Wired bicycle computers

Having dealt with many many bicycle computers since the 80's, I've encountered quite a few anomalies, wired and wireless, with engine and without.
TV stations, radio stations & wireless network routers put out EMI/RFI as well as the typical sources that have always been around: the Northern Lights and even the sun.
The quality of bicycle computers have gotten a lot higher over the years, but so has the level of interference!

EMI (electromagnetic interference) and RFI (radio frequency interference) can cause a lot of odd problems in even semi-shielded devices.
The most important thing is to recognize is when you have multiple electric devices running on the same ground - especially with magneto-equipped engines.
Isolating the ground on one or all sources will go a long way towards solving any EMI/RFI woes. Noise filters (as mentioned by allen standley above and by others in this forum a few times over the years) help quite a bit, and oftentimes are the only way to solve an EMI/RFI problem if you are unable to isolate the engine ground from the bicycle frame.

The magneto on our 2-stroke and 4-stroke engines puts out a significant magnetic field and so is the largest source of "local" EMI/RFI.
If possible, shield the engine ground from the frame. If you need to use the engine as a ground source (such as running lights off the white wire/aftermarket engine-powered charging system) on a chinagirl or similar, simply attach the light/charging system ground wire to the engine block.

Most of the time, running a resistor spark plug and wire will take care of the problem. For more persistent EMI woes, it will take ground isolation and/or noise filter(s).

Hey Tom, on my first and second build, I ran chinagirls and had bottle gennies. My first build also used 12v turn signals that would blink at an odd rate when the engine was running. After isolating the engine, they returned to normal blink rate, but I went ahead and isolated the ground for the signals and gennie as well.

What I did to isolate the engine itself was use a short (~3 inch) piece of old inner tube (not a thorn-resistant, must be thin) between the engine mount and frame before bolting the engine down. For the clutch lever and kit throttle I simply used used a wrap or two of PVC/electrical tape.

I ran wireless computers on those first two builds and on my 3rd build (no bottle gennie on 3rd build) using the engine isolation method and never had any problems with the computer from the engine. Other sources, like railroad crossings in California or AutoZones, still make my wireless computers go crazy tho.

I've since gone back to a wired. I got sick of my maximum daily speed being displayed as 99.9MPH because I crossed some railroad tracks

They wont work in the immediate field of an Ebike controller either. Mine would indicate 0 when the motor was running. As soon as I zeroed the throttle I got the current road speed. I moved it a foot away(on the down tube) and it works fine. Don't seem to be bothered by the motor, just controllers.

I use a Bell wired computer and it works flawlessly on days that are above 32f. Otherwise the LCD does not get warm enough to display all of the segments. I also run a dynohub that should interfere too, it doesn't. Just remember that Bell's memory system is faulty leaving you with a reset computer every once in a while.