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Written by BMWVMCA NEWS   
Sunday, 26 June 2011

 

by Kevin Brooks

 

        Necessity is the Mother of invention, they say; but time and opportunity must be available to play their part as well. And as the Flying Fox Racing Team efforts have expanded into new areas and multiple Record categories our need for Tachometers on our racebikes has increased. And over the past couple of years I have stolen an hour or two from other projects to develop a pet project of my own: an accurate Reproduction of the Camshaft Driven Tachometer system.

 

     Some background…Most of the 1950s and 1960s era BMW Twins can be fitted with a Tachometer made for them by VDO. In the early years the crankcase design was such that the Tachometer was driven by a right angle gearbox attached to the front timing chest cover and driven by the camshaft. These gearboxes were of basically two designs, both by VDO. One is a pair of box shaped castings dovetailed together and very angular in appearance; I call these the ‘coffin box’ drives. The other is slightly smaller and is of an identical design but the castings are now of a rounded appearance resembling a pair of barrels nested together. I refer to these as the ‘barrel drive boxes’.  Of the two styles the coffin box drive is the one more commonly seen, if the word ‘common’ can ever be used in referring to units as rare as these.  The function and method of drive of both of these units is identical, with the coffin box drive being somewhat larger than the barrel drive. Personally I prefer the appearance of the barrel drive unit as its shape echos the rounded contours of so many design elements of the Earles Fork twins such as the Shock shrouds front and back and the engine casting itself.

 

     Sometime in mid 1966 BMW modified the front engine casting and installed a small triangular cover near the bottom. This simplified the installation of a VDO Tachometer which could now be driven by a spur gear off the oil pump. But partial disassembly of the motor was still required to install the spur gear. Many of the latest motors came from the factory with the spur gear already installed, making the total installation very straightforward. These are referred to as ‘Oil Pump Drive’ Tachometers.

 

     Both of these systems are very rare and high on the ‘want to have’ list of desirable accessories for most owners of /2 era and earlier machines. We have been fortunate to find several of these units over the years, and very much enjoy having them on our machines. Barb is very adamant about having one on each bike she rides regularly as she rides ‘by the tach’ and not the speedo! But then she thinks that the Red Zone is where the power begins and the FUN starts!

 

     On a more practical level I have found that having a tach on the bike actually changes how I ride. Since these motors are so smooth and quiet to begin with it is nice to have an instrument to let you know where you are in the powerband. But most of our regular Tachometers have been allocated to the racebikes where they are really needed, leaving some of our streetbikes wanting for an instrument which we couldn’t seem to find anywhere.

 

     So we have developed an almost exact replica of the original VDO tack.

 

     Our system uses a commercially available right angle gearbox, very similar to the barrel type, which we then modify for our application. The Instrument head is an almost exact replica of an original VDO, lacking only the tiny ‘VDO’ letters barely visible on the top of the instrument face. All other dimensions and functions of the instrument are identical to an original VDO. The drive from the camshaft is very similar in appearance, but more robust and much easier to install. Our hexdrive engages the existing bolt which holds the centrifugal advance to the magneto, so no changes are necessary below the cover.

 

     Extensive roadtesting has shown these instruments to be accurate and responsive, just as one would expect in a modern instrument. The view from the topside is almost indistinguishable from an original VDO and the drive unit is very similar as well.

 

     The cover itself must be drilled to accommodate the drive unit, just as it is in the original VDO design. This is a simple matter for any competent machine shop. Or we can perform the task for you at nominal charge.

 

     Our first run of 6 units is already sold out and we are now preparing an additional 10. The cost for the complete unit is $695.00 – Tachometer instrument with mount, drive box and cable. I can offer drilling of your chestpiece for $100.00, insured return shipping included. I ask that the hold-down nuts be removed.

 

     To see more visit our website at http://www.Brooksmotorworks.com and go to the Race Videos.

 

Last Updated ( Friday, 25 January 2013 )
 
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