Ladies frame size questions

For ladies' bikes frame sizing, am I right in thinking the measurement is the length of the tube from the middle of the bottom bracket to the top where it meets the seat post?
My wife has ordered a new ladies hybrid bike and the shop advised an 18inch frame. She is 5 foot 5 inches tall - does that size frame sound about right?
My wife has ordered a new ladies hybrid bike and the shop advised an 18inch frame. She is 5 foot 5 inches tall - does that size frame sound about right?
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As for whether an 18" is rights impossible to say. Some hybrids have road bike like frames, some have mtb like frames, on an mtb 18 inches is likely to be a little large, whereas on a road bike it would be in the ball park.
What bike is it?
Sorry if I am stating the obvious, but the seat should probably be level or above the handlebars, with a few cm of seatpost showing, and the seat height set so that the leg is almost straight when on the down stroke of the pedal - this means that both feet can't be on the ground when sitting on the seat).
http://www.dawescycles.com/p-120-sonoran-ladies.aspx
As it's a ladies bike the top tube meets the seat post tube about half way down, so I think the measurement must be to the top of the seat post tube.
However the diagram in your other post seems to have a seat tube of 49cm which is much longer than the 17 inch size quoted for the frame size in that example, so I am a bit confused.
anyway, the geometry for the bike you are choosing is here:
http://78.129.162.115/SonoranL.pdf
Really, it would be best to try the bike in both sizes in the shop first. If the shop has ordered one only, don't buy it unless the fit is perfect. There is no obligation and there are plenty of (better / different) bikes around.
I see that on the geometry for the Ladies Sonoran (impressed that you found that diagram) the total seat post length is 18 inches right to the top as I thought. We will test the bike and my wife will only buy it if the fit feels right - only sometimes you only realise the fit is not quite right when you get the bike home and ride it a bit more.
I myself have a Dawes Audax with a 57cm frame, which is too big for my 5ft 8in height according to what I read in size charts. That maybe explains why my shoulders hurt as I think the stretch to the drop bars is a bit too long for me despite the fact that I have the bars raised higher than normal.
Carbon 456
456 lefty
Pompino
White Inbred
I do think there is a tendency to go too large when buying bikes.
Even if they only have one bike of that model, there may be other bikes in the shop that your wife can try and then compare the geometry with the two sizes of the one that has been ordered.
Funnily enough, I too have a Dawes Audax - an old (1997) model. It took me several years to get it really just right for me. I think the top tube is quite long. It is a 54cm frame and the stretch to the bars was too much (I am 5'10"). . I now have it with a short 80mm stem with a lot of rise (its a Specialized stem that has a shim allowing 4 different rises between 0 and 24 degrees - very good), I also have the bars rotated up a bit. There is also a fair amount of space under the stem to raise the bars. It is at last supremely comfortable! You can see the setup here, if it helps: