Polar speed sensor over-reading

[mike]

I'd love for it to be true, but somehow dont think I managed to average 26mph over 3 hours last night (with a max. of 65mph  ).

Any ideas why my polar wireless thing is over-reading so much?   I dont think i've changed the battery in 3 years - could that be it?

(no, I havent fitted 2 magnets)

[andrew_s]

Interference from something else, like an LED light?

[Greenbank]

Dynohub?

[mike]

nope.  The only thing fitted to the bike is the polar thing...

[Jakob]

Mobile phones

[tiermat]

Power lines?

when I was working a staying over in that direction I had no end of problems with wireless kit (HRM and computer) due to the proliferation of low slung power lines.

A bit better now over in sunny Hampshire, but I did manage 104.4 mph as a max last week!

[mike]

nope, and nope.  It was overreading pretty much all the way through the 3 hour ride.  My phone was in my back pocket like always.

[tiermat]

One thing I remember seeing on Sheldon's site (IIRC) is that for some bike computers, if you have the magnet too close to the sensor then you get a "double bounce" effect with the sensor.  In the case I read about it meant that at speeds below about 10mph it read as double the speed, but once the magent was travelling fast enough so the sensor only saw it once instead of twice it reaad correctly.  Could that be happening with you?

Oh yes it was Sheldon, here is the link:

http://sheldonbrown.com/cyclecomputer-magnet.html

[border-rider]

Mobile phones

Not unless the handset itself is within a few cm of the receiver when it handshakes.

Base station signals won't affect bike computers

The usual culprit is lights, but failing that it's likely bounce as tiermat said.

[mike]

One thing I remember seeing on Sheldon's site (IIRC) is that for some bike computers, if you have the magnet too close to the sensor then you get a "double bounce" effect with the sensor.  In the case I read about it meant that at speeds below about 10mph it read as double the speed, but once the magent was travelling fast enough so the sensor only saw it once instead of twice it reaad correctly.  Could that be happening with you?

Oh yes it was Sheldon, here is the link:

http://sheldonbrown.com/cyclecomputer-magnet.html

that could well be it, I was using a different wheel to normal. P'raps I really did go over 60mph..

Will take a look tonight.

[Biggsy]

One thing I remember seeing on Sheldon's site (IIRC) is that for some bike computers, if you have the magnet too close to the sensor then you get a "double bounce" effect with the sensor.  In the case I read about it meant that at speeds below about 10mph it read as double the speed, but once the magent was travelling fast enough so the sensor only saw it once instead of twice it reaad correctly.  Could that be happening with you?

I think this is quite likely, and I've experienced it too.

[nuttycyclist]

...
Base station signals won't affect bike computers
...

I know you're the expert and have knowledge in this area, but I stopped using wireless when I realised that I hit 300mph every time I rode up the same hill.

Note the proximity of that hill to the marked feature

[Tynan]

One thing I remember seeing on Sheldon's site (IIRC) is that for some bike computers, if you have the magnet too close to the sensor then you get a "double bounce" effect with the sensor.  In the case I read about it meant that at speeds below about 10mph it read as double the speed, but once the magent was travelling fast enough so the sensor only saw it once instead of twice it reaad correctly.  Could that be happening with you?

Oh yes it was Sheldon, here is the link:

http://sheldonbrown.com/cyclecomputer-magnet.html

yes yes yes

so exactly how slowly was he really going then?

[border-rider]

...
Base station signals won't affect bike computers
...

I know you're the expert and have knowledge in this area, but I stopped using wireless when I realised that I hit 300mph every time I rode up the same hill.

Note the proximity of that hill to the marked feature

Ye canna change the laws of physics. 

It is of course quite possible that there's a sod-off big underground power line under the road in the same location.  Or a radar on a nearby airfield.

Either way, mobile phone base station signals can't interfere with wireless computers (at least, at any position where you aren't in danger yourself of over-exposure )

[gonzo]

You've checked that you've only got one wheel magnet on there right?

[Biggsy]

You've checked that you've only got one wheel magnet on there right?

You've checked that he's already checked, right?

(See original message)

[rower40]

Related but only tenuously:
My Polar Heart-rate monitor gets sent loopy by the radio curtain used by some supermarkets to prevent trolley theft.  The radio beam activates a brake on the trolley wheels, making them even more difficult to control than usual.  But same radio beam also corrupts my HRM.

Probably not the cause of your over-read, unless you're doing laps of your supermarket carpark.

("What's the difference between a Supermarket Trolley and an Essex Girl?"  "A Supermarket Trolley has a mind of its own.")

[gonzo]

You've checked that he's already checked, right?

Ah.....hmmmm.....ooops!

[alecstilleyedye]

i once rode a wireless computered bike over the forth road bridge. i was maintaining an exact 15mph, when at the crown of the bridge my speed dipped to 9mpg and then shot to 97. we were passing a mobile phone transmitter at the time.

i don't use wireless any more.

[border-rider]

we were passing a mobile phone transmitter at the time.

And presumably you were wearing socks too ? 

[TimO]

Either way, mobile phone base station signals can't interfere with wireless computers (at least, at any position where you aren't in danger yourself of over-exposure )

I'm not sure I agree (and this is from a perspective of having worked in a job related to EMC/EMI).

A well designed wireless computer shouldn't be affected by transmissions on mobile phone frequencies (either from base stations or the mobile transmitters themselves), but a well designed wireless computer shouldn't behave as erratically as some seem to anyway.  The only thing a well designed wireless computer should do in the presence of excessive interference is fail to register anything, since in effect this will be jamming.

Of course, a computer which is immune to interference issues will cost more, probably more than most people are willing to pay.

I can never be arsed with wireless computers.  It took me longer to deal with mounting the computer on the handlebars than dealing with the wire, with the last two computers I've set up.  I don't see the advantage of increasing the complexity, cost, and number of batteries.

[border-rider]

A well designed wireless computer shouldn't be affected by transmissions on mobile phone frequencies (either from base stations or the mobile transmitters themselves), but a well designed wireless computer shouldn't behave as erratically as some seem to anyway.  The only thing a well designed wireless computer should do in the presence of excessive interference is fail to register anything, since in effect this will be jamming.

Wireless computers communicate using audio-frequency magnetic  fields.  The only time they should react to/be overwhelmed by a microwave signal is if is big enough that the microwave-deaf antenna can pick it up sufficiently to pass it on to the electronics.  That could happen with AM radio, for sure; it might happen with FM radio or TV if you were close to the antenna.  Mobile phone base station signals (at positions of public access) are vanishingly weak on this scale.  Much more likely is interference from a power line (also low frequency magnetic field), and shop antitheft/access control system (likewise) or an airport/miltary radar, which is also microwave but with a massive peak:mean power ratio which makes it a possibility.

TETRA handsets also are likely to interfere, but not TETRA base stations.

Think of a car radio: it can pick up your mobile handset handshaking and it can pick up radars, and the corona from power lines.   It doesn't pick up base stations.

And: the world is full of mobile phone base stations.  There is signal pretty-much everywhere now.  Just because you can see one nearby doesn't mean it's a higher signal strength - quite the opposite sometimes.

I'm not sure I agree (and this is from a perspective of having worked in a job related to EMC/EMI).

It's (part of) what I do for a living.

[TimO]

Wireless computers communicate using audio-frequency magnetic  fields. ...

Fair enough, I assumed that they just had a small transmitter operating on an MPT1340 frequency, or something of that ilk.  Low frequency B-Field stuff shouldn't easily be interfered with, but I guess part of the problem is that with that sort of design is that it's probably relatively difficult to modulate something distinct onto the carrier, to limit false triggers.

I still think they're a bad idea.  Unnecessary complexity to jump a gap of a few inches to a few feet, when in the majority of cases a wire link is perfectly adequate.

[border-rider]

Wireless computers communicate using audio-frequency magnetic  fields. ...

Fair enough, I assumed that they just had a small transmitter operating on an MPT1340 frequency, or something of that ilk.  Low frequency B-Field stuff shouldn't easily be interfered with, but I guess part of the problem is that with that sort of design is that it's probably relatively difficult to modulate something distinct onto the carrier, to limit false triggers.

Cheap technology.

The cheapo ones are unmodulated - the receiver is just a pulse counter.  Hence their propensity to be jammed by, well,  almost any low frequency magnetic field.  The field strength at the receiver is pretty small too - and the sensors usually are small chokes so they're pretty rubbish all round, really.

I still think they're a bad idea.  Unnecessary complexity to jump a gap of a few inches to a few feet, when in the majority of cases a wire link is perfectly adequate.

Agreed.  I had one on my Pomp for a couple of years that seemed pretty good, but the replacement (after it fell off on the National 400 in 2005) was awful. I use wired ones on the audax bikes now.

The only real advantage of wireless is that they (mostly) keep going in torrential rain whereas some wired ones pack up if water shorts out the contacts on the read-out unit.

Having said all that, I have a £9.99 Lidl wireless computer on the Zep, and that works very well.