Speedo calibration

Status
Not open for further replies.
D

dimitri

Hi all,

First of all, i like this forum a lot! It was a big help getting my commando sorted out over the last couple of months. It took me some work this summer to get it running perfectly, but it is now. My main goal was to get it ready before the winter. Just to know i can enjoy every possible day on it from next year on.
In fact, i enjoyed many days on it already this year.

It starts and runs perfect now. As for running, i don't have any strange side effects. However, there are a few things to sort to get it ready for next season:

• One of the exhaust nuts vibrated loose a couple of times the last weeks. The nut was locked in place, so i assume the thread is damaged. I will take no risk and ship the head for a repair during wintertime.
• When i got the bike, the speedo was not working. I repaired it by following a step by step guide on the internet. That went perfect. Installed a new bezel and is looking as new now. I shortened the cable. After that, it worked better, but fluctuating hard. So i repaired the speedo gearbox. Better, but still fluctuating. Then i just replaced the speedo gearbox by a NOS les harris one (correct gear ratio for a commando 850), and the cable by a NOS Alpha cable. It was a perfect fit. The outer cable was a better length, and the inner cable did not need trimming. Now, the speedo is not fluctuating anymore, but i notice it is too far off. When doing, let's say 120km/h, it reads 90-100km/h.
I did count the turns of the calibrating screw in the back of the gauge, and set it like it was before. Maybe it was at some point calibrated to compensate for a fault... i don't know? Would this kind of difference be possible to correct with the screw? And if so, would i need to turn the screw clockwards or counter clockwards to get a higher reading? I understand it is not really possible to set the screw without disassembly of the gauge, or carefully drilling a hole in the body. I would like the second option for future adjustment or fine-tuning. I managed to get the new bezel on pretty good. So if possible, i would like to keep it on. Any thoughts on that?

Thanks in advance!
 
The speedo drive at the rear wheel is "very" common and used in a variety of bikes... They do have different drive ratio's for different wheel size's etc.. You may need to check what ratio you are running and what a standard Norton runs.. ????. I had the same problem but opposite.... Speedo indicated 70 mph and feels like 40mph..
 
I bought the gearbox as a 15/12. The old one is 15/12 for sure. I have read that is the correct one for the 850 MK2 (1974). But i did not double check the numbers on the new gear. I guess they could have sold me a wrong one, or a wrong one in the right box.

Does the difference seem to high to calibrate?
 
Tire size can also affect speedo calibration. A larger diameter tire than originally fitted will result in faster than indicated speed, and smaller Dia tire than original slower than indicated.

In your case, there is about 20% difference (120 vs 100).....this is more than what would be caused by a one size difference in tires. A rough estimate of changing a 4.00-18 tire to a 6.00-19 is a 10 % increase in speed.

If your rear tire has been altered from original size, it may be PART of your error.

Slick
 
Do you still have the original speedo gearbox? If I remember correctly, the GB on my Atlas had the turns ratio stamped on the body....just compare numbers if ratio is stamped on original and new gearboxes. Otherwise, count the turns of the cable for one turn of the drive dogs. If turns ratio is same, then calibration error lies in speedo.

If error is in instrument, I would send it to a rebuilder for re-calibration.

Slick
 
I have the gearbox that was fitted previously. But it was not an original Smiths either. The one that was fitted is 15/12 gear ratio (checked the number on the gear, the body itself is blanc). You are right, i can count the turns on the cable. I forgot to check the stamp when i installed the new one. That is one possibility.

Would a gauge rebuilder do anything other than adjusting the screw to calibrate (how common is it for the magnet to need magnetizing)? I understand that dry grease etc has impact on the speed. but mine is as new inside. If it is just turning the screw, i can easily calibrate it myself by adjusting it and comparing step by step with a gps speedo.

Now that i write about it, i am thinking of the reason i had to repair it in the first place. The previous cable pushed the internals too far into the speedo, causing friction between the magnet and pickup disc on the needle shaft. I don't recall exactly how it works anymore. Either way, i had to press a part back to it's original location, so the friction between the 2 parts was solved. What the calibrating screw does, is adjusting the distance between the 2, for what i can recall.
When following the step by step, it said to count the turns of the screw. I did that, and set it as it was prior to disassembly. But now, i think the 2 parts might be not exactly positioned as they were from the factory. That way, the original calibration screw setting could just be wrong.

Not trying to be a hard learner here, or cheap. But i like it when i can fix things without sending them to pro's.
The input is greatly appreciated!
 
If the speedo drive gearbox was the wrong ratio, the odometer wouldn't read right. I bet it's right enough, and 1.25 : 1 (or 15/12) ratio is normal for a 150 mph speedo.
There is a little screw inside the speedo that can change the speed reading. It's not easily accessible.
 
Yes, if not opening up the gauge, i would need to drill the body to adjust it properly. If i can do that safely, i don't mind. But don't want to get metal parts inside.
 
Here is the back of a Smiths 150mph green ball dialed speedometer.
The gold screw on the right side of the photo can be seen when in the case through the light socket hole.
This one looks like there is some glue over the screw head to secure the setting.
May be difficult to move, if this is the adjustment screw that is being discussed.

I did confirm that the cable turns anti-clockwise in the speedo head if looked at this rear view.
Looking down on the cable itself, the rotation will be clockwise.
Speedo calibration
 
i just drilled a hole in the back of mine then strapped a satnav on the handle bars and adjusted the screw until the speedo was spot on with the sat nav as long as you are not going up or down a mountain i think this is a fairly accurate way to set it ,,,,,,,,,,,,baz
 
73 commando 750. Speedo Gauge needle is steady but reads about 8 mph too fast (reads 58 when going 50) I understand that this means the little spring that returns the needle to 0 when you stop is weak.

Folks in this thread mention accessing a calibration adjustment screw. My gauge does not have any adjustment screw that I can see. I pulled out the bulb and looked, but there's no screw in there. Some folks mentioned drilling a hole to access a calibration adjustment screw, but I didn't see anyone say where to drill this hole or how big a hole to drill.

I've ordered the book (Smiths Magnetic Speedometer Speedo Repair Tachometer Revcounter Repair) from the UK (via EBay) that details repairing and calibrating these Smiths gauges, so I expect that will answer all questions. http://www.ebay.co.uk/itm/Smiths-Ma...ain:email&cal=6f567628ae10&cust=LMZ6PFNKELNBL On EBay, the author says it tells you how to calibrate with the gauge on the bike, or with the gauge off the bike and in a jig. If you can do it on the bike, then there must be an adjustment screw that is, or can be made, accessible without taking the gauge apart.

I downloaded an app that uses GPS to turn your phone into a speedometer, so I can tape the phone to the handlebars for an accurate reading. I think all I need now is to know how to access the adjustment screw (if it exists on my gauge).

The book should answer my questions, but thought folks here might already know about reaching the adjustment screw without having to have the gauge disassembled. Any help would be much appreciated!
 
Do you have a Smiths or a Veglia speedometer, all the Smiths I have seen have the adjustment screw, never looked inside a Veglia to see if it has one.
 
Thanks for your reply. It's a Smiths. the bottom of the face says "SSM 3001/09 1000". Where have you seen adjustment screws on Smiths Gauges? Did you have to drill a hole to get access?
 
The website I used has disappeared, there is a small brass screw under the mechanism but to get to it you need to drill a hole on the underside or remove the bezel and glass. Removing the bezel will split it and a new one needs to be rolled into position.
 
Thanks! I recently had the glass and bezel replaced by an expert, so I really want to avoid trying to open it up that way. I know folks say it's not rocket science to remove the bezel and glass if you are slow and patient about bending open the bezel where it grips the case. I might have to do that - but I was hoping someone (or the book on it's way from the UK) will tell me where to drill the underside to access the screw. I haven't found a picture or diagram of the innards that would show the screw so I'd know where to drill. I'll search again

These pictures are from the web, but they're the same as what's on my bike -


Speedo calibration



Speedo calibration
 
Status
Not open for further replies.
Back
Top