Halfords Cycle Computer - PROBLEM

[robert b 2]

Howdy.

I've just bought an 8-function cycle computer from Halfords for £12.99.

I've got it all fitted up fine but I'm a little confused about something. The instructions say nothing about how far up the wheel the magnet and sensor should be, i.e. how far along the spoke from the centre.

I have put the magnet and sensor just over half way towards the tyre as that is where the required separation distance (<5mm) could be achieved. I have input the wheel circumfrence into the computer.

But I don't see how it cannot matter where on the wheel the sensor is placed. Surely the difference between being near the pivot and being near the tyre would amount to a lot of difference in accuracy?

HELP!

 

[nosherduke996]

How does that work, the sensor is on a spoke. The wheel revolution is the same surely so it would pass the magnet at the same rate wouldn,t it ?

 

[robert b 2]

I want the readings it gives to be as accurate as possible. How can moving it to different parts of the wheel ensure that??

 

[Trumpettom001]

It makes no difference to the accuracy of the Speedo where you put the magnet/sensor... HOWEVER!!! put the magnet and sensor as close to the hub as possible (within reason) - as at high speeds, the magnet will start to pass by the sensor too quickly for the sensor to register it, and so while you are doing a neat 30mph, the computer reads a neat ZERO miles per hour, effectively limiting your speed to 30mph, as you don't want to be riding, and not have your miles logged do you? Hope that makes some sense...

 

[nosherduke996]

The best way to calibrate it is to pick a point on the tyre i.e where the valve is or a mark on the tyre.
Then walk the bike in a straight line until that point touches the ground again.
Measure the distance between the two points in centimeters and that is the figure you want to put in to the computer.

 

[robert b 2]

When you say "the hub" do you mean the pivot of the wheel? No, the manual says nothing about where on the wheel it should go, only that a given distance between sensor and magnet should not be exceeded.

Why would being closer to the hub reduce interference? Surely at high speed the magnet will pass the sensor the same number of times regardless - otherwise it would matter where on the spoke it was place in terms of accuracy...

 

[peanut]

this question comes up occasionally and it never ceases to amaze me how dim some people can be

Its like giving a teenager a £20 note at the Co-op and watching them try to calculate your change

 

[Trumpettom001]

Assuming you have a 26 inch wheel, then the approx circumference of the wheel will be:
26x2.5 (convert to cm) = 65cm

65cm x pi = 204cm

If you are travelling at 10 metres a second, then every second, your wheel will rotate 1000cm/204cm = approx 5 revs/second

If you place the magnet/sensor at halfway up a spoke, then you are effectively making a mini wheel - in this case 13"x2.5 (convert to cm) = 32.5 (this is the diameter of the wheel)

32.5 x Pi = 102.1 (circumference of magnet's wheel of rotation)

102.1 x 5 revs a second = 510.5cm/s or effectively 5.105m/s, so therefore the magnet is travelling at around about half the speed of the tread...

Obviously, the speed gets smaller, as the magnets "wheel of rotation" gets smaller - i.e. closer to the hub, so there is less likelyhood of the sensor missing the magnet's revolution.





Or you could just take my word for it.....

 

[jay clock]

the key thing is that the sensor knows that each time the magnet whizzes past, the wheel as travelled once round and therefore you have moved x cm forwards.

If it is wireless (which that one is not) nearer the top might give better reception.

I have one of the 8 function ones by the way and think they are excellent

 

[Trumpettom001]

Ahh yes, I forgot to add that to me ridiculous explanation of wheel magnets... (although I had a cheap halfords cycle computer for a while, and I stuck with the magnet next to the hub, as the reception was never a problem (I broke the darn thing before either battery ran out....)

 

[robert b 2]

Thanks for all helpful comments guys (i.e. everyone but peanut.)

 

[Trumpettom001]

Look at a door.

the hinge of the door represents the hub of the wheel.

Open the door.

close the door.

notice how the bit near the hinge dosen't move that fast, but the bit away from the hinge does move fairly fast.

I hope you like my similie/metaphor/comparison (delete as applicable)

 

[robert b 2]

the key thing is that the sensor knows that each time the magnet whizzes past, the wheel as travelled once round and therefore you have moved x cm forwards.

This is the key thing I should have thought about before asking and why it is irrelevant where on the wheel it goes in terms of accuracy. I was thinking that placing it at different points on the spoke would mean (effectively) different sized wheels and therefore different readings. But it makes no difference as the computer knows the wheel size and then just counts frequency of revolutions.

I blame my jet lag

Thanks again people.

 

[gavintc]

1 x revolution = 1 revolution. Nothing more, nothing less.

 

[robert b 2]

Look at a door.

the hinge of the door represents the hub of the wheel.

Open the door.

close the door.

notice how the bit near the hinge dosen't move that fast, but the bit away from the hinge does move fairly fast.

I hope you like my similie/metaphor/comparison (delete as applicable)

[none of the above] thanks for the analogy it works well.

It all makes sense, I was just being a retard when I posted the thread.