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Thread: Caster ?

  1. #1
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    Caster ?

    Definition please (it's not in my engineering manual).

    Comparing the R1200GS , R1200RT, and R1200S - the BMW specs lists "castor" for each as 101, 116, and 87 mm, respectively.

    I think this relates to "trail", but I'm not sure. If that's so wouldn't that make the "S" a little spooky?

    Help?

    Bob

  2. #2
    Benchwrenching PGlaves's Avatar
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    Caster is the difference between a vertical line and the steering axis. Bikes have positive caster in that the steering axis is angled forward from vertical. If measured as an angle it is called rake. I have no idea where BMW is measuring from to express it in millimeters but less caster is less rake.

    And yes, the S has more "upright" steering and turns in more quickly. Not surprising for a sport bike.
    Paul Glaves - "Big Bend", Texas U.S.A
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    Nickname: Droid ANDYVH's Avatar
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    Those must be values for trail, in milimeters. Caster, is essentially the same as trail. Plus those numbers make sense for trail for each respective chassis. I wonder if my 94 R1100RS is between the S and RT.
    Get trained! The best "performance" upgrade you can get is YOU. Visit msf-usa.org for training info.

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    RS Caster

    The Clymer manual cites 111 mm Caster for the R1100 RS.

    R=127 mm
    GS=115 mm
    RS=111 mm
    RT=122 mm
    S= 100 mm

    It does not distinguish model year.

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    Lucky motorradmike's Avatar
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    Quote Originally Posted by bobh41 View Post
    Definition please (it's not in my engineering manual).
    "Castor" is French for beaver.
    Mike Marr
    1978 Yamaha XS750 (Needs rings), 1996 BMW R1100RS, 2004 Honda CRF230F

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    Registered User 36654's Avatar
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    Quote Originally Posted by MotorradMike View Post
    "Castor" is French for beaver.
    Less beaver will make me quicker?
    Cave contents: 99 R11RS, 2013 Toyota Tacoma, 03 Simplicity Legacy, 97 Stihl FS75, Dewalt DW625 & DW744

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    not so retired henzilla's Avatar
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    Quote Originally Posted by 36654 View Post
    Less beaver will make me quicker?
    That's what my HS Track coach told us
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    SABMWRA MOA Club#62's Flat Fixer/ current forum moderator
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    Track

    Funny, my HS coach never mentioned that. I ran the mile in 11 minutes.

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    Registered User 36654's Avatar
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    Quote Originally Posted by bobh41 View Post
    Funny, my HS coach never mentioned that. I ran the mile in 11 minutes.
    Any regrets?
    Cave contents: 99 R11RS, 2013 Toyota Tacoma, 03 Simplicity Legacy, 97 Stihl FS75, Dewalt DW625 & DW744

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    Castor is something you take when you eat too much cheese.

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    "Any regrets?"

    Yeh... that 26-inch inseam (then).

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    look out!!! Visian's Avatar
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    castor is what you burn in 2-stroke racing engines.

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    Quote Originally Posted by PGlaves View Post
    Caster is the difference between a vertical line and the steering axis. Bikes have positive caster in that the steering axis is angled forward from vertical. If measured as an angle it is called rake. I have no idea where BMW is measuring from to express it in millimeters but less caster is less rake.

    And yes, the S has more "upright" steering and turns in more quickly. Not surprising for a sport bike.
    There is no way I would attempt to challenge your authority, Paul, when it comes to technical issues on motorcycles, but I must point out one small issue:

    While rake and trail (caster) are related, they are not the same. Rake is the angle of the steering neck from vertical. Trail is produced by the offset of the fork tubes relative to the steering neck. If you increase the rake you can decrease the trail by reducing the offset. This is done often by custom bike builders of a certain brand of motorcycles. Any bike in order to have halfway decent steering needs a positive trail. If you don't have one, you would have the "broken wheel on a shopping cart" syndrome. Below is a picture to illustrate the relation. A large trail makes steering heavy and "slow" but increases high speed and straight line stability. A small trail makes steering quick and easy but produces a very "tippy" reaction and can be dangerous at low speeds and tight turns.
    Attached Images Attached Images

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    Benchwrenching PGlaves's Avatar
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    Quote Originally Posted by ima4nr View Post
    There is no way I would attempt to challenge your authority, Paul, when it comes to technical issues on motorcycles, but I must point out one small issue:

    While rake and trail (caster) are related, they are not the same. Rake is the angle of the steering neck from vertical. Trail is produced by the offset of the fork tubes relative to the steering neck. If you increase the rake you can decrease the trail by reducing the offset. This is done often by custom bike builders of a certain brand of motorcycles. Any bike in order to have halfway decent steering needs a positive trail. If you don't have one, you would have the "broken wheel on a shopping cart" syndrome. Below is a picture to illustrate the relation. A large trail makes steering heavy and "slow" but increases high speed and straight line stability. A small trail makes steering quick and easy but produces a very "tippy" reaction and can be dangerous at low speeds and tight turns.
    We are not disagreeing. And it was BMW that described an angular measurement (caster) as a dimension in mm, apparently trail. And subsequent posters who said BMW meant trail.

    What I said was "Caster is the difference between a vertical line and the steering axis." Which is what you just said rake was. And I said caster and rake were the same thing. We agree. They are an angular measurement. Trail is a distance measurement.
    Last edited by PGlaves; 05-18-2010 at 06:18 PM.
    Paul Glaves - "Big Bend", Texas U.S.A
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    Quote Originally Posted by PGlaves View Post
    We are not disagreeing. .
    We are indeed not, Paul. After reading it again, I saw that you explained caster as rake, not trail. Makes sense, as caster in a car suspension is expressed in an angle measurement.

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