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Author Topic: Non-contact tachometer  (Read 1203 times)
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Hey everyone,

The speed feature of my bicycle computer does not work when the bike is mounted to the trainer. The sensor is on the front wheel, and the front wheel does not turn while on the trainer.

Options considered so far include an encoder that spins with (contacts) the trainer cylinder (a ~3 in. diameter cylinder that contacts the rear wheel and rotates positively with the rear wheel). Also, I am thinking about some sort of non-contact tachometer.

The trainer cylinder is very shiny. Could I paint a black mark on the cylinder and reflect some sort of light off of it and count the number of times that the black mark interupts the reflection?

I have no experience with light sensors. Would the cylindrical reflecting surface create a insurmountable problem with this type of sensor?
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Should be fine with reflective method, you can check on SFE, they have some sensors for line following robots.
Other way, attach a magnet and do a reading with hall sensor, similar to bicycle computer does.
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can you adhere a small magnet on the wheel?  if so you can use a hall effect sensor.  it'll act as a switch everytime the magent gets close.  you just count the pulses at that point. 
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Are you trying to make your existing bicycle computer work when the bike is on the trainer, or add something completely separate?
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@scott_fx gave the best answer. Thats also how its done professionally.

The light reflecting method has it own problems. You have to account for dust, ambient light,  reflective-ness of the material.

A magnet will take all the guess work out.
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You should have gotten proper rollers, then the front wheel would work and you'd get better training as far as balance and steering.

Too bad you can mount the speedometer stuff on the rear wheel.

You should be able to do the light sensor/black mark thing. But test your sensor & wiring with black mark on paper first. A photodiode or phototransistor should do great, just make sure you know what light band it's sensitive to (most I see cheap are for IR, filter out visible light and need an IR led though red led might do in a pinch as they put out a lot of IR). You might want to put the sensor at one end of a tube so it only sees a small spot especially if sunlight is present. Get it to be able to change a pin read as digital input from LOW to HIGH and back and then you should be set to instrumenting your roller.
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Thanks for all the replies everyone.

I am going to try to find a way to attach a magnet to the trainer cylinder. There is some clearance issues that I will have to address. I have heard of magnetic paint, that would be a godsend. McMaster sells it for 30 USD a quart, though. I just need a few drops!

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Are you trying to make your existing bicycle computer work when the bike is on the trainer, or add something completely separate?
I am trying to make something separate. I want to make a few visualizations in Processing to show time remaining on particular training intervals and I am going to have the bike speed sent wirelessly to the screen. I am still in concept stage, so let me know if anyone has any suggestions.

For example, certain regimes call for 20 min warm up, 15 min hard push, 3 min rest, 15 hard push, etc...

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You should have gotten proper rollers
DEATH TRAP
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Pittsburgh, PA, USA
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Nahhhhhh! Getting used to rollers, set them up in a semi-narrow doorway so your shoulders will hit the sides before your go off, LOL! Once you're cranking along you'll hardly ever touch, if you're much good.

Once you get used to riding on rollers you don't just ride down the white line on the edge of the road, you ride down the -center- of the white line. Really, what you learn is balance and control beyond hack riding. You will not only never get that with a fork stand, you will likely get worse using a fork stand.

There is another way you might pick up on RPM's. On the rear forks you might clip on a setup to have the valve stem interrupt a light beam wide enough that the spokes won't.



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I am going to try to find a way to attach a magnet to the trainer cylinder. There is some clearance issues that I will have to address. I have heard of magnetic paint, that would be a godsend. McMaster sells it for 30 USD a quart, though. I just need a few drops!

Get one of these small neodym magnets (which look like watch-cells) and stick it with 2-component-glue onto the wheel.
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