Wednesday, April 20, 2011
BMW Motometer instruments ..... the "twin instrument" used on BMW/5 motorcycles....1969-1973....
Following the radical change around 1969 by BMW with the introduction of the /5 series...R50/5, R60/5 and R75/5.
BMW introduced instruments made by another German instrument manufacturer...Motometer, rather than VDO who they had obtained speedometers from since the early 1950's.
These were a 100mm diameter "twin instrument", that is they had a speedometer in some 3/4 of the binnacle with a smaller tachometer in the lower half.
The speedometers were either 120mph non trip or 200kph non trip, using some four speedo ratios depending on the final drive in the motorcycle.
The R50/5 used a w=800 for the 200kph dial and w= 1300 for the 120mph dial with 3.56:1 final drive.
The R60/5 used a w=760 for the 200kph dial and w= 1215 for the 120mph dial with 3.36:1 final drive.
The R75/5 used a w=660 for the 200kph dial and w= 1050 for the 120mph dial with 2.91:1 final drive.
Later a lower ratio final drive was introduced for the R75/5, the 3.20:1 final drive.
The R75/5 used a w=715 for the 200kph dial and w= 1155 for the 120mph dial with 3.20:1 final drive.
BMW/5 spares list page for the instruments.
Odometer gearing data....ex DQ..sorry about the "scrappy notes"..these were done many years back.
Warning light display.
The tachometer was the same ratio for the R50/5, R60/5 and R75/5.... 2:1
All German vehicles seem to use what we call a "captive" drive cable, that is the inner cable is secured into the outer at the bottom of the cable by a washer that is staked over in four places under the bottom cable ferrule.
The difficulty with this is that you cannot lubricate the inner...so it has to run with only the meager lubricant supplied when the cable was assembled.
So I hear you say...
Consider the speedo, say the R60/5......lets say it has 78000km on the speedo.
The the cable will have rotated 78000 x 760 ( the calibration factor)
This equals 59,280,000 rotations..at varying rpm and with the scant lubrication I mentioned previously.
This can go part of the way in explaining the pointer "dancing around" when the motorcycle speed is constant.
Following are some details from Motometer...
Left click on the images to enlarge...
The speedcup mechanism used to indicate the speed was not damped on this instrument, for either the speedometer or tachometer.
This means any slight wear in the top or bottom bush that the speedcup shaft pivots in and or an inner speedo or tacho cable with a "set" in it will cause an unsteady pointer despite a constant road or engine speed.
So it is possible to fit the top frame and the damper pot from the BMW/7 series and use silicon damping fluid...the result is a steady pointer.....
Undamped top frame assembly.
Damped top frame assembly...part of the damping mechanism can be seen rivetted to the top bridge piece.
A plastic cup is slid down the speedcap shaft to the top of the speedcup bell and filled with silicon damping fluid. Part of the damping piece fits into the cup when assembled and the speedcup shaft has to rotate through this fluid to indicate the speed and this stops the pointer oscillating.
Undamped speedcup assemblies.
Damping mechanism...the damper bar and the speedcup plastic cup.
The bezel rim on these instruments is a rolled on/crimped on chromed bezel.
Tricky to remove and replace.
My business, KTT Services made these and they are still available...best rolled on by an instrument repair firm, but you can do it yourself if you make up the tooling. Email me see address above.
Illustrated below is a removal method and re-rolling for a Smiths SSM speedo that uses the same principle of crimped on/rolled on bezel. If you've access to a lathe you can make suitable tooling.
Careful use of a screwdriver going around the bezel several times with an oscillating movement will peel the rim back...my wife does the demo...
You can find a suitable pair of pliers and grind the end to allow you to also peel the rim edge back.
Plier end ground to a suitable shape.
Pliers I used...made by the German company Knipex, although they appear to be no longer available. However they are a glass/mirror breaking set of pliers and the end is suitable to grind to shape.
Friend, Gert Boll has brought to my attention that similar mirror breaking pliers are listed on the Knipex site....they may well be suitable for grinding to the shape needed in my illustration above....
A Smiths bezel rolling on tool to illustrate a re rolling on operation...
Tooling you can make...a ballrace attached to a bar to do the rolling on...an aluminium plate with a suitable "V" of the correct diameter to lock the assembly while the bezel is rolled over. The large block has holes drilled in it to fit over the back of the speedo and allow you to push against the seals/glass for a water-tight seal as you roll over the rim.
Rolling the rim over ...you turn the lathe chuck by hand slowly to fold the rim edge over the 90 degrees necessary to seal the instrument.
The tooling before use...
You can investigate more the SSM bezel replacement earlier in my blog...use the search facility on the RHS of the blog...type in SSM or bezel...
Detail of the speedcup shaft bottom pivot length. This wears on the end stone in the bush at the top of the mainshaft and the inside of the aluminium speedcup "poles" on the magnet, eventually destroying it.
You can lightly stone the worn end of the shaft and then tap it though the brass piece in the speedcup to the correct setting...0.140" protruding.