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Speed as a Function of Engine RPM (operating with speedometer out of service)


BlueRidgeBoy

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My speedo is not working and I'm not sure when I will get it fixed. I want to keep riding but I would like to have some idea of my Mach number. Has anyone ever put together a chart of speed (either by speedometer or something more accurate like GPS) vs. RPM every 1k, or even 0.5k, RPM in top gear (5th)?

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Joe Frickin' Friday
My speedo is not working and I'm not sure when I will get it fixed. I want to keep riding but I would like to have some idea of my Mach number. Has anyone ever put together a chart of speed (either by speedometer or something more accurate like GPS) vs. RPM every 1k, or even 0.5k, RPM in top gear (5th)?

 

Yup. It's at home; will try to post it tonight.

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ShovelStrokeEd

MPH = (60*(Circ/5280)*RPM)/(Final*Gear*Primary)

 

Where

Circ=Measured tire circumference in feet

Final=Final drive ratio - stamped on your final drive 33/11 maybe

Gear=Fifth gear ratio - 0.89 or something like that.

Primary=Engine primary reduction ratio - 1.81, I think, for an 1100.

 

Or, you could just wait for Mitch.

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Joe Frickin' Friday
MPH = (60*(Circ/5280)*RPM)/(Final*Gear*Primary)

 

Where

Circ=Measured tire circumference in feet

Final=Final drive ratio - stamped on your final drive 33/11 maybe

Gear=Fifth gear ratio - 0.89 or something like that.

Primary=Engine primary reduction ratio - 1.81, I think, for an 1100.

 

Or, you could just wait for Mitch.

 

I'm not aware of a primary reduction on the oilheads. See attached; no primary included, and the revs match the RPM's pretty well. In fact, if you jack up the speeds by ~5%, you see what the overly optimistic BMW speedo shows you.

765054-speed-RPM.zip

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If I am not mistaken you have a R110RT. In top gear, your 1000 engine rpm should equate to 30 km/hr.

 

Might be easier to have a GPS on board, that way instantaneous speed readouts are available without having to do quick mental calculations on the move.

 

Tio

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ShovelStrokeEd

The primary reduction is in the trans. The ratio between the input shaft gear and the layshaft gear. I don't have the spreadsheet for the 1100S on this laptop and I'm 850 miles from home at the moment but it is there.

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Joe Frickin' Friday
The primary reduction is in the trans. The ratio between the input shaft gear and the layshaft gear. I don't have the spreadsheet for the 1100S on this laptop and I'm 850 miles from home at the moment but it is there.

 

The gear ratio numbers in my spreadsheet are straight out of the BMW manual. There's no separate number in the manual for a primary reduction, but the sectioned drawing in the manual shows an intermediate shaft, and the relative size of the gears on input and intermediate (in the drawing) suggests there is a primary reduction. But again, my RPM-vs.-MPH numbers seem to match up pretty well; best guess is that the gear ratio numbers in the manual include that primary reduction already.

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Mitch,

 

Y = MX + B!! Perfect. Thanks so much. This should keep me out of trouble, or at least aware of the magnitude of the trouble, until I can replace the cable. My 1987 R65 had that same chart in the owner's manual which came in very handy. With 130,000 miles on the odometer the speedometer was just about worthless. It read about 20% + 5mph over so I memorized a few important speeds based on tach readings.

 

Paul,

 

Your intuition is amazing. Unfortunately the very friendly Virginia State Trooper that I passed was in an unmarked car. Fortunately he let me off with a warning. He didn't even run my plate or driver's license so I don't think he was really interested in traffic enforcement that morning.

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Thanks again Mitch. With your help, and a label maker, I've turned my tach into a temporary speedo (see attached). I put 50, 75 and 100 over the 3, 4.5 and 6 hash marks (it doesn't look like it because of the angle of the picture). Then I just add or subtract 8.3 mph for every 0.5 krpm hash mark. This should get me by until I replace the speedometer cable (p/n 62 12 2 306 079).

766236-DSC00017.JPG.28297b0991b6d11b730c543b3a25c6c2.JPG

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