Good old speedo drive.

Information relating to the Matchless G12 or AJS Model 31 650cc twin
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Columbo John
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Good old speedo drive.

Post by Columbo John »

I know this has discussed many times before, but I just cannot understand where I am going wrong.

A brief history..... I have just finished rebuilding my 1961 G12CSR, I took it out for a 5 mile spin last week and then gave it a check over this morning before embarking on ride number 2. One of the things I noticed was the speedo drive had moved slightly, not by much, maybe 20/30 degrees. Knowing the issues with the speedo drive, I took the back wheel out and double checked the various washers and seals etc. I used this illustrated guide to assist me. http://archives.jampot.dk/technical/Whe ... odruff.pdf

I was pleased insofar as I had assembled it exactly as per the guide above but then confused that the speedo drive had moved. I re-assembled everything and then went for a slightly longer ride. After 4/5 miles I pulled over to see the drive had turned slightly again, I loosened the nut, turned the drive to the correct position and retightened it. The speedo drive was too hot to touch without gloves.

I went straight home very carefully and within a mile of home the speedo stopped working. I pulled over and saw what is in the pics attached. To say I wasn't happy would be an understatement. :headbang: :headbang: :headbang:

So apart from the fact that I need a new cable and speedo drive, how do I assemble the thing?
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Taid
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Re: Good old speedo drive.

Post by Taid »

There is a good pic of a sectioned hub here ... https://www.jampot.com//pic_display.asp?id=820

shows the arrangement very clearly

Beware the are a number of apparently identical speedo gearboxes .. the correct one is as rare as hen's teeth (as I found when I did the same thing with my 16ms) the correct one has a bigger bore diameter than the others ..
___

Now sold ... 1956 AJS 16MS Bitsa .. HSU 414 .. rebuilt/re-registered 1987
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clive
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Re: Good old speedo drive.

Post by clive »

I reckon that incorrect assembly is not the only reason the drives rotate. The wheel drives a worm drive in the speedo box. As this wears it is possible for the worm teeth to come to a fine point. At this stage they may slightly misalign and snag occasionally but still continue working. This snagging might explain the partial rotation of the drive. After a bit the wear allows the worm to completely cross thread and the whole box jams and rotates. If the cable does not pull off and allow the drive to rotate with the wheel the bearing adjuster can be driven in locking the rear wheel.
Original Smiths drive boxes have a sticker on the flat area advising greasing every 1000 miles. This is necessary to stop rapid wear. Unfortunately the nipple on many is the early small type which you can only force grease into if you have an early grease gun. Yours appears to have a later type nipple fitted but if it is an original drivebox i wonder if it was abused before someone added the later type nipple (they are the same thread)
clive
if it ain't broke don't fix
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les ward
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Re: Good old speedo drive.

Post by les ward »

I would agree with everything Clive has said, but would add that the nut on the Speedo gearbox is the first thing to undo when removing the wheel and the last thing to tighten. Also check the bearing adjuster has just touched the bearing and then back off slightly so there is just a hint of play.

The only time it happened to me was shortly after repairing a puncture and I didn't touch the nut on the Speedo gearbox. My theory is that if the spindle is tightened more than previously, then it forces the Speedo gearbox into the bearing adjuster/drive with the results you have.

In some ways you were lucky, because left for much longer it can lock the back wheel. When it happened to me, a nearby pedestrian thought I had run over a pig it squealed so much as I slalomed across the road.
Groily
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Re: Good old speedo drive.

Post by Groily »

les ward wrote: Thu Feb 02, 2023 9:08 pm My theory is that if the spindle is tightened more than previously, then it forces the Speedo gearbox into the bearing adjuster/drive with the results you have.
That's my strong belief too. Always worth checking a spacer isn't needed, especially when replacing a drive 'box. Over nearly 50 years I've had it happen three times. Twice for no obvious reason out of the blue, once was my own stupid fault. No dire results in terms of locking up or falling off, just broken bits and curly-wurly cables wrapped round the axle.
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clive
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Re: Good old speedo drive.

Post by clive »

Yes I have had it too, at speed two up with camping gear. In my case the thread on the hub gave way and the only reason I stopped was because the passenger was alerted by a following motorist who had noticed the broken speedo cable thrashing round and round with the wheel. All I had noticed was the speedo had stopped working! Made it to the Jampot the next day in the car and secured a replacement hub in the autojumble. Mine was definitely a worn out and jammed speedo drivebox.
clive
if it ain't broke don't fix
Columbo John
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Re: Good old speedo drive.

Post by Columbo John »

Thanks for all you replies, there are some helpful hints and tips in there!

I've come to a couple of conclusions.............
1) This issue is not purely down to incorrect assembly
2) I brought it upon myself

When restoring the bike I wanted to retain as many of the original parts as possible, one of which was the speedo drive. It was pretty stiff when I first looked at it so lots of cleaning, soaking in diesel and removing the old grease got it moving again. I fitted a later type grease nipple (as someone mentioned above) and pumped in some grease. In hindsight, I had no idea how worn the drive was (also mentioned above), so although I assembled everything in the correct order, the drive could have been too worn to be reliable.

Another issue I have found, which I'm not sure if it contributed to the problem, is that the spacer on the right hand side (which butts up against the swing arm does not go though the nut holding the speedo drive on. I only noticed this as I decided to replace all the nuts and spacers to be on the safe side and the new spacer DOES fit inside the nut.

I intend re-assembling it all this morning and going for a spin later on. Wish me luck :?
G15 Roy
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Re: Good old speedo drive.

Post by G15 Roy »

You can see that the complete spindle has moved as the Tommy bar is hitting the bottom of the leg and that spacer looks not to be as original.
Roy
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