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Motorcycle Steering Dynamics Investigation
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BMW R1100GS
Honda CB-1
Michael Roeder

Abstract

Position and rotation measuring instruments and data acquisition systems will be placed on a motorcycle to gather data which will then be quantitatively and qualitatively analyzed in an effort to understand the nature of motorcycle steering.

Motivation

This project is motivated by the endless arguments on newsgroups such as ba.motorcyles and rec.motorcycles over the true nature of motorcycle steering and by the chance to do some fun and interesting tinkering with electronic gizmos, computers, and motorcycles.

Motorcycle Steering 101

Motorcycles are steered counter-intuitively with a method called "countersteering." To make a right turn, rather than turning the handle bars to the right, the rider pushes the bar away on the right side. The Motorcycle Safety Foundation teaches the mantras "Press right -- lean right -- go right" and "Press left -- lean left -- go left."

There are two basic models of motorcycle steering: gyroscopic precession and outtracking. The precession model holds that countersteering causes the front wheel (which is, of course, spinning) to precess and lean into the direction of the intended turn. Then the tires roll like a cone instead of like a cylinder and the motorcycle makes the turn. Centripetal acceleration acts to hold the motorcycle up and carry it through the turn.

The outtracking model holds that countersteering causes the front wheel's contact patch to deflect opposite the intended direction of the turn. The motorcycle, reacting to this sideways force below its center of mass, rotates longitudinally about its center of mass leans into the intended direction of turn. The fork pivot inclination then causes the front wheel to deflect back to the intended direction of turn, and the motorcycle turns. Centripetal acceleration acts to hold the motorcycle up and carry it through the turn.

There are obvious similarities in these two models. The differences in the models do not exclude each other -- they are compatible. I believe that the actual behavior is a serendipitous combination of these two models. I hope that this project will settle the matter by providing details and data that cannot be observed by a rider concentrating on the job of steering the motorcycle.

In Balancing Act, Tony Foale gives an excellent explanation of motorcycle steering. But that's no reason for me to stop this project.

Data Acquisition

Quantities

Four primary and four secondary quantities will be meausred:
  • steering angle
  • road speed
  • lean angle ("roll" or "bank" in aeronautic terms)
  • rate of turn

    Four additional states will be measured:
  • gear 1 - gearshift sense line 1 from transmission
  • gear 2 - gearshift sense line 2 from transmission
  • gear 3 - gearshift sense line 3 from transmission
  • brake (steady-state)

    Latitude, longitude, and altitude data from a Garmin GPS III will be gathered in real time and post-processed for two-second average speed measurements to provide verification for other measurements.

    Test bed

    The subject of this investigation is a 1998 BMW R1100GS:

    1998 BMW R1100GS

    The running gear of this motorcycle is stock. Equipment not pictured here that will be on the motorcycle includes a Garmin GPS III and BMW System cases to house the computer and a video camera.

    This particular motorcycle is an excellent choice as a test bed for two important reasons:
  • It is a "naked" motorcycle without a lot of bothersome fairings to get in the way of mounting data-acquisition hardware.
  • It's mine, and I was pretty easy to convince to allow it to be used for these investigations.

    Buffoons who dismiss the investigation out of hand because the motorcycle is shaft-driven will be ignored. The data will show that shaft-driven motorcyles countersteer.

    Track

    To keep costs down, I will use appropriate curvy roads in my own neighborhood. As it happens, Diamond Heights Boulevard, Elk Street, O'Shaughnessy, Clipper Street, and Portola Boulevard in San Francisco make a nice 3.0 mile loop convenient to my apartment. Additional data can be gathered and video tapes filmed on any of the interstate freeways in the Bay Area.

    Diamond Heights Boulevard, Elk Street, O'Shaughnessy, Clipper Street, and Portola Boulevard

    Instruments

    To keep expenses small, instruments will be kept as simple as practical. Steering angle will be measured with a potentiometer mounted coaxially with the steering head bearings. The body of the potentiometer will be mounted on the steering head (which pivots on the steering head bearings); the shaft will be linked to the motorcycle chassis.

    Road speed can be measured in a number of ways. A simple solution is to investigate the electronic tachometer and find a point in its circuit that provides a voltage proportional to engine speed. Data from the transmission will indicate which gear the motorcycle is in.

    Lean angle and rate of turn can only be measured with gyroscopes. I hope to find a two-axis electronic gyroscope with suitable output voltage through a surplus dealer somewhere in Silicon Valley. The gyroscope will be mounted on the fuel tank in a tank bag or on the rear seat.

    Shifting will be sensed by monitoring the states of the BMW's built-in shifter sense wires..

    Braking is easily monitored by sensing the voltage on the stop light wire.

    Instantaneous 2-second average speed can be downloaded through a Garmin GPS III connected through the PowerBook's serial port.

    Data Recorder

    A Beehive ADB I/O data acquisition and control device will be used to convert voltages from the instruments to a form usable by a computer. The ADB I/O will be configured to have four 8-bit analog and four 1-bit digital input lines.

    An Apple Computer Macintosh PowerBook 100 running custom data-acquisition software will be used to gather and store the data.

    A regulated power supply for the PowerBook (and thus the ADB I/O) and instruments will be acquired or built.

    All these components will be housed in saddlebags.

    Additional Sources of Data

    Certain effects of motorcycle steering can be observed simply by following the motorcycle and watching what it does. As a supplement to this investigaiton, I will try to get some friends to follow the test motorcycle in a car and make a video tape of it. It may also be worthwhile to mount a video camera directly on the motorcycle itself. Watch for QuickTime movies on this site.

    Data Reduction

    The custom data acquisition software will be kept simple and reliable. It will gather data from the ADB I/O device and store it in a text file (in a format yet to be determined).

    Microsoft Excel running on a Centris 610 will be used to reduce and display the data gathered.

    The motorcycle's lean angle converts through a simple formula to lateral acceleration. As long as the motorcycle is in dynamic equilibrium -- traveling on the road with the shiny side up -- it can be assumed that it is balanced. Thus the cosine of the angle from vertical indicates lateral acceleration.

    The gearshift and stoplight sense lines can be unambiguously interpreted to reveal the number of the gear selected. Safety and practicality require that the engine be running at operating temperature when the data acquisition software is started. The transmisison should also be in neutral. Thus the initial recorded data will show this state, followed by movements in the gyroscope line showing the motorcycle being pushed off its center stand. The clutch will be engaged, first gear will be selected, the brake will be released, and engine speed will increase.

    As the motorcycle accelerates, changes in the shift lines and reductions in engine speed indicate gear shifts.

    A simple calculation using engine speed, gear ratios, and tire circumference will yield road speed. This will be verified by readings from the GPS receiver.

    Data Interpretation

    Graphs generated from the data will be displayed here.
    Motorcycle Steering Dynamics Investigation. Revised 1/21/1999
    Copyright © 1998 by Michael Roeder.
    URL: http://www.timberwoof.com/motorcyle/dynamics.html
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