Teach an old guy about "new frame sizing"



Bob wrote:
> OK... I'm a 70's guy. Back then we bought a frame on the large side
> and minimized seat post and stem extension - not that there were many
> options since they were not very long. Recently the trend definitely
> seems to be toward smaller frames with more seat post and stem
> extension and/or stems and bars with lots of rise ? But, for any given
> fitting desired (i.i. seat surface to pedal surface, seat position to
> stem/bar location) I think we can use either approach to "fit" a bike
> (I think; comments welcome).
>
> So, what's the advantage in a smaller frame and more extension of the
> other components? I would think that in general, a larger frame with
> smaller extensions will be stiffer and more desirable.
>
> Enlightening appreciated,
>
>
>

I think the only advantage is that it allows you to get the bars lower,
which is the opposite of the problem that many riders have.

In the old days, bigger frames didn't have longer toptubes, so a smaller
frame with a long seat post and stem fit about the same. These days,
although manufacturers are offering fewer sizes, the larger sizes have
(a bit) more toptube length.

The critical aspect of bike fit is how flat a back you want. The more
horizontal you get, the more "cockpit" length you need, but you also
need lower bars. Since the first goal steers you to a larger frame, and
the second to a smaller, it can be tricky.
 
Ron Ruff wrote:
> Jay S. Hill wrote:
> > Actually, when I went from a slightly too-small frame to
> > one that fit me very well, I could corner much better, because my center
> > of gravity was in a better place and I was more stable. I was racing
> > criteriums at the time, and there was a marked difference.
>
> You center of gravity will move *very* slightly upward and rearward
> with a larger frame... and I don't see either of these being beneficial
> to handling. For even weight distribution on the wheels, a smaller
> frame is better... you'll still have more weight on the rear, anyway.

The center of gravity (CoG) of the frame may move slightly upward w/ a
larger frame, but the CoG of the frame is a minute aspect in the
overall CoG of the bike & rider combined.

Slight changes in the frame's CoG, which is often touted as a benefit
of a compact-geometry frame, is completely irrelevant as a factor in
how the bike & rider interact, as the CoG of the bike & rider is
clearly defined by the CoG of the rider, which will be placed at the
same point above the BB, regardless of where the top tube intersects
the seat tube.

If you think that lowering the point where the top tube intersects the
seat tube by a few centimeters significantly changes the CoG of the
bike & rider together, then I've got some really primo beachfront
property in southern AZ for you.
 
Phil, Squid-in-Training wrote:

> Longer stem? Have makers begun to use lengthwise-shorter frames?
>

A lot of makes have started making fewer sizes. ;-)
 
Qui si parla Campagnolo wrote:
> Go to a bike shop that actually does bike fits, on a fit cycle and have
> them do a fit on you. No such thing as a test ride around the parking
> lot fit...they are just trying to sell ya something
>
> Ideally, a bike shop tnat starts with frames, and then builds the bike
> you want.

Yeah, at all the local bike shops around here, you can buy any bike you
want, as long as it's a Trek, Specialized, or Cannondale.
 
Peter Cole wrote:


>
> In the old days, bigger frames didn't have longer toptubes, so a smaller
> frame with a long seat post and stem fit about the same.

A few months ago, I was looking at some NOS frames from the '70s. The
55cm (c-t) frame had a 55cm TT, as did the 60cm. But the 62cm jumped to
a 60cm TT. The 5cm increment seemed a bit much.





> These days,
> although manufacturers are offering fewer sizes, the larger sizes have
> (a bit) more toptube length.
>
> The critical aspect of bike fit is how flat a back you want. The more
> horizontal you get, the more "cockpit" length you need, but you also
> need lower bars. Since the first goal steers you to a larger frame, and
> the second to a smaller, it can be tricky.
 
On 23 Mar 2006 14:31:45 +0000 (GMT), David Damerell
<[email protected]> wrote:

>Quoting John Forrest Tomlinson <[email protected]>:
>>Flex in the seat post is a good thing. Flex in the stem, sort of good
>>(for comfort), sort of bad (for handling and accelleration). Flex in
>>the frame, bad (for control and power transmission).
>
>Out of interest, how can your legs tell the difference between the
>seatpost flexing, which is apparently good, and the seat tube flexing,
>which is apparently bad?

I don't think my legs can tell where flex is coming from. But here's
the explanation. Lateral flex in a bike is generally bad -- for
handling and for high-power situations. Fore-aft flex at the seat is
good. It translates into a little bit of up and down flext which
helps comfort.

A seat post can flex fore-aft -- the seat tube can't becasue it's
braced by the top tube and seatstays. So the fore-aft flex is coming
from the post (and seat), not the seat tube.

JT


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Quoting John Forrest Tomlinson <[email protected]>:
><[email protected]> wrote:
>>Quoting John Forrest Tomlinson <[email protected]>:
>>>Flex in the seat post is a good thing. Flex in the stem, sort of good
>>>(for comfort), sort of bad (for handling and accelleration). Flex in
>>>the frame, bad (for control and power transmission).
>>Out of interest, how can your legs tell the difference between the
>>seatpost flexing, which is apparently good, and the seat tube flexing,
>>which is apparently bad?
>I don't think my legs can tell where flex is coming from. But here's
>the explanation. Lateral flex in a bike is generally bad -- for
>handling and for high-power situations.

And is there any reason to suppose that a compact frame with a long
seatpost has less of it?

>Fore-aft flex at the seat is
>good. It translates into a little bit of up and down flext which
>helps comfort.

If that's significant, my dad's old Brompton folder (with a seatpost
extending about 3 feet upwards) ought to be the most comfortable bike I
ever rode. It's... not.
--
David Damerell <[email protected]> flcl?
Today is Second Gouday, March.
 
On 24 Mar 2006 15:06:09 +0000 (GMT), David Damerell
<[email protected]> wrote:

>Quoting John Forrest Tomlinson <[email protected]>:
>><[email protected]> wrote:
>>>Quoting John Forrest Tomlinson <[email protected]>:
>>>>Flex in the seat post is a good thing. Flex in the stem, sort of good
>>>>(for comfort), sort of bad (for handling and accelleration). Flex in
>>>>the frame, bad (for control and power transmission).
>>>Out of interest, how can your legs tell the difference between the
>>>seatpost flexing, which is apparently good, and the seat tube flexing,
>>>which is apparently bad?
>>I don't think my legs can tell where flex is coming from. But here's
>>the explanation. Lateral flex in a bike is generally bad -- for
>>handling and for high-power situations.
>
>And is there any reason to suppose that a compact frame with a long
>seatpost has less of it?

Yes, the seat tube is shorter and braced lower down.
>
>>Fore-aft flex at the seat is
>>good. It translates into a little bit of up and down flext which
>>helps comfort.
>
>If that's significant, my dad's old Brompton folder (with a seatpost
>extending about 3 feet upwards) ought to be the most comfortable bike I
>ever rode. It's... not.

Is everything else about the bike same as the bikes you're comparing
it to?

JT


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In article <[email protected]>,
John Forrest Tomlinson <[email protected]> wrote:

> On 24 Mar 2006 15:06:09 +0000 (GMT), David Damerell
> <[email protected]> wrote:
>
> >Quoting John Forrest Tomlinson <[email protected]>:
> >><[email protected]> wrote:
> >>>Quoting John Forrest Tomlinson <[email protected]>:
> >>>>Flex in the seat post is a good thing. Flex in the stem, sort of good
> >>>>(for comfort), sort of bad (for handling and accelleration). Flex in
> >>>>the frame, bad (for control and power transmission).
> >>>Out of interest, how can your legs tell the difference between the
> >>>seatpost flexing, which is apparently good, and the seat tube flexing,
> >>>which is apparently bad?
> >>I don't think my legs can tell where flex is coming from. But here's
> >>the explanation. Lateral flex in a bike is generally bad -- for
> >>handling and for high-power situations.
> >
> >And is there any reason to suppose that a compact frame with a long
> >seatpost has less of it?
>
> Yes, the seat tube is shorter and braced lower down.
> >
> >>Fore-aft flex at the seat is
> >>good. It translates into a little bit of up and down flext which
> >>helps comfort.
> >
> >If that's significant, my dad's old Brompton folder (with a seatpost
> >extending about 3 feet upwards) ought to be the most comfortable bike I
> >ever rode. It's... not.
>
> Is everything else about the bike same as the bikes you're comparing
> it to?

Most often, noticeable frame flexure is when the rider is
pedaling very strongly. The flex is torsional: the bottom
bracket twists relative to the chain stays on a horizontal
axis parallel to the wheel base. Frame makers invest
considerable effort on this joint.

I am not yet persuaded that compact and horizontal top
tube frames have contrasting ride characteristics.

--
Michael Press
 
David Damerell wrote:
> Quoting John Forrest Tomlinson <[email protected]>:
> ><[email protected]> wrote:
> >>Quoting John Forrest Tomlinson <[email protected]>:
> >>>Flex in the seat post is a good thing. Flex in the stem, sort of good
> >>>(for comfort), sort of bad (for handling and accelleration). Flex in
> >>>the frame, bad (for control and power transmission).
> >>Out of interest, how can your legs tell the difference between the
> >>seatpost flexing, which is apparently good, and the seat tube flexing,
> >>which is apparently bad?
> >I don't think my legs can tell where flex is coming from. But here's
> >the explanation. Lateral flex in a bike is generally bad -- for
> >handling and for high-power situations.
>
> And is there any reason to suppose that a compact frame with a long
> seatpost has less of it?
>
> >Fore-aft flex at the seat is
> >good. It translates into a little bit of up and down flext which
> >helps comfort.
>
> If that's significant,

Of course it's not significant, common sense tells us that. Just
another myth of the gullible and mis-informed.


> my dad's old Brompton folder (with a seatpost
> extending about 3 feet upwards) ought to be the most comfortable bike I
> ever rode. It's... not.
>
 
On 24 Mar 2006 18:09:45 -0800, "Ozark Bicycle"
<[email protected]> wrote:

>Of course it's not significant, common sense tells us that. Just
>another myth of the gullible and mis-informed.

Right, I forgot, the long seatpost thing lacks "character."

JT

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John Forrest Tomlinson wrote:
> On 24 Mar 2006 18:09:45 -0800, "Ozark Bicycle"
> <[email protected]> wrote:
>
> >Of course it's not significant, common sense tells us that. Just
> >another myth of the gullible and mis-informed.
>
> Right, I forgot, the long seatpost thing lacks "character."
>

Oh, that's okay, JT.

I was wondering how anyone could possibly believe that drivel about
fore and aft seatpost flex providing comfort.

Then I remembered, you lack "common sense" and "intelligence".
 
On Fri, 24 Mar 2006 18:06:00 -0500, John Forrest Tomlinson
<[email protected]> wrote:

>>And is there any reason to suppose that a compact frame with a long
>>seatpost has less of it?
>
>Yes, the seat tube is shorter and braced lower down.

But therefore the seat *post* is longer and unbraced. From an
engineering viewpoint, the setup with the seat closest to the stay and
tube attachment point is the most rigid (minimal seatpost). As the
seatpost rises, you increase the leverage on the structure of the
frame and are more likely to flex it, not less.
 
Quoting John Forrest Tomlinson <[email protected]>:
><[email protected]> wrote:
>>Quoting John Forrest Tomlinson <[email protected]>:
>>>I don't think my legs can tell where flex is coming from. But here's
>>>the explanation. Lateral flex in a bike is generally bad -- for
>>>handling and for high-power situations.
>>And is there any reason to suppose that a compact frame with a long
>>seatpost has less of it?
>Yes, the seat tube is shorter and braced lower down.

Is the seatpost somehow completely inflexible laterally?

>>>Fore-aft flex at the seat is
>>>good. It translates into a little bit of up and down flext which
>>>helps comfort.
>>If that's significant, my dad's old Brompton folder (with a seatpost
>>extending about 3 feet upwards) ought to be the most comfortable bike I
>>ever rode. It's... not.
>Is everything else about the bike same as the bikes you're comparing
>it to?

Of course not; that's a cheap shot; but the underlying question about the
magnitude of that fore-aft flex remains. How big is it?
--
David Damerell <[email protected]> Distortion Field!
Today is First Potmos, April.
 
On 27 Mar 2006 14:47:14 +0100 (BST), David Damerell
<[email protected]> wrote:

>> >Is everything else about the bike
> > same as the bikes you're comparing
>> it to?
>Of course not; that's a cheap shot; but the underlying question about the
>magnitude of that fore-aft flex remains. How big is it?

I'm sorry you take that as a cheap shot -- I was merely asking a
question. I meant, does it has the same sort of wheels, tires, bars
and seat as the bikes you compare it to? Those things affect comfort.
Maybe stems too. The frame must obviously be different on that bike
from most other bikes and I doubt differences in the drivetrain have
any effect.

> the underlying question about the
> magnitude of that fore-aft flex remains.

Small. But bigger than any vertical flex in the frame. Maybe close to
flex in the fork.

JT

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On Wed, 22 Mar 2006 18:02:59 GMT, Bob <[email protected]> wrote:

>OK... I'm a 70's guy. Back then we bought a frame on the large side
>and minimized seat post and stem extension - not that there were many
>options since they were not very long. Recently the trend definitely
>seems to be toward smaller frames with more seat post and stem
>extension and/or stems and bars with lots of rise ? But, for any given
>fitting desired (i.i. seat surface to pedal surface, seat position to
>stem/bar location) I think we can use either approach to "fit" a bike
>(I think; comments welcome).
>

You need to think first what kind of bike fit you want the bike to do
for you?

Bike fit is very generic, because not everyone rides for the same
goals. That is why, I think it's pointless to explain any approach to
fitting a bike without knowing who you are, what you do mostly with
your bike and your phsyical flexibility.

>So, what's the advantage in a smaller frame and more extension of the
>other components? I would think that in general, a larger frame with
>smaller extensions will be stiffer and more desirable.
>

Compact geometry refers to the rearward sloping, reduced rear triangle
configuration that became popular a few years ago. It is an attempt to
reduce weight, increase stiffness and, for some companies, simplify
fit. Compact geometry started largely as an idea (or was popularized)
by Giant Bicycles as a design by Mike Burroughs. Some of the concepts
used in mountain bike frame design were translated to the road and
compact geometry was born. Compact Geometry has its advantages: If you
have a super long torso and never get enough stand over height then
compact geometry may work for you. But beware, it isn't for everyone.

It also has drawbacks. The back wheel can be lifted off the ground
way too easily during accelerations. The bike really wasn't any
lighter than a standard bike and the long top tube was nice but the
corresponding wheelbase can be way too long and the bike may handle
like a wet sponge! An enormous amount of seatpost protruding above
the top tube made the bike feel like a turd and it didn't help that
the company that made those bikes come in three sizes (small, medium
and large) would be enough -- you know who.
If you are not sure what you are getting into, I suggest that you pay
a visit to a road store staffed with experienced people (preferably
those who had been selling road bikes for sometime). Make sure they
don't try to fit your compact geometry road bike like a mountain bike.
David.
 
Quoting John Forrest Tomlinson <[email protected]>:
><[email protected]> wrote:

I notice you have not replied to the observation about the longer seatpost
also flexing laterally.

>>>Is everything else about the bike same as the bikes you're comparing
>>>it to?
>>Of course not; that's a cheap shot; but the underlying question about the
>>magnitude of that fore-aft flex remains. How big is it?
>I'm sorry you take that as a cheap shot

I'm not; I was describing what I wrote as a cheap shot. The Brompton is
nothing like a normal bike.

>>the underlying question about the
>>magnitude of that fore-aft flex remains.
>Small. But bigger than any vertical flex in the frame. Maybe close to
>flex in the fork.

So completely insignificant next to tyre flex.
--
David Damerell <[email protected]> Distortion Field!
Today is First Teleute, April.
 
Quoting John Forrest Tomlinson <[email protected]>:
><[email protected]> wrote:
>>I notice you have not replied to the observation about the longer seatpost
>>also flexing laterally.
>It probably does.

So actually there is no reason to believe that compact frames reduce
lateral flex.
--
David Damerell <[email protected]> Distortion Field!
Today is First Teleute, April.
 
David Damerell wrote:
> Quoting John Forrest Tomlinson <[email protected]>:
> ><[email protected]> wrote:
> >>I notice you have not replied to the observation about the longer seatpost
> >>also flexing laterally.
> >It probably does.
>
> So actually there is no reason to believe that compact frames reduce
> lateral flex.
>

There you have it.
 
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