WaveyGravey

Question concerning measurement. What 2 co-ordinates are used to determine bike size? Thanks

delbiker1 

Here is a diagram the tubes on the bike and a listing of the measurements for the various tubes. Note how the top tube measurement is done.

DescriptionXS(42cm)S(46cm)M(50cm)L(54cm)XL(58cm)ASEAT TUBE, CENTER TO TOP420.0460.0500.0540.0580.0C-2EFFECTIVE TOP TUBE LENGTH510.0530.0545.0565.0590.0DCHAIN STAY420.0420.0420.0420.0420.0EBB DROP71.171.171.171.171.1FFORK OFFSET45.045.045.045.045.0GHEAD TUBE ANGLE71.071.572.573.073.0HSEAT TUBE ANGLE76.074.573.573.073.0IWHEEL BASE993.1996.6994.01004.41029.3JSTAND OVER HEIGHT710.9739.4773.7795.3831.4HEAD TUBE LENGTH105.0126.0156.0161.0197.0reach379.5375.8376.0390.0404.0stack512.0535.6565.9572.5607.0trail69.5 63.260.057.056.9STEM LENGTH90.090.0100.0100.0110.0STEM ANGLE-7-7-7-7-7HANDLEBAR WIDTH400.0400.0420.0440.0440.0HANDLEBAR RISENANANANANACRANK LENGTH170.0170.0175.0175.0175.0TOE CLIP SIZESSSMMSEAT POST DIAMETER27.227.227.227.227.2

delbiker1 

Most bikes are going to be sized by the measurement of the effective top tube.
This is just a guideline for height of rider and size of bike. A bike shop can give you more accurate appropriate sizing.
48cm - 5'5" to 5'7"
51cm - 5'7" to 5'9"
53cm - 5'9" to 5'11"
56cm - 5'11" to 6'1"
59cm - 6'1" to 6'3"This is a general guide, please consult the geometry chart.
Sizing is a personal preference, the final choice is up to you.
Between sizes? Size up for higher bars, more casual riding / longer arms/torso.
Size down for lower bars, more aggressive riding; short arms/torso

Bill in VA

Is this the length of the top tube? I was always under the impression that sizing was the length of the seat tube measured from centr of BB to the top of the seat tube or where the tube would end it the top tube was parallel to the ground. Top tube length was taken into consideration.

If the TT length is the new sizing, it would explain why a shop showing me an older bike recommended a 51cm frame. I believe they applied the TT length to the marked ST height. The bike was way too small and I felt like my head was in front of the front hub. At 5'8.5", I ride a 54-55cm frame using ST measurement.

I also notice many bikes have a gap in sizing of 49, 51, (gap) 54-55, 57, 59. Is this a TT measurement sizing thing.

delbiker1 

There are bikes frames produced in those missing numbers. I have a couple of bikes that are 53 cm and also a couple that are 54cm. There are also bikes that measure 52 cm,56 cm. 58 cm and upward.I am 5'8" and have both 53 cm and 54 cm bikes. They are what fits me best.There are a lot of bikes now that have the "compact frame", meaning they have a sloping top tube, shorter seat tubes, and longer head tubes. The sloping top tube is why there is now the " effective top tube measurement". Look at the diagram and you can see how that measurement is done. This gives a more upright,and for a lot of riders, a more forgiving and more comfortable position on the bike. In the end, you are the one riding the bike and you should get one that fits you. It may not be the size someone else recommends for you. Test ride as much as you can.

DaveSSS 

Bike sizing has to take into account both the effective TT length and the head tube length, or better yet, the stack and reach. Going by the TT length alone could result in getting a bike with a head tube that is too tall or too short and would require a goofy stem/spacer setup.

Even the TT length can't be evaluated without considering the seat tube angle. The steeper the STA, the longer the effective TT length, with the saddle in the same position, relative to the BB. Each degree adds about 1cm of TT length.

WaveyGravey

So, in a nut shell, when a size is given for a bike, they are referring to the length of just the top tube.

BlazingPedals

If just one number is used, it's usually the seat tube length. Seat tubes are measured differently for different makers, sometimes C-C, sometimes C-T, sometimes to an imaginary top of the top tube in the case of sloping tubes. The other measurements are important, too; but it's usually up to the buyer to ferret them out.

Flip Flop Rider

center of the crank to top of the seat tube

srode1

Reach and Stack are the only numbers I look at to see if a frame will fit me.

Maelochs

Actually, when a frame size is listed as a number it is .... meaningless.

because everyone measures their frames a little differently, and because seat- and head-tube angles vary, and because so many measurements use "virtual" seat and top tube lengths .... one can never be sure that two frames each advertised as "56-cm" are going to fit well, let alone the same.

Figures are ball-park. And usually the "virtual" or "effective" seat- and top tubes are similar in length .... but the only real way to know if a frame might fit you involves two things: A.) reading the geometry chart, and B.) knowing your own proportions.

As a rule of thumb, it is safe to assume a "56" frame has approximately a 56-cm "virtual" or "effective" seat tube and approximately a 56-cm "virtual" or "effective" top tube ... it is likely the case within a centimeter. What that means according to the other dimensions of that specific frame are not inferrable from those numbers.

DaveSSS 

Seat tube length is the dimension that means little these days, when most bikes have sloping top tubes. The sloping top tube may take 4-5cm off the seat tube length. You can hardly go wrong with the head tube length, the virtual (horizontal) TT length and seat tube angle. Reach and stack are even better, if listed. When looking at head tube length, the type of headset used must be considered, to know how much length it adds to the head tube.

I've never seen a modern frame that didn't list the horizontal TT length.

DaveSSS 

That would be a no. Years ago, when all top tubes were horizontal, frame size was the length of the seat tube, measured from the center of the BB to either the center or top of the top tube, depending on the brand.