
Written by BMWVMCA NEWS

Thursday, 31 July 2008 
Wegdrehzahl:
German noun, in this case it describes the number of turns of the speedometer input shaft per given distance. It is used to define the ratio found on both the face and bottom of the speedometer.
The correct ratio for a speedometer in miles indicates the number of turns the cable entering the speedometer (and input shaft) makes per minute at sixty miles per hour, or the total number of rotations in one mile. A 1,52 ratio would mean 1520 rotations per minute at sixty mph, or a total of 1520 rotations in one mile. The same speedometer with a km face will have 0,95 stamped on the bottom, because at 100 kph it rotates 950 times in one minute.
The ratio conversion in miles versus kilometers is as follows: If a ratio is known in km, for example 1,0 multiply by a factor of 1.6 to obtain the appropriate ratio in miles. Conversely, to convert a known ratio from miles into km, divide by the same factor: 1.6. To check which ratio is needed for a bike, measure off 52’ 9 1/2” on a paved surface and mark the spots. (this is 1/100^{th} of a mile, or 1/100^{th} of 5280 feet). Then, with the speedometer cable disconnected and visible, with a paper clip or piece of tape attached to the visible end, wheel the bike up until the front axle is lined up with the first mark and note the position of the paper clip as it would point to a position on a clock. Tires should be inflated as they would be for highway use. Roll the bike along forward, never backward, keeping count of the number of turns until the next mark is reached. Ten turns within this distance means a 1,0 ratio, sixteen turns means a 1,6 ratio and so forth. For metric ratios, measure off either ten or one hundred meters and go from there. (This formula should hold true for any motor vehicle)
Prewar parts manuals show the speedometers cross referenced with the final drive ratios and their corresponding part numbers, but not the ratios stamped on the bottoms of the speedometer can. Comparisons with post war combinations should help.
There were at least four different manufacturers of BMW speedometers: Veigel, JSGUS (pronounced iskooss), VDO and SWF. I’ve seen several JSGUS speedometers with dates in the 1956’59 range. I also have several VDO speedometers dated 1954 for an R25/3, and 1949 for an R24, both km and miles. Just because it’s a plunger model, does not necessarily mean it came with a Veigel. I’m sure there must have been other manufacturers. If anyone has further information, please share it.
Please note: This research is not complete. Here’s an example: Not all R69 solo machines were built with a 25:8 rear drive. The same applies to the entire line up of machines BMW built. Exercise caution when using this list as a guide to aid in locating an appropriate part for your bike. It is by no means gospel and should not be regarded as such. There might also be differences in the rear drive ratios and speedometer combinations between the singles and twins.
I have seen a W=1,6 be appropriate on a twin with a 25:6 rear drive, while the same speedometer ratio would apply to an R26 with a 26:6 rear drive. There are differences between the singles, twins and plunger models in the speedometer cable take off gear at the rear of the transmission.
Credit for the following chart must go to Chris Betjemann who has been compiling this list over the years. John Harper and Ed Van Rossen have also generously contributed information to help fill in the gaps.
RS
Model & Ratio   miles  km      R51/2, /3 solo 35:9   W=1,52  W=0,95      R51/2, /3 S/C 32:7   W=1,84  W=1,15      R67/2, 3 solo 32:9   W=1,44  W=0,9      R67/2, /3 S/C 35:8   W=1,76  W=1,1      R68 solo 35:9   W=1,52  W=0,95      R68 S/C 32:7   W=1,84  W=1,15      R24 34:8 = 4.25   unknown  unknown      R25, R25/2 solo 27:6   W=1,68  W= 1,05      R25, R25/2 S/C 36:7   W=1,92  W=1,2      R 25/3 solo 25:6   W=1,6  W=1,0      R25/3/C   W=1,84  W=1,15      R50 solo 25:8   W=1,2  W=0,75      R50 S/C 25/6   W=1,6  W=1,0      R60 solo 32:11   W=1,12  W=0,7      R60 S/C 27:7   W=1,44  W=0,9      R69 solo 25:8   W=1,2  W=0,75      R69 S/C 25:6   W=1,6  W=1,0      R50/2 & R60/2 solo 25:8   W=1,2  W=0,75      R50/2 & R60/2 S/C 26:6   unknown  unknown      R50S solo 25:7   W=1.38  W=0,86      R69S solo 25:8   W=1,2  W=0,75      R69S &50S 27:8   W=1,25  W=0,78      R50US, 60US, 69US 25:7   W=1,38  W=0,86      R26 & R27 solo 25:6   W=1,6  W=1,0      R26 & R27 27:6   W=1,74  W=1,08      R26 & R27 S/C 27:8   W=2,0  W=1,25      Rear drive comparisons:         32:11=2.91:1         25:8=3.125:1         27:8=3.375:1         25:7=3.57:1         27:7=3.86:1         25:6=4.17:1         26:6=4.33:1         27:6=4.50:1    

Last Updated ( Thursday, 31 July 2008 )


