felt q720 fitting a halfords computer

[goldenbrown]

need some help please i have a felt q720 21.5" i have just fitted a halfords 8 function computer wired,on the instructions it tells me to program the size of the wheel ok i have done that but then sitting here and thinking about it if the magnet was half way down the spoke then it would not read correctly? any give some advice on how accurate these are.

atb goldebrown

 

[RedBike]

As the magnet passes the sensor it trips a little reed switch.
The computer times how long it takes between passes/ how long one wheel revolution takes. It then multiplys everything up by your wheel size to get your speed.

To get the distance it counts how many times the magnet passes the sensor.

It doesn't really matter where the maget is. It will still take just as long to go all the way around and trip the sensor / switch!

 

[goldenbrown]

ok so as long as i have told the computer that the wheel is 26" then it wont make any difference where the magnet is as i cant get it any higher than half way up the fork?

atb goldenbrown

 

[gbb]

ok so as long as i have told the computer that the wheel is 26" then it wont make any difference where the magnet is as i cant get it any higher than half way up the fork?

atb goldenbrown

No...it doesnt matter where the magnet / sensor is. It simpy reads every time the wheel rotates..one revolution is one revolution. As you have said, the most important thing is to program in the correct size wheel. Hope that helps

 

[robz400]

I had a simalar brain fart when setting mine up. I was getting confused cos i was thinking that the higher the magnet was on the spoke the faster it would be travelling past the sensor, but as people have said it makes no difference as it counts the number of rotations not the speed.

 

[RedBike]

Half way up the spoke will be fine.

If you want your computer to be accurate then it is important that you get the wheel size correct.

When you set the computer up you probably had to input a 'code' for the wheel size. This code is more than likely the wheels circumference in mm. Obviously not all tyres are the same size. In fact generally as the tyre gets wider the circumference of the tyre increases proportionately. The standard circumference of a 26x1.5 tyre is 2025; whereas a 26x2.0 tyre will probably have a circumference of 2115, some 90mm more. It all depends on the tyres and pressure in question!

Obviously when your dealing in km a few mm here and there isn't going to make much of a difference but 90mm /3.5” (which is the difference between a 1.5 and 2.0 tyre) per wheel revolution will soon clock up.


To measure the actual circumference of your wheel put a chalk line on the floor. Line up the valve with the line then roll the bike forward (with your weight on it) until the valve has completed one revolution. Put another chalk line. Measure how far apart your lines are.

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