jmsmech-obra@yahoo.com

> 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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