Setback on Stock Serotta Geometry

[dbrk]

Serotta's stock geometry chart has all sorts of good information but absents the one that I find particularly helpful: setback. This is the distance from the center of the seat tube to the point on the level top tube that intersects the perpendicular through the bottom bracket. If anyone has a 59 or 60cm stock bike that uses the current geometry I would very much appreciate the setback number. (I am particularly keen on comparing the set back of a stock 59cm Pegoretti with a 60cm Serotta.) Setback ---so defined--- is crucial to the way Europeans fit, particularly because of it's relationship to the knee/pedal position. Anyway, I'd love to know and someone out there must have a stock bike (not the Fierte)!

dbrk

[pjm]

My '99 stock CSI is a 60 w/ a 60c top tube. I believe Serotta has shortened the top tube on newer models. The setback on my frame is approximately
173 mm or 6-3/4 inches.

[Jeff N.]

Seems like just a variation of the seat tube angle. Yes, No? Jeff N.

[dirtdigger88]

17.54 on my Legend

Jason

[christian]

Seems like just a variation of the seat tube angle. Yes, No? Jeff N.

Yes. But some people find it easier to just measure it than to calculate based on the seat tube angle.

In order to calculate the setback, you need two variables:
1) The c-c seattube length (say, 58.5cm in DBRK's example)
2) The seat tube angle (say, 73 degrees)

For a 73 degree angle the sin is: 0.95630 and the cos is: .29237

In this case, the setback is the can be calculated using the cosine to be about 17.1cm.

The vertical height of the toptube (or a virtual toptube on a compact frame) can be calculated using the sin to be about 55.9cm.

- Christian

[christian]

Here's an Excel calculator that does this for all seat tube lengths and seat tube angles. It doesn't seem terribly useful to me, but it only took a minute.

Make sure you enter the center-to-center seat tube measurement, if you want setback to the centerline of the top tube.

- Christian

[Ozz]

a2 + b2 = c2

Doesn't this work for this problem? If you know length of seat tube and height to top tube, you can calculate the set back pretty easily.

[christian]

Sure, but Serotta doesn't publish the length of the vertical line extending from the center of the bottom bracket to the center of the horizontal top tube, either. So I'm not sure how DBRK could use that to calculate setback.

What they do publish is the angle and the length of the seattube. The Pythagorean theorem won't readily help there. You'll need to use sine and cosine. So Pythagoras might not have been able to solve this in 500 BCE. But it's likely that Ptolemy could have solved it right around 110 CE, though he wouldn't have use the words sine and cosine. I think the method I used would have been first used by Aryabhata the Elder sometime around 500 CE.

I like math,
- Christian

[weisan]

I like math,
- Christian

You're really weird...<end quote>
~ Willy Wonka says to one of the parents on a tour of his candy factory in the new movie, Charlie and the Chocolate Factory.

[dbrk]

I think the method I used would have been first used by Aryabhata the Elder sometime around 500 CE.

I like math,
- Christian

Nice going, Christian, only an Indologist or mathematician usually knows about Aryabhata (the Former, c.550 C.E.). He was also the first to notice the diurnal motion of the heavens as a feature of the earth's axial rotation. I have looked into these texts in Sanskrit and can tell you that if you think math is hard, well, math in Sanskrit is waaay hard (b/c rather ordinary words are used in technical ways and most notational ideas are expressed in words rather than symbols!) Aryabhata flourished at a time when Indian civilization was among the most literate and sophisticated on the planet (Rome having fallen a little more than a century before and the sub-continent's intellectual rivals, the Chinese, living in a relative isolation, albeit one that accomplishes more than virtually all others despite the absent of input from "outside" sources.)

As for set back, I think you nailed it. Surprisingly small number on the 60cm stock Serotta. Gratuitous bike content...

dbrk

[Climb01742]

i was at a history class when a cycling forum broke out. :rolleyes:

[Ozz]

Sure, but Serotta doesn't publish the length of the vertical line extending from the center of the bottom bracket to the center of the horizontal top tube, either. ...I like math,
- Christian
Yeah, I thought of that after I made my post...I guess it would work if you had access to the bike and could measure it...probably still not as accurate as your method.

I like math too....just wish I liked it more when I was in grade school! :rolleyes: