Announcement

Collapse
No announcement yet.

Anybody know about simple speedometers?

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Anybody know about simple speedometers?

    I've got this speedometer off a junked exercise bike:



    I want to use it as a crude tachometer on my engine-driven winch, so I made this crude adapter:



    that fits in the end of the crankshaft:



    With the engine idling, my model plane optical tach shows 1100 rpm and the bike tach reads 350 rpm.

    When the optical tach shows 2100 rpm, the bike tach reads 650 rpm.

    When the optical tach shows 3100 rpm (top speed), the bike tach swings all the way around to 25 rpm. That would be about 1025 if you extended the rpm scale around.

    It didn't appear to hurt the bike tach to run it that fast, but I hate to go past full scale on any kind of instrument. I'm assuming it's a simple eddy-current magnet/cup/hairspring arrangement, in which case the answer is probably no, but I haven't opened it up.

    Has anyone here been inside one of these tach/speedometers? Is there anything that'll be ruined by running it past its full-scale speed?

    If it's not going to blow up, I'll just make a new face for the dial with the appropriate markings. Otherwise, I'll have to recalibrate it or use a speed reduction system of some sort.

    I removed the engine and winch from the tractor and made this support for mounting it directly on the trailer. I can adjust the position of the winch to get the line to wind evenly without having to jockey the trailer around. It's a lot easier to tow the winch around and set it up at the field.



    Roger
    Last edited by winchman; 08-08-2008, 05:54 PM.
    Any products mentioned in my posts have been endorsed by their manufacturer.

  • #2
    The controls are on conveniently mounted on the end of the winch away from the engine exhaust.



    The nut next to the handle locks the support arm in position. It's not that hard to move, but I'm thinking about making a jackscrew arrangement to move the support back and forth.

    Roger
    Last edited by winchman; 08-08-2008, 05:58 PM.
    Any products mentioned in my posts have been endorsed by their manufacturer.

    Comment


    • #3
      What is wrong with just using the optical tach?
      Gene

      Comment


      • #4
        Geez Roger, you've come a long way from those 12v wenches you were building. Planes getting bigger?

        It's been years since I had one of those speedos apart but as I recall, the spring has an adjustable tension. It's used during calibration and for the old bicycle types, set the wheel size. If you tighten the spring it will reduce the swing then you can calibrate a new face for the rpm. Not real accurate but beats nothing.

        Comment


        • #5
          I've played with these things a little in the past. It should not hurt it to run it past its top point, up to the point where the hairspring is wound so tight it starts to overlap or distort. Probably not a problem within a reasonable distance of the speedo's range, but if you keep going it will eventually foul the spring. However, its already questionable accuracy will probably get even worse as you get out of its usual range. If you're only using it to find a few target RPM's it doesn't matter much how linear it is, but don't count on being able to extrapolate accurately from a couple of known readings. If you want accuracy you'll probably have to use the optical tach to set points throughout the scale.

          Comment


          • #6
            The exposed rotating parts are either at the other end of the winch or between the drum and the engine. Neither are easily accessible while adjusting the throttle, and I'm not comfortable having other people reaching around next to the hot muffler and moving parts. It would be much more convenient to just look at the dial rather than a digital readout.

            My friends have some planes larger than the electric winches I've built can comfortably handle. Good motors suitable for the large 12V electric winches are getting expensive at $300+. Then you have to have the large capacity batteries and solenoid relays.

            After using the engine-driven winch some, we've found it much easier to use than the electric winches, IF it's set at the proper speed. I can do a pretty good job of getting it right by ear, but the other fellows I fly with have a little trouble setting it consistently at the correct speed for their planes. So, I want to make it easier by having a tach with a simple readout mounted right on the winch.

            For my purpose, I think I'll just make a new card to put behind the dial with rpms from 0 to 3000 spaced around 360 degrees.

            Mounting the drive end will be the hard part. I'll have to give that some thought. The first step is getting the thread size for the knurled ring that holds it in place.

            The ratio of optical tach to bike tach for the three measurements I took earlier is 3.14, 3.23, and 3.02. That's pretty consistent considering I wasn't being really careful about the readings.

            Roger
            Last edited by winchman; 08-08-2008, 09:13 PM.
            Any products mentioned in my posts have been endorsed by their manufacturer.

            Comment


            • #7
              Vintage tachometer

              On eBay as we speak------Look for vintage tachometers, Starrett manual jobby, 7 bucks plus postage, two days to go. Just like you used in physics lab. Accurate down to the gnats whatever, no estimating.

              Also about a dozen other more modern tachs which will go for less than ten bucks, name brands: Stewart Instrument, etc.

              G

              Comment


              • #8
                Pre-sets

                Why not take the "other user" out of the equation altogether?

                Pre-set a number of stops on the throttle linkage and (re)set to the stop required. These stops can be (pre)set using your digital tachometer.

                Comment


                • #9
                  I did take a speedometer apart once. I believe it was a bicycle type. It was indeed a a simple eddy-current magnet/cup/hairspring arrangement type device and almost impossible to damage with any speed you would manage with a bike, motor bike, auto, or even trying for the land speed record. I would think your exercise bike device is of similar construction.

                  One thing to consider is that often such instruments on bicylces or exercise bikes are not calibrated directly. Often they have a small wheel that runs against the tire and the RPM calibration is actually much slower than the actual RPMs of it's shaft due to the step up in speed due to the relative diameters of the pickup wheel vs the actual tire. If the pickup cable attached directly to the axel of the exercise bike, this will not be the case. I have seen it done both ways on exercise bikes I have used.
                  Paul A.
                  SE Texas

                  Make it fit.
                  You can't win and there is a penalty for trying!

                  Comment


                  • #10
                    Roger, this won't satisfy the creative urge and the "make it fit" attitude, but digital bike speedo/tacho/odo "computers" are available for a few bucks and should be easy to adapt.

                    Try dealsextreme for instance.
                    Just got my head together
                    now my body's falling apart

                    Comment


                    • #11
                      Or something like this,
                      http://www.manddsmallengine.com/tinytach/gasoline.html
                      I think I have seen them cheaper than this I cant remember where.
                      Gene

                      Comment


                      • #12
                        I think oldtiffie has the answer. There's too much play in the throttle cable to make a labelled quadrant next to the throttle lever, but I can install one right on top of the engine where the governor is located. The mechanism there is very stable and repeatable, and it's right in front of the person setting the speed.



                        It'll be simple enough to fabricate an indicator there.

                        Thanks for the good suggestion!!

                        Roger
                        Any products mentioned in my posts have been endorsed by their manufacturer.

                        Comment


                        • #13
                          nice to see the KISS principle still applies
                          Just got my head together
                          now my body's falling apart

                          Comment


                          • #14
                            So, here's the indicator I came up with. I bent a wire that went through some unused holes in the bellcrank, and epoxied it in place. Then I added the curved support made from part of a large plastic jar lid. The scale is part of an old flexible metric ruler, which I epoxied to the support.

                            It really has numbers on it, but they don't show in the pics.

                            Low throttle:


                            High throttle:


                            The pictures are taken from the operator's position.

                            Roger
                            Any products mentioned in my posts have been endorsed by their manufacturer.

                            Comment

                            Working...
                            X