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Caster ?

B

bobh41

Guest
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
 
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.
 
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.
 
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.
 
Track

Funny, my HS coach never mentioned that. I ran the mile in 11 minutes.
 
PGlaves said:
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.
Click to expand...

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.
 

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  • rake and trail.JPG
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ima4nr said:
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.
Click to expand...

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:
ima4nr said:
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.
Click to expand...

It appears to me from a variety of sources that for cars the angular lean of the steering axis fore and aft is named caster and for motorcycles the same angular lean of the steering axis fore and aft is called rake. I have never heard the term rake applied to a car however, nor the term trail for that matter.
 
Thanks for all the excellent responses. BTW the term "castor" came from the BMW marketing news feed I receive often. I really puzzled over the possibility that the term was a corollary to "caster". This is all very good stuff -even the witty hijacks.
 
bobh41 said:
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
Click to expand...

Castor is a small furry mammal that builds dams across streams.....Regards, Rod.:D
 
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