Bouncing/Inaccurate Speedometer

I have a lovely 1974 Commando MK2A with a very erratic speedo needle that bounces even when I roll the bike with the motor off AND reads 20-30mph over the actual speed when riding the bike. Obviously, the problem could lie with: (1) the speedo drive, which appears to be an aftermarket replacement on my cycle (no obvious stamped "Smiths" mark, though I am unsure if my model had a Smiths); (2) the speedo cable, which appears to be in good shape; or (3) the speedo itself.

I would like to address this problem soon, especially as it is driving up the mileage on my odometer, but where should I begin? Does anybody have a good way to diagnose the root cause of the bouncing/inaccurate needle? I would prefer not to just replace everything if there is a way to identify the real problem. I thought of removing the speedo cable and spinning it at a steady speed in a drill to determine whether the needle still bounces, thereby eliminating (or implicating) the drive as the culprit. Is that the best way to do this?

Assuming the problem is the drive, is there a way to fix this, or should I get a replacement (if so, where)? Assuming the problem is the speedo, can anybody recommend a good place to send it for repair?

Thanks!
-Keith

Stuffing a bit in a drill and running the speedo with it will tell you if it's the clock. Easy to pull the gearbox off the wheel to see if teeth or somethings wrong with the gearbox. Chances it's the cable, pull it apart and clean and oil it up and see if you have the correct ends or if something's not connecting right. Could be the old clock though.

Dave
69S

My speedo bounces at low speed but settles down at high speed. My tach is steady at low speed but bounces at high speed. Or vice versa, I can't remember . I consider it a feature.

My speedo used to bounce wildly all the time so I changed the Smiths drive for a new non-Smiths one, fitted a new cable and then had the speedo itself serviced. I ensured the cable wasn't pushed too far into the speedo which can put pressure on, and damage the internal workings - and still the needle bounced all over the place! It was only when I cable tied the speedo cable to the swing arm that it was cured, suddenly for the first time in eight years I could tell what speed I was doing. Don't put the tie on too close too the speedo drive - try to make the line of the cable as smooth a curve as possible and avoiding having any unsupported speedo cable that can wiggle about.

nomadwarmachine said:

reads 20-30mph over the actual speed

the speedo drive, which appears to be an aftermarket replacement on my cycle

especially as it is driving up the mileage on my odometer,

Click to expand...

Do you know if there is a number and/or drive ratio stamped on the inside ring of unit? From your description it sounds as if it could be a 2:1 ratio drive gearbox instead of 1.25:1 (15/12). If in doubt, count how many cable turns per wheel turn.

nomadwarmachine said:

(no obvious stamped "Smiths" mark, though I am unsure if my model had a Smiths)

Click to expand...

It would have been Smiths originally.

nomadwarmachine said:

I would like to address this problem soon, especially as it is driving up the mileage on my odometer, but where should I begin? Does anybody have a good way to diagnose the root cause of the bouncing/inaccurate needle?

Click to expand...

A bouncing needle is often caused by the speedo cable.

nomadwarmachine said:

I thought of removing the speedo cable and spinning it at a steady speed in a drill to determine whether the needle still bounces, thereby eliminating (or implicating) the drive as the culprit. Is that the best way to do this?

Click to expand...

I use a small thread tap held in a battery drill chuck, as the 1/8" square end is an ideal size to drive a speedo/tacho.

The drill rotation must be set to reverse.

I have a 74 Roadster that the needle also bounces.I have read various replies that tells you to tie the cable to the
swing-arm.Could someone post a picture showing where this is done as my cable does not come anywhere near the
swing-arm.The last place it is supported is just under the gearbox on the frame.
Thanks,
YING

Clock end of cable should come about flush with the cap nut rim so it don't stick into speedo too far. Read hub drive has a number of failure modes, one being the drive end cap crimps get loose so swing arm motion tugs in cable in and out of engagement or the cogs slip a track. There are some wear items in the clock that allow magnetic to drag. I don't like to put a 'sharp' bend it cable tying it to swing arm. Maybe with a stand off, but most all of us tie off on frame tube. Some of us have had the drive lock up inside itself and pull the cable out of the clock and then flail and wrap around the the axle which tends to act like locked rear brake on grass and Gravel. On 2 Combat's over a decade, I seem to get about 5000 miles between each speedo ha$$le then needle starts to jump/twitch then wildly swings till a dead needle over next 1000 miles or so. My Trixie Combat is now about 7000 miles after last rebuild and lass 1000 miles no speedo working again. Not sure which end to start on yet.

Here's how I route mine. It's pretty much parallel to the swing arm, attaches to the swing arm right in front of the passenger peg and then along the inside of the right side frame. Seems to work for me. It keeps the angle going into the speedo box pretty much straight.



Dave
69S

Just another thought, the inner cable can "wind up" and jump inside the outer cable if it is poorly routed or needs lubrication.

Installed new cable, still had bouncing needle especially at low speeds. Checked speedometer on bench with drill (using red robinson screw bit - it's the square head bit for screws, I think in Canada only), ran smooth all through range of speeds. The cable I bought was from India, so I ordered new one made in UK. Waiting for arrival to install on bike. Looks like crap cable gave me an unreadable speedo... Couldn't take cable apart without destroying it - wanted to grease it up.

Don't use grease on the cable. You need the thinnest oil possible, or ideally some dry lubricant. I tried all sorts of things to make my speedo needle smooth and the cure was to remove the inner cable and pour petrol through the outer to clean it out, then clean the inner with petrol. I then used some cycle lubricant which contains teflon and is a "dry" lubricant. Needle is rock steady now.

You should be able to test the cable performance by connecting the drill to the drive end and running through various speeds watching the needle. If still twitchy then either cable or speedo is reason. If all good, then speedo drive gear is the cause.

Canada has its own experts for cables, highly rated.