21 March 2010

a reliable and sleek bicycle odometer

Knowing how far you've gone is very useful when cycling. It can help you find your way with the help of a map (for ex. to “turn right after 1.5 km”), find which of a set of alternative routes is shorter, know how much workout you had, and estimate how far you still have to go on your trip. A classical odometer offers two counters, one resettable “trip” counter and one non-resettable “lifetime” counter, both having their own scale. The odometer proposed here shows just those two counters in the form of black digits printed on a white background, the digits sliding behind a little window. This has the advantage of being very readable, high-contrast, good-looking, robust and not using any power to display the numbers. The counting up is done using impulses from the bicycles hub dynamo and uses almost no power from the wheels. Resetting the trip counter is done mechanically with the user's gesture being the power-source. This construction ensures that the counter always displays the current mileage and is resettable even when the bike is not running or has not been running for a long time.

Here are some advantages of the proposed odometer:
  • high readability, always-on display
  • ease of use with only one dial (or button) to reset the counter
  • no parts attached to the bike wheels; only a small cable branching of the lighting system
  • no battery needed, therefore no need to swap batteries
  • very little energy use which does not noticeably increase the loss power of a normal hub dynamo
  • handle-bar mounting in-between brake lever and grips for sleek design, robustness, and discouraging theft

Here's a very sketchy drawing of how the odometer integrates with the handlebars. The top line shows XXX.X km and the bottom XX XX0 km.

And here some more detailed design information: the trip counter has four digits, the smallest counting 100 m. The life-time counter also has four digits with the smallest counting each 10 km. The longest trip will therefore be 999.9 km, which has been chosen because some people ride more than 100 km in a day and other might want to count trips of more than a day. The life-time counter will relapse to 0 after showing 99'990 km which is enough for most bicycles, although not enough for people who really ride a lot and keep the bike for more than a decade. (For an example see Rohloff Wanted day.) For those people there will be a luxury version of the counter with two times five digits: the trip counter measures each 10 m and the life-time counter goes up to 999'990 km. I would think that some people would buy this extended counter just to show off how far their bike is designed to go!

The two photos above show historic counters with little disks carrying the digits. To make the counter slimmer to integrate it better with the handlebars, the disks can be replaced by a little strip that turns around two pins as shown in the pseudo model on the right. Designing the electronics for this device will not be totally trivial because it has to deal with a wide variety of currents and voltages coming from the dynamo at different speeds and with lights on or off. There will several hidden digits to account for fractions of the distance displayed. Alternatively those values could be stored in some digital solid-state memory of a couple bits. In any case, storing this information won't use any power. The only power supply will be to increase the invisible counter which at every full interval will increase the visible counter. I wish I knew somebody who could figure those electronics out for me!

A variant with speedometer would use a classical needle to show the bike's current speed on a scale and it would additionally have a counter for the time of the current trip (accumulated time with speed >0). Then a second needle would show the average speed of the trip (which is the dividend of trip length and trip time so far). This second needle would be held in place with a gearing mechanism that doesn't need any power either. So when you stop the bike, the current speed will drop to 0 and stay there while the average will just stay where it was. When you sleep during a cold night that would drain any batteries and come back the next morning, the needle will still show your trip's average speed. I think that's reliability! And that's cool!

PS: Germersheim, I come!


Robert Jack Wild said...

Just want to add some motivation for this idea. I think that on a bicycle, just like in a car or on a motorbike, the odometer should be part of the vehicle and not just an add-on. I also don't like the fiddly plastic “bike computers” because they are fragile and stop working when the battery is empty. And I don't like dynamo-powered electronics, because it stops when the bike stops rolling (or at least when it's tiny self-charging battery is emptied).
Therefore, I want something that's part of the bike and just as durable as the bike – good for several decades! Of course the part will be manufactured as a module that can be installed on most bikes, but I want to install it at build-time of the bike (just like the pedals, lights, seats) and then never changed. And never stolen, of course.

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