speedometer speed off, fixed

My speedometer has been off, since I got the bike back on the road. I've been using a GPS speed app on my phone.

I have an '84 KZ700, that has all the gauges in a single housing. I already rebuilt the tach (40 year old electrolytic capacitors weren't happy about being woken from their slumber).

I did a little bit of google-fu on how to fix an out of calibration speedometer. Mine uses a magnetized disc, inside what appears to be an aluminum cup. Research says the magnetism fades over the years.
Decided to bite the bullet and try fixing it. Can't really make it worse if its already off at anything over 15mph.

Supplies:
Stack of neodymium magnets (i had 5, about 0.100" thick x 0.300" diameter)
handheld compass
hand tools to get to and remove speedometer from housing.
a somewhat constant speed drill
square drive bit

Mine is easy. 3 screws to separate the housing halves, and 2 holding back of speedometer in.
Once removed, I put the square drive bit in the cable drive hole, and ran the drill as fast as it would go (full trigger).
I took note of the speed shown. In my case, it was about 51 mph. Per the GPS app, this should actually be about 65 mph.
This step is to get a reference on where the needle *should* show, at max drill speed. (NOTE: I had to run the drill in reverse)
Next, I took my compass and held it to the magnet ring. I slowly rotated the ring to show the "poles", and their "polarity".
Mine had 4 poles. 2 north, and 2 south. I marked each pole. A + for "north" and a - for "south".
I did the same, for the stack of magnets.

Now this is the "get your tongue at the right angle" part.
Hold the mag ring so a marked polarity is facing out. If north is facing out, swipe the SOUTH end of the magnet stack against it a few times.
The polarity of the mag ring pole and the polarity of the magnet stack pole should be opposite, to make the ring poles stronger.

I repeated this for the other 3 poles. I'm not concerned about unequal strength between them.

after each round with the magnets, I checked it on the drill. Each time, my speed got closer.
I had to do 3 passes, to get it to within 1-2 mph.


Took it for a short blitz this morning. At 30 mph on speedo, i had 30 mph on GPS app.
At 55 mph on speedo, I had 53~54 mph on app.

Hope this helps someone. If not, I still learned something, so still a win.
If anyone needs pictures of any of this, I can take a few.

Well done! I attempt not to pass up a chance to learn something new and your repair/fix certainly feel like I just did.

Cool approach to a common problem.

Some pictures would be nice, too.

I will take some pics this weekend. It's supposed to rain anyways.

My experience is most motorcycle speedometers read high. Maybe this procedure would work in reverse?

I learned something too. Great hack, solid physics, and a great example of applied problem-solving!

@ckahleer
As far as using this method in reverse to bring the indicated speed down, i don't see why not. i would proceed with great caution though. It might be easier to "reduce" the existing magnetism.
If you try it, and it does work, I'd love to hear about it.

It seems most of them read to high compared to GPS. I have been doing some work on the gages this winter, most of which consisted in refilling the damper cups but was thinking of the calibration. On the Z1 gages you can adjust the spring tension which should bring down the indicated speed.I would assume most gages have that capability That will be my next experiment 

@Cra-z1
from research and trial/error, i would stay away from the spring. the spring and magnet ring work together. the faster the magnet ring spins, the more force its going to impart on the cup. but also, the further the cup rotates, the more back pressure from the spring. Adjusting the spring will change the ratio between the two. When the magnetism fades, it makes the spring "too strong", the further it rotates. If you try to reduce the spring pressure only, it will make the lower speeds read higher. If adjusted too light, the needle may not even reach zero.

clock spring:
the further it's "wound", the spring force progressively increases. this is controlled by the spring material, length, etc. Adjusting the spring only changes the "preload", in a sense. it will still progressively increase as the shaft is turned, but at a different point range. 

magnetic ring/driven cup:
stationary, no force is imparted on cup. a static magnetic field is effectively inert in regards to the aluminum cup.
a moving magnetic field will induce a complementary field in anything conductive that is nearby. the faster the field is moving, the stronger the induced field will be.
when the strength of the field is also considered, a stronger field, at the same speed, will also induce more force on the cup.
This is called Lenz Law. Since we can't change the drive ratio of the speedo cable, we are left with proximity and strength. I couldn't see any way to get the cup physically closer to the magnetic ring. that leaves changing the strength/intensity of the field

see this for a demonstration:  Lenz's Law Magnet Falling through A Copper Tube or Pipe

I guess you could aim for accuracy at a "nominal" speed, and accept any over/under at other speeds.

I agree but yours was reading too low and was 15 mph off. I would be adjusting for a gauge that is reading to high by 8 MPH or so at around 60. Yes you may have some accuracy issues ( and you always will since that spring will never be linear) at lower and higher speeds  but better at 40-60 where you spend most of the time riding. Short of removing material from the magnet I would say the spring would be the best answer in a hight reading unit.