Frame Sizes - Trad v Compact

A few months ago I had a computerised bike fit for the (trad framed) road
bike I was then using and the bike was adjusted to suit my 'ideal' fit.
After a few tweaks here and there I arrived at what was a very comfortable
fit at a set up not too far from that recommended by the computerised bike
fit.

I now have the opportunity to buy a compact framed bike which basically
cocks up the computer fit, as this is aimed at trad frames. From the
measurements below, both recommended and actual from the compact, how much
can I allow for in terms of the compact frame (i.e. how much smaller would
be acceptable without the bike being deemed too small?)

Top Tube

Recommended (Trad Frame) - 576mm
Compact actual - 540mm


BB - Top Seat Tube

Recommended - 558mm
Compact actual - 515mm


On the compact to arrive at a comfortable riding position I need to have
about 204mm of seatpost tube out.

The reason I ask is that I have absolutely no idea what the 'official' size
of the compact is as I am buying second hand and can't find the frame
geometry anywhere. It's therefore a little different to gauge whether the
frame will be too small or not, although first impressions are comfy enough.

For the record I am 5 feet 11 inches with a 31 inside leg.

Thoughts anyone?

Cheers

Paul

Paul wrote:

I now have the opportunity to buy a compact framed bike which basically
cocks up the computer fit, as this is aimed at trad frames. From the
measurements below, both recommended and actual from the compact, how much
can I allow for in terms of the compact frame (i.e. how much smaller would
be acceptable without the bike being deemed too small?)

Top Tube

Recommended (Trad Frame) - 576mm

[snip]

For the record I am 5 feet 11 inches with a 31 inside leg.

Thoughts anyone?

You really need to sit on the thing. Although a "58cm compact frame"
will often mean "58cm if this were a trad frame" - i.e. they measure
a hypothetical horizontal top tube length.


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jc

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I'm the same height, with slightly longer legs. I ride a 73 degree
parallel 56.5 cm top tube traditional frame. It so happens with this
geometry that the seat tube can be the same length. There are at least
two methods of measuring seat tube length (c-c, c-t). Your fitting
methods have shown that you fit the norm for this too.

Most compact frames now quote a "virtual" top tube length. This is the
horizontal distance from centre of steerer to centre of the seat pin.
The length of the down tube, which is the normal measure of frame size,
for a compact frame is irrelevant, it is virtual top tube length that
matters. Compact frames have longer real top tubes and shorter down
tubes to help standover. The most compact compact frame is of course
the women's mixte frame.

Length of exposed seatpost is irrelevant, I ride a Brompton!

From the details provided, if the virtual top tube is 540mm then the
frame is too small. If the actual top tube is 540mm, then the frame is
much too small.

regards,
daren
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