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I've been thinking about this for manual transmissions, to ease RPM matching.

Has anyone ever seen an additional dial (on the same tachometer) that shows the clutch plate RPM? In order to match RPM, one would line up the two tach dials.

Benefits:

  • Smoother shifting (no bringing engine down/up to speed)
  • More reliable downshift (RPM match while dropping the clutch)
  • Easy to diagnose transmission issues (synchro clutches worn etc)

Cons:

  • Needs an additional tachometer

Thoughts? I think this would be a cool idea and useful in many cases, however rebuilding the dial cluster seems like the most difficult part (will probably need specially machined parts and nested shafts for both dials)

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  • Never been needed on all the vehicles I have driven / worked on and been passenger in. In fact the foreman (when I was an apprentice) showed that it was possible to change up (and down) on a 6-speed box on a large tow truck without using the clutch - he did this just to emphasize the point about matching engine and box speeds...
    – Solar Mike
    Sep 19, 2018 at 15:14
  • With a digital display the dual reading would be easy... Just need a sensor for the gearbox input shaft, and that is not difficult once the box is out...
    – Solar Mike
    Sep 19, 2018 at 15:16
  • Agree with Mike on the digital displays. The 2nd tach input is where you're going to have fun, though. You'd have to have some type of input from off of the input shaft in the transmission in order to get your clutch disk rpm, because there is no way to get it from the plate itself (without extraordinary means), considering it is completely encased in the pressure plate and flywheel. The easiest place would most likely be attaching it somewhere on the snout the throwout bearing rides on without interfering with its operation ... just a thought.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Sep 19, 2018 at 15:22
  • Yeah that seems like quite a challenge to figure out - especially with a transaxle. The would also require making the entire dash digital which while cool (and a computing project I'd willingly take on), I don't think is worth the cost without further justification
    – Brydon Gibson
    Sep 19, 2018 at 15:26
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    You don't need any additional sensors, except something to input the position of the gear stick. You already have the road speed and the engine RPM. You can calculate the clutch RPM from the road speed, if you know the gear ratios.
    – alephzero
    Sep 19, 2018 at 15:48

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This is a bit long to fit in a comment, so:

It would be cool, but, ultimately, you've got to get a feel for the rev differences between gears in your particular vehicle. Once you get in the habit, you won't be looking at the tach, anyway.

That said, you could always put a couple of magnets on the flywheel (180* apart) and use a Hall effect sensor. Plug that into an Arduino. Calculate the RPM from the sensor, and update whatever gauge you have.

There are a number of examples out there of doing this with stepper motors, etc.:

http://engineerexperiences.com/tachometer-using-arduino.html

https://www.youtube.com/watch?v=0lwu3mAKyXc

If you get a good sensor, you could potentially do this without the magnets by reading the teeth on the outer edge of the flywheel. (And, I'd be really, really careful about attaching anything to the flywheel or clutch plate.)

Edit That'll give you engine speed, not input shaft speed. Oops.

Another option would be to put the magnets on your driveshaft. If you had a way to tell what gear the car is in - which you can calculate based on engine RPM and vehicle speed or driveshaft speed - you can do some basic arithmetic and extrapolate the flywheel RPM.

There are a lot of options here, but, ultimately, I don't see this being useful beyond the bling factor.

Now, if you wanted to build an "automatic" manual, and have the computer control the clutch and rev match for you, this isn't a bad place to start.

Update

Ultimately, the best place for the sensor is near the input shaft, but you'd need something on the shaft to sense. You could also do this optically, I suppose. :/

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