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Discussion Starter #1
I was checking a geometry chart, and it seems that the 56cm C50 has a 55.0 top tube. The 57cm C50 has a 55.6. But, the STA is .5 less on the 57cm, so the effective top tube is .5 to .6cm shorter. Therefore, the top tube effectively on each size will be identical.

Why is this so? I was thinking that perhaps the 56cm is designed for a slighly shorter rider, while they expect the rider on the 57cm to be slightly taller or longer, and therefore strech out with a 1cm longer stem (maybe 120mm on the 57cm vs.110mm on the 56cm).

Otherwise, why build two frames that are the same size, except that one has a raised top tube of 1cm and an 8mm longer head tube? With a head tube length of 15.0 on the 56cm (plus another 24mm gained with a headset) it will have a longer head tube than most bikes of its size (for comparison, a Dogma in the equivalent size (54cm) has a 163mm head tube, including the conical spacer, Fondriest Carbon Lex is something like 162mm). The only advantage I can see here is that the 57cm would fit the long legs/short torso rider a bit better. Perhaps the handling changes somewhat between these two sizes?
 

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STA is measured from the back

I'm looking at the C50 frame geometry chart at Competitive Cyclist, so assuming the accuracy of those numbers and diagrams ...

Maybe I misunderstand the post, but the STA is measured from the back, that is, the angle up from the rear horizontal (i.e., by the chain stay) to the seat tube. The 57 cm has a lower STA of 73 deg (vs 73.5 for the 56 cm frame), and is therefore "flatter" or "lower down" to the horizontal than the ST of the 56 cm, thus lengthening the span of the TT and the set back.

In any case, the lengths of the top tube and set back are what they are, so to speak, as shown in those charts. Note that the set back length is also proportionately longer.
 

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Discussion Starter #3
Hillen said:
I'm looking at the C50 frame geometry chart at Competitive Cyclist, so assuming the accuracy of those numbers and diagrams ...

Maybe I misunderstand the post, but the STA is measured from the back, that is, the angle up from the rear horizontal (i.e., by the chain stay) to the seat tube. The 57 cm has a lower STA of 73 deg (vs 73.5 for the 56 cm frame), and is therefore "flatter" or "lower down" to the horizontal than the ST of the 56 cm, thus lengthening the span of the TT and the set back.

In any case, the lengths of the top tube and set back are what they are, so to speak, as shown in those charts. Note that the set back length is also proportionately longer.
If you assume a rider will put his saddle in the same spot with relation to BB no matter what frame he rides (a pretty valid assumption: a frame shouldn't change how far behind the BB a rider is positioned), the slacker STA on the 57cm will mean that a rider would have his seat further forward than the on 56cm to achieve the same set-back with relation to BB. In this case, the difference would be about 5mm. This would make the top tube lengths of each bike identical, no?
 

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Drawing

I made a simple CAD drawing to illustrate.
Blue is for 56-cm size measurements.
Red is for 57-cm size measurements.
Seat tube is drawn c-c.
Top tube is drawn c-c.
Dotted line is BB vertical.

So the author is correct, the end of both top tube have the same distance from the bottom bracket vertical.

The question is: Does the longer saddle set-back (15.3 vs. 16.1 = 0.8 cm) is enough to compensate for the longer torso for someone who rides a 57-cm size frame? The difference in a person's height for these two frames is probably less than an inch. Thus the portion of torso difference is even less. Looking at 1-cm difference in seat tube size, the 0.8cm longer set-back might be sufficient.
 

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Dawg - I had both

I had a 57 C-50 earlier this year. My past Colnagos were all 56's. The LBS talked me into the 57 because i wasn't getting any younger and the longer headtube would help as my flexibility decreased. There was a 60 day guarantee on the fit. The 57 rode well but I ended up swapping it for a 56. Part of the reason was aesthetic and the other was a perceived difference in handling. It was probably all in my head but I pulled the trigger on the swap and am happy I did.

Here are picks of both, similary equiped. The first is a 57 and the 2nd is the 56.
 

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gorgeous 2nd pic, the classic post stem and rise proportions all show a neutral cassic positioning compared to the 1st pic which shows somewhat of a short post and stem slammed to the lower limit.

my two cents, it is not all in your head, and fit should be transferred from a known value,
to avoid the (put kindly) unknown variable of the bike shop fit.

disclaimer:
my first fitting was done by a director of wheelsmith palo alto, so please do not think i slam all shops, i was on too large then too small a bike left to my own devices. that was a while ago though, over the years i have actually dialed the fit (gradualy, slightly) as a result of being more, not less, flexible...

oneslowmofo said:
I had a 57 C-50 earlier this year. My past Colnagos were all 56's. The LBS talked me into the 57 because i wasn't getting any younger and the longer headtube would help as my flexibility decreased. There was a 60 day guarantee on the fit. The 57 rode well but I ended up swapping it for a 56. Part of the reason was aesthetic and the other was a perceived difference in handling. It was probably all in my head but I pulled the trigger on the swap and am happy I did.

Here are picks of both, similary equiped. The first is a 57 and the 2nd is the 56.
 

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Discussion Starter #7
oneslowmofo said:
I had a 57 C-50 earlier this year. My past Colnagos were all 56's. The LBS talked me into the 57 because i wasn't getting any younger and the longer headtube would help as my flexibility decreased. There was a 60 day guarantee on the fit. The 57 rode well but I ended up swapping it for a 56. Part of the reason was aesthetic and the other was a perceived difference in handling. It was probably all in my head but I pulled the trigger on the swap and am happy I did.

Here are picks of both, similary equiped. The first is a 57 and the 2nd is the 56.
I am curious: how tall are you, and what is your cycling inseam? Just trying to get a feel for where you are compared to me. I have only owned a 56cm Colnago (CT1, Dream Plus) but Mike at Maestro, based on my 85.5cm pelvic-bone to floor inseam (I am 5 foot 9, 175cm) thought I should ride a 57cm.
 

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Dawg

I'm 5'11". I don't remember my inseem but recall that it's around 33 inches.

At 5'9" I would think that the 57 would be far too large for you. There's always a lot of discussion on colnago fitting. I'm a believer in the longer stem theory. I've had a number of other bikes that have more traditional geometry (slight variation of the 73.5/73.5 HT and St angles). Each of those were 56 cm frames and I usually ran a 110 stem. On the C50, I run a 120 or 130 (depends on my flexibility) and the bike handles great. Any bigger and the bike feels a little sluggish. I think the slacker HT angle and shorter top tube really are the key too it all. I don't claim to be an expert fitter but I've done a lot of trial and error.
 

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Discussion Starter #9
oneslowmofo said:
I'm 5'11". I don't remember my inseem but recall that it's around 33 inches.

At 5'9" I would think that the 57 would be far too large for you. There's always a lot of discussion on colnago fitting. I'm a believer in the longer stem theory. I've had a number of other bikes that have more traditional geometry (slight variation of the 73.5/73.5 HT and St angles). Each of those were 56 cm frames and I usually ran a 110 stem. On the C50, I run a 120 or 130 (depends on my flexibility) and the bike handles great. Any bigger and the bike feels a little sluggish. I think the slacker HT angle and shorter top tube really are the key too it all. I don't claim to be an expert fitter but I've done a lot of trial and error.
My inseam converts to 32.9 inches, so it is on the long side for somebody my size. Being shorter than you, I would be better off with a 55 and slightly longer (120) stem as a race bike I believe. Actually, most of the bikes I have ridden recently seem to handle very well with a longer 120 stem: more precision in the front end for sure. I can't always ride a 120 stem early in the season: I usually start with a 110 for a 54.5cm TT early in the season, until I stretch out (I am also running more drop than you: 10.5cm at the moment, due to my long legs) The Colnago, with the slack HT, would probably be more stable in this configuration than most, which could only be a good thing (the bikes I am comparing it to are otherwise the same, save for the steeper 72.5 HTA).

Thanks for the info in the thread everyone. It seems to confirm what I was thinking with regards to sizing.
 

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Interesting because when SlowMoFo was asking for advise on this board as to whether he should swap for the 56, most people said stay with the 57.
 

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Elviento

elviento said:
Interesting because when SlowMoFo was asking for advise on this board as to whether he should swap for the 56, most people said stay with the 57.
You're right. Most people thought the 57 looked fine. I even prefered the PR00 color scheme of the 57 over the PR02. But, at the end of the day, it did come down to handling first and looks second. Having had two 56's in the past made the biggest difference. Just a little quicker when I needed it to be.
 
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