify, breaking contact between the friction-generating surfaces and reducing the conversion of motion to heat.

I’ve been reading a lot about downspeeding engines (operating at lower engine rpm to save fuel), and I’d like to try it. My truck is a 2015 International with a 450 horsepower Cummins ISX15. It has an Eaton 13-speed overdrive and a 3.36 rear end. It’s geared for 1,350 at 65 mph. I can get a Meritor 2.73 tandem pretty cheap from a friend’s wrecked truck. The front was mangled when a four-wheeler stopped in front of him, but his drives were OK.

What do you think of making the switch?

My first thought was to see how the numbers work out: 2.73 is about 82 percent of 3.36, so you’ll be dropping rpm at cruise about 18 percent.

You’ll be running 1,096 rpm at 65. Your engine is rated for peak torque at 1,200 rpm, so you’ll be operating 100 rpm below engine rated speed. While you may be able to cruise on a level road at less than 1,100 rpm, you’ll have problems getting there and even minor grades will be difficult.

Downspeeding is a good idea, but not in the aftermarket. For every 100 rpm that engine speed is reduced, there is an average 1 percent gain in fuel economy, but to make those 100 rpm increments work, they must be part of an integrated system. That involves more than just changing the drive axle ratios. Every phase of the drivetrain is involved, from air induction to fuel injection, transmission ratios and shift points, drive shaft design and construction and, of course, rear-end ratios. The changes are so great that Cummins changed their engine designations from “ISX” to just “X” to differentiate the newest models.

In order to get torque to peak at 1,000 rpm or, as is predicted for future engines, 900 or even 800, injectors must be redesigned and timing must be altered, requiring re-engineered electronic controls. Higher torque must be created for each revolution, resulting in higher loads on each component per revolution. Everything, from oil pump driveshafts to inter-axle driveshafts, must be beefier to take these greater loads. And, for optimum results, shifting should be computer controlled using an automated manual transmission engineered and programmed for that specific truck and operation.

I applaud your desire to downspeed, but doing it without integrating all the parts will result in a truck that will be virtually undrivable. Best save it for your next truck.

I read that diesels operate more efficiently when they’re warmer. Should I get a replacement thermostat that opens at a higher temperature?

At too low a temperature, combustion will be inefficient, but go too high and the engine will wear out more quickly. Consult your owner’s manual and get the thermostat recommended by the engine maker. It’s the result of a great deal of research and testing. LL

(Editor's note: When sending in questions for Maintenance Q&A, please include the make and model of your truck and your vehicle identification number (VIN) as well as your contact information. Paul Abelson tries to respond to every question, whether it's published or not. Send questions to TruckWriter@WowAccess.net)