jim beam wrote:
> Tom Kunich wrote:
>
>> <[email protected]> wrote in message
>> news:[email protected]...
>>
>>> how much wear on the rim's braking area before the rim weakens?
>>> does excessive wear beyond safety limits show up in the truing process?
>>> to measure-span the high outsides and slip a credit card thickness
>>> underneath?
>>
>>
>>
>> On unmachines rims and whole lot.
>>
>> On machined rims there are failures fairly early in the wear life
>> because there are wave patterns in the extrusion and machining makes
>> the low spots in those waves much thinner than the tops of the waves.
>> The make the extrusion thicker than they think is necessary but they
>> are constrained by the buyer's habit of always wanting a lighter rim.
>> This leads them to cut it as close as possible to meet parket
>> expectations.
>>
>>
> more jobstian drivel thoughtlessly parroted. true, there are
> differences in wall thickness in an extrusion, but it's untrue that
> machining makes the slightest difference to their wear-through variance.
First of all, you misunderstand the concept behind the argument; an
extrusion that has a constant wall thickness but is not perfectly
straight, as in the case of bicycle rims, produces a "wavy" wall profile
that needs to be straightened so as to have an essentially "flat-walled"
rim. Manufacturers do this by machining the walls flat to get rid of
the "wavy" profile. When this happens, it is obvious that a section of
the "wavy" wall that protrudes from the reference point will get
machined, resulting in a thinner section compared to a section which is
depressed from the reference point, and therefore will not be machined
as much. It follows then that you will have a rim where the walls will
vary in thickness after machining. It's quite plausible that the brakes
can wear away the thinner machined sections to minimum required
thickness for structural integrity much earlier than intended by the
manufacturer, had the rim had even wall thickness.
Even if we take your understanding as an argument by itself, if there is
variation in rim wall thickness after extrusion, machining will make the
thinner parts of the wall even thinner. Which part of that do you
misunderstand?
> - the brake pads are essentially "fixed", particularly for dual pivot
> calipers, hence they "machine" a rim relative to the interior face in
> excatly the same way as a lathe does.
So you should not have any problems imagining that thinner sections of
the walls, as produced from the process described above, will wear away
to the minimum faster than the thicker sections.
Your criticism of "Jobstian drivel" is itself thoughtlessly spewed forth.
