speedometer relation to odometer

[Steve]

How would you compare the speedometer against one of those radar speed trailers you see the cops put out then?

My son call those "high-score signs". Some of his friends try to see what numbers they can get to show on them.

They are accurate, but I would only trust it if you are the ONLY thing within sight that is moving.

Traffic radar does not 'lock' on the closest or the fastest, as many people think. It locks on to the best reflector. You could be a couple hundred yards away from that radar trailer, probably not close enough yet to have it read you. If there is a semi-truck about 1/4 mile behind you, it WILL get picked up. You see the truck's speed and think it's yours.

Yeah, it works that way out on the highway, too. That's why officers are using LIDAR (lasers) instead of RADAR. The actual accuracy of speed is about the same, but the accuracy of target is improved to the point that they can pick a motorcycle out of a bunch of cars and/or trucks, where they could not do that with RADAR.

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[Guest]

Install a Sigma brand bicycle computer. Program in the diameter of the wheel assembly that you attach the magnet/sensor to. That should be within about 2%. (Sigmas indicate speeds well over 100 mph. Most others don't.)

[Normk]

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.

[Hacked850]

Here's one for you,is the GS1100L speedo different then the GS850G speedo??.The reason I ask is that I just swapped out my 850 forks for a set of 1100 forks and I had to use the 1100 wheel and as such I had to use the 1100 speedo drive because it was a different dia then the 850 drive.Now I'm wondering if the speedo is going to be off??.By the way my 2007 Vstrom speedo is 17% percent off at 60MPH.!!!!!!

[duaneage]

Yes, the tire diameter will have an effect; larger tires will reduce the rpm's for any given speed. Changes like this are pretty subtle though; you won't see the rpm's drop 750 just by moving a tire size or two.

I did. I'd prove that a tire which is 3/4" smaller in installed diameter caused the engine to turn a lot faster but I don't feel like buying a 110 and swapping it out.
You'll just have to trust me. Or do it yourself.

[Guest]

Speedo and Tach

Just was reading the thread on this. My speedo is about 10-12 mph too slow and my tack is way off too, probably by 2500 to 3000 rpms. So if I understand this right, I can reposition the needle to some what make it what it should be? I tried prying both off with no luck, maybe I wasn't using enough force? Any suggestions? I'm actually looking to install a Vapor system on my bike and just get rid of the garbage there now.

[Normk]

IME, (In My Experience) a tach which is off calibration by much is best replaced. The obvious issue is what can happen if you are even a small percentage off when over red line, especially with old machinery. But you already knew that so apologise for mentioning.

Sometimes the needle can be tough to pull. I have had to place some blocking across the face and to use two fork shaped panel clip tools. I have used a solder gun placed against the shaft while pulling on the needle hub to loosen but one has, obviously, to be quick.

It is always a bit frightening to recommend pulling & prying, especially on old parts, plastic in particular, as I do it by "guess and by-golly" and have no way to convey an impression of my own best judgments or that they are well taken.

One acqauintence mentioned that he was unable to release a speedo needle hub and so cracked the hub axially in order to effect the release. He placed a notched fulcrum (wood likely) against one side of the hub's lower section and then used a home-made tiny chisel to crack the hub parallel to the needle shaft (axial manner). He was then able to withdraw the hib from needle. Can't recall having done that but may have used the technique as it is common trade practice for removing seized nuts.

Before trying that on an old piece of plastic, I think I'd be using a Dremel and tiny burr to groove the hub in order to release the grip.

Hoping some of the above is useful. Will you please keep us posted as to your thoughts and progress?

Just was reading the thread on this. My speedo is about 10-12 mph too slow and my tack is way off too, probably by 2500 to 3000 rpms. So if I understand this right, I can reposition the needle to some what make it what it should be? I tried prying both off with no luck, maybe I wasn't using enough force? Any suggestions? I'm actually looking to install a Vapor system on my bike and just get rid of the garbage there now.

[hampshirehog]

A fork (type you eat with) can be useful for prying a needle off and keeping the pressure even.

[Normk]

Maybe not the type of fork I use....you have no idea of the wrath my wife would visit, and since I'm much more afraid of her than I am of you, I'll ignore the recommendation.

A fork (type you eat with) can be useful for prying a needle off and keeping the pressure even.

Kidding aside, did forget to mention earlier, if one is correcting/recalibrating an instrument it is well to consider whether the calibration is off by a fairly uniform percentage which will be addressed by repositioning or whether there is some other issue which would not support this solution. I like to use a GPS or other, known good, reference to compare the calibration throughout the useful range of the instrument.

Far a speedo, very low speeds are ignored in my view as the need for an accurate measure is low. Speed measure is always a compromise when using a mass produced mechanical device such as the traditional units used on GS and older automobiles. One needs to consider at which speed the speedo is best to be the most correct and so how much compromise is able to be borne within other speed ranges.

Having the speedo read very accurately at 10 mph or 125 mph with poor accuracy at 50 mph would, IMO, be poor practice. Bike speedos which I have checked are always wildly optomistic at higher speeds bu who cares? The court transcript will contain accurate and well vaidated figures for reference.

Fuel gauges are typically a compromise but are easily referenced by simply measuring the actual fuel at two or three important levels.

[Guest]

Speedo/Tach repair

Thanks for the suggestions, though after looking at this Vapor unit, I may just go that route. Looks easy enough to set up though not sure how I'm going to mount it inside my existing cluster. Gonna study on it over the winter then tackle it unless y'all have some suggestions for that as well, all are welcomed.