VDO Speedometers and Drive Ratios

[Mark Annan]

   I have been messing around with VDO Enduro speedometers in an effort to fit one to my Penton.  I have figured out a few things that may be of interest to others trying to fit a Speedometer-Odometer to their motorcycle.

1.  The speedometer "ratio"  refers to the number of revolutions of the speedometer / odometer  input drive required to register one (1) mile.
   For example:
   1.15 = 1150 revolutions make 1 mile
   1.42 =  1420 rev/mile
   1.6   = 1600 rev/mile
   2.15 = 2150 rev/mile
   (yes, I actually spun and counted to figure this out.  To much time on my hands or just nuts, you make the call)

   The ratio is usually marked (stamped) on the housing as W1.15, W1.95, etc.

2.  In order to get an accurate speed or mileage reading (assuming that the speedometer/odometer units are accurate, I don't know if they actually are) you need to get a speedometer that coordinates with the drive mechanism built into your motorcycle, or fit an external accessory drive unit.  How do you determine this?  Let me try to explain how I did it.

3.  First I figured out how many revolutions my wheel makes in a mile.  I measured the circumference of the tire mounted on it's wheel.  In this case 3.00x21 =    87 inches.  One mile is 5280 feet or 63,360 inches.  
63,360 divided by 87 = 728 revolutions per mile

4.  Next I determined the drive ratio in the front wheel.  I had easy access to the drive gears so I just counted them.  The drive gear has 13 teeth and the driven gear has 7 teeth.  13/7 = 1.857.

If you don't have easy access, or just don't want to take your front end apart, you can spin the wheel and count the revolutions of the drive output.  To make it easier elevate the front wheel.  Then put a stripe of tape across the tire.  Make an indicator to fit into the cable drive.  I used a small (1/8"diameter) piece of wood dowle rod whittled to a square taper with a piece of tape as a flag.  Spin the wheel 10 (or better 100) times and count the number of times the flag on your indicator goes around.  In my case 100 revolutions of the wheel makes 187.5 revolutions of the flag.  All of this spinning and counting goes much easier with two people.

5.  Now just a little math and you will know what ratio you need.
(revolutions of the wheel per mile) x (drive ratio) = the revolutions of your speedometer drive per mile, which divided by 1000 gives you the desired speedometer drive ratio.

Ex.1  728 x 1.857 = 1352  / 1000 =  1.35
Ex.2  728 x 1.57 = 1143 / 1000 = 1.14  

   If you don't have, or can not get, a speedometer/odometer of the correct ratio there are a couple of solutions.  
   First, and probably best, is to change the internal drive ratio of your speedometer.  This was at one time a fairly common procedure.  I haven't (yet) found a place that can do it to these old VDO units.
   Second is to get a ratio correction box.  A speedometer repair facility should be able to make up one up for you.  It is a small gear set (available in many ratios) installed in the drive cable.  They are typically used to correct for changes in tire size or rear end gear ratios in cars and trucks.  To find a speedometer shop in your area look in the yellow pages under speedometer, drive train, transmission, or look for a shop that services over the road trucks.  Often they can also make up and/or repair drive cables.  Some may also be able to change the internal ratio of your speedometer.

6.  Another option is to use an external drive unit.  These can be hard to come by.  You may get lucky and find a complete kit (speedo, cable, and drive) intended for another bike and adapt it to your bike.  I currently have a kit for early Yamaha's that I will use if I can't find a 1.35 speedo.

7.  When the speedometer is driven by the counter shaft (as on Sachs engines), if you change the  gearing to something other than stock you will need to compensate for the change.  A correction box can handy for this also.

PS  For those of you that may not have heard about it yet, check out the Vinduro eGroup  http://www.egroups.com/group/vinduro  It is free and full of Vintage enduro nuts.

 

[Doug Wilford]

Mark;
You have done a lot of work.   All you need is to measure in a straight line 1/10th of a mile or 100th of a mile, I usually did 104ft.
put a paper clip on the speedo drive cable up by the handle bars and count the revolutions in the above distance (like 14.25) send this count with the VDO to VDO in California and they will put that ratio in the speedometer.   If I haven't forgotten I am sure that all Sachs engines and a 14/54 drive sprockets was 14.2   I may be wrong on the ratio, it has been a lng time.
Doug Wilford  


 

[Mark Annan]

Doug,  Do you have an address, phone#, or web site for VDO in CA. ?