beatngu Posted February 14, 2006 Share Posted February 14, 2006 what exactly is double clutching? Quote Link to comment Share on other sites More sharing options...
rsmartyn Posted February 14, 2006 Share Posted February 14, 2006 At first when I saw your question I thought what the hell is this. But when I saw your age I said okay, a new generation that loves Macks and B's in particular so here's your answer. In heavier trucks without sycronized transmissions internal gear rpm's have to match the driveshaft rpm's when you shift. To up shift sometimes you have to let out on the clutch while in neutral between gears to slow down internal gears to accomplish the shift (usually when your clutch brake ain't workin'). Most of the time though it's when you're down shifting. In that case while shifting from the higher gear to the lower one you have to let out on the clutch and speed up the engine (while in neutral) to match internal trans rpm's so you don't grind the gears off. It's kinda funny tryin' to put in words something you do all the time without thinking. Anyway, when you get on the road you'll learn fast (hopefully not at the expense of a trans, rear or driveshaft). Quote Ain't a Mack? Take it back! Link to comment Share on other sites More sharing options...
convoyduel Posted February 14, 2006 Share Posted February 14, 2006 Basically, to add to what RS was saying, all cars and light duty pick-ups have synchronizers in the transmission that activate when you push the clutch pedal. The synchronizers align the speed of the gears in the transmission to the speed of the engine and the driveshaft. This was primarily done so that absolute morons with no mechanical aptitude can put on make-up or yap on the cell phone while bearing only a minimal responsibility for the 3000 or 4000 pound piece of machinery they are operating.On heavy trucks, there are no synchronizers, so it is up to the driver to match the RPM's and gear selection with the driveshaft speed. As soon as the driver runs through a gear, the shift to neutral begins a process where the driveshaft speed will increase or decrease with the speed of the vehicle (depending if it is on flat ground, or up/downhill). Also, the gears in the transmission fall out of synch as part of them turn with the engine rpm input speed and others turn with the driveshaft output speed. To get into the next gear, the driver has to select a gear that can bring the two speeds back into synch, and make the final adjustment to complete the match using the engine RPM's. Many drivers will float gears, shifting progressively up or down without using the clutch. This process is accomplished by moving the gear selector to the neutral position at the split second that the drivetrain "slacks" as the driver eases up on the accelerator. As the next gear is selected, the driver brings the engine RPM's to a speed that puts the revolutions of the gears on the input side of the transmission to the speed of the revolutions of the output side of the transmission from the driveshaft. As everything falls into synch, the gear level is eased into the next gear. Think of a farm tractor pulling a wagon hitched together by a pin. When the tractor is pulling, there is tension on the pin. If the tractor slows, the wagon drawbar will ease forward and compress against the pin, but there is a precise moment when the pin is being neither pushed nor pulled. At that exact moment, the pin can be pulled, much like the gear selector when floating gears.Double clutching is often used when a transmission is tight or less forgiving for floating gears. By pushing the clutch when leaving a gear, it eases the tension in the driveline, but it releases the link between engine RPM's and the gears on the input side of the transmission. Releasing the clutch puts power back to the front side of the transmission to keep turning the input gears. By applying the clutch a second time as you select the next gear, it allows the drivetrain some slack again to make matching the next gear a bit easier. Mack transmissions are notoriously tight and not as conducive to floating gears as other transmissions such as Eaton Fullers or Rockwells. Quote Link to comment Share on other sites More sharing options...
Freightrain Posted February 14, 2006 Share Posted February 14, 2006 If you go find the twin stickin question,you'll find the video I made driving my B model. I was doing the "floating" gears routine(no clutch usage). You match the speed of motor to drivetrain speed and the gears just fall together without a click.I'm lucky...my old Mack tranny is more than loose....LOL!! It floats gears easily. Quote Larry 1959 B61 Liv'n Large...................... Charter member of the "MACK PACK" Link to comment Share on other sites More sharing options...
Dick815th Posted February 15, 2006 Share Posted February 15, 2006 The old military M123A1C 10 ton tractors, with the Mack TRDXT72 transmission.......my experience with floating was learned in them.After I got out of the service, for several years I drove a '57 Chevy with a 327 and Muncie 4 speed. I lost the clutch in it once, out in the country several miles from home. Not wanting to pay a wrecker bill, I put the Muncie in 1st and hit the key. The 327 started and I was headed home, drawing on my "floating" experience in the 10 tons. Ease off the throttle and change gears, not rushing things. Ran one or two rural stop signs but got it home and in the garage. Quote Link to comment Share on other sites More sharing options...
David Posted February 15, 2006 Share Posted February 15, 2006 The old military M123A1C 10 ton tractors, with the Mack TRDXT72 transmission.......my experience with floating was learned in them.After I got out of the service, for several years I drove a '57 Chevy with a 327 and Muncie 4 speed. I lost the clutch in it once, out in the country several miles from home. Not wanting to pay a wrecker bill, I put the Muncie in 1st and hit the key. The 327 started and I was headed home, drawing on my "floating" experience in the 10 tons. Ease off the throttle and change gears, not rushing things. Ran one or two rural stop signs but got it home and in the garage. I did this once with a R model when the clutch cable broke. Quote Link to comment Share on other sites More sharing options...
Lmackattack Posted February 15, 2006 Share Posted February 15, 2006 I did this once with a R model when the clutch cable broke.My first day driving an R model went like this!. I had Just got my CDL the day before. I was at a stop light, put the truck in first gear as the light was about to change green. next thing I know the truck starts to move foward with the cluch "released" . I call the boss to tell him I think I tore up his truck and he says "put the compound in low and bang it into first.....figure it out...click" I drove the rest of the day like that. learned alot about the 2 stick 6 speeds that day.LOL Quote Link to comment Share on other sites More sharing options...
rsmartyn Posted February 16, 2006 Share Posted February 16, 2006 Wait a minute, I thought the clutch on a 2 stick 6 speed (mud stick) was just for red lights or reverse or puttin' in the PTO. You mean I was supposed to be using it to shift that DM686SX all this time? Quote Ain't a Mack? Take it back! Link to comment Share on other sites More sharing options...
Lmackattack Posted February 16, 2006 Share Posted February 16, 2006 you dont need the clutch for shifting just to start from a dead stop or for the clutch brake pto etc... Quote Link to comment Share on other sites More sharing options...
rsmartyn Posted February 17, 2006 Share Posted February 17, 2006 you dont need the clutch for shifting just to start from a dead stop or for the clutch brake pto etc...I was kidding. Quote Ain't a Mack? Take it back! Link to comment Share on other sites More sharing options...
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