Mark J. Ginsberg
Hey, this post was on point, interesting and useful.
You sure you meant to send it to obra chat?
Mark J. Ginsberg
--- On Wed, 2/18/09, jmsmech-obra@yahoo.com wrote:
From: jmsmech-obra@yahoo.com
Subject: Re: [OBRA Chat] Reach around
To: obra@list.obra.org
Date: Wednesday, February 18, 2009, 5:39 PM
> Whenever you're making holes, especially in areas of
> vibration, remember to chamfer the holes on both sides to
> relieve stress and smooth the cutting edges
kind of in the weeds, but In this particular case, chamfering holes has no
effect on vibration (or vice versa), and relieving stress is not necessary (well
and there is no stress in an "edge" of a hole, typically worry about
"corners")
The hole at brake mount is clamped under compression, therefore a fatigue crack
cannot grow. The two holes one might put for to attach fender are under only
minuscule stress... but yes you are right do smooth sharp edges to keep edges
from cutting your attachment ziptie, wire or cord, etc...
You might be thinking of the titanic, where if the holes for riveting plates
together had been drilled and then reamed, rather then punched, the ship might
not have sank. (punching put little micro cracks all around the hole, whereas
drill and ream produced a nice clean surface. And rivets, produce unknown
typically fading clamp force. That's why bolts are used now to put bridges
and stuff together. That and bolts and nuts weren't cheap or readily
available 100 odd years ago.. they were still cut, on a screw-cutting lathe,
not rolled.
>That particular bracket is weak due to the slot that runs down the middle
>of it.
>I used to have Bike Gallery do those type of brackets for me, back when I
>didn't have a cx bike as my rain bike, and they would always break. I
>actually thought River City made a better bracket that was solid as
>compared to that slotted one.
Why these brackets break is understandable. Its not so much the slot but also
other reasons... Mainly due a constant bending force in the bracket (cause it
wasn't aligned perfectly on install) and then add on road vibration =
classic fatigue situation.
-The bracket is rather long. stiffness might be such that allow resonance to
occur (vibration). also increases bending force especially at attachment pont.
-Use of two bolts insures mis-alignment forces, moments (torque), from fender
transfer into the bracket..
-Too much weight. Using a thick water bottle with bolt and nut etc at bottom..
too much mass increases force, also helps drop natural frequency..
fixes:
-Use a thicker bracket or double it up ie just make it stronger... and shorten
it.
-make sure bracket is perfectly aligned. In other words, zero bending stress
should be in the bracket when it is just sitting there (this is just like how
you want bicycle spokes... ie no bending stress in the elbows! )
-Less weight hanging off the fender. I use thin plastic some holes and zip
ties
-consider a 'decouple' soft attachement. this has seemed to work for
me, as shown in the link/pic in my original post with some cut up inner tube in
between. zip ties too allow a non-rigid attachment, used this in the past with
either tape or innertube or etc in between ... (carry extra zipties in bag or
wire ties).
Thus no forces/moments are transfered from fender into bracket, but probably
mostly vibration is not transfered, well actually the system
"stiffness" is now way soft, with some damping, so resonance/vibration
doesn't occur. This is probably the way to do it (ie make it soft) because
impact hits (road vibration) excite the whole spectrum, making it almost
impossible to de-tune with increasing stiffness.
jm schmidt, pe
--- On Wed, 2/18/09, Michael O'Hair wrote:
> From: Michael O'Hair
> Subject: Re: [OBRA Chat] Reach around
> To: obra@list.obra.org
> Date: Wednesday, February 18, 2009, 12:03 PM
> Whenever you're making holes, especially in areas of
> vibration, remember to chamfer the holes on both sides to
> relieve stress and smooth the cutting edges. A tapered
> round file (rat tail) works best, but I've used 100 grit
> sandpaper rolled into a cone with success.
>
>
>
>
> ----- Original Message ----- From: jmsmech-obra@yahoo.com
>
>
> making one of these is as simple as flat narrow slightly
> heavy gage sheet metal,some hole drilling and bending. if
> you have the tools, should take about 5-10 minutes to make.
> for simplicity a two holes in the fender matched to two
> holes in your bracket, loop a zip tie through. smooth sharp
> edges or it will cut through tie sooner or later.
>
>
>
>
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