Re: Blades in or straight on a MUni frame around a 3.0 gazz?
On Sat, 4 May 2002 19:44:19 -0500,
gauss <gauss.45n9y@timelimit.unicyclist.com> wrote:
> I really can only provide input as far as saying that the 2nd situation
> you described (frame blades come out at an angle) is probably the one
> you want to avoid. This causes the blade to not be loaded in the axial
> direction… ever.
Err, actually they’ll be loaded axially all the time the unicycle seat is
loaded. They might not be 100% purely axially loaded very often, but then
neither is any part of any unicycle frame, so I’m not sure why you think
this is so terrible.
> Which means that it will always be seeing bending loads.
> This is rough on the frame and could be rough on your bearings
> and axle.
Why? Why is flexural stress more onerous than axial stress? How does the
element of metal know that its compression is down to flexure of the whole
component rather than uniform compression?
> Believe it or not, in this configuration, the bearings will
> always see a load greater than your wait while doing seated riding.
Can you elucidate what it is you’re concerned about? The bearings in
every unicycle that is ridden see a load greater than the statuic weight
the rider applies to the saddle. If the bearings are approipriately
specced this is not a problem. If the bearings are inappropriate it might
be a problem regardless of what weight they get.
I also think there’s an incompatibility in your structure. If the fork
blades resist the eccentricity by bending (which is what you are concerned
about early in the posting) then the bearings could be purely readially
loaded with exactly the weight applied at the saddle on them. However, at
this point in the posting you seem to be suggested the bearings see an
out-of-vertical component of force. This would be a bad thing, not
because of the magnitude of teh force, but because most ball-races don’t
like axial loads much. However, if this is the case, then it implies the
non-vertical component of force is being resisted by an axial load along
the axle, in which case there won’t be bending in the blades due to this
component.
In reality, it probably ought to be a statically indeterminate structure,
because you don’t want the bearings slopping in their housings, so there
will be a small axial load conducted through them into the axial whenever
the fork blades flex together or apart. I would expect that to be
whenever you pedal while also having weight on the saddle. However, if
the blades are adequately designed teh flex will be small, and the load
can be made sufficiently small.
If you were raking the fork blades at some incredible angle, and had
flexible forks and were concerned about it you could use cup-and-cone type
bearings, or ballraces with inclined contact points, which will take an
axial force in one direction (remebering to orientate them appropriately,
of course). However, I don’t think the fork crown would fit between your
legs, and to get the fork legs sufficiently flexible you’d probably buckle
them were you to actually sit on the saddle. I guess you could get the
same effect by hingeing the blades at the crown, but I can’t imagine why
you’d want to do that either.
regards, Ian SMith
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