Shake, Rattle, and Break

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Some replacement clutches last just about forever, while some give out in a month or two, maybe even less. Driver abuse? Possibly, but it could also be . well, a managerial defect. If you’re not getting acceptable life out of your replacement clutches, and if it’s not a question of drivers punishing the poor things, then perhaps you’ve innocently or carelessly spec’d an inappropriate product for the application, or you bought on price and ended up with a poor-quality “will fit” part. They do exist, those will-fits, and not all of them are made in strange offshore places because the patent on the old Spicer angle-spring clutch ran out a few years ago. You can buy them as new products, though not from the original manufacturer, or as remanufactured units.

Pardon the cliché, but you really do get what you pay for.

It’s largely a question of technology’s progress. The simple angle-spring clutches of the 1970s and 1980s were just fine when mated to the engines of that era. Engines that produced their power way up on the tach, engines that drivers knew they had to run out to the pin on just about every shift. And most importantly, engines that produced peak torque in the range of 1200 pound feet at 1400 rpm or more-and died more or less instantly at slower speeds.

Today’s engines, on the other hand, are building torque even at the clutch take-up point that would rival some of those older engines maxed out. That’s an exaggeration, but not by much, because it’s routine to see almost 900 pound feet of torque at 800 or 900 rpm. And by 1100, many of them have almost reached a stretch of the torque “curve”-and that word’s a misnomer if ever there was one-that’s actually flat all the way up to 1800 rpm. Even lesser engines are churning out 1400 or 1500 pound feet of clutch-wrenching torque at pretty low engine speeds, and the ever-more-popular 550s and 600s are shipping 1800 or 2000 pound feet or more direct to the clutch plate for furtherance beyond. Worse yet, they’re creating torsional vibrations that can build devastating spikes of torque.

And people want to buy a 1970s-style angle-spring clutch to deal with all that because it might save them a hundred bucks? If you’re in that category, think again. There are real differences between modern clutches and those old angle-spring types-and the challenges they face in everyday use. The 1970s clutch just doesn’t have the damping ability to deal with the crushing torsional vibrations generated by new low-rpm engines. Modern clutches, on the other hand, play a shock-absorber role. In fact, these so-called “soft” clutches are so necessary that they’ve become standard equipment on almost all heavy trucks.

The need became clear a few years back when it was learned that powertrains can literally self-destruct because of those vibrations, which occur when the natural resonance of the drivetrain is “excited” by the engine’s own resonance at certain frequencies. This amplifies the drivetrain’s dynamic torques and motions by as much as three times the torque-capacity rating of components like gear teeth and axle shafts.

In one test we know about, the driveline spikes arising out of a 1375-pound-feet Caterpillar engine actually reached 4000 pound feet.

Failures of the clutch-plate hub, springs, even the transmission input shaft and driveshaft universal joints-among many others-can all be attributed to operating a truck at or close to these highly damaging natural frequency points for extended periods. The modern clutch’s soft-spring and damping characteristics protect driveline components because they allow more “twist” between the engine’s flywheel and the transmission input shaft. They literally cushion the blows rendered by the engine’s firing pulses and send the destructive resonances to lower operating speeds where they can do less damage.

The cushioning effect is created by the springs in the driven disc that couple the clutch’s driven discs to the input-shaft spline. In softer clutches, they’re more compliant and they offer more travel, thus becoming better shock absorbers. It’s worth noting that spec’ing a soft-damped clutch assures warranty coverage for torsion-vibration-related failures that would not otherwise be covered.

Still, some people continue to buy old-style clutches for new-style work. John Aldrich, Eaton’s clutch aftermarket manager, fears that a lot of end-users are simply confused about the difference between old and new.

The modern “Easy Pedal” Eaton clutch (formerly Spicer) was introduced in 1990, partly to reduce pedal effort, but it also got a higher plate-load clutch to handle torque and horsepower. “Along with that we’ve changed a lot of the damper technology, the driven-disc technology,” Aldrich says. “Back before 1990, everything was an eight-spring damper and that’s about all we offered. Today we offer a six-spring VCT (vibration control technology), an eight-spring, nine-spring, and a 10-spring. There are a lot of variations in damper technology specially tuned for today’s engines.

“I sell reman product too, but my concern is that as a consumer that you get what you pay for and you get what you ask for.

“Say you go in and get a new clutch installed and you went from an Easy Pedal to an angle-spring [without knowing it] and the first time you push down that clutch pedal you think, Well it’s stiffer but maybe the other one was really bad. This is the way the other one was supposed to have been, and you just accept it. You think, Well, they really did fix it. It’s stiffer now so it must be stronger and more robust. But it doesn’t have to be.

“The damper technology in my opinion is the key,” Aldrich continues. “If somebody puts a clutch back in your truck that wasn’t approved as the standard original equipment, what are you doing to your transmission gears and synchronizers, or your driveshafts or axles? If the clutch wasn’t designed for that truck, what are you doing to the rest of your drivetrain? It’s one of those ‘pay me now or pay me later’ things. You’ll save a couple of bucks now with that cheaper clutch, but what are you going to spend for a transmission overhaul, or a driveline or an axle overhaul?”

Or maybe even engines? It can be argued that when you go to a stiffer pedal like the angle-spring, you put more load on the thrust washers in the bottom of the engine because you’re putting a load on the crank trying to pull the clutch back. Technology has come a long way in terms of dampers, with special designs for new fuel-efficient engines.

Charlie Allen, ZF Meritor’s director of sales and marketing, agrees that there’s confusion out there when it comes to clutch replacement and is concerned that people are buying on price alone when it comes time to replace a clutch.

“You have to ask yourself what you should put back in your truck,” he says. “You need to install something equivalent to what was there. If there was a long-travel, LTD-type clutch [ZF Meritor’s “soft” model], you need to put that back in. We-and I know our competitor does the same-go to great lengths to tune the system to specify the springs, to control the hysterisis. Some of these rebuilds and will-fits, they take the spring out and put in a spring that’s filled with rubber. They’re not going to perform.”

Why is there a market for these inferior replacements? Price is one argument-but not a very solid one.

“If you look at doing a clutch job, probably the biggest cost is the removal and replacement of the transmission,” says Allen. “Say it’s seven hours at $80 an hour. That’s $560, plus the cost of the clutch. If you want to get like-new performance, it’s going to cost a little more [to buy the right clutch]. But incrementally, the cost of the better clutch isn’t that great because your big cost is removing and replacing the transmission.”

That’s compelling logic, especially if you’re getting poor life out of your replacement clutches by going cheap. It varies by application and driver skill, of course, but an original-equipment-quality clutch should get you in the ballpark of 300,000 or 350,000 miles in ordinary linehaul work. Upwards of 500,000 miles isn’t rare. Aldrich says he even knows a Canadian who managed over a million miles with a faster-wearing organic facing-despite driving the mountains.

So if you’re not getting at least 300,000 miles out of a clutch kit, maybe you should look inward instead of blaming careless drivers and faulty products. If you haven’t taken enough care to spec the right clutch, the blame may be sitting right there on your very own desktop.

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Rolf Lockwood is editor emeritus of Today's Trucking and a regular contributor to

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