Speedo's and gas engines...Help !!

frenchman159

New Member
Apr 19, 2008
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Winnipeg, Manitoba, Canada
It's driving me nuts..... I'm on my second speedo, the electronic one with the magnet, and i'm starting to think that they are not compadible with these engines running !! Always cutting out at a certain speed and sometimes going blank. I tryed a new battery and a different location, with no luck. I'm hoping sombody may be having the same problem and found a solution. Must have something to do with harmonics from the magneto Or ???????
 

Bikeguy Joe

Godfather of Motorized Bicycles
Jan 8, 2008
11,843
236
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up north now
Simply put, they don't work very well due to the unsheilded ignition on our bicycle motors. You can get the cable drive speedos from a lot of sources on the web, including ebay.
 

cityevader

New Member
May 11, 2008
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Santa Cruiz, CA
If is is EMF, one could do what carmakers do for vital sensors that measure engine rpm very nearly exactly the same way our bicycle computers do... Twist the wire. About two twists per inch is more than enough. (Maybe even less, say one twist per 3/4") Then wrap it in a continuos spiral of foil along with a thin-gauge uninsulated wire and ground it to the frame. When the magnetic wave from the ignition hits the twisted pair wire, the voltage it induces into the wires will now be opposite each other, cancelling them out. Also, the sheilding will carry away some of it before reaches the wire. Note: the shielding isn't bulk related. You don't need more than a single thin layer (of course each spiral will overlap onto the previous one.) and it doesn't help any more to was it up around the wire, as the fashion police will stop you. Take a long strip, say, 8 feet or so, cut into a 2 inch strip. Lay it on the floor with the uninsulated wire taped every few spots to keep it from wandering, alongside your twisted wire computer pickup just a bit off of parallel. Roll the wire into the foil at a relatively shallow angle...say 30 degrees, overlapping each wrap about half the strip width, or an inch or so. After all wrapped, spiral on a layer of electrical tape to prevent foil tearing. Make sure it's grounded to frame using the uninsulated wire!

It's a lot of work, and twisting alone may be enough. If not, go for the foil.
 

Ghost0

New Member
Mar 7, 2008
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Bellingham, WA
Two things I have done that seem to have alleviated some of the problems are:

Something similar to mentioned above but a little simpler. Take whatever excess wire you have on the speedo wire and fold it back on itself as shown in the picture. It helps cancel out some of the EMI.

The other thing I have done is use a shielded plug wire, basically standard automotive wire and boot.

Seems to have solved the problem.
 

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Saddletramp1200

Custom MB Buiilder
May 7, 2008
1,439
40
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Houston, Texas
All of you are right:) However if you happen to live near an airport and can ask an aircraft mechanic he might just have some used electronic shield cable
laying around. It wraps around cable like a spring. It kills emf by about 60-80%. If that don't work a mechanical speedo is the way to go. (unless your bikes Haunted)rotfl Couldn't help it. Ride safe All (c)
 

frenchman159

New Member
Apr 19, 2008
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Winnipeg, Manitoba, Canada
SUCCESS !!!! I ran the pick up cable in a a manner that it completely blocked ( out of line of sight you might say ). Up the front of the fork and handle bar. I also did 2 loops in two different places, again in front only. Did 2 miles and it worked perfectly !!! :) I will now be clocked with my car to caliberate it by moving the senser up or down as needed to get it closer to acurate. Thanks again everyone !!!!!!!
 

zebratruck

New Member
May 26, 2008
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Pittsburgh, Pa.
if you want an accurate speed from your bike computer, do a roll out measurement and enter that # in mm into the computer. Air valve @ 6 o'clock, you on it with tires pumped up, roll one wheel revolution until the valve is @ 6 o'clock again, measure in mm. Most every modern cycling computer will let you enter a custom # for a more accurate speed.
 

frenchman159

New Member
Apr 19, 2008
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Winnipeg, Manitoba, Canada
This one is a CCM. It asks you to input the wheel size or a wheel factor # from a chart it comes with, But it does not exactly specify how far from the hub to install p/u and magnet. So, I cannot use this chart as theres no way it can be accurate when speed of magnet pulses changes with distance from hub.......What do you think..?
 

cityevader

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May 11, 2008
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Santa Cruiz, CA
YAY!!!

First, it matters NOT how far away from the hub the magnet is mounted. Whether it is close to the hub or close to the rim, it still spins at the same rpm. The computer is simply a counter, counting how many times the magnet passes the pickup. It doesn't measure how fast the magnet is moving, but rather, simply counts pulses and then the computer calculates speed etc..

The ONLY way to get an accurate measurment of tire "size" is to have it at anticipated operating pressure, load (panniers, lunch, water bottles, backpack, what-have-you, and on a level concrete surface, while seated on the bike, roll it forward very straight, one full wheel revolution (oh yeah, dab a bit of grease or paint or lipstick or water or whatever onto one single knob of tire tread, and measure very carefully from leading edge to leading edge, or trailing edge to trailing edge, i mean, really be accurate, a 1/16 inch difference times a great many many many many many revolution in a trip can throw things off considerably.

Note: cars aren't "accurate" either. Tire tread thickness and psi affects speed reading. Modern Fords ( which I work on, are biased to read 5mph higher at freeway speeds on purpose...(without increasing odometer reading).