From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: Number of pawls in a shimano (lx) freehub?
Date: 3 Mar 1998 21:25:51 GMT

Tim Andersen writes:

> I recently decided to service and re-grease my Shimano lx hub.  I
> took it apart and carefully placed the parts on a cloth on the
> table, at which point I had to run an errand.  When I came back I
> noticed to my dismay that the piece of the freehub that holds the
> pawls only had two pawls attached to it.  Is this normal?  There are
> definitely places to put three pawls, so with only two pawls it
> doesn't seem like it would work correctly (they aren't exactly
> opposite of each other).

With cassette hubs, in which the escapement is constrained to be
inside the splined body on which the sprockets mount, the active
diameter is restricted.  Older freewheels had a larger diameter and
the lower gears (larger sprockets) had larger bores, allowing the
ratchet mechanism to be larger.  The smaller size increases leverage
so the pawls and detents must be larger.  To achieve the added
strength, the number of detents was reduced and the pawls made larger.

Regina freewheels of old had 21 detents and two tiny diametrically
opposed pawls giving 42 clicks per revolution or 8.6 degrees backlash
as each pawl engaged alternately.  Early cassettes were made with 16
detents and two large pawls to give 16 clicks or 22.5 degrees
backlash.  This was not good and also a waste because both pawls can
never divide the load.  Only one pawl is working at some point of
rotation.  I think you can convince yourself of that if you draw two
concentric circles with a pawl connecting them at one place.  Now let
the chain pull tangent to the outer circle and observe what forces
arise.

In any case, the large backlash and the absence of any advantage of
two pawls engaged seems to have made an impression on the engineers at
Shimano and other places.  By moving the one of the pawls 1/2 space
away from being opposite the other, they instantly cut the backlash in
two.  Now we are stuck with 11.25 degrees that still isn't great but
it doesn't hurt.

You may have noticed that various manufacturers tries no backlash
sprag clutches as are common in automatic transmission, but these
cannot hold the forces in the confines of a cassette hub.  A larger
unit is not practical, sprag clutches are expensive, and the
advantages are insignificant... but it feels good.

> I figured that I must have lost a tooth somehow, so I went to the
> LBS and they gave me an old Shimano freehub to extract teeth out of.
> I carefully opened up the old freehub, and it only had two out of
> three possible teeth in it too (and unfortunately they weren't the
> same kind of teeth as in my lx hub).

I'm not familiar with this hub.  Does it have three pawl recesses?
How are they distributed in the circle.  I know only of the old
Campagnolo aluminum body freewheel that had tree pawls.

Jobst Brandt      <jbrandt@hpl.hp.com>