Cycle computer question...

madtheory

Surely the closer you put the magnet to the axle, the shorter the circumference? So it passes the sensor more often. So why is it that you're only required to dial in the circumference according to the tyres you have, and not take the position of the sensor into account?

I've read the manual, googled around, and I'm still flummoxed. I've set the computer for the tyre diameter, but I'm not at all certain it's reading right.

g0g

It'll be same number of revolutions either way surely!?

Lemag

I works by counting the number and frequency of your wheels rotations. It assumes your wheel to be of the same diameter for each rotation.

Notch000

Its a bit of a blonde question but il put you out of your missery

The circumfrance is the distance around the wheel in a circle, the distance from the magnet to the center a distance and is irrevelant,

The computer will calculate the speed based on the number of rotations and the circumfrance of the wheels, this will not change no matter where you place the magnet.....

Rovi

All it's doing is counting revolutions of the wheel, where it's actually located on the wheel doesn't matter.
The computer takes the number of revolutions/minute along with the tyre diameter you've manually supplied to do the sums to provide the speed/distance data.

Sr. Assumpta

madtheory wrote: »

Surely the closer you put the magnet to the axle, the shorter the circumference? So it passes the sensor more often. So why is it that you're only required to dial in the circumference according to the tyres you have, and not take the position of the sensor into account?

I've read the manual, googled around, and I'm still flummoxed. I've set the computer for the tyre diameter, but I'm not at all certain it's reading right.

You're overthinking it. Welcome to The Cycling Forum

stevemac

can I jump in with another question. Is it hard to install them. New and got a computer, it was only 20 quid I think and it gives you speed and distance and time of day. Was thinking of upgrading already to get one that gives you cadence and has a stop watch. Wanted to get it offline as the shops are a nice bit dearer. And if it is easy enough to install any sugestions?

cdaly_

Yep, easy enough to install. For cadence types you have an extra sensor and an extra magnet to install. I found mine was not picking up the crank magnet as it was too far away from the chainstay. I ended up putting it on a packing piece.

Wireless computers are easier to install as there's no wiring to deal with. However you have two batteries to worry about and the possibility of radio interference causing them to misread at times.

madtheory

OK so it measure frequency, not speed directly, yes I'm blonde.

triggermortis

madtheory wrote: »

OK so it measure frequency, not speed directly, yes I'm blonde.

frequency is speed.

If the diameter of your wheel is a metre, and you cyle for an hour and the computer has counted the wheel doing 20,000 revolutions then in an hour you have travelled 20,000 metres (20Km) and so your average speed is 20Kmh.

Where you put the magnet makes no difference as it is going to turn once every wheel revolution wherever it is put

cdaly_

triggermortis wrote: »

frequency is speed.

If the diameter of your wheel is a metre, and you cyle for an hour and the computer has counted the wheel doing 20,000 revolutions then in an hour you have travelled 20,000 metres (20Km) and so your average speed is 20Kmh.

Where you put the magnet makes no difference as it is going to turn once every wheel revolution wherever it is put

[Pedant mode] If the circumference of your wheel...

Circumference of a 700c wheel is just over 2 metres.
[/Pedant mode]

madtheory

OK, my hair colour is slowly darkening.

Lemag

What might bare consideration regarding the positioning of the magnet and the sensor is that the magnet will pass the sensor at a greater speeds the further away from the centre they are positioned. This could lead to the sensor not detecting the magnet passing if you are at crazy speeds (or if the battery is low?). Placing the magnet further away from the hub will also slightly contribute to a greater rotational mass although this would hardly be noticable. And finally, placing the sensor behind the fork is more aerodnamic.

Btw, if you ignore all of the above you'll probably perform just as well.

madtheory

triggermortis wrote: »

frequency is speed.

Not really, because it depends on exactly what you're measuring, and how you define speed...as mentioned above velocity varies because close to the hub the distance travelled by the magnet is shorter than the outside. But here simply counting frequency works as a speedometer when the wheel circumference is known.

For example a CD varies its rpm to keep velocity constant. On an LP, frequency response is better on the outside compared to the inside.

So as I said, it depends on the application!

Lemag

^^^^^^^^^^^^^^

10 days later..

madtheory

Gone blond with all the sun.

G2ECE

madtheory wrote: »

Not really, because it depends on exactly what you're measuring, and how you define speed...as mentioned above velocity varies because close to the hub the distance travelled by the magnet is shorter than the outside. But here simply counting frequency works as a speedometer when the wheel circumference is known.

For example a CD varies its rpm to keep velocity constant. On an LP, frequency response is better on the outside compared to the inside.

So as I said, it depends on the application!

Except in this case (on the bike) there is a direct correlation between frequency and speed!!!

I've had a good laugh reading this thread! great to see nobody got up on a high horse.

damoz

G2ECE wrote: »

Except in this case (on the bike) there is a direct correlation between frequency and speed!!!

I've had a good laugh reading this thread! great to see nobody got up on a high horse.

smacl

High once, but i fear this horse can take no more