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How about a digital speedomoter display for my bracket corvette — Parallax Forums

How about a digital speedomoter display for my bracket corvette

vettezr1vettezr1 Posts: 77
edited 2011-08-29 22:00 in Propeller 1
Hey guys its been a while I have been busy long story short I drag a 1982 corvette the speedometer used to work but that was before I fixed it,lol I shopped around for ready made stuff very expensive and my vette has a mechanical cable I was wondering since I have so many prop boards hydras could someone think of a way that maybe I could use the tach output off the magneto and calibrate it that way? I do drive this on the street sometimes and believe it or not going slower than the posted speed limit will get you pulled over faster than speeding they think you have something to hide then they start with the pipes the roll cage the nitrous bottles all the bs I can not just replace the old speed dash becuase I cut it up to add my auto tach with shift light trust me there is no room in a vette so I have to use in dash you can not let stuff obstruck view in 9 second class and guess who gets to decide your view!
anyway one other thought was to buy a GPS unit with speed display and build it into dash but the numbers are pretty small I have a ton of display for prop boards that would look awesome in dash Oh this is a carb no computer straight up bracket 10.5 so no can or odb to pull signals off of thanks guys

Comments

  • FranklinFranklin Posts: 4,747
    edited 2011-08-28 08:51
    What do you have for inputs? speedo cable, electrical pulse, what? You need to build/find some device that provides a n input from the wheels of driveshaft. You could wrap a magnet to the drive shaft and read that with a hall sensor or read the passage of the front universal with the same device and a magnet.
  • vettezr1vettezr1 Posts: 77
    edited 2011-08-28 13:03
    yea that would not really work I paid a smal fortune to balance drive shaft,, the only thing I have beside the mech cable off the tranny is the magneto has a tach wire that drives a tach I was thinkng somwhow use tach pulses to calibrate to actual speed but problem would be RPM differant RPMS for each gear so 5K in 2nd gear would be entirely differant than 5K in 4th MPH wise I might just go with my GPS IDEA Nuvi makes one thats 5 inches it just seems like such a waste to see mph
    I just saw some cool ones at the track the guys did not use prop chips prob
    pics but no one will tell thier secrets
  • Cole LoganCole Logan Posts: 196
    edited 2011-08-28 13:04
    What type of magneto do you have in your car. I would figure if it has a tach output it shouldn't be that hard


    P.S. Just wondering what type of motor are you running.
  • kwinnkwinn Posts: 8,697
    edited 2011-08-28 16:54
    vettezr1, as you noted in post #3 the speed for a given rpm would depend on what gear you are in. It would be trivial for the Prop to calculate the speed from the rpm if you can provide the gear information. That is possible, but it would be simpler to use an optical encoder on the end of the speedometer cable. That would also be a good start on a digital dashboard.
  • Cole LoganCole Logan Posts: 196
    edited 2011-08-28 17:10
    Depending on what transmission you have in your car I would bet that you can buy an electrical speed sensor to go where the current mechanical cable is plugged in at.

    One problem I just thought of is that using the tach signal you would have to know what gear you are in.
  • Duane C. JohnsonDuane C. Johnson Posts: 955
    edited 2011-08-28 17:28
    Put a magnet on both sides of the drive shaft.
    That way it would remain balanced.

    Duane
  • idbruceidbruce Posts: 6,197
    edited 2011-08-28 17:31
    vettezr1

    Nope that won't work. There are too many factors. Firstly, you have different gears in the transmission that vary the output. Secondly, monitoring the driveshaft won't work either, becuase you also have gearing in the rear end. In most cases, or at least this is how it was done in the past, the speedo gear that goes into the trans matches the output on the gear ratio of the rear differential, plus I would assume tire size was accounted for.

    On the other hand, if you know the actual gear ratio of the differential, then you could monitor the drive shaft and apply the ratio to your arithmetic, but you must also compensate for the diameter or circumference of your tires to obtain MPH.

    You could monitor you MPH electronically with a hall effect on your wheel to measure RPM, and then apply the math for the diameter or circumference of your tires to obtain MPH.

    Bruce
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2011-08-28 17:54
    You need to retrofit your car with a pulse generator.

    This guy shows how he did that on his Jag; it's all the same, a thousand variations on a theme.

    Hope this get Propellery soon.
  • idbruceidbruce Posts: 6,197
    edited 2011-08-28 19:02
    @PJ

    That looks pretty cool and they claim:
    Dakota Digital speedometers can be calibrated for nearly any tire/gearing combination. Once the gauges are installed you will calibrate them for your combination using a simple push button calibration routine.
  • Cole LoganCole Logan Posts: 196
    edited 2011-08-28 20:07
    I've looked in to useing them before on project cars. Its cool the way the work basically you hit the calibration button, drive a known distance ( I think its a mile), Then it hit the button again. That way it can learn how many pulses are in one mile which it can use to tell you your speed.
  • Mark_TMark_T Posts: 1,981
    edited 2011-08-29 08:10
    Duane C. Johnson wrote: »
    Put a magnet on both sides of the drive shaft.
    That way it would remain balanced.

    Duane

    Or paint one side white and one side black and use an optical sensor? The paint is guaranteed not to throw the shaft off the balance (and can't come loose at high speed ;)

    Oh and with two optical sensors it can tell you if you are reversing or not!
  • kwinnkwinn Posts: 8,697
    edited 2011-08-29 21:59
    Mark_T wrote: »
    Or paint one side white and one side black and use an optical sensor? The paint is guaranteed not to throw the shaft off the balance (and can't come loose at high speed ;)

    Oh and with two optical sensors it can tell you if you are reversing or not!

    The only problem with that is that the drive shaft is exposed to a very dirty environment so the optical sensor and paint marks would eventually be covered. Simpler to use a speedometer cable and put the sensor in the cabin.
  • idbruceidbruce Posts: 6,197
    edited 2011-08-29 22:00
    @kwinn

    He must have been joking, hence the smiley. :)

    Bruce
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