Speedometer

[Weldangrind]

I finally got a chance to try my new speedo today. I took a somewhat simpler approach to installation; the supplied magnet has a small bracket that it screws into for spoke attachment. Rather than use a rare earth magnet, I tossed out the bracket, drilled and tapped a hole in the driven sprocket and screwed the magnet in with a dab of loctite (despite LynnEdwards guidance about not fastening the magnet to a steel surface). I attached the reed switch sensor to the swingarm, adjacent to the sprocket. I presume that the plastic housing around the magnet must have insulated the magnet from the sprocket, because it works well.

I had the Beast up to 65 km/h (40mph) on a fire road today, and I don't really care to go much faster.

[LynnEdwards]

Originally Posted by Weldangrind

... I tossed out the bracket, drilled and tapped a hole in the driven sprocket and screwed the magnet in with a dab of loctite (despite LynnEdwards guidance about not fastening the magnet to a steel surface)....

Actually (if I understand your post correctly) you did nothing wrong. One pole (north or south) of your magnet is still sticking out away from the other pole which is attatched to the steel base of the sprocket. Thus any lines of magnetic force from the free end of the magnet must still travel through air to get the the other pole. Air is a terrible conductor of magnetic lines of force, so they spread out in space and can therefore be detected much further away.

In earlier posts it was suggested that the magnet be embedded in a drilled steel bolt making the whole bolt magnetic. This is wrong. Now both poles of the magnet are surrounded by steel. Steel conducts magnetic lines of force much better (about 10 times better for mild steel than air) so the lines of force from one pole to the other are shunted away from returning through the air. The're bottled up within the steel resulting in a much weaker external field. The external field is all the pickup sensor will see.

Look at it another way: Imagine a horseshoe magnet picking up a bunch of ball bearings. Pickup as many as you can and count them. Now repeat experiment, but put a steel bolt across the north/south poles of the magnet first. Now pick up as many ball bearings as you can. Obviously it will be less, because there are less external lines of force outside the bolt to suck up the ball bearings.

You want the triggering magnet to make as large an external field as possible. At the same time, the sensing element wants to have a core that is much more permeable than air and sucks in an concentrates external magnetic fields.

[TurboT]

Originally Posted by LynnEdwards

Actually (if I understand your post correctly) you did nothing wrong. One pole (north or south) of your magnet is still sticking out away from the other pole which is attatched to the steel base of the sprocket. Thus any lines of magnetic force from the free end of the magnet must still travel through air to get the the other pole. Air is a terrible conductor of magnetic lines of force, so they spread out in space and can therefore be detected much further away.

In earlier posts it was suggested that the magnet be embedded in a drilled steel bolt making the whole bolt magnetic. This is wrong. Now both poles of the magnet are surrounded by steel. Steel conducts magnetic lines of force much better (about 10 times better for mild steel than air) so the lines of force from one pole to the other are shunted away from returning through the air. The're bottled up within the steel resulting in a much weaker external field. The external field is all the pickup sensor will see.

Look at it another way: Imagine a horseshoe magnet picking up a bunch of ball bearings. Pickup as many as you can and count them. Now repeat experiment, but put a steel bolt across the north/south poles of the magnet first. Now pick up as many ball bearings as you can. Obviously it will be less, because there are less external lines of force outside the bolt to suck up the ball bearings.

You want the triggering magnet to make as large an external field as possible. At the same time, the sensing element wants to have a core that is much more permeable than air and sucks in an concentrates external magnetic fields.

Lynn Edwards,

This is like a science journal read. Well done! I am curious as to your background?!

[Weldangrind]

LynnEdwards, are you saying that there is a path of magnetic force that exists in an otherwise static magnet? In other words, the field is travelling through the air from north to south (or perhaps vice versa) at all times? I remember something about that, but I thought it only occurred in dynamic applications, like an electric motor.

[LynnEdwards]

Originally Posted by TurboT

This is like a science journal read. Well done! I am curious as to your background?!

Thanks!

As far as my background, I have a high school diploma, and I barely got that. I left home the day I turned 18 and could sign a lease on my own apartment. I was anxious to be on my own. Since then I have had a long time to learn new things. I have had a lot of training in electronics over the years.

I had to do a lot of reading about magnets and magnetic fields before I understood it enough to decide how to mount my magnet. Magnetics is a much more complicated subject than first meets the eye.

[LynnEdwards]

Originally Posted by Weldangrind

LynnEdwards, are you saying that there is a path of magnetic force that exists in an otherwise static magnet? In other words, the field is travelling through the air from north to south (or perhaps vice versa) at all times? I remember something about that, but I thought it only occurred in dynamic applications, like an electric motor.

Yes, and you can see (and probably have seen) this field. Take a bar magnet and cover it with a piece of paper. Sprinkle iron or steel filings (the debris from a grinding wheel after grinding steel works well) onto the paper and you will see the powder line up with the static lines of force from the magnet. Take a steel bolt and place it it the vicinity of the magnet and you will see how the pattern of the magnetic field gets distorted by its presence.

[Weldangrind]

I'm every bit as amazed by magnets now as when I was a child, especially rare earth magnets. Thanks for your explanation.