12 replies to this topic

[JamesC]

Hi

 

The miles covered odometer in my centre speedo Smith clock has stopped working. I have taken the speedo out, and apart only to discover that the speed calibration will now be lost! 

 

 

Can anyone recommend a company capable of fixing and recalibrating the clock?

 

After a quick search I have found http://www.speedogra....com/index.html

 

 

Thanks in advance

 

James

 

 

 

[cal844]

Guessworks

[russo]

Speedo calibration lost??????????

[Spider]

Speedy Cables also do calibration, but why do you think the Calibration will now be out?

[JamesC]

Speedy Cables also do calibration, but why do you think the Calibration will now be out?

Correct me if I'm wrong - The main needle tension that springs it back to 0mph is lost when taken apart.

[timmy850]

The spring tension can be done by using the dot on the edge of the speedo face. A quote from the links below:

 

I had one of my speedos fixed today. I had stripped it apart to get the odometer working. The instrument tech then showed me something that I thought might be useful to other members.
To get the correct calibration and degree of tension on the speedo spring, the needle has to be fitted back onto the fine shaft so it lines up with a tiny dot under the oil pressure light (in my case).
The fine needle stop is spring loaded (I didn't know that) and you simply push it in to move the needle to rest on the other side.
He then tested it at 1408 tpm and it was spot on at 30 and 60mph.

 

 

http://mk1-forum.net...t=speedo needle

 

& from a gauge repair guy:

I agree The spot if it is there is used for setting pointer backlash and as a starting point for linear calibration.
Often it equates to the same distance as that between the pointer stop and the first major graduation.
From there things like wear, hairspring tension, height of the speed cup and particularly magnetism come into play.
I trust the documents help people.

 

http://www.ausmini.c...41&hilit=speedo

 Speedo-dot.jpg   23.88K   10 downloads

[JamesC]

The spring tension can be done by using the dot on the edge of the speedo face. A quote from the links below:

 

I had one of my speedos fixed today. I had stripped it apart to get the odometer working. The instrument tech then showed me something that I thought might be useful to other members.
To get the correct calibration and degree of tension on the speedo spring, the needle has to be fitted back onto the fine shaft so it lines up with a tiny dot under the oil pressure light (in my case).
The fine needle stop is spring loaded (I didn't know that) and you simply push it in to move the needle to rest on the other side.
He then tested it at 1408 tpm and it was spot on at 30 and 60mph.

 

 

http://mk1-forum.net...t=speedo needle

 

& from a gauge repair guy:

I agree The spot if it is there is used for setting pointer backlash and as a starting point for linear calibration.
Often it equates to the same distance as that between the pointer stop and the first major graduation.
From there things like wear, hairspring tension, height of the speed cup and particularly magnetism come into play.
I trust the documents help people.

 

http://www.ausmini.c...41&hilit=speedo

Speedo-dot.jpg

 

Hi Tim

 

Thank you very much for this information, this sounds like just the fix I was looking for.

 

As I may now be able to set the spring tension correctly myself its well worth a go. Then if it all fails I will just send it off for a full rebuild.

 

James

[JamesC]

@timmy850

 

I have just checked the speedo and found that the little mark just below the H/B light is present.

[MrJenkinsEsq]

I can highly recommend Speedograph Richfield if you're still looking for opinions.  I sent off my VERY stuttery and clunky Mk1 speedo with a trip that wasn't working, and it was back with me in less than two weeks, reconditioned and calibrated, for £75.  It's now beautifully smooth and I have a working odometer for the first time in years!

 

 

[tiger99]

Both suggested companies have a good reputation. However I am sure that you will get it right yourself. The only time that it would be irrevocably lost would be if the magnet lost its magnetism, and that would mean remagnetising it on a jig.

 

You may know already that the speedo basically is a crude and simple dynamo with a fixed magnetic field and solid copper armature.  (Well, actually, either the magnet or the armature could be the rotating part.)The rotating field generates a current in the armature, into an almost dead short, because it is a lightweight copper cup. The current is proportional to voltage induced, which is proportional to field, so the drive torque to our little dynamo ends up being proportional to speed and the part fixed to the pointer rotates proportional to torque, and therefore speed, till the linear force reacted by the spring, due to Hooke's law, balances the torque. so the needle deflection is linearly proportional to shaft speed. Knowing that, you will see that you set the range by changing the magnetic field (which is how they do the calibration, usually) or by selecting different springs, while the starting point, zero, is set by moving the spring anchor.

 

So there is not much to go wrong, except for very delicate bearings due to the small torques involved. Most speedo problems are caused by the cable, or in your case a simple but fiddly mechanical counter.

 

I have to say that the modern electronic speedos are better but need different skills to fix them, and in many cases will have to be thrown away and replaced by something different when the eventually break. There is always something like a Digidash, or equivalent made at home, if the supply of parts for modern classics runs out. I actually think that way back in 1959, Alec was thinking well ahead, because one of his original sketches showed a small horizontal speedo under the upper dash rail, right in front of the driver, and a full width, unobstructed parcel shelf with no centre binnacle. Presumably Smiths could not do it for a reasonable price, so we are where we are, with a speedo design that was seen in the Moggy (slightly different warning lights and scale) and probably much earlier, but still repairable..

[midridge2]

Why oh why is it that most of your replies to questions asked end up being a science lesson that has nothing to do with the question asked.
If I need a science lesson I can do a Google search.

[timmy850]

Also, when I was talking to one of the gauge repair places here, he said one of the big problems with the speedo needle waving up and down was that some new speedo cables are eccentric. When they add the blob of plastic to the cable where it meets the speedo it can sometimes be off centre. If this is the issue normally it'll be worse at low speed and smooth out a bit as you go faster.

[Magneto]

Tiger...you can't teach people who don't want to learn, but please keep on giving us "science lessons", some of us really appreciate the depth of your knowledge.

 

Timmy 850, interesting point, but on my cables the cable wire itself is inserted into the speedo drives, both ends, how would an offset plastic bushing affect that? I guess if it caused drag, and windup and release of the spring cable?

Edited by Magneto, 04 June 2017 - 02:34 PM.