Tuesday, April 14, 2009

Using the Clutch on a Big Rig

I've talked before about how to shift a "big rig," and I did mention the clutch - but only a little bit about it. Clutches in a truck/tractor are different than one in a car with a manual transmission.

The main difference is the clutch brake. Since the heavy duty gearboxes do not have synchronizers, when the engine is idling, the truck is parked and the transmission is in neutral, the gears all spin. So, the clutch is locked in, the gears are spinning, and the driveline isn't moving. When the clutch is pushed in, the gears still spin. The transmission will not shift into gear until the spinning gears speed matches the non spinning driveshaft. Which means the gears have to be stopped by grinding them until they do. Or, you can use the clutch brake. When the clutch pedal is pushed all the way to the floor (Peterbilt or Volkswagen Beetle) or firewall (pendant style), the brake is engaged and stops the gears from spinning. The operator usually has the gearshift "riding" the gears waiting to "catch" the moment when the teeth mesh - it is just as likely that the gears will stop in a position where they will not mesh than if they do. So, as the gears slow to a stop, one tries to slip it into gear without grinding - just enough to help the clutch finally stop the spinning, successfully getting the thing into gear. If the gears don't line up, one eases off the brake to allow the tranny to roll over a bit until the gears do mesh.

These brakes do wear out, and they are adjustable. I hear a lot of gears getting rounded off in truckstops because of out of adjustment brakes - or they are just worn out. Years ago, the transmission had to be pulled to replace the brake - it is on the input shaft right by the clutch. But, a good mechanic can take a "blue wrench" (cutting torch) to the old brake and a install a two piece unit. The skill is not cutting up other things whilst waving the torch around in the small access hole.

So, a good driver gets into the habit of only using the bottom end of the clutch travel to stop the tranny when getting the thing into gear - if said driver likes to use the clutch to shift, then only the travel required to disengage the clutch is used.

The other thing that is different about using the clutch in a big rig is that starting the truck rolling when idling is the recommended course of action. In a gasoline engine, we all gun the motor just enough to reach the necessary torque level required to launch the vehicle in the proper gear, feathering the clutch so the motor keeps running. Well, in a diesel truck, the motors are all about torque - considerable amounts of torque. Comparing torque curves of a gasoline motor vs a diesel would show the gas motor peaking at a fairly high rpm level, while the diesel will have a fairly flat curve that spreads over a narrow rpm range.

So, winding up the motor and dumping the clutch is a very very bad idea. A gas powered vehicle might smoke the tires, but a diesel? With eight huge contact patches compared to two small ones? Not so much. Plus, we are talking a lot of torque, multiplied by the lower gears. Enough torque to twist driveshafts in two, or break universal joints, strip splines on axles or just plain shear them, or break teeth in a rear end. In short, there are a lot of bad possibilities.

So, a good driver selects a lower gear that experience has taught him/her that will start the truck and it's load easily without undue stress or excessively slipping the clutch. It isn't necessary or all that great on equipment to start a truck rolling in the lowest possible gear each and every time - that super low gear is generally specced for moving the truck as a very slow speed for some special reason rather than sprinting from a stop light.

So, a driver starts the truck in a lower gear without slipping the clutch much, the truck is launched and rolling. What next?

Well, it isn't "gouging" on the loud pedal and winding the motor to it's cutoff, like I see so often in truck stops. Yep, pulling out of the fuel island in first gear, winding the motor, twisting the chassis and shifting six or seven times is less than impressive. Because they are unduly stressing the driveline. All a good driver should try to do is ease into the throttle enough to roll the truck up to speed just enough to be able to shift gently into the next gear, until the transmission is in some of the higher gears. Then, the torque isn't multiplied so much, and the driveline can stand the full measure of the power the motor has available. Only "pour the coals to 'er" in the higher gears. Unless, you as the consummate trucker, enjoy sitting in the waiting room of the truck repair shop cooling your heels until the parts are shipped in from across the country.

One of my pet peeves is a "throttle blipper." You know the type. They can't do anything with a manual transmission vehicle without stabbing the throttle several times before taking off. This is actually pretty fun with a Harley equipped with straight pipes, and most riders can't seem to resist. But, a diesel isn't a freaking Harley, and blipping the throttle while trying to get the tranny to stop is counter productive, plus having the motor wound up when starting to release the clutch is going to cause excessive clutch wear, among the other hazards mentioned above. Most guys that do it have it so ingrained they don't even realize what they are doing - it's like a dog turning around three times before it lies down.

What inspired this post was a dumbass at the west Flying J in OKC the other day. When the throttle is opened on a lot of the newer motors, you can really hear the air being sucked in. The sound is louder than the exhaust - and it almost sounds like a turbine engine in some respects. So, I hear this truck at the fuel island working up rpms sounding exactly like a plane building thrust before takeoff. Suddenly, he dumped the clutch and the truck leaped forward - until he cleared the pumps, then he stood hard on the brakes, stopping the rig just past the pumps so he could go inside to get his fuel receipt. I had to wonder what was the point? He only had to move the length of the rig - why wind 'er up, launch like a drag racer about two or three gears too high, and shut it down all in about ninety feet?

But, like "they" say, be careful what you ask for. I'm pretty sure I don't want a look inside a mind like that.


Anonymous said...

Good info, that jackass must be at ever hook i fuel at to! T.B

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