Seat tube angle vs offset...

[oldguy00]

If two frames both have a 73 degree seat angle, shouldn't their 'offset' be the same?? If not, is it because some frame manufacturers aren't measuring the offset based on a virtually level top tube, instead going by the c-c seat tube measurement of a compact geometry to measure the offset? eRitchie??

I'm looking at the time frames, and they seem to have an offset of about 15.5cm in a size small, which seems like a cm more than most other frames in that size range...

[BdaGhisallo]

All things being equal, which they rarely are of course, the setback for frames with the same sta should be identical at a given vertical height from the bottom bracket. It's simple trigonometry.

At a given height from the bb along the seattube, you simply multiply that height by the cosine of the sta. So at 55cm, the setback is 55(cos73) for a 73 deg sta.

The issue is exactly how far up the seattube from the bb, the manufacturer is using as their height in that calculation.

[dirtdigger88]

by looking at spec sheets off my old legend and my dks- I believe serotta measures set back at the ACTUAL ST measurment- not the virtual one-

my dks is listed as having a 15.2 set back- but that because is a 53cm seat tube-

jason

[Dave]

As already noted, setback will be the same for a given angle, if it's measured at the same distance along the seat tube from the ceter of the BB (not vertical). Then the setback is traditionally the c-c frame size times the cosine of the STA.

The mistake most people make is comparing frames that are a different size and expecting the setback to be the same, with the same STA. They will not be the same.

It would be highly unusual not to use the c-c distance for a horizontal TT, when figuring the setback, even if the frame has a sloping TT.

[Ray]

As already noted, setback will be the same for a given angle, if it's measured at the same distance along the seat tube from the ceter of the BB (not vertical). Then the setback is traditionally the c-c frame size times the cosine of the STA.

The mistake most people make is comparing frames that are a different size and expecting the setback to be the same, with the same STA. They will not be the same.

This is one area where I don't get the insistence on the part of some that setback is a more important measurement than STA. A given STA gives you the same effective/functional setback regardless of whether you've got a sloping top tube or a level one, whether you're measuring c-c or c-t - it tells me all I need to know. For example, my long experience with Brooks saddles and not quite as long experience with Fizik Aliantes tells me that I need a 72 degree STA to get my Brooks where I need it with commonly available setback seatposts. With the Aliante, I'm fine at 73 degrees and would probably be ok with 73.5. When I was getting a custom built that I planned to use with a Brooks, I let the designer know that I needed a 72 degree STA to make the frame work with the saddle I was going to be using - that was one of the key beginning parameters. He made all of the decisions about where the wheels were gonna be under me and how long a tt, stem, what kind of fork rake, etc were needed to give me the kind of ride I was asking for.

It just seems that STA is an easier number to work with than setback. Am I missing something here?

-Ray

[Dave]

This is one area where I don't get the insistence on the part of some that setback is a more important measurement than STA. A given STA gives you the same effective/functional setback regardless of whether you've got a sloping top tube or a level one, whether you're measuring c-c or c-t - it tells me all I need to know. For example, my long experience with Brooks saddles and not quite as long experience with Fizik Aliantes tells me that I need a 72 degree STA to get my Brooks where I need it with commonly available setback seatposts. With the Aliante, I'm fine at 73 degrees and would probably be ok with 73.5. When I was getting a custom built that I planned to use with a Brooks, I let the designer know that I needed a 72 degree STA to make the frame work with the saddle I was going to be using - that was one of the key beginning parameters. He made all of the decisions about where the wheels were gonna be under me and how long a tt, stem, what kind of fork rake, etc were needed to give me the kind of ride I was asking for.

It just seems that STA is an easier number to work with than setback. Am I missing something here?

-Ray

While you're entirely right that knowing the STA will automatically tell you if the frame will work with a particular seat post and saddle that you have been using, knowing the setback can also be of value. The frame reach is calculated as the TT length minus the setback. The difference in reach will tell you the difference in the stem length required between two frames. Once again, the comparison must be made between frames of the same c-c size. Otherwise you'll get the wrong idea about the fit.

Another method that's good enough for most of us is to just add 8-10mm per degree to the TT length of the frame with the steeper STA and then compare TT lengths to get the stem length difference. It's accurate enough even if the frames are up to 2cm different in size.