Need help on doing tug test and clutch

Yeah but the only way your output shaft speed will change is if you add throttle. Its speed is directly proportional to input shaft speed. Unless you folks are using the clutch to shift? Because then I guess I could see it but that's gotta be hard on the clutch because you're trying to accelerate the truck by using the clutch.

Proper double clutching is just enough to break engine torque. There is minimal speed variance between the clutch discs and flywheel.

 

No, the output shaft is tied directly to the rear end and is effected ONLY by the speed of the vehicle. The throttle changes the input shaft RPMs. Once you start moving the output shaft spins faster. The output shaft is tied to the trucks driveshaft and the input shaft is tied to the engine's crankshaft.

In a set gear, these shafts are all connected and the engine torque is transmitted to the back axle. As the vehicle gains speed the tach will show a rise in the RPMs. This is when you up shift. These big diesel engines put out so much torque they can continue to gain speed at idle. Honestly try it, you will be surprised at what you see that tach show.

 

No I completely understand how these transmissions work. I've rebuilt a few of them. I just really can not see how the speed of the driveline can change.

Walk me through this:

You put the truck in 1st, let the clutch out and idle along.

Shift to neutral. Road speed drops off slightly because there is no longer power to the wheels. Truck is essentially coasting.

Move the stick to 2nd. How do you manage to get the stick into 2nd gear when there's now an approximate 400 RPM difference between the main shaft and the 2nd speed gear?


It just does not compute in my mind unless you shift using the clutch. And now the clutch has to bear the brunt of making up that speed difference between the mainshaft and the gear so it can fall in.

 

It's called momentum. Remember low gear = more power, higher gears = more speed. As you start off the engine will provide more than enough POWER to speed the drivetrain. As this happens the engine RPMs will increase because of the ever-increasing change from power to speed. In fact, if you don't upshift you will at some point redline the engine and never touch the throttle. I have done it many times. I never used the clutch either. All I can do is tell you it can be done as I and several others have said. You really can't do this in a car or light gas power vehicles. You can do it in a truck because of the almost massive power and torque that big diesel puts out. In fact, I know a guy that is an instructor at a school near me. He teaches this to his trainees to help them understand just how powerful that engine is.

 

No, you are confusing the process. The RPMs are going to change, In fact, that's how you know to change gears. If your tractor is still showing basic starting RPMs to change to a higher gear is to lug the engine. The output shaft through the driveshaft will cause the RPMs to increase NOT the throttle. Unless you have never actually done this I can see how it can be confusing, just try it someday, you might be surprised at how incredibly easy it is.

 

Now I am totally lost. How the blue fuzzy #### can you redline that engine without touching the throttle? The ECM wants that engine to stay at a desired RPM (which with no throttle input would be idle). The ECM will command zero fuel injection if it needs to in order to keep the RPM at idle. It takes torque to move the truck so unless you're aiming the truck down a grade while doing this I can't see how its possible for the engine RPM to rise up as you grab gears.

 

In all honesty, please don't take my word for it. Just go test it out. I along with several others have stated we have done this, and it is not only possible it is a fact. I can see how it is hard to believe, I understand how you feel, just go do it sometime. I can't think of anything else to add.

 

This is kinda true but only in the perfect conditions. Yes if you have the right weight and slope of the road and start in low low you could theoretically get the truck moving with the right momentum to raise through the gears without throttle.
I spend a lot of time in NYC traffic and I'm very familiar with the lower half of my gear box. On a flat even road in a ten speed you'd be really taxing the clutch and motor trying to get north of 3rd gear without adding fuel. 1st and second I agree you can almost always let out the clutch with out fuel regardless of grade or weight. Whether you'll gain rpm has nothing to do with motor torque just weight and momentum. A truck with 1650 tq or 2050 is going to lope along at 600rpm until a grade is encountered. Going from 2 to 3 at 600 rpm will just bring the road speed up to whatever 600rpm is geared at in third. Above that gear your probably stalling the motor unless your going down hill which would mean momentum, not the motor got you above 600rpm.
Sure you can red line a motor without giving it fuel. You can even do it with the motor off. Next time your at a stop getting ready to go down a hill shut off your truck release the brakes and let out the clutch and watch her red line and explode.

 

Nobody said anything about pointing the truck downhill......

 

Exactly!

Now if going down a steep enough hill to let the truck accelerate in neutral, then yes what it increases in speed enough to go into the next higher gear and match the engines idle rpm you could. But on flat ground no way in hell. Have owned numerous trucks with 13 speed, 15, 15 rto, and 18 speeds, never going to happen in any of them, unless while shifting you turned the key off to, and just as the engine was about to stop turning you engaged the next gear and turned the key back on. This is so ridiculous that I am not going to post anymore on the topic.