Geometry question

[jpw]

I've had my Serotta CX frame built and built up. It's great.

I was looking at my frames spec sheet. Is there a quick way to accurately calculate the angle of the seat stay tubes off the horizontal plane?

[echelon_john]

digital angle finder. tell us why? i'm curious what you're trying to mount to the thing! :beer:

[jpw]

digital angle finder. tell us why? i'm curious what you're trying to mount to the thing! :beer:

I'm designing my custom rack. I want the twin rear most vertical supporting struts of the rack to be the exact oppositional angle to the seat stay angle when viewed side on, creating a symmetrical 'v' shape. Hence I need the seat stay angle.

[palincss]

Usually the way this is done is (for a front rack) you send the fork to the rack builder, and leave the design details to them.

[jpw]

Usually the way this is done is (for a front rack) you send the fork to the rack builder, and leave the design details to them.

Well, I'm not usual :-) (ask my Serotta dealer!), and where's the suffering in that? :beer:

[Mike748]

Draw the rear triangle out on paper and make right triangles out of it. Use the BB drop and chainstay length to locate the rear dropout in x and y with the BB as the origin. Then use the STA and ST length to locate the top of the seattubes and seatstays in x and y. Then from those coordinates you can determine the angle of the seatstay.

My Serotta build sheet in excel format shows the x and y position of each of the joints... also shows seatstay angle. If you want I can email you a copy and you can plug in your values.

[echelon_john]

assuming the bike's built and has wheels on it:

- digital angle finder direct on seatstay with bike standing straight will get you close
- adjustable protractors are cheap (at least the cheap ones) and could tell you the same thing, esp. if you had something vertical (like a doorframe) to reference against

another fun exercise could be taking a picture of your bike and 'tracing' it into bikecad or similar. if you know your seat tube angle (from serotta) and use that as a reference you should be able to figure out the seatstay angle.

final suggestion, off the top of my head: take a thin stainless steel ruler or straightedge. tuck it between the quick release and the dropout, getting it visually close to the right position. then measure your triangle (the straightedge, the seatstay (to the point where they'd virtually intersect) and the distance between the two (level). this would also get you close, and no stinkin' angle finder or protractor necessary!

good luck!
jc