Wild speedo

I know that wild speedo needles are pretty common on Airheads. Mine was no exception; wavering from 10 mph all they way up to peg.

My question is that suddenly the needle has pegged all the way to the right and lays there (looks pretty impressive to be coasting through a school zone at a registered 85MPH).

What might have happened?

Not sure how the later airhead speedos were driven, but on my '75 /6, it is mechanically driven, so that would be the speedo cable. It either needs lube, or to be replaced.

Weasel said:

Not sure how the later airhead speedos were driven, but on my '75 /6, it is mechanically driven, so that would be the speedo cable. It either needs lube, or to be replaced.

Click to expand...

You had better hope that's all it is. Mechanical failure can also happen within the meter. Those who repair these devices get a pretty penny for their work.

They're mechanical, yes.... but there is no direct drive to the needle. The cable is routed into some gears which turn the odometer, and spin a little magnet in the middle.... the speedo needle reads whatever the spinning magnet vs. the coiled hairspring can find as common ground..... not as precise, by far, as one would suppose with a "mechanical" speedo....

Most likely, pegged at the high end, the coil spring broke or came undone somehow. If it were resting at zero, it might be a broken cable, disconnected gear or some such.

An interesting look into the innards is here. .(..how to repair the odo drive gear.... take a look at photo 9 for the "spinning magnet" that drives the needle itself.)



edit: PS: the mechanical tachos (/5 and /6) are driven the same way, so they can be off as well. Electric/electronic speedos and tachs tend to be more accurate.)

I kinda wondered if it was that hair spring getting loose that ended its wild wavering. From illustrations the thing looks really tiny.

Is this an 'owner fix' possibility? Or should I just judge speed by tach like I have been doing.

Of course, the comment is :" wow..this bike looks like it is doing 85 just sitting here"....

I would suggest that you try replacing the cable first, seems unlikely tho. Next you might, depending on your mechanical skills, remove the speedometer and see if there is anything obviously wrong that appears within your skill level to correct. A suspect might be the tension spring under the face plate (looks like an old fashioned radial watch spring, it places tension to keep the needle at zero and resist upward movement of the needle.

Contact one of the agencies (I used North Hollywood) to see if they will sell you parts, or inquire as to the repair price (Usually a set amount regardless of the fix)

If you wish to continue with the Tachometer as speedometer for a time this link will take you to Snowbum's site where he has an RPM vs speed chart that may be helpful.


http://bmwmotorcycletech.info/ringgears.htm

Anton has taken the gearing information to the next level and provides a cool tool to figure out speeds in various gears:

http://www.largiader.com/articles/gearing/

He references Snowbum's website, but it appears he has the URL wrong. Maybe he'll see this and update the link...I think he needs to remove the ",etc" part as indicated by the previous post.