Motorcycles and Traffic Lights

Try shutting the bike off and restarting it while stopped on the loop (providing you have an electric start bike and not a T500 from the 50's). I'm sure a thousand people will immediately jump on here and tell me I'm wrong, but it works for me every single time at a particularly recalcitrant stoplight on my way home.

ETA: Just read shawmutt's post. Not an urban legend in my experience, but I know anecdotes aren't evidence.
 
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A friend of mine always used to refer to "weights" when this sublect would come up. He would say something like, "I wasn't stopped over the weights". Finally I said, "You know, those things aren't triggered by the weight of a vehicle, right?" He looked at me like I was insane and said, "Of course I know that." I said, "Then why do you call them 'weights'?"

His reply?

"Because they make you wait!!!"
 
If anyone wants to know why permanent magnets don't work here...

1) When you're stopped at the light the magnet isn't moving relative to the wire in the street. No relative movement, no induced current. The wire just doesn't see that magnetic field.

2) The loop isn't even set to detect induced current. Instead it detects a change in *its own* magnetic field as this field is continuously built up and collapsed by the alternating current in the wire. A nearby (or large) conductor or ferric material will alter that field, and this is typically sensed by it changing the frequency of an oscillator using this loop inductance, analogous to changing the length of a pendulum.
 
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I rode a bicycle a lot, and I triggered the loops by tipping the bike down about 45. So I guess they work by having an AC current running through them, inducing a current in the frame of the bike, and detecting the impedance change by the change in the current of the loop. I suppose it could be a tank circuit detecting a change in the frequency, but that would be more sensitive to the size of the loop and also to rain.

Tipping a motorcycle is probably not practical, but driving to one of the sides of the loop might increase the probability of triggering it. Or just switch to a Fat Boy.
 
Pulvinar said:
If anyone wants to know why permanent magnets don't work here...

1) When you're stopped at the light the magnet isn't moving relative to the wire in the street. No relative movement, no induced current. The wire just doesn't see that magnetic field.

2) The loop isn't even set to detect induced current. Instead it detects a change in *its own* magnetic field as this field is continuously built up and collapsed by the alternating current in the wire. A nearby (or large) conductor or ferric material will alter that field, and this is typically sensed by it changing the frequency of an oscillator using this loop inductance, analogous to changing the length of a pendulum.
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The magnet will be moving before you stop. A magnet should change another magnetic field, in a similar way as metal will, as long as it is strong enough.
 
rjh01 said:
The magnet will be moving before you stop. A magnet should change another magnetic field, in a similar way as metal will, as long as it is strong enough.
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Hey, you've invented the DC transformer! And a generator where you don't have to turn the shaft!

Back to reality... The wire won't see the magnet's field because that field's lines aren't being "cut" by the wire. A conductor on the other hand can have an effect because in that case it's the wire's field lines that are moving (in an expanding or collapsing field due to AC), and the conductor cuts those lines (a change of flux linkage, to be precise), inducing a changing current in the conductor. That current produces an opposing magnetic field (Lenz's law) that the wire *does* see because it's also changing.
 
Pulvinar said:
Hey, you've invented the DC transformer! And a generator where you don't have to turn the shaft!

Back to reality... The wire won't see the magnet's field because that field's lines aren't being "cut" by the wire. A conductor on the other hand can have an effect because in that case it's the wire's field lines that are moving (in an expanding or collapsing field due to AC), and the conductor cuts those lines (a change of flux linkage, to be precise), inducing a changing current in the conductor. That current produces an opposing magnetic field (Lenz's law) that the wire *does* see because it's also changing.
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That sounds very much like how transformers work.

Lenz's law explained a bit better than above here http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html (scroll down a little and you will see it)

I guess I will have to buy a rare earth magnet and see if I can convince some traffic light that it is a car. Then I will know.
 
Pulvinar said:
Hey, you've invented the DC transformer! And a generator where you don't have to turn the shaft!

Back to reality... The wire won't see the magnet's field because that field's lines aren't being "cut" by the wire. A conductor on the other hand can have an effect because in that case it's the wire's field lines that are moving (in an expanding or collapsing field due to AC), and the conductor cuts those lines (a change of flux linkage, to be precise), inducing a changing current in the conductor. That current produces an opposing magnetic field (Lenz's law) that the wire *does* see because it's also changing.
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rjh01 said:
That sounds very much like how transformers work.

Lenz's law explained a bit better than above here http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html (scroll down a little and you will see it)

I guess I will have to buy a rare earth magnet and see if I can convince some traffic light that it is a car. Then I will know.
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Thanks, this is exactly what I was after with the OP - if the theory behind using a magnet was sound. I still am not clear on the answer, but I will take one of my big rare earth magnets and walk out into an intersection with no traffic and see if I can trigger the light with it.
 
If the starter motor running does it, a motor surrogate ought to too?

An electric motor would make a flux field that would vary in frequency as it changes speed. We don't as yet know the particular freq used by the coils in the road, but would it be real critical?

A small motor stuck to the bottom of the bike, and a push button switch ? Just give it a couple pulses when you pull up to the light? Remember, gain/lose of speed is a good idea, not length of pulse.

If freq not critical, hook the motor to the brake light switch? You use the brake when nearing the light

Starters use many watts. Maybe the motor would need a load? use a heater blower out of a car, with the fan on it?

If we knew the exact freq used, you could build a transmitter, but in the mean time, KISS. Second stage development might be a motor with a speed controller?
 
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madurobob said:
Hmmm.... I've heard some emergency vehicles have a device to trigger lights for them. Perhaps this is what they use?
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It depends on the area you're in, these triggered lights are generally only around intersections that have high volumes of emergency personnel. I don't believe they're very common otherwise, perhaps in big cities they are but I haven't paid too much attention while driving around in NYC or Boston. It's pretty obvious to see when it's being used because flashing light goes off when these types of lights are triggered to let the emergency personnel know that the light has been triggered for them.

It would also be very illegal.
 
I just stumbled on to this. I ride a lot and when ever I encouter a light that will not work I treat it like a stop sign. The worst lights are ones where you need to make a left hand turn . Nine out of ten times I end up running them. I will sit thru a cycle and if it does not go green I will wait until I have a clear shot and go . I have never been stopped or pulled over by the police for doing this either .
 
This doesn't really address the effectiveness of magnets, but I found this link last summer and the info improved my success at triggering lights: http://www.humantransport.org/bicycledriving/library/signals/detection.htm. At least for bicycles, the rims are what matter (and aluminum rims are better than steel), and you've got to position them in a "sweet spot" that can be pretty narrow, especially in older dipole loops (newer quadrupole loops are better).

There are a few more tips on this page: http://www.humantransport.org/bicycledriving/library/signals/green.htm

Your local traffic authorities might be interested in hearing about intersections that are consistently problematic-- the sensitivity of these things can be adjusted.
 
LTC8K6 said:
IIRC, some states allow motorcycles to treat automated stop lights as stop signs.
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In California it is revenue generator. Police hide near lights that don't work for motos when there is virtually no traffic. Moto arrives, waits for multiple iterations of lights in other directions to change, and realizes this bike isn't tripping the detector. After waiting for a safe gap (frequently not a single other vehichle in sight), proceeds cautiously through the red light (usually arrow). Gets ticket and loses in court. Since red light tickets are around $500, there is little motivation to change the law despite some serious efforts to do so (CA is broke). Safety is obviously a non-issue, since they find it perfectly reasonable to force a moto to make 3 rights and 2 u-turns to avoid a ticket due to faulty signal equipment.

The distance from the loop is the key factor. My stock height Jeep Wrangler doesn't trigger a few of these signals either.
 
casebro said:
If we knew the exact freq used, you could build a transmitter, but in the mean time, KISS. Second stage development might be a motor with a speed controller?
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You can determine the exact frequency by receiving it, as with a radio antenna. You could then duplicate that, reverse the direction, and re-transmit it back to the wire. All of that would require... A CONDUCTOR!

KISS, indeed.
 
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