Double-Clutching Transmission Shift Commercial Semi-Truck
What Is Double-Clutching?
The shortest description I can give is that you are matching the engine rpms to the rpms that the rear tires are going, as that rotational speed is brought into the transmission. This is needed so that you may match gears without scrapping through them, during the process of downshifting and upshifting. In my video, I show you both upshifting and downshifting.
Proper Car Transmission Technique
If you know how to properly shift a standard car manual transmission, this will be very easy. It not, you will need to understand the logic in that first, before moving on to double-clutching.
During a standard transmission shift in a car, say if you were going from 1st to 2nd gear, you will need to simply go through 1st gear until you are at a high rpm, push the clutch pedal to the floor, and change to 2nd gear, which will be at a lower rpm level. But since the two gears are so similar, the rotational speeds are almost the same, so double-clutching is not needed. Newer synchronized transmissions also help with this.
Proper Semi-Truck Upshifting Technique
During a semi-truck transmission shift, the gears are wider apart in rpm speeds, meaning that you cannot simply shift from 1st to 2nd gear easily, because the rotational speeds of those two gears are not as close together as it would be in a car.
Instead of simply pushing the clutch pedal to the floor and shifting to the next higher gear, you need to drop the clutch back out, while out of gear, so that the flywheel and clutch can touch momentarily, and the rpms of the engine and the rpms of the rotation speed of the tires can match correctly. You then push the clutch pedal back down the floor, and you can easily shift to the higher gear without scraping or grinding. It sounds a lot hard than it is, and my video above shows just how easy it is.
Proper Semi-Truck Downshifting Technique
The same holds true as the above paragraph, but instead of only releasing the clutch pedal back out as the truck is out of gear, you must rev the engine to a higher rpm level, to match that of the lower gear, because it is turning much faster. This technique is harder because if you over-rev the engine, the flywheel and clutch will be spinning faster than the rotational speed of the rear wheels and it will grind the gears. But if you under-rev the engine, you still are not high enough to match the rotational gears of the rear wheels, and you will still grind the gears. There is definitely a sweet spot and fine art to this. But once you get it, you will have it forever, the same goes for riding a bike.
As I wrote the gear diagram down below, it seemed to help me understand.
Every gear has a low and a high side, meaning lower rpms, to high turning rpms. So, you must bridge the gap in the middle, and make those rpms match more correctly so that you can have easier shifts without scraping gears.
Especially while downshifting, say if you were going from 3rd gear back down to 2nd gear, you must rev the engine back UP to the higher rpms, so that you can match the LOW side of 3rd gear to the HIGH side of 2nd gear. You must match the engine rpms as you come back down in gears.
Thank-you for taking the time to read my hub. Any ratings and/or comments are welcome.
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.