Sunday, 13 October 2013

Day 1: Saturday

Bike Fitting and CAD 

The course kicked off with an intro to the workshops, tools, tubing etc.

First task was to use an Italian bike fitting jig (imagine a turbo trainer bike with no front wheel and adjustable frame, saddle, and bars) to ascertain the ideal frame geometry for each of the three of us doing the course.

To give you the basic method, you set up the seat tube to be about the right length and get measuring!  The ideal is that when the crank is very nearly at bottom dead centre (perhaps 5 degrees before), the angle from the ankle to knee to hip bone should be around 35 degrees.  Once you've achieved this by adjusting the seatpost height, you then check to see if the front of the knee is above the pedal axle.  If it's not, move the saddle backwards or forwards as required, then re-measure the leg angle.  Once both variables are within tolerances, you can measure the distance between the centre of the crank and top of the saddle.

Next thing is to adjust the length of the frame and height of the bars to the point where the rider is comfortable - again by trial and error and a lot of it is down to feel and an experienced eye.

You end up with a series of measurements (here are mine):

Drop from saddle to hoods: 60 mm

Centre of crank to top of saddle: 710 mm
Saddle set back: 40 mm
Centre of seat clamp to centre of bars: 640 mm
Centre of  seat post to centre of bars: 610 mm
Front of saddle to centre of bars: 470 mm
Front of saddle to Hoods: 630 mm

Jig Settings
Angle: 73 Degrees
Seat Tube Length: 560 mm
Top Tube Length: 530 mm

Once in possession of the critical dimensions, it's a relatively easy job to transfer it to BikeCAD Pro.  Here's my first attempt at a bike frame:

I chose to go with a horizontal top tube to keep things classic-looking.  I always think that road bikes look odd with sloping top tubes...  This frame still needs some work on it as the head tube is too short (mainly because I'm insisting on a horizontal top tube!) but as long as the three points where the rider connects with the bike are in the right place a lot of things can be changes around.

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