J
Ben C? writes:
>>>>> "erd" is simple enough - it's what "effective" means that's
>>>>> unclear. maybe that's why mavic call it "spoke support" - since
>>>>> they clearly haven't read jobst's book and are untainted by its
>>>>> terminological inexactitudes.
>>> "erd" is calculation input. you decide the result you want it to
>>> give.
>> But the output is the spoke length, if I knew the spoke length I
>> could calculate the ERD, but I don't know the spoke length, so
>> which ERD do I choose?
> You decide what point in the rim cross-section you want the spoke
> ends to reach. Then you measure the distance from there to the
> corresponding opposite point on the other side. That's the ERD.
>> My point was, surely it's defined?
> It is defined, but in terms of what you personally want! This is
> what Jim Beam is saying, also Damon Rinard on the link posted
> earlier.
http://sheldonbrown.com/rinard/spocalc.htm
# "Effective Rim Diameter (ERD) is the diameter on which you want
# the ends of the spokes to lie. Most people prefer it near the
# end of the spoke nipple."
> But, in Sheldon's glossary, a slightly different definition:
http://www.sheldonbrown.com/gloss_e-f.html
# "Effective Rim Diameter. This is the rim diameter measured at
# the nipple seats in the spoke holes, plus the thickness of the
# two nipple heads. The E.R.D. is needed for calculating the
# correct spoke length."
> This second definition implies ERD is defined as distance between
> nipple seats. But according to the first definition it's only that
> distance if _that's how long you personally like your spokes_. Some
> people might add 3mm to that.
Not so. Both of these are essentially the distance over spoke nipple
ends measured across the diameter of the rim. Only the wording is
different.
>> Actually, I'm ****** and I'm going to bed, thoroughly confused
>> about whether a manufacturers ERD is to be trusted or is just a
>> number they dreamed up.
> It would be much better if manufacturers always just quoted spoke
> support diameter. Then you know what the number actually is supposed
> to correspond to and can add 3mm if you like longer spokes, or not
> if you don't.
That is not my perception of the dimension. You can do anything you
want with an ERD that spans the tops of spoke nipples, the place where
spokes should reach from the hub.
> If 3mm ended up getting added twice due to these misunderstandings
> the spokes could be too long by enough to cause a real problem.
3mm is roughly the height of a spoke nipple head, so it needs to be
added twice to the bed of the rim support surface, there being two
spoke nipples on the diameter.
Jobst Brandt
>>>>> "erd" is simple enough - it's what "effective" means that's
>>>>> unclear. maybe that's why mavic call it "spoke support" - since
>>>>> they clearly haven't read jobst's book and are untainted by its
>>>>> terminological inexactitudes.
>>> "erd" is calculation input. you decide the result you want it to
>>> give.
>> But the output is the spoke length, if I knew the spoke length I
>> could calculate the ERD, but I don't know the spoke length, so
>> which ERD do I choose?
> You decide what point in the rim cross-section you want the spoke
> ends to reach. Then you measure the distance from there to the
> corresponding opposite point on the other side. That's the ERD.
>> My point was, surely it's defined?
> It is defined, but in terms of what you personally want! This is
> what Jim Beam is saying, also Damon Rinard on the link posted
> earlier.
http://sheldonbrown.com/rinard/spocalc.htm
# "Effective Rim Diameter (ERD) is the diameter on which you want
# the ends of the spokes to lie. Most people prefer it near the
# end of the spoke nipple."
> But, in Sheldon's glossary, a slightly different definition:
http://www.sheldonbrown.com/gloss_e-f.html
# "Effective Rim Diameter. This is the rim diameter measured at
# the nipple seats in the spoke holes, plus the thickness of the
# two nipple heads. The E.R.D. is needed for calculating the
# correct spoke length."
> This second definition implies ERD is defined as distance between
> nipple seats. But according to the first definition it's only that
> distance if _that's how long you personally like your spokes_. Some
> people might add 3mm to that.
Not so. Both of these are essentially the distance over spoke nipple
ends measured across the diameter of the rim. Only the wording is
different.
>> Actually, I'm ****** and I'm going to bed, thoroughly confused
>> about whether a manufacturers ERD is to be trusted or is just a
>> number they dreamed up.
> It would be much better if manufacturers always just quoted spoke
> support diameter. Then you know what the number actually is supposed
> to correspond to and can add 3mm if you like longer spokes, or not
> if you don't.
That is not my perception of the dimension. You can do anything you
want with an ERD that spans the tops of spoke nipples, the place where
spokes should reach from the hub.
> If 3mm ended up getting added twice due to these misunderstandings
> the spokes could be too long by enough to cause a real problem.
3mm is roughly the height of a spoke nipple head, so it needs to be
added twice to the bed of the rim support surface, there being two
spoke nipples on the diameter.
Jobst Brandt