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Ladies frame size questions

ForumNewbieForumNewbie Posts: 1,664
edited February 2009 in Road beginners
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?

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  • alfabluealfablue Posts: 8,497
    This all depends, they tend to be measured either c-c (centre of bottom bracket to centre of top tube) or c-t (top of top tube). Note that the end of the seat tube, where it meets the seatpost, may well be a few cm beyond the top of the top tube - this length is not the one usually quoted for seat tube sizes. But the length of the top tube is probably the most important measure as this effects reach to the bars and the overall feel of the bike. The "effective" or "virtual" top tube measurement is the one to go by, this is what length the top tube would be if it were horizontal (most bikes these days have sloping top tubes). You can usually get detailed geometry for most bike models from the manufacturer's web sites and they will give all the tube dimensions for each size, and how they are measured.

    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?
  • alfabluealfablue Posts: 8,497
    Here is a typical manufacturer's geometry diagram, just as an example - dimensions will vary depending on particular brand, model and frame size (note they say seat tube is centre to centre, even though the lines suggest otherwise).

    geo.jpg

    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).
  • ForumNewbieForumNewbie Posts: 1,664
    alfablue wrote:
    This all depends, they tend to be measured either c-c (centre of bottom bracket to centre of top tube) or c-t (top of top tube). Note that the end of the seat tube, where it meets the seatpost, may well be a few cm beyond the top of the top tube - this length is not the one usually quoted for seat tube sizes. But the length of the top tube is probably the most important measure as this effects reach to the bars and the overall feel of the bike. The "effective" or "virtual" top tube measurement is the one to go by, this is what length the top tube would be if it were horizontal (most bikes these days have sloping top tubes). You can usually get detailed geometry for most bike models from the manufacturer's web sites and they will give all the tube dimensions for each size, and how they are measured.

    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?
    Hi alphablue - it is a 2008 Dawes Sonoran Ladies bike as seen at link below:
    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.
  • alfabluealfablue Posts: 8,497
    Okay, the above example is confusing - the 17" is centre to centre, whilst the actual seat tube length to the top is 49cm (19") implying a 2" portion above the top tube,

    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.
  • ForumNewbieForumNewbie Posts: 1,664
    Thanks alphablue,

    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.
  • She really needs to try it, they should have had other models on the smae frame in other sizes to get the fit right, it's just good practice really. Or tried a smaller one and found it a bit small.
  • alfabluealfablue Posts: 8,497
    Yes, I agree, first impressions about bike fit and comfort can be misleading, you only really tell after a good ride. When buying a road bike for my girlfriend my LBS let her ride it before purchase, but also said that they would exchange it after a few days if undamaged if she wasn't happy (John's Bikes, Bath - great service!). It would be nice to think your bike shop would do the same, though realistically it might be too much to expect.

    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:

    dawes.jpg
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