Thread: Geometry and lengths

1. Geometry and lengths

Heya,
I'm completely new to frame building. So new in fact, that I've never actually built a frame!
I'm about to buy a fairly inexpensive set of tubes with their appropriate lugs, and braze em all together. The first hurdle I've seemed to have hit is seat tube length. While reading the Paterek Manual, a formula is given to help find an approx. seat tube length. I'm completely confused and not sure how to get a result which seems about right.

A = Inseam length in centimeters with stocking feet
B = Bottom bracket height in centimeters
C = Crotch Clearance 4 centimeters for touring and 6 centimeters for racing
D = A-(B+C)
E = Seat tube angle
F = Complement of angle E
After measuring the rider's inseam and finding the other variables (B & E will be covered later), 2 sides and three angles will be known. With that knowledge, the Pythagorean theorum can be applied to find the seat tube length. The following formula will give the seat tube length. All that is needed is a hand held scientific calculator:
D [sec (F)] = Seat Tube Length
So for myself:
A= 85cm
B=6cm
C=6cm
D=73cm
E=72 degrees
F=18 degrees

So using the formula:

73 [sec (18)] = seat tube length.

My result = 76.76

This is wayyyy too large for a seat tube. Not sure where I'm going wrong.

Any suggestions?

2. Take a look at B. looks like you have input your BB drop instead of height.

3. What are you riding now? What would you change about it? Example: I built my frame based on the specs of my Pinarello. It was the best frame I had ridden. I am a little long in the torso so, I adjusted the TT & DT lengths. My choices in tubing were limited but, the bike turned out great. Fits me better & handles like a dream.
Calculators are great, don't get me wrong. Fitting to an actual device or bike is like measuring twice/cutting once.

4. If you are reading Paterek, which is a mixed blessing, you know about drawing out the frame. A good starting point is the frame size. So if you are building say a 56 cm, that is from center of BB to top of top tube. By looking at bikes you have ridden, and hardware you have seen (and best case will source before you start building), you can take a stab at how far above the horizontal top tube you will trim the tube. If you are going to make the tube slope, then just make those adjustments in your drawing. A build drawing can be very sketchy, you just need the build info. When addressing a client there is a marketing function to the drawing, and that is where the fancier computer graphics come in.

I fully agree with the idea of modifying frames you have actual experience riding to get to the base numbers rather than pursuing some system. Obviously to fit clients a broader grasp of theory is necessary, but for one's own frame, I think modelling from known frames and making obvious changes that would improve fit, is a better method.

5. Originally Posted by busdriver1959
Take a look at B. looks like you have input your BB drop instead of height.
I think that's a big part of his problem. A 6cm BB height wouldn't let the crank arms get much past horizontal! Paterek is careful to define BB Height as "This measurement is from the ground to the center of the bottom bracket spindle. If working with bottom bracket drop, convert it to bottom bracket height for this formula by subtracting bottom bracket drop from the wheel radius."

Wheel radius should include an inflated tire of the size that's going to be used.

Common sense and practice would suggest a frame with ~58cm-60cm seat tube for a rider with an 85cm (33.5") leg length.

6. Good catch. And this is one of those cases where having actual parts on hand to measure, in this case wheels, is nice. You can get variations in tire size that while not that great, lead to discrepancies that are at the physically tangible level (not thous).

7. Thanks a lot everyone for your replies!
Ya, I definitely took BB drop instead of height. I'll put in the new measurements and let ya'll know what I come up with.

Thanks again,
Gav

8. Originally Posted by busdriver1959
Take a look at B. looks like you have input your BB drop instead of height.
Yes. If I use BB drop = 8 cm (like the way this pedals!), then BB height is 25.5. Working through I then get 583.6 mm seat tube, c-t not including a brake cable routed on top of the TT. For my height (A=81.8 cm), the ST would be 547.3 mm c-t, or 534.6 mm c-c assuming 25.4 mm diameter of the TT.

As a check, I just got a bike with a 54 cm frame and 7.5 cm BB drop, and I like the fit a lot on 700c tubulars (about 670 mm diameter). I have another one 56 cm and 8 cm drop, and it is marginally large with Hetre tires (about 678 mm diameter).

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