Someone mentioned recalibrating the speedo so thought to reference in a separate post from the other one containing the magnetic reference.
Obviously, changing the speed of the front wheel by changing tire diameter/tire & wheel diameter will change indicated speed as related to actual speed over the ground. Changing the gear ratio between the drive and driven speedo gears will affect speedometer indication also but this is a technique common to automobiles but difficult to apply to bikes in practice.
The rate of needle progression versus speed (RPM of the speedo cable) is a function of the torque transmitted from the magnetic rotor (driven by the cable) to the reaction drum which is connected to the needle. The rotation of the needle is opposed by the clock spring which links the needle or needle shaft to a fixed mounting.
In order to change the relationship of cable RPM to needle indicated position one can change the strength of the clock spring, magnetic rotor but these are also difficult to do in practice.
In my experience, bike speedometers tend to be out by a fairly consistent percentage which is most noticeable at higher speeds because the difference in distance covered seems to be more evident. Testing around town, for example, and shown that the percentage tends to be consistent.
For this reason, I have been less concerned with attempting to "re-map" the speedometer's relationship and have concentrated on simply repositioning the needle within its existing range.
Those who have serviced speedometers, tachometers and other mechanical indicating devices will be well aware of all of the above, agree with much of it and have valid differences in view point. I apologize to these people for taking so much band width but do try to place my information within as much context as possible for those who have not been involved in the subject at hand.
One could, for example, reposition/retension the clock spring in order to create a different spring rate in order to affect needle movement versus cable RPM but I have found it to be most workable to simply reposition the needle on the needle shaft to point the needle to a more accurate representation of actual speed.
I usually use a procedure similar to the following:
1) Using an electric drill, turn the speedometer cable and observe the indicated speed on the speedometer head. Note that this is best if a speed in the 50 mph/80 kph range and that the drill speed must be able to be dupicated. For this reason I prefer a mains powered drill rather than a cordless model. Let's use 50 mph as the example speed although it is unlikely to be actually some even number.
2) Use a GPS or measured distance in order to determine the actual ground speed at this indicated speed. Again example 50 mph.
3) Remove speedo and acess the needle. Some machines have a sealed/encapsilated speedo head such as the GS while others have a speedo which is covered by a removeable common dash lens.
4) Refer to the actual speed and to the indicated speed when the speedo is driven. Let's say that the speedo indicates 55 mph when it is actually driven at an RPM which reflects a true ground speed of 50 mph. We want to reposition the needle so that the new position is pointing to 50 mph when driven by the drill.
5) In almost every case, and have done a lot of speedos and tachs (although less of an issue with a tach and one has to be seriously anal to care
) it has been possible to carefully pry the needle hub outward by wedging from both sides evenly. Sometimes one has to be innovative to avoid too much force on the face plate etc. but judgment calls are difficult to describe. This has never been rocket science however.
6) When the needle as been removed from the shaft, one can slip it partially onto the needle shaft in the estimated new position, run the drill and recheck/reposition. This works well enough.
7) In most cases it is convenient to reach one finger onto the needle drum. In these cases, I simply rotate the needle to the correct indicate position, hold with one finger, use a marker to indicate the alignment position of the drum relative to some fixed point near the drum. When the needle has been removed, rotate the drum to the marked position, hold with one finger and press the needle partially into place.
8) Run the drill and check. Note since the needle hub is only partially onto the shaft, it is simple to hold the drum and to rotate the needle in the direction required in order to correct indication.
9) When satisfied with alignment, press the needle hub into position on the shaft, recheck and done!
What have I forgotten to mention...??....
Oh, yes. Take care not to attempt to use the needle as a means to apply torque to the drum. Especially in old speedometers, the needle may not be as strong as one expects and may be very brittle with age. Don't ask!
In some cases it may not be practical to pry the needle hub from the needle shaft or the needle may be snapped off from accidental handling. No biggie! Simply use your die grinder, small file or other to remove the covering paint/coating from the intended needle position and glue the needle to the hub. I....I mean someone told me.... that they found it best to rotate and to hold the drum with a finger and then to place the needle to the correct indicate position.
A piece of spacer material of the correct needle to face plate thickness should be placed across the indicate zone and anchored at each end by a bit of masking tape. The needle can then be dabbed with glue, positioned and held to the spacer with a small bit of masking tape. When dry the needle can be held against the anchor and tape rolled off. Or at least that's what a guy who wasn't careful enough and snapped a needle told me that he did.
I later used the same practice when couldn't risk any more force to remove a needle. Most pry off quite readily and in a few cases, have simply held the drum into the correct position and used the needle hub to rotate the hub while still in place.
Another very workable method is to create and print a new face plate which reflects a more accurate positioning of the speed under the existing needle position. Despite that this seems to be a more involved procedure....try it out and I think that you will agree....
As an aside, an acquintance made and shared by email, a new face plate to indicate the operating temperature in degrees F, of the KLR650 coolant temperature guage. Interesting but got some peoples' socks in a knot, however that's another story.
Anyone who is disassembling speedo heads for other service is over qualified for the above recalibration so it might not be best to confess to your buddy how simple it was for you to do this. You will both be happier if he thinks you performed some magic procedure which is impossible for ordinary mortals.
However I soom noticed that when at 90 indicated, utility truck pulling trailers, and women in ford focuses were passing me.
So now I know the spedometer is optimistic. Im still trying to gain a reference point as to actual mph. Im waiting to pull along side a car with a digital speedo. I could go 60 ind and count off between mile markers but going that slow is no fun and im probably going more like 50. I did observe my odometer between 2 known mile markers and it was spot on. Am i to assume that the odometer and speedometer arent running off of separate mechanisms?:-k Id like to think my speedometer is relatively correct and there are just alot of lead foots out on the highway. thanks.
