Bicycle Mechanics - Wierd max speed on cyclometer

This has happened to me when using both wireless and wired Cateye cyclometers. Now and again, I'll return from a ride, check to see what my max speed was for the ride, and find a reading that defies reality - like 89 mph, 97 mph.

What might be causing these erratic readings?

Thanks.

Caruso

(I, of course, try to maintain forever those readings when they occur to show my non-biking fans just how tough a rider I really can be!!)

Electric interference causes this problem.

I've heard of problems like that happening with wireless cyclometers, though not wired ones... Perhaps you rode through something very magnetic? Past an electrical substation? Under high-tension power lines??

Where is your celphone when this happens?

Older model Cat-Eyes like the Solar were prone to this problem even though they were wired. A friend had one and he would get occasional readings like you report.

I have experience with at least a dozen newer wired Cat-Eyes (Enduro, Astrale and Mity in various series) on my own and friend's bikes and have never seen or heard of the problem with any of them.

Where is your celphone when this happens?

My cell phone stays in my bag at the back of the bike. I would be surprised if, from that location, the cell phone could be causing a problem - but your troubleshooting suggestions hints at a way I could replicate the effect and enter my own outrageous max speeds, LOL.

Caruso

I noticed this problem, too, recently. I've got a Cateye wireless (a little more than a year old), and after descending a long hill the other day, I wanted to check to see what my max speed had been. Usually, I hit around 40-43mph coming down this particular hill. But when I checked it this time, it said 77mph! I've never had any problems with this cyclometer, I didn't have a cell phone with me, etc. Weird.

The sensor on the fork is just a little switch that gets pulled over by the magnet as it goes by. They can bounce and give extra pulses making it seem like you have a higher speed. It happens more at high speed because the bumps are harder and higher frequency. It can happen more as the switch gets older. The return spring can weaken with age. It's nothing to worry about, no damage has occurred.

I have seen this with the computer in the back seat of my car with antilock breaks. The wheel lock up is detected in much the same way as the cyclometer. A magnet and a switch.