Mechanical speedo calibration

[rdhunt1]

Does anyone know how?  

 

Having disassembled, cleaned and recal'd smiths motorcycle speedos I was not concerned with removing the bezel from my caterham speedo to see if any adjustment was possible. I do not see the 'gap adjustment' screw like I was expecting.  The speedo is labeled sn5234-16 with a small 2696 sticker on the back. 

 

The issue I have is the speedo is reading very low. For instance,  Phone gps says 60 mph, odometer seems to be correct turning a mile every minute,  but speedo only says 30ish mph. As a second test out of the car, i drove the speedo with a foredom (variable speed electric porting tool) and held it at 60 mph and timed the odometer mile.  It was still much less than one minute. Has anyone experienced this, fixed it, or had it fixed by a reputable shop?  Open to any and all reasonable suggestions. 

 

Thanks, 

Ryan

 

Car is a 1996 supersprint, four speed, 3.9:1 rear. Speedo is 160 mph.

Edited January 23, 2023 by rdhunt1
Added car and speedo info

[MV8]

Has it always read low? Calibration speed for most is 1000 rpm at the cable for a 60 mph indication. The plastic driven gear on the transmission end of the speedometer cable is usually where calibration to a good instrument is made. Each tooth is typically a 5 mph change. When gears to change the ratio are not available, a ratio changing box is used. The boxes and gears are universal and assembled to order. The box goes between two cables or direct attach to the transmission the same way the cable does. The sometimes include a 90 degree drive for clearance and better cable routing. Costs run $75-150.  

If it is a speedo issue, it sounds like the magnet has lost strength or there is too much friction. The adjustment you are referring to sounds like the end of the clock spring.

[rdhunt1]

It is has always been low for the few years I have owned the car.   It sure looks to be the original speedo to the car and the speedo vs odo discrepancy makes me think it is a magnetic issue.  Reading through my assembly manual us see where it should be a labeled as w=1.0 for mph, and should have the 24 tooth driven gear in the Trans.  I will have to see if I can chi inform the with the teams in the car.  

 

I did see the ratio modification boxes you mention.  I may have to learn more about them.  I did find a dedicated speedo shop in Milwaukee that I may ring as well.

 

My speedo is labeled caerbo instruments Ltd on the speedo can.   I have been reading many topics of failed caerbont speedos, but most are the electronic where mine is not. 

 

Keep the hints tips, tricks coming!

 

Thanks,

Ryan

[rdhunt1]

Reading MANY posts about remagnetizing the armature.  Sounds simple enough,  I just hate removing the needle if I don't have to.  But, the speedo is currently of no use the way it is!

Edited January 24, 2023 by rdhunt1

[MV8]

You could just pop in a similar face cheap ebay gps speedo for now and cap the trans end. No noise, no cal, adds lightness, etc. Then you can spend as much time as you want on the old speedo and have the hole in the dash filled.

[Rosteri]

We made instruments for our boat project (Vesper) just as MV8 proposed, i.e. purchased new KUS GPS instruments, that have a removable screw on bezel and made new dials. I tried to find shops that would make the dials, but no luck.
 

In the end we used a UV printer available for free at a local library and the base was a 1 mm clear acrylic. We had the dials still attached to a sheed with tiny tabs for alingment and printed directly on the acrylic. The quality is great, not good enough for a watch, but worked for the dials.
 

Design:

 

Prints:

 

Assembly:

 

 

For the bezels we might still make custom ones. And yes, we also made all the small ones, like oil pressure etc…

 

[rdhunt1]

On 2/11/2023 at 5:37 AM, Rosteri said:

 

For the bezels we might still make custom ones. And yes, we also made all the small ones, like oil pressure etc…

 

 

[rdhunt1]

Rosteri- that's beautiful!  Very well done!

 

 I think I have successfully remagnetized the oe speedo.  Similar to magnetized a screwdriver, I used very strong magnets to remagnetize the rotor.  Thankfully i believe the oe would magnetize to saturation, so i am not sure it can be overdone.  Previously it read about 35 mph when it should have been 76.  At the same rpm ( fed via cable from my lathe) it now reads about 75 mph.  I will get it in the car and validate with a gps before rolling the bezel back on.  While I tried very hard to get the needle back in the identical position it may be a mph off.