Hello,

Writing this on behalf of my wife. The cbb to top of seat tube dimension doesn't seem to be published anywhere. Can someone with a 48 provide me with that measurement?

We are trying to determine frame size for a S1. She is 5'5", rides with 67 cm cbb to top of saddle and is comfortable with the 51.5 tt (100mm stem) on her Specialized. Her handlebar to saddle differential is good with 3 to 4 cm drop. We are thinking S1 for part-time tt-ing, so a deeper drop would be necessary for that. It would be helpful to hear from someone of the same height to find out how you're doing with your 48 or regrets for not going 51cm. For those on the 51, do you feel hands down, a rider 5'5" is too small to be on the 51? I'm thinking 51 is too long with the 53 tt and the 74-ish standover. Thanks

I am 5' 6" and ride a 51 R3SL. My cbb to top of saddle is 70.5 cm and it fits me perfect with a 100cm stem. Your right the question is top tube.  She seems to be right on the edge between the two. Of coarse the best would be to test ride each size and have her get a fit done at a good shop.

There is a long thread on this Forum that shows pictures of Cervelos. Buried in that thread is a lovely picture of the smallest sized S-type Cervelo posted by MessyParrot. The bicycle is distinctive and so is the name of the poster. Perhaps you can email MessyParrot directly and get some feedback on their determination of size. You could post what you learn here.

I am a stocky 5'5'' (more like a soccer player than the lean cyclist look) and my interest is the size 51 RS. My cbb to top of saddle is almost 70 cm. I have measured the stack and reach of my current bike and conclude that for me the size 51 is the more likely fit.

As your wife is interested in a tri-fit, you can not make comparison with those who are looking/using a road fit, which is why MessyParrot may have better insight as the bike was used for triathlons.

About 10 years ago, I used a small sized Cannondale Multisport tri-type bike whose seat tube was 78 degrees. Its top tube was 51.5 cm. I found the top tube to be a little too long for comfortable use with aero bars; and, for comfortable road riding the top tube was a little too short when the bike was equipped with drop bars. Possibly, I would have found the size 48 Cervelo to be more comfortable for a tri fit given the adjustability of the seat angle and the consequent shortening of the top tube. Initially, my Cannondale Multisport was equipped with drop bars but I subsequently found bull horn bars to be preferable. The STI controls were put on the bull horn bars and the aero bars had no brake or shifters. Today, my son uses the Cannondale for long distance triathlons. He is about an inch or so taller than I am.

Your wife needs a 48cm bike. I'm 5'6 and my friend who is 5'7 both ride 48.  Her legs are 4cm longer so she has her saddle up higher and runs more spacers under a 9cm stem. I have short legs and a long torso, so I have a less spacers under a longer stem on the bike.  I have a cbb to top of saddle distance of 66cm, much like your wife. The numbers you put for her Specialized are almost the same as a Soloist 48cm (both come with a 51.5cm TT). Keep in mind, if she wants to get the front end lower, the 48cm with a longer stem will give you a lower front than a 51cm with a shorter stem due to the shorter head tube. You'll just have more seatpost showing with the 48cm.

 Here is a list of the CSC riders and their bike sizes. Pedersen is the smallest at 5'7 and rides a 48cm. Sastre is the next smallest at 5'8 and is the only rider on a 51cm. 

http://forums.cervelo.com/forums/p/1852/10431.aspx

 Pro's tend to get a smaller bike with a longer stem to get the bigger difference in saddle to handlebar height they use. Many recreational riders tend to get one size bigger to bring the bars up a bit.  Keep in mind, a smaller bike should be slightly stiffer and lighter.

 Cervelo has the right idea with reach based geometry. The inseam length is not really that important as you can adjust for this by moving the seatpost up and down (like my friend and I). The headtube is a static measurement and you can only adjust your stem by a few centimetres before it starts to affect the handling. It's a little counter-intuitive. For instance, a 51cm would fit me better than it would my friend, even though she is taller than me and has longer legs.  That's because my reach is longer. I would just run the seat down a bit on a 51cm and get a 9-10cm stem (but then I wouldn't be able to get the drop I want - hence the 48cm!).  If she got on a 51cm, the reach would be too long for her no matter what she did and she would never get a comfortable fit.

Hope that makes sense!

Gracie, thanks for the link...very interesting.

One thing I definitely know about bike sizing....IGNORE what pro riders do!  Talk to a fit professional if you are not sure. 

I think noting what size the pros ride is very applicable.  After all, they are the best bike handlers and most demanding in terms of performance.  Now if you're not flexible enough to comfortably adopt their position on the bike, fair enough . . .

As for "professional" fitters, most aren't familiar with the special needs of shorter riders.  for that matter, most bike manufacturers aren't particularly good at addressing our needs.  Notice how other manufacturers rake out the front ends with 71 degree head tube angles and steepen up the seat tubes in order to avoid toe overlap?  The result is a bike that under steers.  Also, I find these kind of bikes act nervous due to not having enough weight distributed over the front wheel. 

 I'm 5'7" and prefer the 48 with a 110 stem versus a 51 with a 100 stem.  The 48 rails turns and feels very planted on fast downhill sweepers.  

 The best option is to demo both, but I would recommend a 48.  

Agreed! Cervelo's handle better in their small sizes because they keep the seat tube angle constant and accept the toe overlap by not slackening the head tube angle.  I too like a longer stem on a smaller bike, the handling is less twitchy.

 Grazie mille to all. The question was answered without doubt. 48 it is, and I concur with confidence.