how does a wireless speedo work?
 

How exactly does a wireless speedo work? I'm about to buy a cat-eye micro-wireless speedo online and it says its "wireless", but that doesnt seem possible?? --what is wireless about it?

There's a magnet attached to a spoke, a sensor attached to the fork (in line with the magnet), the sensor senses each time the magnet passes by (one wheel revolution) and sends a signal to the receiver (mounted on or near the handlebars). The "receiver" is also the brains of the unit, you enter wheel size info into it and it computes speed, distance, etc. based on that-

Yeah i knew that, i thought that those were called magnetic speedo. So if those are called wireless, then what is a nonwireless speedo? thanks

AAhhhh, one with the sensor and 'receiver' connected with a wire.
What's not to understand? Wireless=no wire.
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Yep, that's it, it sends the signal from the fork to the receiving unit without a wire, therefore it's wireless. Follow the directions carefully when setting it up, they can be a little finicky, but once working, they're great-

i have one and those things are nice; very accurate. i had the LBS install mine; no troubles with it at all

I've got several "wireless" computers and I don't really see the point.

Less reliability, more weight, higher cost and twice as many batteries to replace. Give me some advantages.

Not really. The wireless units will always be suceptible to outside interference, especially moreso in the city than in country.

I ride in the city with mine every time I ride, never a problem. Have you ever used one?

No wire?

very carefully
;)

string?

and maybe a rolled sock.

You don't need the wire, which could get in the way of things, it has a cleaner "look" without the wire, and
you can use the same computer on two bikes (you just need a second sender unit, tuned to the same frequency).

Looks cool, no wires to break. If I lived in the city, i'd never use wireless - interference from every single stoplight sensor.

To answer the OP in better detail.

A wired cyclometer has a wheel magnet that passes a pickup placed near it on the fork. The pickup generates an electrical pulse every time the magnet passes it. That pulse travels up the wire to the cyclometer head where the pulse is counted and the interval between pulses timed. That information allows the cyclometer to calculate your speed and distance.

A wireless cyclometer has the same magnet except the pickup not only reads the magnetic pulse, it contains a small battery powered transmitter that sends that pulse as a radio signal. The cyclometer head has a receiver that reads that signal and does the same calculations as a wired cyclometer.

The tricky part is the sending unit and cyclometer head have to be aligned properly so the head "sees" the signal. Also, there are two batteries to be concerned with as both the sending unit and receiver have their own. And as noted, outside radio frequency sources can interrupt or interfer with the signal.

As for these claims of problematic interference, I must be lucky and live in a wireless cyclometer-friendly city. I've used them for about five years and never had a problem with that. What happens during these periods of interference?

no contacts to get wet and fail. my cateye wireless works flawlessly...i change the battery once a year like any other computer and at this point in life body weight is a bigger issue than a couple of grams of computer weight. :eek: .

ed rader

i live in the city. my cateye wireless has never failed or shown any signs of being influenced by outside interference.

ed rader

i think these claims are made by the same sorts who argued against index shifting. my wireless works flawlessly :eek: .

ed rader

I use a wireless computer, live in the city, and yes I'm plagued by interferance on almost every ride. The only thing affected is my max speed which reads 74.9 most of the time. For $10? And I can easily swich it between bikes? Yeh, I can live with that.


P.S. I think indexing is overrated.
P.P.S. The max speed spike seams to be so brief that even though it jumps to a very high speed it doesn't affect my average speed, which I do tend to be interested in.

Most do but it depends on where you live and how new your cyclometer is. Newer designs are much better at ignoring rf interference than the first models.

My funniest (funny=strange, not funny=hahaha) experience with RF interference was the heart rate monitor a friend was using at his doctor's recommendation after heart surgery a few years ago. It was a cheap, non-coded Nashbar unit since he didn't want to put out lot of money for temporary use.

We were riding easily and his heart rate was in the desired range when we passed an electrical substation. All of the sudden his heart rate reading spikes to 250 and then falls to zero! Talk about panic. It took a moment for us to realize why the reading suddenly got so alarming and it returned to normal about 1/4 mile later. His next purchase was an expensive but coded Polar monitor.

I'm about to leap to the improbable conclusion that we live in different cities. Either that or your city has a population of 2.

You can also easily buy a second wired mount/sensor for a second bike. I suspect it is cheaper for a second wired mount/sensor than a wireless sender unit (but I may be wrong).

I've ridden under 13800 volt transmission lines and never ever experienced any inteference with my wireless. The only thing that ever affected it was the antilock wheel sensor, which guess what uses a magnet to generate a pulse. The first time I saw that I couldn't understand how I could have ridden at 60mph and never even got on the bike. I had the computer sitting in a bag on the back seat of the car.

torgrot

Marginally. The DB4l wired unit is $7.50 and the 4LW wireless is $9. Not a huge difference. For that price i'd just buy the whole package as the wired one is only $12.50.

Never had any interference issues with my Shimano wireless setup with the exception of the L&M ARC light. Has anyone had any luck with shielding?

I prefer wireless for easier install.

It's very simple, like a wired speedo, you have a magnet, and a reed switch, the magnet trips the switch, sending a pulse. Inside the switch unit is a very small, extremely low power radio transmitter, so low in fact that from more then about 2m away, you would not be able to pick up the signal. Inside the speedo unit is a radio receiver tuned to the same frequency. Now each time the switch gives a pulse, the transmitter sends a signal, that the receiver gets, it gives a pulse to the speedo. Each pulse means you have covered the distance of one wheel rotation, from that it can track distance, given that speed is a relationship of distance over time, it can figure that out as well.

Like any other radio receiver the speedo is subject to radio frequency interference, RFI for short, there are thousands of electrical and electronic devices that generate RFI, including the speedo's internal computer. Now different speedos can be affected different ways, for example one may use amplitude modulation, where another uses frequency modulation, they can use different frequencies, even the information sent, one might use a simple tick, another might send a set of tones. One may use the minimum transmitter power possible -- to save on batteries, another might use the maximum power legal, to get a nice clear signal.

Now, RFI can do two things, it can overpower the signal from the transmitter - blocking it, meaning that some pulses are missed, or it can appear as extra pulses, either way your speedo has the wrong count, so the computed distance, and speed are now inaccurate.

If you need absolute accurate results, like with a training program, stick with the wired units, if your only looking for an approximation, then look at reviews for various units, and see if they had trouble. Realistically though, these units have been around for a while, I would expect as the technology matures, that manufacturers will be trying to deal with the issue of RFI. However the cheap $10 unit, based on a 5 year old design, is more likely to have an issue, then a $100 unit, based on last years design.

One of the biggest issues is pagers, they use powerful transmitters, and may drift frequency wise, they tend to be powerful enough that the pager will still work, even though the transmitter has drifted off a good deal. Because pagers use a short burst of signal, and it may be hours between pages, it's hard for regulators to catch them at it.

i live in san jose. the conclusion i have come to is you use a crap computer....you know they even make crap computers that are wired :eek: .

ed rader

Not a brifter fan, then, are you? ;) No indexing = No Brifters

This is not even close to correct. Wired computers have a very simple magnetically actuated reed switch in the sensor. Every time the magnet passes the sensor, the switch is closed for that moment. There is no "pulse". It's a very straightforward open or closed circuit. It's a digital circuit in the very simplest form - on, or off. The computer simply measures the period between the "on" states (assume "on" = switch closed) and calculates the speed from the tire circumference you enter.

If you ever suspect your wired sensor, or the 2 conductor cable (i.e, the "wire"), is bad then all you need do is pass the magnet by the sensor while monitoring the two cable conductor ends (which normally terminate at two contact pads on the computer head mounting bracket) with an ohmmeter. The circuit will show "open" or "closed" as you pass the magnet by the sensor (remember, it's a reed switch).

In the case of a wireless computer, you DO have a pulse generated by the sensor. A carrier wave is established between the sensor and the computer head and the carrier is modulated (ok, if it's frequency modulated then you'll get a momentary change in the frequency rather than a pulse on the carrier wave) every time the magnet passes the sensor. The sensor contains a Hall effect device that senses changes in a magnetic field. When cycling computers go screwy at traffic intersections, it's most likely that it's the Hall effect sensor - not the carrier - that's being affected. Trip sensors at intersections are, in effect, like a very big Hall effect sensor. They set up a steady magnetic field and when a large metal object is passed over it, the field sensor detects this and says "car over trip sensor" to the traffic computer.

One of the disadvantages of wireless computers is that, when riding in a tight group, your computer can get false inputs from other riders sensors that may be on the same carrier frequency as what your computer is using. One of my wireless computers will read the sensor as long as it's within 70cm, or 27.5", so it's easy to see why you could get a false input from someone on your shoulder. This is precisely why you see a lot of pros riding with wired units. All that being said, the algorithms in the head units are averaging the inputs out so, over time, they are still very accurate even if they do get the occasional false input. If you're doing a lot of group riding and having computer problems, then a wired computer probably makes sense.