Friday, February 10, 2012

Mechanical Speedometer / Odometer. A look inside

Hello and Welcome,
On the last blog post I was working on getting my vintage Schwinn Exerciser working a little smoother and quieter. If you read the post you know I replaced the noisy Speedometer with a quieter one that was a near perfect match. Checking the comments afterwards, one of my regular readers and supporters "Ryan" commented that I would probably repair and re-use the original Speedometer. I laughed to myself when I read it thinking, "What the #@$* do I know about speedometers?" Answer: Less than nothing. But I sure would like to get a look inside of this old speedometer/odometer. Just to see how it is put together. And maybe get a "general idea" of how it works. So reef the sails! Here we go into uncharted waters again!
Above: The whole process starts here. The actual worm drive unit on the Exerciser. The spinning wheel turns the large gear by way of a tang. (you will see the tang in a moment) The large gear turns the worm gear which the cable is attached or inserted in to. The cable is held in place by a threaded retainer collar.
Above: This is a side view of a nearly identical worm drive unit. The next pic (a break-down) will give you an idea of how it all goes together.
Above: A worm drive unit broke down. This clearly shows the worm gear (white) inside the housing. Also you can clearly see one of the four notches in the large gear that fit the tabs on the drive-tang which are also clearly visible. Now that we have covered where the process starts, lets move on (or up) to the Speedometer/Odometer unit.
Above: First thing is to remove the thin nut. Then remove the threaded collar. The threaded collar holds the speedometer in place inside the housing or cover. Next I will remove the bezel & glass.

Above: The bezel and glass are secured to the housing by these metal tabs that are folded over the lip that goes all the way around the face. These tabs will need to be pried up and straightened.
Above: Using a small screwdriver I pry each tab up and as straight as I can get them.
Above: Now the bezel and glass can be removed. I want to do this with the glass facing upwards. I do not want the speedometer falling out and getting damaged. After reviewing this, I think it would be better to remove the threaded collar after removing the bezel and glass. Less chance of dropping or damaging the speedometer/odometer.
Above: Judging by how clean the inside of the housing looks, I would say it did a fine job keeping the speedometer protected from the elements. Now I can get my first look at the inner workings of a mechanical speedometer.
Above: The face looks fine, no signs of rust or dirt. The odometer wheels look clean and the speedometer needle looks undamaged. No obvious problems here.
Above: Notice the white wheel has a gear on the end. This makes sense since the white is the "tenths of a mile", and the first wheel to turn. I suspect that there is some kind of pin or peg system, that one wheel turns the next after a complete rotation. I will find out latter if I am on the right track. But for now I want to see how the rotation makes it from the cable to the first white odometer wheel.
Above: The thing that looks like an auger (marked A1) is a worm gear. Gear (A1) is inline with the cable input. It turns a more normal looking gear with angled teeth. Each time the worm gear completes a rotation, it turns the angle-toothed gear one tooth. This is called "gear reduction". But this gear-reduction takes place more than once on the way to the first (white) odometer wheel.
Above: Ok.... The (W) marks the far end of the slanted tooth gears shaft that is turned one tooth per rotation by the steel worm gear (A1) in the last photograph. At this end of the axis or gear shaft (W) is a worm gear (not seen) that turns gear (A) one click per rotation. Now (B)at the opposite end of gear A's axis or gear shaft turns gear (C) one tooth per one rotation. (C) is the gear attached to the 1/10th mile white wheel. This is how gear reduction works. Each time a "worm gear" turns a "slanted tooth gear" rotation is slowed. And by the time the rotation reaches the "white 1/10th mile wheel" it is rotating at the proper speed to turn the white wheel one digit per 1/10th mile that the front wheel of the bike travels. I get a headache just thinking about how they calibrate all this correctly. This is why God made Engineers!
The above photo is a different speedometer. Used for example only
Above: First I turned the mechanism by inserting the small hand screwdriver in the cable receiver as I did to the speedometer/odometer that was seized up in the last post. That is when I heard the noise.
The above photo is a different speedometer. Used for example only
Above: Although this speedometer was not seized up, it did need lubrication in the port hole on the side of the threaded cable receiver post. So I gave it a drop of Easy Lube oil and the sound went away.
Above: So now I`m going to hook up my power drill so I can watch the mechanism work. I have asked my volunteer photographer to take a pic of the speedometer working.
Above: It`s Alive!!! My favorite line from Young Frankenstein (see video section) Next I am going to attempt to explain how the mechanical speedometer mechanism works.
Above: Known as an eddy-current speedometer. First, The cable runs straight through the first worm gear directly to the speedometer mechanism. The speed of the cable is not reduced
along the way (like the odometer). The magnet at the speedometer end of the cable is rotating.
Here is a shot of the actual mechanism. As you can see it is basically the same as the diagram I found doing an image search.
Following Quote is from How Stuff Works "As the magnet spins, it sets up a rotating magnetic field, creating forces that act on the speedcup. These forces cause electrical current to flow in the cup in small rotating eddies, known as eddy currents". I knew that....LTMS Yeah right, I had no idea!

Above: A shot of the actual magnet and speed cup. I can't wait to see how it works from here!
Ok Me again, In the above pic of the speedometer you can see the copper colored spring located directly behind the black face plate. Now lets hear (read) what How Stuff Works has to say about it.
Another quote from how stuff works.
"The eddy currents create a drag torque that does work on the speed cup. The cup and its attached needle turn in the same direction that the magnetic field is turning -- but only as far as the hairspring will allow it. The needle on the speedcup comes to a rest where the opposing force of the hairspring balances the force created by the revolving magnet."
Above: Who would have thought all that was going on inside this simple looking device. As the front wheel spins faster so does the magnet inside the "speed cup".
This makes the magnetic field stronger and those eddy currents bigger. And when all this happens the speedometer needle reacts. The faster the speed the faster the magnet spins and the needle reacts to all this accordingly. So when you slow the rotation of the front wheel, the magnet slows decreasing the magnetic field and the eddy currents become smaller and the needle drops. It is sort of like a compass except the magnetic field is created by the mechanism. And the needle reacts to that field similar to how the needle on a compass reacts to the Earths magnetic field. Amazing Stuff!

PS: I was unable to locate a photograph of the inventor of the eddy speedometer Otto Schulze who patented it in 1902. His design (or variations of it) were used in virtually all automobiles until the all-electronic speedometer appeared in 1993. If anyone can locate his photograph Please post it on the face-book page.

Above: Every once in a while I get to refurbish or restore a bike for someone special to me.
This tiny Trek is for someone very special to me and to all who know her. And tomorrow is her third birthday, I hope she likes it!
That`s all I have for now. I`m still trying to figure out what my next project will be. I have a bout 25 projects to choose from. So until next time, Please RIDE SAFE and remember to Always RESCUE, RESTORE & RECYCLE
Cheers, Hugh


  1. HAPPY-BIRTHDAY!!! and best wishes on their next trip around the sun. Good post. I think that plastic worm gear in the wheel assy could be an engineering failure at attempting to limit the life of the speedo, I refer to it as planned obsolescence. The conversion of mechanical energy to electromotive force seems little high tech for the bicycling universe. I guess it led to the bottle-dyno,and eventually the handlebar mounted GPS.

  2. Thanks John,
    The birthday party was great, and she loves the bike :) Seeing the modernization of bicycle accessories over the years has been incredible.
    Gone from hanging front lanterns to handlebar mounted GPS units. But the basic design of the bicycle itself really has not changed much at all. Which is a testament to what a great invention the bicycle was and still is today.
    Cheers, Hugh

  3. Well it didn't take long for you to get into the old Speedo! LOL. Great breakdown of the inner workings. Kind of funny that the speedo "accessory" is about 10 times more complex than the bicycle itself. Great 3rd birthday present I am sure she will be thrilled not to mention riding in style.

  4. Posts such as this are precious gems.

  5. Hey Ryan,
    Thanks for the idea. I was surprised by what I found in side that speedometer/odometer. I find the rotating magnet in the speed-cup fascinating. And the resulting magnetic field and eddy currents equally fascinating. Not at all what I expected.
    And you make a good point, the speedometer is much more complex than the bicycle itself.
    By the way, my little friend loved her birthday present :) Which really made my day.
    She always makes me smile, so it was nice to return the favor.
    Thanks again for another good idea!
    Cheers, Hugh

  6. Hey Steve,
    I really do appreciate that. Knowing I have a few readers who are engineers by trade, I tried very hard to get this one right. I always look forward to reading your comments, especially on the technical stuff. Thanks again.
    Cheers, Hugh

  7. Hi Hugh -

    Here's an idea for your next project:

  8. Hey Jay,
    Thanks for the lead. You must be psychic! I have been working on a unicycle related post all week. It is not about building your own unicycle, but this is still amazing.
    And I have been considering a post about building something in which unicycle parts are used. In fact, it was going to be part of my next post. But it was not turning out right so I shelved that part of it for now.
    I will be posting my unicycle related post tonight or tomorrow. I just need to finish final assembly. Ok I`m thinking of a number? LTMS
    I will put the unicycle build link on the links list.
    Cheers, Hugh

  9. Great post. I found you in my search for a mechanical odo/speedo for a friend's bike. The newer ones seem to be cheaper construction than the old. Anyone have an older, working speedo/odo (not digital!) for an adult bicycle? Thanks!

  10. Hey Tom,
    I suggest that you search on E Bay for a NOS Vintage Bicycle Speedometer. And you can also place your request on my blogs (Hugh's bicycle blog) face book page. The link is in the right column just below the followers. Then you can jump over to "old ten speed gallery`s" face book page and post your request there as well. Good Luck
    Cheers, Hugh

  11. Been looking all over for a mechanical odometer, without speedometer. I used to see them on Schwinn bikes from the '70s. A peg fixed to one of the spokes would turn a small wheel on the odometer which was fixed to the frame. The unit itself was small, simple, no rotating cables, it just turned a count of wheel rotations into mileage. Would like to get ahold of one for 27 inch wheel.


  12. High Hugh, how do you break down the wheel hub part, mine need a cleaning and was wondering how to open it. I was able to remove the clip ring and the Drive Tang, but I cant seem to remove the seal (mine is metal) - any suggestions?


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