How comparable are bike sizes

[sfscott]

Of the few bikes that I have owned, only a couple were standard sized and not custom. All of the off the peg bikes were recommended by the shop, and were "fit" with trial and errors on stems, seats, post, bars, etc.

I'm looking for a used frame for a project, and since I can't test bikes on the 'Bay or classifieds, how comparable are measurements from one brand to another?

For example, I have a C59 in a 50s size, which is a seat tube of 50cm center to top, with an effective top tube of 54cm but not sure, without consulting old geometry charts, how comparable one frame would be to another?

In the example above, I'd probably get a 50H Colnago to facilitate a more normal angled stem than what I'm using..and maybe size down as well. In my customs, I've always had head tube extensions and want to make sure something I buy used will fit without an extreme stem angle. On the C59, I have a +12 degree, which, aesthetics aside, is hard to find. Ended up there after being fit after buying the C59.

[colker]

Originally Posted by sfscott

Of the few bikes that I have owned, only a couple were standard sized and not custom. All of the off the peg bikes were recommended by the shop, and were "fit" with trial and errors on stems, seats, post, bars, etc.

I'm looking for a used frame for a project, and since I can't test bikes on the 'Bay or classifieds, how comparable are measurements from one brand to another?

For example, I have a C59 in a 50s size, which is a seat tube of 50cm center to top, with an effective top tube of 54cm but not sure, without consulting old geometry charts, how comparable one frame would be to another?

In the example above, I'd probably get a 50H Colnago to facilitate a more normal angled stem than what I'm using..and maybe size down as well. In my customs, I've always had head tube extensions and want to make sure something I buy used will fit without an extreme stem angle. On the C59, I have a +12 degree, which, aesthetics aside, is hard to find. Ended up there after being fit after buying the C59.

Old school sizing was about leg length dictating seat tube size. Since you had lugs and a level top tube, it was easy to assume the head tube size which in turn dictates the height of your hbar. You can always have a longer or shorter stem but there is a limit in stem height so that weight distribution works fine.
one of the main numbers to watch is head tube length. To be precise, front center length and seat tube set back are also important.

Don´t size down just because it looks cool.

There are lots of variables when you are buying an expensive frame and want it to fit precisely.

[R3awak3n]

forget sizes, go by stack and reach. If stack and reach is the same or comparable to your current bikes, you will be ok.

[Kontact]

Stack and Reach are only useful if you understand that Reach is only applicable to bikes on nearly identical stack.


Most bikes that come in a virtual size (a number that doesn't necessarily correspond to a point on the seat tube) are going to be somewhat similar in TT measure, enough so that someone with four 56 frames is probably somewhat safe buying another.

That said, Colnago's Sloping size frames, like 50S, don't correspond to any other sizing system. And as Colker alludes, two frames of the same size with similar TT reach may have fairly different head tube heights (Stack), because they might be intended for different kinds of riders.

There is enough online geometry information that you shouldn't have to guess about how a bike is sized. For older bikes you can look for catalog scans. There is a little art to reading and comparing charts, like how to rectify TT lengths with different seat tube angles, but the information is all out there so you don't have to guess.

[cmg]

i'd recommend consulting old geometry charts and calculating stack and reach for projected frames. do you have a frame you'd like to match? if your fav needs a shorter or longer top tube calculate S&R and use that as a guide. there is no general rule for comparing makers. do your research.

[wallymann]

Originally Posted by sfscott

...without consulting old geometry charts...

impossible to compare without comparing.

Originally Posted by cmg

there is no general rule for comparing makers. do your research.

truth.

[simonov]

Originally Posted by Kontact

Stack and Reach are only useful if you understand that Reach is only applicable to bikes on nearly identical stack.


Most bikes that come in a virtual size (a number that doesn't necessarily correspond to a point on the seat tube) are going to be somewhat similar in TT measure, enough so that someone with four 56 frames is probably somewhat safe buying another.

That said, Colnago's Sloping size frames, like 50S, don't correspond to any other sizing system. And as Colker alludes, two frames of the same size with similar TT reach may have fairly different head tube heights (Stack), because they might be intended for different kinds of riders.

There is enough online geometry information that you shouldn't have to guess about how a bike is sized. For older bikes you can look for catalog scans. There is a little art to reading and comparing charts, like how to rectify TT lengths with different seat tube angles, but the information is all out there so you don't have to guess.

Yep. If you want to be confident in how two bikes will compare, learn how the angles and lengths relate and don't rely on nominal sizing. Stack and reach are good starting points, but without understanding the other measurements, they can still lead you astray.

[oldpotatoe]

Originally Posted by sfscott

Of the few bikes that I have owned, only a couple were standard sized and not custom. All of the off the peg bikes were recommended by the shop, and were "fit" with trial and errors on stems, seats, post, bars, etc.

I'm looking for a used frame for a project, and since I can't test bikes on the 'Bay or classifieds, how comparable are measurements from one brand to another?

For example, I have a C59 in a 50s size, which is a seat tube of 50cm center to top, with an effective top tube of 54cm but not sure, without consulting old geometry charts, how comparable one frame would be to another?

In the example above, I'd probably get a 50H Colnago to facilitate a more normal angled stem than what I'm using..and maybe size down as well. In my customs, I've always had head tube extensions and want to make sure something I buy used will fit without an extreme stem angle. On the C59, I have a +12 degree, which, aesthetics aside, is hard to find. Ended up there after being fit after buying the C59.

Not at all, unless you are lucky. Have a fit guy there with a fit cycle? Do a fit, get the seat tube angle, top tube length, stem length, etc., that 'fit's you and go from there to the frames you are looking at. If the fit person is decent, take the GEO to him to see if it will 'fit'.

[paredown]

If this helps, here's a pretty recent Colnago geometry chart--I think around the C50 era--others may know if there were much in the way of changes...

Nominal size as advertised by manufacturer or seller often has very little basis in reality, and certainly does not work well for cross brand comparison.

My personal approach--I work within a range of top tube length--I know I like about a 57 square in a standard (Italianesque racing) frame, but will size down a little. In Colnago (after re-selling two frames that were not as advertised) I settled on the Colnago '59' in a standard frame (56.9 TT) or 54S. I have never really assimilated stack and reach into my understanding of fit, so I tend to assume that anything around that TT length will be usable (this kind of ignores the effect of seat tube angles of course). And because I started out on relatively 'tall' frames (short TT in relation to ST)--I am even willing to go larger than a modern fitter would likely put me on, because I find standover unimportant (within reason).

Then the fun part starts--what you've described as the bar/stem trials/seatpost height / position fore and aft etc. I will generally mock up by measuring a bike that is comfortable for me, set saddle height (rough drop measured with a level) and position, reach, and bar location with a tape, and then fine tune a little after riding. Often the hardest part since we lost the convenience of quill stems is getting the right rise & length on the stem.

But I personally don't see a way of making the occasional mistake--IME, most eBay sellers can't measure properly, and manufacturers are not consistent with their published figures since there are often running changes and/or slippage in manufacturing tolerances.

I just look on the re-selling of mistakes part of the cost of doing business!

[cmg]

Here's a stack and reach calculator, http://www.bikegeo.net/ . Plug in the known dimensions and compare, yea it's a pain but so is getting the wrong frame.

[Kontact]

Originally Posted by cmg

Here's a stack and reach calculator, http://www.bikegeo.net/ . Plug in the known dimensions and compare, yea it's a pain but so is getting the wrong frame.

The problem with this comparison is that some head tubes include the headset, and external headset tubes do not, which causes them to be off by several centimeters. They don't even start at the same point above the fork crown.

[Kirk007]

Originally Posted by Kontact

The problem with this comparison is that some head tubes include the headset, and external headset tubes do not, which causes them to be off by several centimeters. They don't even start at the same point above the fork crown.

yeah this is a pain but you can use 32 mm for an external King and most headset manufacturers, like King show stack for lower and uppers for their headset. A little guess work for sure but I feel like I've been able to get within less than a centimeter, and then your within the range of variability that difference in spacers and stem angle can adjust for.

Definitely not perfect.

[wallymann]

Originally Posted by Kontact

The problem with this comparison is that some head tubes include the headset, and external headset tubes do not, which causes them to be off by several centimeters. They don't even start at the same point above the fork crown.

this is why i stick with STA and TT...it's pretty easy to make mental (effective) TT adjustments based on +/- STA. e.g., steeper STA --> longer (e)TT, slacker STA --> shorter (e)TT. etc.