Need Help with Sloping Geometry
 

Hi all,

I ride a CAAD9 in size 54. I need a 54-53 top tube for sizing. When looking at several other bikes with a sloping TT, to get a 54 or 53 TT, I need a size small. Pinerello has a size 51.5 with a TT effective length of 54. Am I reading this correctly? In order to get a 54 TT, I need a size 51.5? Same for Ridley, Specialized, and other brands with the sloping TT.

thanks for the input.

RD

You need a 54.5 ETT to get the same top tube length as your CAAD9. In some brands that is a medium or small/medium, in Ridley it is a small.

thanks, yeah, the Noah was one bike I was looking at, and it said size small. I just wondered if that was correct.

Short answer? yes. That's how it works.

That makes sense.

I looked up two bikes - Pinarello Paris and the CAAD10. The Paris, 51.5, has a 53.5 ETT, 13 cm head tube. The CAAD10, 54, has a 54.4 ETT and a 14 cm head tube. You need to take into account the head tube so your stem doesn't end up too high or too low.

I'm not sure what your goal is with fit - is the CAAD9 a bit off or are you trying to replicate that fit?

With head tube heights I consider any integrated head tube length about equal in terms of functional length to other integrated head tube lengths. This is based on the fact that most of them require a few mm minimum of spacers/caps on top and most of them hold the fork in about the same place. In other words a 14 cm integrated head tube is about the same size as a 14 cm integrated head tube across a wide range of manufacturers.

For me when I was looking to replicate a position (actually I wanted the longest possible top tube) I rode, depending on the year, a size S Giant TCR or a 52 cm SystemSix (aka similar in geometry to a CAAD9). Both had 53.5 cm ETT, both had similar head tube lengths (the Cannondale was shorter I think), but the TCR had a much shorter seat tube height (40 cm to the top of the top tube on the aluminum frame, 44 cm to the top of the top tube on the carbon one).

You should read up about Reach and Stack, which is how you can compare the sizes between bikes.

Just be sure to distinguish between nominal size and actual seat tube length. Nominal size may be actual seat tube length, effective seat tube length, or nothing real at all, just a number out of the designer's head. Same for top tube. Example: a Giant M (medium) has a listed seat tube length of about 50 cm (I forget exactly, but that is close), but that size Giant is essentially a perfect 54 cm bike in old time horizontal top tube terms. If you look on Giant's geometry chart, and see 50 cm, man that can be confusing. Make sure you know what you are looking at. The best approach is to speak only in terms of effective tube lengths, i.e. think as if all bikes had horizontal top tubes. Just establish what the manufacturer means by its nominal bike sizes, then establish which of its measurements are the effective tube lengths. I hope this is more help than hindrance. This stuff does get to be confusing.