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# geometry formulas for first steel bike frame projects

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# geometry formulas for first steel bike frame projects

05-16-13, 01:42 AM
#1
Alfred31
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geometry formulas for first steel bike frame projects

Hey folks,

first of all I want to apologize if a similar thread already exists but the search just gives me error messages so I decided to open a new thread.

I'm at the very begining of my first selfmade steel frame for a road bike and I need some help about the geometry.
I'm using rattleCAD to design the frame measurments but I get still a bit confused about how to calculate things like saddle distance, handlebar distance and so on. So I was wondering if you guys would have some handy formulas for me to calculate the most important sizes, like tube lengths, angles and so on.

the design I've in mind will be for agressive fixie road bike with negative top tube angle (TT intersects higher with ST than with HT) and very narrow distances between the tires and the tubes. I wanna make it very "condensed" looking but also effective to right (as far as that's possible)

To give some data. my body measurments are:
body height: 168 cm
torso (shoulders to ground): 140cm
shoulder weight: 41cm
inseam: 74cm
arm length: 58cm

what I basicly looking for would be formulas to calculate the ST and HT length as well as saddle and handlebar distance (center of saddle/handlebar to center of HT) and some suggestions about Seat-angles. I was thinking about 74° here.

I hope you don't feel offended by my basic questions but I'd really appreciate your help here, because most of the information I found in the internet are either for riding adjusments or already very specific which are more confusing as helpfull.
05-16-13, 03:04 AM
#2
ftwelder
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I don't know if any such formula exists. Some people will look at your height/inseam and just call it out and others will measure you all over or put you on a fit bike and measure your power output.

I started by duplicating a frame I liked with minor changes and went from there.
05-16-13, 03:43 AM
#3
MassiveD
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+1.

If you want to make your own frame you have to become an expert in it. This could be theory, but more practically it is to familiarize yourself with the numbers. Since you are designing for yourself you have an easier task in many regards than making a frame for someone else.

The known stuff is dimensions of your wheels. For a first effort you should have the wheels already. Their dimensions affect the amount you can tuck the rear wheel, the BB height, and steering geometry, though fairly slightly. Still, no need to just guess on a custom.

Another known should be the bottom bracket height for your style of bike.

Also, your seat tune angle, and height for the style of bike you ride. This is the main issue, both for determining position in the legs power unit, and also for the frame build.

Then you should know what fork you are using, you should own it, and you should know typically what head tube angle.

Wheel base will probably be specified in catalogs so you want to look at that, but also it is determined by the reach you will need, and this is an item you have to work out from known rides.

You need to study the bikes you own, or can otherwise get a hold of and figure out what the fit is for you. You can also study, or redraw several key frames you are imitating.

You then need to draw out the frame. I think stuff like rattle cad makes it too easy since it is supposed to do some of the work for you. You aren't drawing bike pictures, you are determining the position of parts and the shape of the joints. This does not require a rendered drawing, it requires a full size drawing of the locations of junctions, parts and their relative positions. What you need also is determined by how you build. I only need a few numbers to set up my jig.

Once you start to put this stuff together it pretty much determines itself. Your BB height and seat post angle and length goes in; you postion the rear wheel as close as you want it, then draw the stays. Your front wheel and fork/headtube, headset assembly is drawn. About all you then need to know is how forward or back to move it, then you can draw in the top and down tube. Then you just keep working out the details and coming back at them until you are satisfied, and then leave it and try again later.
05-16-13, 08:43 AM
#4
Scooper
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One place to start is part II, chapter 5 of Cycling published in Rome by C.O.N.I. in 1972. The English version is on-line. Part II, Chapter 5 is titled "Modalities for constructing a frame to measure", and is downloadable as an 11 MB pdf file.

http://www.sandcreeksports.com/coni.htm

There are some errors in the tables, but they are pretty obvious (like a couple of places where digits are transposed).
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05-16-13, 11:10 AM
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Scooper
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Another route for obtaining frame dimensions from body measurements is by using the Competitive Cyclist Fit Calculator. There are three styles of fit to choose from: 1) the competitive fit, 2) the Eddy fit, and 3) the French fit. The three syles are explained on their website. This fit calculator provides a good starting point.
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05-16-13, 11:36 AM
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Italuminium
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Check Dave Moulton's blog, and read all the articles under the bike design and framebuilding tags. I've learned a lot there.
http://davesbikeblog.squarespace.com...bicycle-design http://davesbikeblog.squarespace.com.../framebuilding
05-17-13, 12:26 PM
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Alfred31
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Thanks a lot!
05-20-13, 09:11 AM
#8
ksisler
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OP: You would also want to figure in your total rider weight, shoe size, arm length, and shoulder/chest width into the design.

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