R75/6 Speedo consistantly 10mph over...

[f11at125]

Hi all,

I am driving a 1976 R75/6 that I bought from its original owner, a nice enough guy whose mechanical inclinations I have been busy improving upon (he used a hacksaw to take off the horn mount and turn signals to fit a Vetter, and was really good at rounding out Allen heads. But at least he was adept enough to keep the lubes fresh, and anal enough to log every change of them. You gotta take the good with the bad, and with this bike's stellar compression and incredible overall health, I'm happy to replace a few bolts as needed).

One of the items he recently replaced was the speedo cable. The bike is 10mph generous at all speeds I've been comfortable comparing to the stationary radar in a nearby town ("Speed limit: 30mph - your Speed: XXX"). 25 MPH on the speedo shows as 15. 30 on the speedo shows as 20. 40 as 30, and 50 as 40. I can detect no increase or decrease in the discrepancy at high versus low speeds.

Yesterday, I saw a cop with a radar gun in a 40 mph zone and kept it right at 50. I pulled over and asked him what my speed registered as, and he said "40, dead on."

So I lie in bed at night trying to be like Pirsig. What do I KNOW? I know the speedo is off by 10 MPH. I know the former owner whose mechanical ability I disparage replaced the cable. Those are the only two things I am certain of.

I THINK that if the gearing ratio were off, that the discrepancy out to be different at higher versus lower speeds - based on the articles I've read linked here on the forums. I WONDER if it's possible that there was at some point a dissassembly of the speedo that wound up with the mechanism rotated 10 MPH-worth clockwise - or I guess I should start by wondering if that's even possible?

The easiest solution would be to just subtract 10 mph from the readout and live happily ever after, but this is a BMW forum, and we all know that solution would only work on a rice burner.

Could it be as simple as the wrong cable?

Any advice would be appreciated, and thanks in advance.

[schrader7032]

Welcome to BMW motorcycle speedos!! Actually, there's a service bulletin that describes how much the speedos can be off, typically high. You can imagine that it's "better" to have the speedo read high to keep you out of trouble with the law. But, as you've done, you're factoring that in by running faster by 10mph to compensate. I've also done the timed mile(s) on the freeway and computed my average speed and compared that to my speedometer. I've also paid attention to the odometer to make sure the mileage is not part of the problem.

I don't think it's the cable. It comes down to a handful of things:

- do you have the correct speedometer for your final drive? I suspect you're aware of the "W" value written on the face of the speedo and that it must be a match for the final drive ratio. Snowbum has various relationships here:

http://bmwmotorcycletech.info/ringgears.htm

- what about your tires? Are they too big, too small for your situation?

The one adjustment that I've heard people do is to take the speedo apart and change the spacing between the speedo needle pickup and the magnet that's being driven by the cable. The spacing results in the magnet influencing the needle and registering the speed. If the spacing is large, less influence; conversely, close spacing, more influence.

I've also heard about repositioning the needle on the shaft, somehow using something like a pickle fork to lift it off and resetting it. Seems like brain surgery to me.

The Airheads.org website has a series of steps to repair the odometer not working problem; it might provide some insight into the speed side of things.

http://www.airheads.org/content/view/160/98/

[f11at125]

I must say I'm puzzled by what I've read about the "built-in" nature of speedometer inaccuracy. These are the same guys that machined my valves and bored my cylinders? Because from what I've seen in the engine block they were rather particular about being exact. Then you get to the speedometer - an instrument, for God's sake - you'd think they would be all over it like hair on a bride's 10th anniversary - but instead, the official BMW position seems to be "meh...it's close enough for government work?!?" In my imagination, Groucho, Gummo and Chico are in charge of everything except the speedometer and turn signal, which they left to Harpo. "Hey, at least it shows what direction you are going accurately" - HONK.

[Darryl.Richman]

If a speedometer, as delivered with a bike, was pessimistic (eg, read lower than true), and the rider got a speeding ticket, and the rider in turn sued BMW, they'd have a hard time defending. So, they came up with a specification for their subcontractor to ensure that the situation wouldn't come up. These speedometers can be set to be completely accurate, and BMW supplied such accurate speedometers to police forces -- and they still do. (Look at an Authority RT-P and you'll see that they have a different speedo, and it's guaranteed to be very accurate.)

The subcontractor reads the spec and understands that the speedometers need to read high. So they make them and calibrate them high. Because there are certain other inaccuracies in the system, mostly having to do with the tires on the bike (no two tire models are the same shape and height, and the wear on the tires has an effect here as well), the speedometer has to be significantly optimistic to overcome all of these factors. This will ensure that you can't sue BMW for getting you into trouble.

[f11at125]

Once I saw that the odometer agreed with Google Earth to within a tenth of a mile (over a 30 mile trip) I knew the problem was speedo-only. So I disassembled it, marked the present position, backed it off by 10 mph worth, and now it works perfectly. At all speeds. I spent half an hour racing up and down a street in town with a stationary police radar trailer ("Speed Limit: 35 - Your Speed: XX") trying to confound it and could not. It's scary accurate now.

The only downside to this solution is that I think I was lucky - very lucky - in estimating the necessary angle of shift. If I'd been off, even by a vexing 2 MPH, I'd have had to completely remove and disassemble the whole shittery to correct it, and if I had over-corrected, well, probably would have given up.

But that didn't happen, and the precision I'm getting now is befitting a German instrument.

Huzzah.

[George]

I just joined this list and I should say I've been owning and working on and riding them since about 1985.
My current rider(R60/5) consistently reads 5MPH high. You did the correct and only way to fix this error.
In a previous post, Darryl.Richman explained the problem accurately. I plan to correct mine the same way AFTER I get my new speedo chrome ring that holds and seals the glass in place and install a new glass, since mine is showing it's age.
It's a real pain to take the speedo off the bike and get into it. I had to to this last year on this bike. Clean and lube all the spinning parts. I didn't know at the time it was reading 5mph too fast. Dangit !! I could've reset the needle then.
A tire size error will result in a "percentage" error. So if it's off by 2.5 mph at a true 50mph, then it is off by only five percent.
If the true speed is 10mph, a five percent error equals to .5mph(point five percent) error or one half MPH. At 100mph, it would be off by 5mph. This error will require and adjustment of the magneto to correct that before the needle is repositioned. Doing it this way require MUCH less brain work.
Huzzah was also correct in assuming that BMW would provide accurate instruments. The speedo IS accurate, minus about five miles per hour.

George

[Bruce Frey]

Here is an interesting article on speedos and their adjustment.
http://home.jtan.com/~joe/speedo.htm

[f11at125]

Maybe I got lucky; I just went in there all ham-fisted with a pickle fork, but the results were similar (at least according to the PD's "Speed Limit/YOUR SPEED" signs and the speedometer on my GPS).

[gapl1953]

I don't worry about the speedo error. The speedometers on all three of my BMW's (R75/7, R80RT, R90/6) jump around so much that I'm lucky when they are not covering 10mph in a swing. I have tried to fix the bouncing needle but have not been successful to date.
~X(

[f11at125]

...just re-grease the magnet's axle. Prollum solved.

I recently broke my needle off the peg (don't ask/don't tell/don't hit 7,000 going downhill). So... since BMW no longer stocks speedos for my bike, I had to remove the needle and superglue it. Which no doubt changed the offset weight to some minute degree. But having some experience in this, here's the fool-proof method. Compare this to the algebra here:http://bmwmotorcycletech.info/ringgears.htm and, if you're at all like me, you'll note a substantial difference in efficacy.

1. Make sure your tires are fully inflated.

2. Familiarize yourself with dissasembly of speedo in a controlled environment, down to where you have separated the magnet from the needle's drive disk.You really only need a 10mm wrench, a small and a large flathead, and a small Philips head screwdriver. But get to the point where you can remember at a glance what screws go where the way a Mujaheddin knows his AK47. LUBE THAT AXLE SO IT STOPS BOUNCING!

3. With your four tool in your pocket, go find a radar sign ("Speed Limit: XX Your speed: YY"). Compare "your speed" to what your speedo is telling you. Try it at different speeds, and rejoice that it always is off by the same number of MPH.

4. On the side of the road, disassemble your speedo down to separating the magnet from the needle's drive disk. Turn the drive disk until the speedo points to whatever it said you were doing on the last pass and hold the drive disc firmly in place with your left index and middle fingers. Twist the needle's disk on the front of the speedo so it points to what the radar said. So for example, you pass the sign and it says "Your speed: 55" but your speedo said 45 - point the needle to 45, hold the drive disk firm, and twist the needle'd disk - the black one right in front of the speedo's face - to 55.

5. Put it back together and run past the sign again at various speeds, delight in your stellar mechanical ability, and whistle a happy tune.

That's it!