Fitting a Bike Speedo to Locost for Under a Fiver

Ok why do it ?


How much will it cost ?

Bike computer speedos vary in price but mine cost me less than a fiver from my local Woolworth. A better quality one will cost more but be aware design features such as the size of the and clarity digits and the physical size of the tranducer are more important than additional features.

Ok what do I get in the box ?

  1. A sensor/transducer usually a simple magnetic reed switch.

  2. A small magnet or magnets

  3. A Display unit with battery.

  4. A cable & mounting bracket.

  5. Usually a few tyraps to clip the cable.

How does it all work ?

Simple the magnet is attached to the wheel or wheel hub and the reed switch sensor mounted so that the magnet passes close to it once per revolution. As the magnet passes the reed switch it pulses it on and off, the electronics simply convert the pulse rate into wheel RPM and converts this into speed.

Ok How Do I Fit It ?

The first thing is to decide where to fit the reed switch sensor and magnet, for best accuracy I chose to fit mine to a front wheel. On my Woolworth Cycle Computer the both the reed switch and the magnet are vey compact this enabled me to fit to the magnet to the front brake disc (Cortina mkV) on the inside of flange where it bolts on to up. The reed swich was simply attached to the suspension upright/stub axle using one of the holes provided in the upright provided to mount the splash guards. The wires should be carefully routed and will require extending, to reach the dash. Mount the display on the dash then follow the instruction in the box to program the wheel size and thats all !

How accurate is it ?

Not tested on the car yet but bench testing using an osilloscope the results the Woolworth unit appears almost 100% accuarate for measuring wheel RPM but the MPH reading is less accurate because tyre rolling diameter can only be entered to the nearest inch this implies a built in error of just over 2% -- not too bad. The specification sheet of the unit only states it will work up to 99kph (62mph) however it appears to work up to 99mph when switched to the mph scale.

Next Step ?

Once the car is up and running I might fit a better unit such as the Avocet Cyclometer 35 which has a bigger display (0375") and uses more magnets improving low speed accuarcy more importantly it allows the tyre rolling diameter to be entered exactly greatly reducing the built in error. Another company Sigma also make some very nice high quality computers.

What I would realy like is a proper backlit unit with a decent sized display without having to pay the 200+.

Handy Links

Mounting a Sigma Computer in a Caterham

Avocet Cycle Computer Homepage

Avocet Cycle Computer Homepage

My Home Page