Frame design question (no, not compact frames)

Appkiller

So, after a group ride the other day a dude was checking out my ride
and asked if it was stiff. Hell yes it is, especially in the bb. He
looked at my chainstays and commented that the large profile of the
chainstay was responsible for this stiffness. Hmmm. These are
lenticular (Foco) in shape with the long axis in the vertical plane
and not exceptionally wide in the horizontal plane.

I was about to contradict him, but decided not to as it really didn't
matter whether or not he understood bicycle design. AFAIK, the
horizontal width of the chainstay will determine the relative
stiffness of the bb, kinda like the concept behind "bi-ovalized" down
tubes. I would agree the addition of more material in the vertical
plane will add a wee bit of stiffness in the horizontal plane, but not
nearly as much as if the same amount of material was added in the
horizontal plane.

I would also assert that the stiffness in the bb had much more to do
with the advent of the super strong air-hardened steels (thermacrom,
853, S3) than anything else.

Was I smart to not contradict him because:

A. I would have looked like an idiot and the above blather is all
wrong?

-OR-

B. Direct contradiction (even if correct) of an assertion of someone
you just met can alienate them?

Just wondering,

App

Bruni

You are right about the minor axis being paramount for stiffness, which is
why I "deovalize" such stays and make them longer to get my tire/ring
clearance back. If this seems to contradict the shorter/stiffer trend, it
does. Shorter/stiffer is a direct inverse proportion, so 2cm shorter only
gets you 5%. OTOH, fattening an 18mm minor axis to 22mm is 50% stiffer
(measured, not calculated). As to 853, its elastic modulus is nearly the
same as other steels and since it is to my knowledge only 22.2mm ROR (as
opposed to the 24mm oval I prefer, it would have to be nearly round to get
the 22mm lateral dimension. This is why steel is still a competitive mat'l.,
it packs a lot of stiffness into the small space afforded a designer between
tires and rings. Low density alloys only have a stiffness/weight advantage
when diameters are large,and walls are thin.

--
Bruni Bicycles
"Where art meets science"
brunibicycles.com
410.426.3420
Appkiller <petengail@yahoo.com> wrote in message
news:ac4af1d8.0404300941.de87981@posting.google.com...
> So, after a group ride the other day a dude was checking out my ride
> and asked if it was stiff. Hell yes it is, especially in the bb. He
> looked at my chainstays and commented that the large profile of the
> chainstay was responsible for this stiffness. Hmmm. These are
> lenticular (Foco) in shape with the long axis in the vertical plane
> and not exceptionally wide in the horizontal plane.
>
> I was about to contradict him, but decided not to as it really didn't
> matter whether or not he understood bicycle design. AFAIK, the
> horizontal width of the chainstay will determine the relative
> stiffness of the bb, kinda like the concept behind "bi-ovalized" down
> tubes. I would agree the addition of more material in the vertical
> plane will add a wee bit of stiffness in the horizontal plane, but not
> nearly as much as if the same amount of material was added in the
> horizontal plane.
>
> I would also assert that the stiffness in the bb had much more to do
> with the advent of the super strong air-hardened steels (thermacrom,
> 853, S3) than anything else.
>
> Was I smart to not contradict him because:
>
> A. I would have looked like an idiot and the above blather is all
> wrong?
>
> -OR-
>
> B. Direct contradiction (even if correct) of an assertion of someone
> you just met can alienate them?
>
> Just wondering,
>
> App

Zog The Undeniable

Appkiller wrote:

> So, after a group ride the other day a dude was checking out my ride
> and asked if it was stiff. Hell yes it is, especially in the bb. He
> looked at my chainstays and commented that the large profile of the
> chainstay was responsible for this stiffness. Hmmm. These are
> lenticular (Foco) in shape with the long axis in the vertical plane
> and not exceptionally wide in the horizontal plane.
>
> I was about to contradict him, but decided not to as it really didn't
> matter whether or not he understood bicycle design. AFAIK, the
> horizontal width of the chainstay will determine the relative
> stiffness of the bb, kinda like the concept behind "bi-ovalized" down
> tubes. I would agree the addition of more material in the vertical
> plane will add a wee bit of stiffness in the horizontal plane, but not
> nearly as much as if the same amount of material was added in the
> horizontal plane.

True.

> I would also assert that the stiffness in the bb had much more to do
> with the advent of the super strong air-hardened steels (thermacrom,
> 853, S3) than anything else.

Sadly not. The stiffness (Young's modulus) of different steels is much
of a muchness - around 200,000 MPa [1]. It's the strength that varies,
which allows you to use thinner tubing with stronger steels. Thinner
tubing will actually be less stiff as a manufactured tube. The way to
get stiff tubes, asusming we're talking about plain round ones, is by
making the diameter bigger or the gauge heavier. The former is more
acceptable for a bike.

[1] think of it as the stress that would stretch a rod of the material
to twice its original length (in the real world it would break first).
https://vault2.secured-url.com/reyn...properties.html

Paul Kopit

On 30 Apr 2004 10:41:49 -0700, petengail@yahoo.com (Appkiller) wrote:

>I would also assert that the stiffness in the bb had much more to do
>with the advent of the super strong air-hardened steels (thermacrom,
>853, S3) than anything else.

How do you measure the stiffness of the BB?

Phil Brown

>>I would also assert that the stiffness in the bb had much more to do
>>with the advent of the super strong air-hardened steels (thermacrom,
>>853, S3) than anything else.

Nope. It has to do with the shell itself and the brazing technique. A cast
shell is stiffer and a good brazing job is stiffer.
phil Brown