Fitting a cycle computer to a Birdy raises three special concerns: will the computer work with the smaller wheels, can the sensor be fitted properly to the forks and can it be fitted without interfering with the folding?
I've used quite a number of different types of computer with other small wheeled bicycles over the years, and all the ones I've tried could be calibrated to cope with 16 inch wheels, so the 18 inch ones should not present any problems. However, I've heard a rumour, which I haven't personally confirmed, that the new Shimano system available for some of their groupsets, which integrates gear change information, speedometer and other functions, won't handle smaller wheel sizes, including the Birdy size. That excepted, it seems likely that most others will, but it's worth checking before you spend your money. The Sigma (700 and 800), Cateye (Mity 2 and cordless), Vetta (C500 cordless), and a couple of early Avocet models are the ones I have personally used successfully with small wheelers (Moultons, Brompton and Bike Fridays as well as the Birdy).
The calibration factors listed in the manuals with the computers rarely include our wheel sizes, but an equation is provided to allow you to work out the factor for yourself. It usually involves measuring the distance rolled by the wheel in one revolution. Suitably armed with the manual, you then need to work out the combination of buttons to press to set the calibration factor on the computer. Don't lose the manual, or when the time comes to replace the battery you will have real problems setting the calibration factor again, as unfortunately it's usually lost when you do this. I usually write the factor on the manual when I originally calculate it, to save re-measuring the wheel and performing the calculation again when the battery is changed.
In order for the magnet which attaches to a spoke to pass the sensor which attaches to the fork at the right angle, you will need to attach the sensor to the front vertical section of the fork. Like the rest of the fork assembly, this has a rectangular cross section. Many of the computers on the market have a pre-formed mounting for this sensor which is only suitable for circular or elliptical forks, usually of much smaller dimensions than the Birdy forks. While there may well be other makes which can be fitted, the only one I have found with a suitable means of fixing the sensor is the Sigma. In the case of the standard unit (cable operated, rather than cordless), a large rubber ring is wrapped around the fork to hold the sensor against it, and although it is rather a stretch, it will fit the Birdy forks. It doesn't feel a very firm method of fixing, and I don't like it all that much, but at least it works. The cordless version of the Sigma sensor is secured using the equivalent of electrical cable ties, which should also enable it to be used on the Birdy (although I haven't tried it myself).
sensor for a Sigma (corded) computer mounted on the Birdy forks - the
rubber ring fixing will accommodate (just!) the Birdy fork section. The
cordless sensor is rather larger (it houses a battery), and is mounted via
cable ties - I haven't tried it on the Birdy, but I imagine it would fit.
With the corded version, the standard cable is too short to reach from the
bars to the forks, so a rear mounting kit was used to mount the sensor at
the front - the only difference being the longer cable (see comments
Sigma 800 computer mounted on the bars. This is the simple standard corded
mount, with a battery in the computer, but none in the mounting. The
rather smart bell is the nicer of the two designs of Moskee bell sold at
one time by Phoenix Cycles (I believe that they have now sold all of their
latest Sigma 700s have rather neater operating buttons than earlier
models, such as the 800 shown above.
Some additional general comments on the Sigma computers appear in the next section.
The usual place to put the computer readout is on the handlebars, which is the most convenient place for seeing the display and pressing the buttons which control which particular values are displayed. However, this means having a very long cable run from the bars to the wheel sensor, which will need to be mounted on the front part of the forks, running down the stem and round the fork blades. The cable normally supplied with computers is not long enough, but a rear mounting kit is available for some makes of computer (such as the Sigma), and this provides a longer cable, which will probably suit. The one with the Sigma is just the right length. The alternative is to fit a cordless computer, but this is not without problems. The difficulty lies in the distance between the wheel sensor and the computer mounted on the bars, which is much greater than on a bicycle with larger wheels. Many of the cordless computers won't work in these circumstances, including the Cateye Cordless and Vetta C500. One which DOES work (at least on the bikes I have tried it on, although I haven't tried it with the Birdy) is the Sigma. In the case of the Sigma, you use any of the standard Sigma computers, but purchase the remote mounting kit, which replaces the wheel sensor and the handlebar mounting bracket. There are a number of drawbacks, though. Firstly I don't much like the system of mounting the sensor and bar bracket used by Sigma, which involves a not very positive strap. Secondly there are batteries in the sensor, the mounting bracket AND the computer itself. Thirdly, you must remember to press a button on the bracket to start the thing working at the beginning of the ride. Finally, the bar mounting bracket is a clumsy affair, since it contains a sensor as well as the batteries (this is the down side of the wireless system being built into a universal bracket, rather than into the computer itself. I must say that I also don't much like the buttons for selecting he display on the Sigma, which are clumsy to operate, and which are apt to move the not very positively mounted bracket when you press them. On some models it is too easy to zero the unit while removing it from the bike - the latest 700 overcomes this particular problem. Another drawback with the Sigma is that although it is sensitive enough to work with greater wheel to bar distances, this also makes it a bit prone to interference - take it off the bike when travelling by electric train, or you may find that it has recorded very high mileages at amazing speeds during the journey!
I have only been able to write about my own experiences here. I do hope that readers tell us about their experiences, so that we can extend the information provided here.
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Last updated: 24 December 2001