# setback measurement / top tube length



## are (Feb 5, 2005)

On Look's full geometry chart there is a measure of setback, labeled "E".

On a 55cm kg461, this measure is 15.6cm and the top tube is 55.5cm. On a 51cm, the setback is 13.6cm and the top tube is 52.5cm.

If I used a seatpost w/ 2cm setback on a 51cm frame, I'd maintain the same setback and the effective top tube length would become 54.5cm (52.5cm + 2cm), correct?

I ask because I have a 55cm frame which works for me w/ a -17 degree stem (perfectly horizontal) and 0.5cm spacers. If I downsize, it seems I could easily move to a 51cm instead of a 53cm frame; I'd just need a setback seat post, a couple of spacers, and a longer, more normal sloping stem (like a -8 degree stem).

Thanks


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## C-40 (Feb 4, 2004)

*no...*

You're a bit confused here. If you know the seat tube angle of the frame you have now, I can tell you the difference in the reach, which is the TT length minus the setback. To use the concept of reach, both frames must either be the same size or the reach must be calculated for the same size on both frames.

For example, a 51cm 461 has a 74.5 degree STA. Although the setback is 13.6cm, this is measured at the 51cm distance along the seat tube. It is 51 x cos74.5 = 13.6. At 55cm, it would be 14.7cm, not 13.6. The reach would be approximately 37.8cm. In comparison, the 55cm frame would have a reach of 55.5 - 15.6 = 39.9. This means you would need about a 20mm longer stem on the 51cm frame.

You should also carefully compare the head tube length, with the headset and spacers. The HTL on the 51cm LOOK is only 135mm with the headset, compared to 165mm on the 55cm frame. Either can have up to 30mm of spacer addeded to those lengths. "A few spacers" won't make up a 30mm difference in height, but more stem angle, like an 84 degree would make up about 20mm of the difference.

To figure the difference in the seat post setback you may need, you also need the STA of your current bike. The setback difference then becomes (cosA-cosB) times the saddle rail height. Roughly, it's 1.2cm (1/2 inch) per degree.


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## are (Feb 5, 2005)

*perfect!*

Thank you very much - that's very helpful. I'll measure to my saddle rail (from the center of the BB, right?), but I think the setback will be reasonable.

re: the stem/spacers ... that's actually why I'd like a slightly smaller frame. On my 55cm kg461, I have 0.5cm of spacer with a 73 degree, 100mm stem. I'd really prefer a slightly longer stem; I'd consider 120mm to be just right. Accounting for the 1 degree change in headtube angle w/ the 51cm frame, with an 84 degree, 120mm stem I'd get a "run" of ~118mm and a "rise" of ~21mm. So, it ought to work w/ 1.5cm of spacer. Comparing: now = 151 (headtube) + 15 (headset) + 5 (spacer) + 0 (stem) = 171mm total, versus possible: 120 (headtube) + 15 (headset) + 15 (spacer) + 21 (stem) = 171mm total.

The stem/spacer calculation is what actually got me thinking that a 51cm frame might be better for me than a 53cm, even though I'm about 5'9" - with a 53cm I'd have to use an 80 degree stem and I'd still get a "rise" of ~17mm, which would put me higher than my current setup w/o any spacers at all.

Thanks again.


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