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Old 05-19-2012, 02:40 PM   #82
roger 04 rt OP
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Joined: Nov 2011
Location: Massachusetts
Oddometer: 1,695
GS-911 as Intertia Dynomometer

The GS-911 time stamps, to the millisecond, all the data it collects. Somewhere along the line it hit me that it could be used as a Inertia Dynomometer since it collects RPM data and the transmission ratios are printed in the R1150 BMW Repair Manual.

I've also been curious about the 1150 Torque at 2000 RPM at a 13.8:1 mixture. The final piece of the puzzle was using some of the HP physics that I boned-up on for the R1150 Gas Mileage thread.

A couple key (rough) numbers:

--8 to 12 ft/sec-squared is a decent rate of acceleration for town and highway driving.

--R1150RT and rider (in my case) weigh about 800 lbs.

--It takes 25 to 30 horsepower to accelerate bike and rider at 8-12 ft/sec2 plus 2-10 horsepower for aerodynamic drag between 30 and 60 MPH

--25 Horsepower corresponds to 65 ft-lbs of torque required at 2000 RPM. But 40 HP translates to only 52 ft-lbs. at 4000 RPM.

Have a look at the chart and data below which were taken in 4th and 5th gear, starting at 1500 RPM and 2000 RPM respectively and with Wide Open Throttle. It looks like (and felt like) there was just enough torque to meet the HP/Torque/Acceleration targets above at about 2000 RPM in 4th gear and about 2500 RPM in 5th.

There's lots to discover but, for instance, look at the 81 degree TPS angle and the 60-70 ft-lbs of torque. Notice too, the relatively flat rate of acceleration from 2000/2500 to 4000 RPM. It's making more HP as it accelerates but air-related drag is increasing too.

The graphs are choppy because I didn't use a high enough sampling rate and drop off at 4000 RPM or so because that's where I let go of the throttle on the in-town road where I was trying this out.

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