The Dreaded Speedo Gearbox

[thornebt]

I have fitted a few of these over the years and they never seem to last very long! I've just fitted another one. I've tightened the rear wheel spindle, then tightened the speedo gearbox nut. The speedo gearbox then tightens up and wants to rotate with the wheel. If I leave it slightly loose then it's OK. I know there is a well known problem with parts diagrams showing a part fitted round the wrong way but I made sure I got this right when I replaced the wheel bearings years ago so I am sure that is not the problem. I do believe that my stepped chrome spacer (021591) that is fitted outside the speedo gearbox might be shorter than it should be as it was previously burred on one end and shows signs of wear there. I don't think that would cause the problem I have but it's a possibility? I can buy one from the Spares Scheme if necessary.

I should mention that it's not a £10 speedo drive from Ebay. It cost about £45 from Pembrokeshire Classic Bikes. I also replaced the bearing adjusting ring in the past so the slots in it are perfect condition.

I've had a look through the forum. Someone fixed the problem by fitting a spacer inside the speedo gearbox. I think they said that some replacement drives are slightly wider than the originals.

So I'm thinking that maybe my worn stepped spacer might be the problem and that a spacer inside the gearbox might be a good next step. Has anyone got any ideas please, particularly in relation to size / thickness of a spacer inside the speedo gearbox?

Cheers. Bruce.

[Plugsnpoints]

Try these, Neville sent me these a couple of months ago and I haven't had a problem

I was petrified it'd lock up. I'm pleased I saw these

Andy



http://www.jampot.com/article_read.asp?id=57

http://www.jampot.com/article_read.asp?id=406

[McMaz]

I too had this problem when rebuilding my G3. After several failed attempts and much head scratching, I finally got out my cheap digital micrometer and discovered that the new replacement gearbox was wider than the original by about (I think) 0.5 mm. I filed this amount off the back of the replacement and voila – everything tightened up as it should.
Sean

[thornebt]

Thanks Andy. I remember seeing those articles in the past although I didn't find them when searching the forum today. I'm sure my wheel spindle and all the associated parts are correctly fitted so I'm not in a rush to start drifting out bearings etc. I don't understand why the drive wants to rotate with the wheel when tightened. The threaded part (021594) that the gearbox nut tightens onto doesn't look as though it should rotate at all when the wheel spindle is tightened. I'm hoping that an internal spacer / shim, although not part of the correct set up, will sort the problem. It was Stuoyb who fitted a 2mm spacer but I don't know the ID or OD of it. Cheers. Bruce.

[thornebt]

I too had this problem when rebuilding my G3. After several failed attempts and much head scratching, I finally got out my cheap digital micrometer and discovered that the new replacement gearbox was wider than the original by about (I think) 0.5 mm. I filed this amount off the back of the replacement and voila – everything tightened up as it should.
Sean

Hi Sean. I guess that your speedo gearbox was rubbing on the rear brakeplate? I've got quite a bit of space between the gearbox and the brakeplate, possibly due to the bearings being quite new. Cheers. Bruce.

[JEAN-NOEL]

i had experienced your problem on my bike (64 with N....n hubs). I had fitted my English made replica speedo drive in the exact way according user's manuel, but it was rubing against the ali plate of the hub. Checking the wideness of the new item, it was 25.1 mm instead of 23.5 for the original one. I had to reverse and shorten the "centralizing spacer" and all was fine. Hope this can help you to solve your problem

[Duncan]

Normally modern replacement speedo drive gearboxes require the central hole reaming if its too tight it may be rubbing internally, I had one that the felt retaining ring came out of the gearbox too much (it hits on the lock ring) so I eased it out, reduced its depth and pushed it back in. Are there any witness marks on where it could be catching?

[thornebt]

The speedo gearbox is nowhere near the brakeplate, there's a good gap. I put a cable tie round the threaded cable boss and the swinging arm and took the bike for a run. It all worked fine. It just needs the speedo gearbox to not be done up tight and something to stop it rotating. So I made an angled bracket that goes onto the threaded speedo cable boss and onto the spare thread on the bottom shock bolt. I know it's a bodge of sorts but it works and I can't see any downside to it. I've got cheesed off with spending so much time on the speedo gearbox. I don't want to fit an electronic speedo as it will be expensive and the bike is still 6V positive earth. Also I think a cable needs to be attached to the fork leg.

Cheers. Bruce.

[Mick D]

So I'm thinking that maybe my worn stepped spacer might be the problem and that a spacer inside the gearbox might be a good next step. Has anyone got any ideas please, particularly in relation to size / thickness of a spacer inside the speedo gearbox?

Hi Bruce

I think you are on the correct lines there, (if I understand you correctly). Tolerance issues could be resulting in the bearing adjuster loading the ring gear as the speedo drive nut is tightened.

I'd try a washer of no more than 1/16" between the gearbox and flange indicated in this snag

Speedo Flange.png

I'd use a copper sealing washer as I have a 'selection box' of them, they are of about the correct profile and they're easy to file to an exact size.

Regards Mick

[thornebt]

So I'm thinking that maybe my worn stepped spacer might be the problem and that a spacer inside the gearbox might be a good next step. Has anyone got any ideas please, particularly in relation to size / thickness of a spacer inside the speedo gearbox?

Hi Bruce

I think you are on the correct lines there, (if I understand you correctly). Tolerance issues could be resulting in the bearing adjuster loading the ring gear as the speedo drive nut is tightened.

I'd try a washer of no more than 1/16" between the gearbox and flange indicated in this snag

Speedo Flange.png

I'd use a copper sealing washer as I have a 'selection box' of them, they are of about the correct profile and they're easy to file to an exact size.

Regards Mick

Thanks very much Mick. I will give that a try and let you know how it goes. I know that's a large thin washer but I think I might have one. Cheers. Bruce.