Cycle Computers

[Guest red7]

Here's my effort. A bit Heath Robinson, but it all works and it cost nothing as I used stuff I had lying around. I'll upgrade to a back lit unit one day.

[Guest yosamite sam]

now thats a good idea!!

[Guest Fred2b]

On a slightly different angle, I use my PDA with GPS occasionally and that has speed readings as well. I wonder how accurate it is? I suppose it depends on the receive rate from the satelites?

The bike speedo sounds like a good idea, here I come Ebay.

Peter

[Guest stu205]

I know my GPS calculates every 2 seconds and in which time when doing 70mph you have traveled quite some distance before it then tells you your doing 65!

 

I also seam to remember reading somewhere that GPS dose not acount for inclines so if your going up or down hill they will be inacurate which is why they still dont use them in aircraft??

[Peter Bell]

I have had one on my car for quite a few years now, if it helps, here is a picture of the sender mounted on the rear drive shaft.

Peter

[enforcer]

Pete, with such a small diameter how accurate do you think your set up is? I was going to fit mine (when it arrives ) to the front offside wheel/track rod end bolt, but your method is obviously a bit tidier.

[Guest salty_monk]

The diameter of what you attach the magnet too does not matter at all!!

 

These are a simple reed switch & only count pulses!! It counts one pulse for every time the magnet passes the sender & then counts that as one wheel revolution.

 

You program the circumference of the WHEEL (incl tyre) yourself. Some ask you for a diameter, some want the actual circumference (Pi x Diameter) entered.

 

As you can accurately measure the wheel, these can be very accurate indeed more so than a normal speedo!!

 

Dan

[Grim]

As you can accurately measure the wheel, these can be very accurate indeed more so than a normal speedo!!

 

You have to be careful with this! You need to measure the distance from the centre of the wheel to the ground (and calculate your diameter / circumferance from there), to account for the fact that the tyre is deformed by the weight of the car. Altering tyre pressures also affects this, as would camber settings (and therefore ride height, although only probably significant if on the rear). If you wanted to be really smart you'ld take an average of the inside and outside of the wheel, after a run (so tyres are at normal operating pressures).

 

It's only likely to be a small difference, and it will cause the speedo to over read.... but you did mention accuracy

[Guest timswait]

I also seam to remember reading somewhere that GPS dose not acount for inclines so if your going up or down hill they will be inacurate which is why they still dont use them in aircraft

GPS can be used to determine altitude, but the resolution in altitude is not as good as in position, so depending on the way your GPS set works it should be possible to take account of the incline, this will be a very small effect in most cases anyway.

GPS is used in new aircraft for positioning (although not to my knowledge for speed or altitude). It is just that it's not generally relied upon as the sole source of navigation information.

[NeilD]

Best way I found to measure the wheel diameter was to put a chalk mark on the wheel - drive forward 10 revolutions and measure the distance and divide by 10. Gives a pretty accurate reading

[Guest timswait]

Yes, and that takes account of the tyre deformation as pointed out by Andy.

[Grim]

i'm always the one to go for the easy method

[enforcer]

All done!

 

 

 

Easy peasy and seems to work fine too. There is quite a difference between the Sierra speedo and the Sigma one - Does everyone rely on the Sigma or re-calibrate to match the Sierra? I wouldn't want to rely on the Sigma at the moment because when it says I am doing 30mph the Sierra says I am doing 34mph Wouldn't want to get caught out by this

[Longboarder]

If you can accurately measure the rolling circumference of the tyre using Neils method,(also how the instros tell you to on my mountain bike) then the bike speedo will be very accurate, far better than the sierra one.

 

Nigel

[Peter Bell]

I set mine up by just rolling the car forward by one wheel revolution as I havent got a 60ft tape or a long driveway. It is a bit disconcerting though driving through a speed camera at 60 when other cars are braking.

Peter.