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Double Tachometer

Tach displaying ideal engine RPM with true RPM
  [vote for,

For my new job, I am required to hold a Commercial Driver's License, to which end I recently attempted to learn to drive a tractor trailer combination vehicle. The training included learning to "double clutch". (I know, I know: all the truckers in the bakery are throwing fish at me now, because real truckers don't double clutch. I had to do it that way to pass the test, OK?) Anyway, the hardest thing for me, which I never mastered, was what to do when I missed the gear I was trying to shift into. This leaves the vehicle coasting. I then had to try to choose the gear which was best for the roadspeed at which the truck was now moving (which is usually a moving target: increasing if you are coasting downhill, decreasing if coasting uphill), and figure out what engine RPM matched the roadspeed in that gear. Often I ended up giving up, coasting down to 5 mph and grabbing 5th gear in idle. Obviously, traffic around me was not happy with me. (Have no fear, fellow drivers: I will never be on the road again in this vehicle.)

I wished for a double tachometer. Due to the complexity of what I want to achieve, it would probably have to be a computer generated "virtual" tach, not physical needles. In addition to the usual engine RPM needle, one or two additional needles would be displayed. These needles would display the appropriate engine RPM for the gear or gears which would match the roadspeed. Under most roadspeed conditions, there would be one "gear" needle displayed, but at the top end of (for example) 4th gear, the 5th gear needle would appear near the bottom of the tach, following the 4th gear needle up. I picture the "gear" needles displaying the gear number at their tip. With the clutch engaged, the "engine" needle and the "gear" needle for the gear you are in would obviously coincide on the gauge.

Now suppose I'm shifting up from 7th to 8th on an uphill grade, but I miss or lose the gear. I am coasting now, losing speed and creating a traffic hazard. (True story.) I can look at the double tach, watch the "gear" needle marked "8" drop off the bottom of the tach, see the "gear" needle marked "7" appear at the high RPM end of the scale, moving counterclockwise. Now instead of guessing or trying to remember the nearest RPM/gear/MPH combination, I simply chase the "7" needle with the "engine" needle, and smoothly shift into 7th gear.

This tool would not be needed once a driver has a certain amount of drving in a particular rig, but it would be a great help to the trainee driver. I'm picturing it as a feature of driver's ed vehicles, truck driver training vehicles, and perhaps in rental trucks too. Rentals are often driven by people who don't drive such vehicles for a full-time job.

Searching the bakery, the idea closest to this one proposed a green/yellow/red display system to alert the driver if engine RPM is close to correct. In the discussion of that idea, supercat proposed something similar to my idea.

gardnertoo, Feb 15 2008

ericscottf's red/yellow/green light Real_20Shift_20Indicator
The closest any other halfbaker has come to the idea presented here. [gardnertoo, Feb 15 2008]

Dual Tach for Helicopters http://www.cybercom...ctures/F28_tach.jpg
Here's a picutre of the dual tach showing engine RPM and rotor RPM (different scales) in a helicopter [gardnertoo, Feb 16 2008]


       So what you're talking about is a tacho for the input shaft of the tranny, in conjunction with the engine tacho. Got it. When the two needles overlap, you're good for full engagement. Got it.   

       Bun. As a hypermiler, this would rock for P&G.
elhigh, Feb 15 2008

       What if you took a roadspeed input and geared it to a bunch of pointers. Each pointer would represent the rpm you would need to match that gear to the engine. Most of the needles would be off the top or clustered near idle, but a few would be in a functional range.   

       This would make a neat digital display for combination gearsets. However I would insist that you need to show each gear individualy so you can choose the apropriate gear and match revs.
WcW, Feb 16 2008

       Turns out this idea is sort-of baked, but not for cars. I've added a link to a photo, showing a dual tach for helicopters. The layout is very much what I am after. There are two needles, with the longer one displaying engine RPM and the shorter one displaying rotor RPM. (There is also a triple tach available for twin engine choppers.) Because engine and rotor are seperated by a gearbox, the scales are different. Some models are scaled 0-100%, so those scales match. The green regions are the appropriate engine RPM for powered flight and the appropriate rotor RPM for either powered flight or autorotation (hence the wider range on the rotor scale).   

       There is at least one "this really happened to me..." story on the web of a pilot goofing up his recovery from autorotation, chasing the rotor RPM needle with the engine RPM needle ("rejoin[ing] the needles" is how he put it), and safely getting the clutch back in gear.
gardnertoo, Feb 16 2008


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