From: markk@tcs.com (Mark Kromer)
Subject: Re: What would you ride on a long distance trip?
Originator: markk@sparcplug
Reply-To: markk@tcs.com (Mark Kromer)
Organization: Teknekron Communications Inc.
Date: Thu, 19 Nov 1992 02:26:18 GMT

In article <BxxoMr.F6@news.iastate.edu>, tomes@iastate () writes:
>In article <1992Nov18.020307.19538@tcsi.com> markk@tcs.com (Mark Kromer)writes:
>>In article <Bxw183.92L@news.iastate.edu>, tomes@iastate () writes:
>
>>>of their sportbike's engine/trans?  Angular momentum is the primary
>>>determinant of handling,  about the pivots described by the tire contact
>>>patches.
>>
>>At turn in, the motorcycle rotates about the cg not the contact
>>patches.  Re: countersteering.  Low polar moment in the transverse
>>plane (and in the longitudinal plane but for different reasons) is the
>>key, not low cg.  Keep the mass concentrated as close to the cg as
>>possible.
>
>Ok, I used the wrong term (polar moment).  But show me how an object
>can pivot about a point above the ground and still remain in contact with
>the ground (assume point touching the ground is the point farthest away
>from the CG in that immediate part of the object eg contact patch).
>The instantaneous roll axis for a singletrack vehicle MUST be the
>line between the two contact patches; otherwise the bike is off the ground
>by definition.  

Try countersteering your bike while maintaining a straight line path.
If you really want to experience motorcycle dynamics, take a hard ride
on an FZR400 - eveything becomes obvious.

WARNING:  The following is written in TechnoGeek [tm].

Say you're initiating turn in to a right turn.  You steer the front
wheel left, the front wheel moves left.  As it does, the rear wheel is
steered and it moves left.  What is the cg doing at this time?  Moving
down and possibly slightly left or right depending...  The influence
of the pure moment produced by gyroscopic presession on cg location
would tend to increase the rate of rotation causing the cg to move
right, but the forces to the left generated by the tires would tend to
pull the cg to the left.  But anyway before you know it, you're
leaning to the right and the tires are generating forces to push the
cg to the right.

If the bike is rotating (without translating) about the contact
patches at turn in (the initiation of lean) then the cg must translate
toward the side you are leaning to.  In order to get the cg to move
like this, you have to apply an external lateral force to the bike.
The only lateral force you are applying at this point is from the
countersteered front wheel and that force is in the *wrong* direction.

>BTW, the same argument I used to defend Wings applies to polar moment also:
>since most of the engine/trans lies within a foot of the axis passing through
>the cg and parallel to the line between the contact patches, the bike handles 
>better than you would think if all you considered was mass.  

I don't follow this.  How is this better than having the heavy bits
(crankshaft, gears, trans shafts etc.) as close to the cg as possible
and therefore as close as possible to this axis.

>I am very interested in a good referance book on motorcycle chassis design,
>but have been unable to find anything real.  I am looking for the motorcycle
>version of the book "How To Make Your Car Handle" by (Fred Puhn?).

I've read "How To Make Your Car Handle".  Excellent book - for cars.
I bought a book on motorcycle chassis dynamics a couple of weeks ago,
unfortuantly, I loaned it to a friend of mine who helps me setup my
race bike, and I haven't seen it since!  Once I get a chance to read
it I'll let you all know (after flameing you all toasty ;-).

--
)V(ark)<               markk@tcs.com                   DoD #400
1988 FZR400           1985 ZX900 Ninja              1977 RD400D
                  "I'd rather shoot back."
Newsgroups: rec.motorcycles