Snowbum's BMW Motorcycle Repair & Information Website

To purchase replacement for the flexible printed circuit board, including LED lamps conversion:   http://katdash.com

Airhead instrument pod overhauls, repairs, parts, etc. Speedometers, Tachometers, can even fix the threaded insert areas.  Highly recommended!   Terry Vrla; 776 SE 3rd Ave; Hillsboro, OR 97123; (503) 421-5782

For ODOMETER gears, all models:
http://www.odometergears.com/products/BMW/R+&+K+Motorcycles/42

Repair and calibration of speedometers, odometers, mechanical & including the electronics type on the K bikes. I think they can do actual circuitry repairs.  Rick Borth   http://www.speedometer.com/bmw_cycles.html

Palo Alto Speedo www.paspeedo.com   in California.

West L.A. Automotive www.speedometerplus.com  877-900-9009   310-838-7350   In Los Angeles

One specific item! ....well known for his driveshaft repairing, ...he also has the small rubbery button that fits over the button on your odometer on later models...a hard to find item. Guy Hendersen (209) 962-7500  guy@hendersenprecision.com http://www.hendersenprecision.com/index.html  Sales@hendersenprecision.com Hendersen Precision; 22105 Feretti Road, Groveland, California 95321
 

NOTE 1.  Variables:

Your actual on-the-road tire diameter varies from that which is in tire manufacturer's books; or, that which is calculated. Most road type tires have a rather close tolerance as to diameter ...but the tire is not round when loaded, it contacts the pavement, and the heavier the load, the less round, and the less the effective radius.  There is also an effect when the tire is new to when well worn.  Even in same size, tires vary a small amount by manufacturer and model, sometimes considerably though. This is usually with off-road tires, like enduro tires, which can be widely variable in the same size as a road tire ...especially in total width, but sometimes in diameter.   Tire pressure & temperature has an effect, so does the rim width; but those are usually small effects.   There is one considerably larger effect, the slippage of tire rubber at the road surface. You do not have to, and usually do not, feel the normal slippage, it is always occurring to some extent except when the motorcycle is not moving.  Because it is the rear tire that drives the motorcycle, and it slips even in normal easy-going, the resultant instrument reading varies with which wheel has the speedometer-odometer drive.  Manufacturer's often try to compensate for that, of course ...but, it almost always results in the speedometer reading equal to or higher than actual speed due to most company policies on never having the speedometer read slower than you are actually moving.  The odometer is usually very accurate; or, may read very slightly higher.

The unloaded to loaded measured diameter of a road-type 4.00-18 rear tire versus a 120/90x18, typically the largest rear tire on Airheads that was ever used as standard, is only about 15 mm in the worst case I know of (that is 47 mm difference in circumference, about 2 inches).  The actual rolling circumference usually differs by only about 2%.

BMW has used 3.25-19, 3.25-18, 90/90-21, 90/90-18, and 100/90-19 front tires.  Rarer is that some were shipped with 3.50-18 front tires.

For the rear, BMW has used, 3.50-18, 4.00-18, 130/80-17, 120/90-18, and 140/80 tires.  The difference between the extreme's in circumference, is ~3.0 inches.  That is ~3.7% difference.  Hence the values shown below in the text and charts are theoretically reasonably accurate, and many are taken from a BMW chart dated 1978; others are calculated; some are actual test data.  Keep in mind the variables. Be sure to read the various notes in this article, regarding such as BMW's WRONG conversion of Kph to Mph, regarding the ratios that the speedometer/odometer use.

In all of the above, I have used standard rim width sizes.  The rim width size variations that BMW has used has a rather small effect on rolling circumference. It's primary effect is, with the roundish shape of the tire, to change the rubber footprint SLIGHTLY.

The values in the AIRHEADS chart, well below, are theoretical rpm & speeds, for original stock size tires, at recommended original inflation pressures, loads, etc.  Where available, I suggest you use the Kph W figures and YOU convert to W Mph figures, & disregard BMW's Mph ratio figures, as those W figures are shown charted here with BMW's own original errors.

It is unlikely that you will have exactly the same results for rpm and speed as in the table, due to accumulated errors including instrument calibration, tires, other tolerances, etc.  It is my belief that BMW did NOT chart for actual on the road, but under some factory controlled setup or calculations, and it is also my belief that you will always have a higher rpm on the road, for a given true speed.  Probably the rpm it takes for the charted speeds will be about 5-8% more. Interestingly, this is about the amount many stock BMW speedometers are in error ...ON PURPOSE by BMW!

NOTE 2.  BMW speedometers are notorious for reading high?
Yes!  (but the 0-85 mph speedometers are often very accurate).  So, what is going on?   BTW: All odometers are usually quite accurate.

BMW, in general, purposely calibrates its speedometers to never read slower than the actual speed the motorcycle is traveling at; taking into account tire variations, temperature, instrument changes with conditions, and so on.  BMW has had bulletins on this, the last one, #2756, is dated 10-21-1996.    The bulletin states that the maximum amount the speedometer will read fast is:  10% of true speed, PLUS 2.4 mph.

NOTE 3. The most accurate methods for YOU to check the calibration of your speedometer and odometer are, in no specific order of accuracy, mile posts and stopwatch; radar gun; GPS.

NOTE 4.  If you are really anal, you can have someone measure the radius of the tire, bike on tires (not side-stand nor center-stand, and do it on a level surface), while you and passenger seated on bike, bike pointed straight ahead, and 100% balanced straight up, preferably with the help of another person.  The bike MUST be very square to the surface!   Your 1st buddy measures the CENTER of rear axle to the floor on both sides, precisely. Doing it all this way is somewhat inconvenient, compared to having your feet help balance the bike, but you get the full effect of loading.  You adjust the lean of the bike with help of buddy #2, until both measurements are identical.  Take that measured true distance from center of axle to floor; multiply it by 2; then multiply that by pi or by 22 divided by 7 ....and that will give the working circumference close enough to the actual in-use number (except for slippage).

The formula for determining the relationships, suitably simplified, is as follows:

Let T = the tach reading
Let M = miles per hour
Let C = circumference in INCHES
Let S = small number in the rear end ratio
Let L = large number in the rear end ratio

Rearrange this formula to find any of the values, like you learned in beginning algebra.

Practical example:
Suppose you want to know the engine rpm in any particular gear, for any particular tire and road speed.   Transmission ratios are in your owners book or on this website.  Rear end ratios are stamped into the rear drive.   You need to have a helper very accurately measure the radius of the tire with you and normal load, bike balanced on its tires as previously noted.     Select a mph speed, multiply that by the overall gear ratio which is the gear ratio of the rear drive and the transmission gear ratio (multiply them).  Multiply the result by 336 and divide by the tire diameter in inches.   The result is the engine rpm.

NOTE 5.   The single sided Monoshock and Paralever models have a limited number of gear ratios available.    3.36 was popular on the ST, G/S, R80RT & late R65;  3.00 (33/11) was on the late R100RS and RT.   For the R80GS, R80R, R100GS, and R100R, you will likely find only the 3.20 and the 3.09.

NOTE 6.   This section is for the dual-pod, which is NOT the R45, R65, ST, GS, G/S, and R80R and R100R.  BMW used different colored dials and pointers depending on model/year.  BMW used different plastic covers for the speedometer-tachometer and lights pods. There have been some anomalies, that may not agree with the below information:

The 1974 only had white squares all around
1974-1977 (/6 series and first /7) had the Hi beam indicating lamp at the bottom of the tachometer face.  Pointers are orange into mid-1977, and markings were in white.  In 1974 only there was a white surrounding area inside the rim edge.   While there was both a /6 and a /7 in 1977, the dial markings were white for all of them. The oil lamp was AMBER color, and the turn signal lamp was also AMBER, located at the bottom of the tachometer face.  The brake failure lamp is the top lamp in the center lamps group, then came the neutral lamp, GEN lamp, Oil lamp.
The /7 series had the turn lamp on the top, and just below it is the Oil lamp.  Neutral lamp is second from the bottom.  The bottom lamp is the Brake Failure lamp.  Dial markings are green, pointers are white.
1978 was the transition year for the electronic tachometer; and BMW changed the numbers and text from white to green, but a white pointer.
1981+ had a blue light plastic lens for the high beam indicator, and it was at the bottom.

Speedometers were 140 mph (with kph in smaller text); or, 220kph for metric countries.  There were 85 mph speedometers for the USA during the Federal 55 mph speed limit years.

Hints: When a rear drive ratio is numerically lowered by one step, the speedometer error, if present, is often eliminated, without any need for speedometer internal work or a new speedometer.  The odometer will usually be off, however.   If the speedometer is driven by the front wheel and not the rear (powered) wheel, the over-all accuracy CAN be more accurate, particularly due to slippage being greatly reduced, which varies by amount of horsepower in use.

Transmission ratios used on Airheads (1970 through 1995 models).
(ratios used on pre-1970 models are in a table chart at the end of this article).