Framebuilders - Please help with frame geometry
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Please look past the crude sketch:
I am designing a frame that is somewhere in between road & track geometry. Theses numbers seem pretty good to me, I'm just curious if anyone has an thoughts. I mainly do short rides (10-20mi) with the occasional century.
Thanks in advance.
Why would you choose track geometry for that range of ride lengths. I don't know track bikes, but I wouldn't personally choose a 75 degree seat tube for the kind of riding you are proposing. Probably not too far off what a tri bike would be (another style of riding I don't know) So it may work ok for a given posture.
I design seat tube angles from my personal preference combined with the specifics of the seat post, rear wheel, and seat needs I have.
I design the front angles from the handling and fork type I will be using. The top tube and downtube relate to my torso size, and features of the bike type. The rear stays are usually long on my bike but different uses will dictate. The design just fills itself in. Ideally you should have on hand as many of the parts as possible, so that specific numbers you assign relate to the real world as much as possible. It is alluring to only spend money on the parts when the frame has been built, but unless you have done the same bike frame before, it makes more sense to actually have the parts on hand.
11-19-09, 05:20 PM
Whoa - 75 is a rather extreme. On the frame geometry project table, only 9 of the 350 or so frames were over 74.5 deg. 72 - 73 would put you much closer to the median.
11-19-09, 06:01 PM
Swap the angles and you'll have a nice geo for a road going fixed gear, albiet one with a bit of toe overlap.
To me, that geometry says you are 5'7" to 5'8" with legs that are shorter than average for your height. If thats not the case, Check out strawberry's site and use the custom measure dealie and the process to measure yourself, draft a frame from these measurements and get back to the forum with what you find. From there you can make adjustements to the geometry to 'quicken' the bike if you like but for 20-30 milers with the occasional century the sketch as it sits my be a little........rigid.
11-23-09, 05:37 PM
Curious. Your numbers are almost a match to a SystemSix 52 cm. A tweak here and there, but literally millimeters off. I know because I've been studying my frame and that's what I have.
I'm 5'7" with short legs, and I'm contemplating a 75.5 degree seat tube frame. 56 TT. 40 chainstay. 10 headtube.
Right now my 52 has the post low enough that I can't fit it in a normal Park clamp. 13 cm stem, 73 deg (-17). No spacers at all, just the short bearing race up top. And although mythical, I can see my whole hub in front of my bars. My arms hang almost vertically down in the drops.
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