Speedometer Head Physics Question for the Masses

[A383Wing]

See pic below (borrowed from a member here just for illustration)

If I cut off the round "ball" at the opposite end of the speedometer needle, (piece circled in blue), would there be any difference in the accuracy of the speedometer readings compared to if it was left on? Does this piece act as a counterweight in any way because of the long needle?

Or am I just overthinking this?

Bryan

[70B5Cuda]

I think you're overthinking it.

[Lennard]

I think you're overthinking it.

  I think it's just for esthetics .

[ws23rt]

The mass of the part you are talking about it way too trivial. Balance is a virtual non-issue.
A calibration will show the important stuff.

[XH29N0G]

agree.  I think most of the calibration is with the hair spring.

[440]

I'd be inclined to say it will have an impact unless you get it recalibrated, some claim the weight of repainting a needle has an effect, how much so? Thats anybody's guess. Would I worry about it? Probably not personally.

[aerolith]

Counterbalance effect is very important on certain dials such as a rev counter' but on a speedo not so much.

I would remove the rest of the pointer to be safe and when you get stopped by the Feds doing 150mph you have the best 'excuse in the world' except for a pregnant wife about to give birth?

Happy New Year and who looks at the speedo anyways???

Fast is Fast...

[c00nhunterjoe]

Fwiw, ive never seen a 60s chrysler speedo that was accurate, regardless of what "correct" gear was in the trans. They may be close in a window, but then either slower or faster in all the other ranges.

[Nacho-RT74]

part of the calibration is on the spring coil and part on the magnetic section.

think on this: The return spring coil will react diferent to the needle weight change. Also the aluminium cup magnetic section will have a diff inertia with the needle mass. They are in fact counterweighted with some drilled holes on cup

NOW... how much can be affected and readjusted JUST giving more or tension to the coil spring ( since the magnetic saction can't be made at home ? Can't tell it.

If you get a diff needle, no matter from a diff car but same shaft diameter, I'd try that first to know the variation

I got one speedo damaged and trying it fo fix it broken the needle too. Fixed with glue. Since  it was damaged at the needle shaft bushing which is the return spring coil retainer too, I had to "calibrate it" once made to fix this bushing. Used a home drill which on specs tells is a 550 rpms. Used it at reverse speed with an old speedo wire end hooked up to and calibrated the spring coil tension setting the speedo up to 53/54 km/h ( the calibration spec is set at 60 miles at 1000 rpms, so I made made the maths ). I guess the glue used will affect the "accuracy" ( if any is real ) because is a small ammount of spoxy, even lighted it up at max as posible. Couldn't use crazy glue because got a tiny minimal piece ( allmost unnoticeable ) missed, and epoxy will filled it. But my car is dissasembled, so can't tell yet how accuracy is at this moment

Odometer won't be affected since that's a direct drive relation, not a inertia relation

[green69rt]

Fwiw, ive never seen a 60s chrysler speedo that was accurate, regardless of what "correct" gear was in the trans. They may be close in a window, but then either slower or faster in all the other ranges.

First of all, I agree with the above.   Old speedos are pretty bad.   You would,probably be better off trying to calibrate the thing then worrying about the pointer weight.

Next, if you look at how the thing works you have the force created by the spinning magnet against the return spring.  So the faster you go the faster the magnet turns and the further the pointer moves until the force of the spring balances it and stops the movement of the pointer.  The weight of the pointer ONLY affects how fast the needle moves (inertia.)  The needle could weigh 10 lbs (balanced) and the force causing it to move would still cause the pointer to move to the point where the spring balances it out.  The 10 lbs would probably cause the pointer to move so slow as to be useless but it would still move (assuming no bearing friction)  and it would still stop at the same point.  Only reason, that I can see, that they are so light is that a fast response is desired, after all you don't want to accelerate up to 60 and the speedo catch up 2 miles down the road!

Edit, forgot to add that the pointer shaft needs to be balanced.   BUT,  would bet that pointer only weighs a couple of grams so any balance issues are masked by friction of the shaft bushing.

Bottom line, don't sweat it.

[green69rt]

Final question, why are you thinking of cutting off the end?