Accurate speedometer alternative for old boxer... - K-Bikes.com - Excellence in Motion
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post #1 of 11 (permalink) Old Jul 6th, 2010, 9:48 pm Thread Starter
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Accurate speedometer alternative for old boxer...

Hi there K bikers,
My 1980 R 100RT was always "a bit off" as far as the speedometer goes. But until I temporarily strapped my wife's car Garmin GPS on there to check it more carefully I was very surprised how far off the speedometer is... 50 km/h is 65 on the speedometer... 125 on the speedometer is 100 km/h real speed. As I ride a lot in the US as well I am looking into getting an alternative for the original speedometer that would get me km/h as well as mph. Doubt that there is an adjustment in the worm drive to the speedometer cable to correct the original speedometer. THAT would be ideal.... So, I am thinking to maybe get a pedal bike speedometer but I'd have to turn it off and on every time I fire up the bike? I'd have to feed it batteries all the time too.... Are the wheels not too small for a pedal bike speedometer OR are there other thoughts anyone can give me? MUCH appreciated as TOO fast is dangerous but TOO SLOW is not safe either...
Jan
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post #2 of 11 (permalink) Old Jul 7th, 2010, 2:27 am
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You will never get an accurate reading speedo on a bike....tire ware is fast and in the corners your tire size ratio is constantly changing ....best you can hope for is close at best....
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post #3 of 11 (permalink) Old Jul 7th, 2010, 7:48 am Thread Starter
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Does not need to be accurate to the mile/h. My speedometer is so far off that it is not funny anymore... I have to keep on converting the speedometer reading to my findings which is not ideal...
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post #4 of 11 (permalink) Old Jul 7th, 2010, 1:23 pm
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Take it a speedometer shop. They can replace the needle and set it back and point where it indicates better. Your odometer may still be off due to gearing, if it is off too, but at least the needle on the face may point to something closer than a 15 MPH difference.

The cop's R1100RT-Ps had some gearing and head changes as they need to rely on them. They even had to stop issuing tickets around 1997 for a brief while when they began using them until BMW came up with a closer reading head. Don't know if they make the parts on yours to be comparable though.


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post #5 of 11 (permalink) Old Jul 8th, 2010, 12:33 am Thread Starter
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Thanks for that thought Mack, repositioning the needle... THAT could be good enough to get me SOME accuracy. I'll have a closer look as to how different speeds relate to real time speeds and how equal or unequal that works out on the needle distances between the two speeds, over the range of the speedometer to guess where I'd end up. But I had not thought of that one...! Could be what I need! Thanks!!
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post #6 of 11 (permalink) Old Jul 9th, 2010, 5:06 am
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You could always try one of these: http://www.dealextreme.com/details.dx/sku.2682
and you may get the added benefit of a trip computer.

Phil Mars

1984 K100RT
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post #7 of 11 (permalink) Old Jul 12th, 2010, 9:57 am Thread Starter
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Thanks Phil from ALL the way down under. I MAY have to go that route but I wonder if there are any pedal bike speedometers with backlighting so that in the winter months with all the riding I'd be doing in the dark, I can see my speed... But I see that these units have motion sensor on/off. So my worry of having to turn it on and off every time is not there. Still looking for a way to get the stock speedometer corrected somehow, like the needle resetting... Thanks!
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post #8 of 11 (permalink) Old Jul 13th, 2010, 6:33 am
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Googling bicycle and motorcycle speedos brings back some really interesting results and there are quite a few accessories available and not too badly priced either.

It is also evident there are quite a few speedo rebuild options and one would assume calibration but like everything it comes down to cost and availability.

The other option is a bit of maths and with the aid of the GPS it would be interesting to see how close the theoretical is to the real world.

This of course assumes the tacho is accurate but if you know your engine revs, gearbox ratios, final drive and rear tyre circumference it is not hard to calculate the speed.

Phil Mars

1984 K100RT
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post #9 of 11 (permalink) Old Jul 27th, 2010, 11:22 pm Thread Starter
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Thanks Phil for the input. I have been scouting around on the internet as well for alternatives. But do not want to go too far with new instruments disrupting the dash. There is lots available but the needle adjusting would be nicest. The simple pedal bike speedometer on the dash would not be too bad. BUT I heard from a cyclist that I may get ignition interference with that solution. I also wonder what happens with winter/ in the dark- riding.. Do pedal bike speedometers have back lights... So I'll have some checking out to do. I am very busy now with work and will post later on what I decided to go with. Any more inputs are ALWAYS appreciated!
Cheers!
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post #10 of 11 (permalink) Old Jul 31st, 2010, 6:30 am
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Yours is an old bike with what I imagine will be the old style either totally mechanical (like clockwork), or it may have a drive cable which gets converted into electronic pulses. I've re-calibrated the last type before in cars I've owned.

There are three types of error problems. The first is an 'offset error' which is when the reading is always the same in widths of the needle at each point around the scale. That is the easiest to get right ( move pointer or adjust hairspring). The second type of error is a percentage non-linear type where the speedo may read correct for the low speeds but then runs rapidly out at higher speeds. The third type is non-linear error where the speedo reads correct over 1/3 of the scale, something different over the next third and another error over the last third - that is the most difficult error to correct.

Yes, tires and the rolling circumference make a difference, but this would be a 'linear error' more easily corrected.

The only way to check and adjust a speedo is on a test bench. If it is completely mechanical it should have good accuracy unless the mechanism is fouled up or dirty. For years the leos here used large 'Chrono' speedos they claimed were linear, giving 1% accuracy and were like clocks inside. Of course on the vehicle that 1% would change with tires and offered a good defense! A lot of the old pre 60's 'Smiths' speedos were made like this. Mechanical speedos are checked with an adjustable speed test motor and accurate rpm meter, electronic speedos are tested with pulse generator equipment.

Then electronics came in to get them cheaper and speedos became part mechanical and part electronic. With some electronic speedos you can attach a pulse generator inside to check each of the marks around the scale. The dial is usually a large angle moving coil meter. That's where the problem starts because the voltage applied may be linear and correct but the meter doesn't move to the right place. So they usually fit a thin circular metal cup around the meter magnet which has finger segments. By bending the segments you can change the magnet field at a dozen points around the scale - that is how they deal with non-linearity. Since these speedo meters are the common moving coil type, there is a hair spring at either end of the moving pivot. Just like mechanical watches, these can be adjusted to offset a zero shift and give the right tension.

Most vehicle speedos are optimistic which gives a 'feel good' factor in lower power cars and builds in a safety marging for speed limits. Perhaps I'm biased, but most cars with engines under 1600 cc seem to have optimistic speedos.

If you want to avoid cluttering up the bike with addons that either have no backlight or aren't that weatherproof, I'd look at the speedo re-calibration option first. The alternative is to add some calibration marks to the speedo face for important speed limits in your local.

It's a brave rider that would ride through radar speed checks several times to see if the calibration was correct!



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