TTL: The key measurement in sizing

Andre Jute wrote:
[...]
I must say I have found it convenient, in a sturdy mountain-bike type
frame at least (where the stiffness seems to be guaranteed by the size
and beef of the ail tubes), to go down one size (2mm, not 2in!) from
the traditional size, and adjust to the desired fit with a longer
seat post and a very upright North Road type handlebar. Several
European dealers told me they now advise people on comfort bikes to
take one size down from the obvious fit.

How much of a real difference can 2-mm in frame size make?

--
Tom Sherman - Holstein-Friesland Bovinia
The weather is here, wish you were beautiful

On May 3, 2:55*am, Tom Sherman
wrote:
Andre Jute wrote:
[...]
I must say I have found it convenient, in a sturdy mountain-bike type
frame at least (where the stiffness seems to be guaranteed by the size
and beef of the ail tubes), to go down one size (2mm, not 2in!) from
the traditional *size, and adjust to the desired fit with a longer
seat post and a very upright North Road type handlebar. Several
European dealers told me they now advise people on comfort bikes to
take one size down from the obvious fit.

How much of a real difference can 2-mm in frame size make?

Some makers think a lot and therefore supply bikes at 2 or 3cm size
intervals, some are not impressed and supply bikes only at 5 or 6cm
intervals.

In this particular case that I have in mind, where it worked
brilliantly for me, there was also a switch between manufacturers. The
nominal 2cm difference on seat tube length is 3cm when measuring under
the same assumptions (hypothetically horizontal seat tube). But with
differences of design outlook and style working through into the
geometry and fittings, the effective reach distance (call it effective
seat tube length) differs between the bikes by 10cm. Now that is four
inches and makes a big difference. It is the difference which allowed
me to make a poorly conceived Trek *more comfortable* than a proper
Gazelle -- an amazing outcome in anyone's book.

So I think a quite small change in nominal size (specified as the
useless seat tube length) can make a big difference in how you set up
the bike, indeed in how you "can" set it up. For instance, if you
recover an ideal (in my case = desired, not what someone else
considers fashionable) seating position by altering the stem (height,
angle, length -- easily done on the adjustable quill stems fitted to
all my bikes) to put the handlebar clamp at a different position, you
change the steering response of the bike by altering the lever length
of the steering arm. On that particular, by itself, all other things
being equal, the smaller frame is a superior choice because the bigger
the frame the shorter the steering arm lever length will be set if the
purpose is, as it is in most instances, to have the rider sit more
upright. (Of course, one of the things threadless headsets and stems
do, to a large extent, is to remove this sort of choice from the hands
of the consumer.)

Also, when one speaks to several experienced dealers in four or five
different countries, and there is a clear consensus of reasoned
opinion, it is worth reporting here. This is the place for an exchange
of opinions, yes?

Andre Jute
http://members.lycos.co.uk/fiultra/B...20CYCLING.html