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TECHNICAL MOTOBRICK WRENCHING In Remembrance of Inge K. => The Motobrick Workshop => Topic started by: Nantucket_Red on August 13, 2015, 05:16:02 PM

Title: Speedometer drive ratio
Post by: Nantucket_Red on August 13, 2015, 05:16:02 PM

Looking into getting a new speedo and tachometer. I'm coming across different "drive ratios." Can someone tell me what that is about and which is the proper ratio for my  87' k100rt?

example: http://www.amazon.com/Bikers-Choice-Mini-Tachometer-76071/dp/B000GZTJOM/ref=pd_rhf_gw_s_cp_14?ie=UTF8&refRID=0HENS9XJ56FX51Z81FBZ

Title: Re: Speedometer drive ratio
Post by: petermarek on August 14, 2015, 08:57:14 AM

I believe the K100 series relies on the "Speed Sensor" mounted to the rear drive. This is a cable driven speedometer if I'm reading correctly. Are you intending to mount a drive gear on the front wheel?

I'm working towards just using a GPS driven speedometer. I have determined that my original is 10% FAST. This also means that the 13,044 on the odometer when I pulled it from the car port it sat in is actually 10% high.

I built the attached to figure every thing out and then did the comparison between my phones GPS Speedo and the bikes to confirm the result.

I zippped the excel sheet, so you'll can use the formulas if you wish. You'll need to measure the circumference of your rear wheel.. I got all the Gear an Drive ratios from my original Rider's Handbook for my 1985 K100RT

Good luck!

Title: Re: Speedometer drive ratio
Post by: rbm on August 14, 2015, 09:23:37 AM

The tach you linked to is electrical.  It needs a signal to tell it the speed of the engine.  Usually this is a signal from the coil primary; the Black/Blue wire to pin 16 of the instrument cluster.

The speedometer is also electrical.  Normally, there is a Variable Reluctance sensor on the rear drive that issues 6 pulses for each revolution of the rear wheel.  This sensor is unusable for modern speedometers (without an amplifier).  So, what is normal is the speedometer you buy will have a magnetic sensor.  This sensor is mounted on the front forks and a magnet is attached to the front wheel.  There is no gear driven speedometer for the K-series.

Title: Re: Speedometer drive ratio
Post by: Nantucket_Red on August 14, 2015, 11:15:03 AM

Thanks for clearing that up for me guys. Damn near lost my mind thinking about it
. :clap:

Title: Re: Speedometer drive ratio
Post by: jay1622 on August 16, 2015, 09:37:25 PM

According to the original BMW shop manual, the ratio for a K 100 RT is 1:2.91.  Number of teeth 11:32.

It does say here that there is also a special version. That ratio is 1:3.0. Number of teeth 11:33.

All tooth backlash on all models of K bikes made should be 0.07 to 0.16 mm.

Granted, this is specific to the final drive gearing. As mentioned above, the speedometer will register speed based off of the pick up sensor.