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Are frames sizes mis-labeled/lugged at times?

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Are frames sizes mis-labeled/lugged at times?

Old 11-03-17, 03:59 PM
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shuru421
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Are frames sizes mis-labeled/lugged at times?

Are the 58, 54, etc. numbers sometimes mis-labeled/lugged underneath the bb?

A 58 can be a 56 ex.?

I think I have a 58 lugged frame which actually is closer to a 56-57.
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Old 11-03-17, 04:02 PM
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Originally Posted by shuru421 View Post
Are the 58, 54, etc. numbers sometimes mis-labeled/lugged underneath the bb?

A 58 can be a 56 ex.?

I think I have a 58 lugged frame which actually is closer to a 56-57.
Depends on how it was measured. Some builders even did top of BB shell to top of seat tube.
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Old 11-03-17, 04:04 PM
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Same sizes really can vary. I use frame size for a general idea until I see the bike in person.
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Old 11-03-17, 04:06 PM
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Only in the real world. Gotta be a crap shoot way more often than anyone would like to admit, changing standards, systems and processes, simple mistakes, not so simple mistakes, Friday after a long lunch at the pub, Monday morning after a long weekend at the pub, etc, etc. Short answer would have to yes.


Originally Posted by shuru421 View Post
Are the 58, 54, etc. numbers sometimes mis-labeled/lugged underneath the bb?

A 58 can be a 56 ex.?

I think I have a 58 lugged frame which actually is closer to a 56-57.
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Old 11-03-17, 04:06 PM
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Top tube length can be even more important than "frame size" in determine whether a bike fits you.
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Old 11-03-17, 05:04 PM
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Colnago, most notably, uses a center BB to top of top tube ( or maybe even top of seat lug) for frame sizing. A 58 Colnago will usually have about a 56.5 c-c top tube. I usually size my bikes by top tube length and use the manufacturer sizing as a ballpark guideline.
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Old 11-03-17, 05:20 PM
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The rules of thumb that I tend to use if no other information is available:
  • Italians and Belgians often measure center-to-center
  • Dutch and French mostly measure center-to-top
  • frames designed for Shimano Dyna Drive (notably early eighties Koga-Miyatas) are ca 1 cm shorter than stated (to compensate for the off-center pedals)
FWIW, YMMV and SGDG.
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Old 11-03-17, 08:00 PM
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The convention of frame sizing has changed over time.
Center to top was pretty standard, with some variation - to the top of what?
Top tangent of the top tube? ( buried in a lug and kind of the default), tip top of the seat lug tip? Masi did it that way.
Later on in the 80's the Italians seemed to migrate to center to centerline intersections of the tubes.
Top tube length was always measured center to center along the horizontal and often still is but a virtual measure now, supplanted by reach and stack.
With oversized top tubes, size became the decision of the mfg. Cannondale and Klein come to mind where you needed to know their convention if you were marginal to straddle the frame without contact.
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Old 11-04-17, 11:55 AM
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So top tube to top head tube is more crucial than center bb to top tube?
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Old 11-04-17, 12:11 PM
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Originally Posted by shuru421 View Post
So top tube to top head tube is more crucial than center bb to top tube?
Depends on the era. When level top tubes were the norm, it was reasonably straightforward. Knowing what basic position works goes a long way, if not being able to straddle the frame safely over the top tube is a basic starting point, most bikes are not outside a reasonable design ratio.

Recently I was asked to advise on the size of a bike to purchase for someone, it took a bit to divine the best size, including a quick lofting of the dimensions on a CAD system to come up with the best rec, once done it took three shops that sold that brand to get the correct stem length exchanged, two advised it was a huge deal! Come on guys, it is a threadless stem / headset system, easy peazy... Felt like I was trying to convince a auto service writer that $70 to install the cabin air filter was a bit extreme... On that car, you drop the glovebox door, slide out a tray, exchange the filter and close things up... No tools required! " well it's buried in under the dash..."
All the bike shops btw wanted big dollars to exchange tires, I advised I will assist later when they wear out.
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Old 11-06-17, 06:13 PM
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Originally Posted by non-fixie View Post
The rules of thumb that I tend to use if no other information is available:
  • Italians and Belgians often measure center-to-center
  • Dutch and French mostly measure center-to-top
  • frames designed for Shimano Dyna Drive (notably early eighties Koga-Miyatas) are ca 1 cm shorter than stated (to compensate for the off-center pedals)
FWIW, YMMV and SGDG.
Good points, I never looked at it that way.

These are bikes not Swiss watch movements. (Flaming those amongst us who fret over continuously inconsistent minutiae )

All frame dimensions are nominal and rounded to the nearest convenient advertising figure. For example a 54cm frame is NOT 21 1/2" - it's 21 .256". Same with a 58cm frame - 22.84" not 22 1/2".

The nominal frame sizes were published by the manufacturer or importer as aids for selling bikes.

Gitane and many other French manufactures listed sizes in the US as 50cm, 54cm, 57cm, 60cm, 62cm and 64cm. Some brands listed their frame sizes as 21" 23" & 24" or with 1/2" increments - 21 1/2" and so on.

Personally I've always measured seat tubes center to top except on frames with sloping top tubes.

In cases where the seat lug is cut square with the end of the tube not scallop cut on the sides, I eyeball estimate where the top of the tube would intersect.

The top of the top tube is where stand over height is measured from - not figuring out inch or metric tube diameter, dividing by 2 and adding that number to the equation. Center to center is like counting cows by their teats and dividing by 4!

BITD, frame sizes were determined by how many pints of ale or glasses of wine the workers had at lunch!

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Last edited by verktyg; 11-06-17 at 06:17 PM.
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Old 11-06-17, 07:55 PM
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Originally Posted by exmechanic89 View Post
Same sizes really can vary. I use frame size for a general idea until I see the bike in person.
...this is how I do it, too.
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Old 11-06-17, 11:58 PM
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Originally Posted by shuru421 View Post
So top tube to top head tube is more crucial than center bb to top tube?
Depends upon the range of stem lengths you are willing to tolerate ?!?
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Old 11-07-17, 12:19 AM
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Originally Posted by shuru421 View Post
..... A 58 can be a 56 ex.?

I think I have a 58 lugged frame which actually is closer to a 56-57.
Yep. That is normal. Some measure top tube for size... others the seat tube... (or the distance of the BB to the effective top tube).
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