bearing, loose bridge pins leading to Wapin considerations

[Tim Coates]

Hi Ron,

Your statement,  "the string on a vertical pin will be less likely to be
seated on the bridge as
the bridge shrinks" isn't clear concerning a Wapin bridge.  You haven't
indicated whether your model has the standard Wapin configuration.

It is not just the front pin that is involved with a standard Wapin Bridge.
The front pin terminates and the trapping pin traps.  The back pin also
traps.  On a conventional bridge the front pin terminates and traps and the
back pin traps.  If your front pin model doesn't have a middle trapping pin it
is an incorrect standard Wapin Bridge configuration.   As far as I know, it
has been the experience of all involved with Wapin that the strings stay
seated to the bridge better with the Wapin configuration.  I don't know about
the relationship between front bridge string grooves and the Wapin Bridge.  I
haven't  looked into that area.  It is food for thought.

As some have been stating in this thread, seating the middle of the bridge is
a desirable thing.  The trapping pin helps the string stay seated to the
middle of the bridge, that seems to be holding true.

 I would like to see your data for a standard Wapin Bridge, not just a
perpendicular front pin.  There is a difference.

As we have some true scientists studying the Wapin Bridge, I myself am not
totally up to speed all the data we at Wapin have.  Michael Wathen is
presently uploading some of Bob Coleman's (NASABob) data onto our website.  I
don't know if the pages are public yet, but should be shortly.  I will be
presenting some of the information at the Central West Regional Seminar.

I agree, I don't think Wapin can compensate for negative downbearing.   That's
asking a bit much <g>.

Tim Coates
Wapin Co., LLP



Ron Nossaman wrote:

> >Ron,
> >
> >Didn't you show me some data at Reno that showed the Wapin bridge had the
> >least amount of stress at some point?  You measured some sort of stress in
> >relation to the angle of the bridge pin.  I apologize for not remembering
> >any specifiics.  This whole line may be completely off topic.
> >
> >Tim Coates
> >Wapin Co., LLP
>
> Hi Tim,
> I did.
>
> String tension = 160lb.
> Stagger angle = 10°
> Downbearing angle = 1°
> Downbearing load = 2.792lb
> Coefficient of friction between pin and string = 0.53
>
>                                 20° pin angle   0° pin angle
> Side bearing induced
> downbearing load on bridge      9.503lb         0lb
>
> Static resistance down  4.335lb         14.725lb
>
> Static resistance up            23.34lb         14.725lb
>
> Where it takes 23.34+2.79= 26.13lb to push a string up a 20° pin, it only
> takes 14.3+2.79=17.52lb with a vertical pin. The cap will suffer less
> crushing from bridge expansion with the vertical pin. Conversely, the
> string on a vertical pin will be less likely to be seated on the bridge as
> the bridge shrinks. The slanted rear pin(s) will help, but there will still
> be a difference.
>
> Ron N