Cannondale's tests of disks and QRs

On Wed, 29 Sep 2004 21:36:48 -0700, jim beam wrote:
Ian Smith wrote:
On Wed, 29 Sep 2004 06:01:30 -0700, jim beam wrote:

http://groups.google.com/groups?selm...&output=gplain

You maintain that fork legs are always soft and axles always embed in
them.

suspension forks, yes.

Possibly you have in mind that fork dropouts are always
aluminium, but that's not true.

correct, they're frequently magnesium alloy. i never stated aluminum,
so please don't make assumptions for me.

Do you know what the word "possibly" means?

Having maintained that the drop-outs
are soft enough that embedment occurs, you then decide that they
require 200 N/mm2 to shear (maintaining that this is a conservative
figure), but assuming a Tresca yield criterion,

Tresca is a fudge. use von Mises instead.

Which will make very little difference, numerically, and is
conceptually no less of a 'fudge' as you call it, since both are
simplified envelopes fitted to a few data points.

You go from a 94mm2 gross contact area, to a 25mm2 area of sheared
material. I'm not sure how you'd predict the area of material sheared
by a given embedment of a given serration pattern, but I'll try and
work out how you came up with that. Suppose a 3mm seration spacing,
implies 31mm length of contact in teh area. Given your 25mm2, that in
turn suggests a shearing plane width of 0.8mm at each serration, which
if we assume a 45 degree plane, suggests teh serrations must bite into
teh dropout by about 0.6mm. Is that compatible with your assumptions?

no. i have no idea where you're getting your shear analysis from. this
is not cupping of a tensile sample, it's simple planar shear. if you
want to get into an analysis based on serration orientation,
differential embedding depths, etc., be my guest, but don't make
assumptions like your others here; get yourself access to a mtb fork and
make some measurements. your comment below seems to indicate that you
don't have direct personal experience.

Actually, if you _not_ making assumptions about serration orientation
and embedding, your figures are even more nonsense. If you're not
looking at these effects, your argument depends upon saying just
because there are serrations, they must embed, and they must then
plastically shear a high proportion of teh overall area of the axle
nut face.

This is clearly nonsense - it would be like taking out a knife,
scratching a cliff-face and claiming you've sheared teh mountain (or
about a third of it, in this case). You don't havew to shear the
whole face of teh drop-out when an axle slips.

It seems excessive to me to assume that every time you tighten teh QR,
you have to embed teh serations 0.6mm into teh material of teh fork
dropout. Can you justify that? I think after a few weeks use, fork
ends would be so chewed to bits you'd need new ones.

that guess makes no allowance for serration patterns to be persistent,
which is what we see in practice. once serrations are made, fork & axle
mesh in the same place time after time.

So you _are_ making assumptions about serration orientation?
Otherwise, what if teh first time teh wheel is fitted the serrations
are parrallel to teh dropout? Thereafter every time teh wheel is
fitted teh serrations will be parallel, and there is no shearing of
material, just friction, and we're back to your figures requiring QR
levers to be shut with 110kg force.

http://home.comcast.net/~carlfogel/d...d/Img_3199.jpg

this is my fork. that pic was taken somewhere north of the 2000 mile
mark. persistent serrations are clearly evident. and the wheel gets
taken out of this fork each and every ride, so it's got to be roughly
100 insertions you see there. not exactly "chewed to bits" is it?

Exactly. Thank you for proving my point.

regards, Ian SMith
--
|\ /| no .sig
|o o|
|/ \|

On Wed, 29 Sep 2004 21:36:48 -0700, jim beam
wrote (more or less):
....
correct, they're frequently magnesium alloy. i never stated aluminum,
so please don't make assumptions for me.

If you don't want folk to document their best guesses at what your
assumptions were, in a post which tries to work out how you arrived at
your conclusion by working backwards from your conclusion, please
state your assumptions and working in your original posts.



--
Cheers,
Euan
Gawnsoft: http://www.gawnsoft.co.sr
Symbian/Epoc wiki: http://html.dnsalias.net:1122
Smalltalk links (harvested from comp.lang.smalltalk) http://html.dnsalias.net/gawnsoft/smalltalk

Tom Sherman wrote in message ...
Ian Smith wrote:

It seems excessive to me to assume that every time you tighten teh QR,
you have to embed teh serations 0.6mm into teh material of teh fork
dropout. Can you justify that? I think after a few weeks use, fork
ends would be so chewed to bits you'd need new ones.

The word "teh" appears seven times. What does "teh" mean?

I would have thought that anyone with the intellectual equipment to
follow, or even read through, Ian's argument would have had little
trouble with such a trivial anagram.

--
Dave...

(Dave Kahn) writes:

Tom Sherman wrote in message ...
Ian Smith wrote:

It seems excessive to me to assume that every time you tighten teh QR,
you have to embed teh serations 0.6mm into teh material of teh fork
dropout. Can you justify that? I think after a few weeks use, fork
ends would be so chewed to bits you'd need new ones.

The word "teh" appears seven times. What does "teh" mean?

I would have thought that anyone with the intellectual equipment to
follow, or even read through, Ian's argument would have had little
trouble with such a trivial anagram.

OK: The word "het" appears seven times. What does "het" mean?



A

"Ambrose Nankivell" wrote in message

(Dave Kahn) writes:

[...]

I would have thought that anyone with the intellectual equipment to
follow, or even read through, Ian's argument would have had little
trouble with such a trivial anagram.

OK: The word "het" appears seven times. What does "het" mean?

"Het" is the neuter form of "the" in Dutch.

HTH

--

A: Top-posters.
Q: What is the most annoying thing on Usenet?

On 30 Sep 2004 15:46:49 +0100, Ambrose Nankivell
wrote in message :

OK: The word "het" appears seven times. What does "het" mean?

Dutch for "teh" ;-)

Guy
--
May contain traces of irony. Contents liable to settle after posting.
http://www.chapmancentral.co.uk

88% of helmet statistics are made up, 65% of them at Washington University

Ambrose says:

OK: The word "het" appears seven times. What does "het" mean?

It means someone is getting "het" up over something. Sure as sh!t seems that
way from here. ;-P

Steve

Ian Smith writes:

So you _are_ making assumptions about serration orientation?
Otherwise, what if teh first time teh wheel is fitted the serrations
are parrallel to teh dropout? Thereafter every time teh wheel is
fitted teh serrations will be parallel, and there is no shearing of
material, just friction, and we're back to your figures requiring QR
levers to be shut with 110kg force.

So what if het second time het wheel is fitted het serrations are not
parallel to het dropout? If thereafter every insertion leaves het
serrations in a new position, does het old indentation yield to het
new position? I don't see that het separation force caused by het
caliper position has any effect on het safety of this design.

Jobst Brandt

Ian Smith wrote:
On Wed, 29 Sep 2004 21:36:48 -0700, jim beam wrote:

Ian Smith wrote:

On Wed, 29 Sep 2004 06:01:30 -0700, jim beam wrote:


http://groups.google.com/groups?selm...&output=gplain

You maintain that fork legs are always soft and axles always embed in
them.


suspension forks, yes.


Possibly you have in mind that fork dropouts are always
aluminium, but that's not true.


correct, they're frequently magnesium alloy. i never stated aluminum,
so please don't make assumptions for me.


Do you know what the word "possibly" means?


Having maintained that the drop-outs
are soft enough that embedment occurs, you then decide that they
require 200 N/mm2 to shear (maintaining that this is a conservative
figure), but assuming a Tresca yield criterion,


Tresca is a fudge. use von Mises instead.


Which will make very little difference, numerically, and is
conceptually no less of a 'fudge' as you call it, since both are
simplified envelopes fitted to a few data points.


You go from a 94mm2 gross contact area, to a 25mm2 area of sheared
material. I'm not sure how you'd predict the area of material sheared
by a given embedment of a given serration pattern, but I'll try and
work out how you came up with that. Suppose a 3mm seration spacing,
implies 31mm length of contact in teh area. Given your 25mm2, that in
turn suggests a shearing plane width of 0.8mm at each serration, which
if we assume a 45 degree plane, suggests teh serrations must bite into
teh dropout by about 0.6mm. Is that compatible with your assumptions?


no. i have no idea where you're getting your shear analysis from. this
is not cupping of a tensile sample, it's simple planar shear. if you
want to get into an analysis based on serration orientation,
differential embedding depths, etc., be my guest, but don't make
assumptions like your others here; get yourself access to a mtb fork and
make some measurements. your comment below seems to indicate that you
don't have direct personal experience.


Actually, if you _not_ making assumptions about serration orientation
and embedding, your figures are even more nonsense. If you're not
looking at these effects, your argument depends upon saying just
because there are serrations, they must embed, and they must then
plastically shear a high proportion of teh overall area of the axle
nut face.

This is clearly nonsense - it would be like taking out a knife,
scratching a cliff-face and claiming you've sheared teh mountain (or
about a third of it, in this case). You don't havew to shear the
whole face of teh drop-out when an axle slips.


It seems excessive to me to assume that every time you tighten teh QR,
you have to embed teh serations 0.6mm into teh material of teh fork
dropout. Can you justify that? I think after a few weeks use, fork
ends would be so chewed to bits you'd need new ones.


that guess makes no allowance for serration patterns to be persistent,
which is what we see in practice. once serrations are made, fork & axle
mesh in the same place time after time.


So you _are_ making assumptions about serration orientation?
Otherwise, what if teh first time teh wheel is fitted the serrations
are parrallel to teh dropout? Thereafter every time teh wheel is
fitted teh serrations will be parallel, and there is no shearing of
material, just friction, and we're back to your figures requiring QR
levers to be shut with 110kg force.


http://home.comcast.net/~carlfogel/d...d/Img_3199.jpg

this is my fork. that pic was taken somewhere north of the 2000 mile
mark. persistent serrations are clearly evident. and the wheel gets
taken out of this fork each and every ride, so it's got to be roughly
100 insertions you see there. not exactly "chewed to bits" is it?


Exactly. Thank you for proving my point.

regards, Ian SMith

ian, honestly, having a ****ing match gets us nowhere. if i'm curt,
it's because i don't understand some of your logical leaps &
assumptions. i probably have a different background to yours, so we may
have different perspectives. example: Tresca is used by engineers, but
materials folk regard it as less rigorous than von Mises. perspectives,
right?

bottom line, i'm open to better modelling, particularly if someone
like yourself has the time & energy to be more rigorous about it. i'm
not completely ignoring serration orientation, i'm merely simplifying
with reasonable assumptions to get a first order approximation. please,
be my guest if you want to do this exercise properly.

On Wed, 29 Sep 2004 21:19:19 -0500, Tom Sherman
() wrote:

The word "teh" appears seven times. What does "teh" mean?

t3H 12 duTcH l33T 5p33K PH0R h3t. D0 k33p Up @ T3h b4cK.


--
Matt K.
"It feels great to wake up and not know what day it is, doesn't
it?"