Please let me out of the parking garage.

November 16, 2007 8:26 AM

Do the green light triggers for motorcycles actually work?

I've been riding my motorcycle to work lately. And while it's a fairly big bike ('93 BMW K75) it doesn't seem to trip the induction loops at the parking garage. I was thinking of getting one of those magnetic doohickeys, like so (basically a big magnet that sticks to the bottom of your bike), but I'd rather not drop $20 if they don't actually work.

So, two questions:

1. Do they work?
2. If not, why not? I'm familiar with the concept of induction, but Physics 262 was a long time ago.

posted by electroboy to travel & transportation (11 comments total)

I've seen DIY examples that seem to work (buy a powerful magnet, put in weatherproof capsule, duct-tape to bottom of bike). If those things have big enough magnets, the price doesn't seem excessive.
posted by Tomorrowful at 8:48 AM on November 16, 2007

Here is a primer on activating different types of sensors on bicycles; I used to have a friend who was a traffic engineer that I discussed this same issue with on occasion. He said that in GA, at least, the signals controlled by quadrupole type sensors were supposed to be sensitive enough to detect even a small motorcycle but that the were sometimes adjusted improperly. A couple of times I let him know that a certain intersection did not detect my bike and he was able to get a crew out there to calibrate it properly. I don't know if a private parking garage has to adhere to any kind of standard, but it might be worth a polite call to the management to see if they can adjust the sensitivity. In some states motorcyclists are allowed to run a red light if they do not trigger the signal and it is safe to do so.
posted by TedW at 9:06 AM on November 16, 2007 [2 favorites has favorites]

You can search Ebay for a large Rare Earth Magnet. This auction looks promising. In this case you are getting 10 magnets that are roughly the same dimension of the product you linked to for roughly the same price. Buy some double sided tape and you are good to go.

That product should work given that the magnet is strong enough.

I believe that there is actually 2 types of sensor systems that control things like parking garage doors. One, as you mentioned is the induction loop and the other one that I have seen was a pressure switch. I know it was a pressure switch because jumping up and down in a certain area would raise the gate :)
posted by remthewanderer at 9:07 AM on November 16, 2007

Yes, kind of (assuming the magnet is large enough, and that your bike is already close enough to the trip point). You can also enter the loop really, really fast which should cause a greater induced current, i.e., trip the sensor. But both of those have problems, you're probably better off asking them nicely to install a manual button that you can just push to be let in/out.
posted by anaelith at 9:20 AM on November 16, 2007

Have you talked to someone at the garage about the possibility of adjusting the sensors? I was having trouble tripping a light with my bike at a local intersection and sent an email to the traffic engineering office. Within two weeks they'd adjusted the sensitivity and I haven't had further problems.

On preview, what TedW said.
posted by cocoagirl at 9:27 AM on November 16, 2007

The best way to trip the sensor is not to place the bike in the middle of the loop. Set it on one of the corners of the loop assuming it was sawed into the concrete and is visible. Put the motor block directly above the corner.
posted by JJ86 at 9:59 AM on November 16, 2007

I don't have any insights but I wanted to thank everyone for theirs. I have this problem at several local stop lights and will put a few of these suggestions into practice.

But I don't think I'll be taping a magnet to the underside of my bike as that just seems like a recipe for ejecting it at high speed and causing damage to my bike or whoever is behind me.
posted by fenriq at 10:19 AM on November 16, 2007

I know it doesn't work with a gate, but as for red lights, even my Harley sometimes doesn't trip the sensor.. Once I've sat through a cycle of the light, I wait until it's clear and go.... I've never been stopped for this.

As for taping a big magnet under the bike, naw..I don't like that idea much... I wouldn't want to be behind me when that sucker comes off at 70 mph and starts bouncing down the road....
posted by HuronBob at 11:52 AM on November 16, 2007

Presumably, the underside of most motorcycles has some type of metallic surface, no? Wouldn't a magnet stick - thus reducing the possibility it would fly off? Perhaps a sufficiently strong magnet wouldn't even need any tape?
posted by GPF at 12:04 PM on November 16, 2007

I bought one for my 96 honda shadow, and I didn't see any marked improvement. I had better luck making sure to stop on the cement cutouts near the edge of the cutout.

Best case scenario, it works
Worst case scenario, you wasted 20 bucks.

Mine came with wire ties, so even if something knocked it, it wouldn't fall off the frame.
posted by garlic at 12:15 PM on November 16, 2007

It's not the magnet, it's the metal. The above humantransport link says:

"Note that it is the shape, size, and net conductivity of the material that matters most to this type of sensor system, not whether or not the object contains iron."

They work, usually, for my steel bicycle although I often have to position the bicycle tangentially to the circle, or hunt around if it's been paved and I don't know where the wires are.

From my experience I can't imagine a motorcycle not having enough metal, but I guess it does happen from what I've been reading.

If you haven't already, read the above link--you might just need to position yourself differently.
posted by RikiTikiTavi at 11:07 PM on November 16, 2007