Choosing the Dimensions of a Custom Made Frame
Excluding fig 1 , all the figures in this page are from the book of Zeno Zani;
Pedalare Bene-Ediciclo-----Zeno Zani-Le tecnopatie nel Ciclismo-Ediciclo-ITALY
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http://www.ediciclo.it
To make a custom-made bicycle frame requires to define the "Saddle Seated Position" (SSP) of the cyclist.;
The 3 cyclist/frame contact points generate the SSP: handlebar, saddle, pedals.;
The pedals rotate around the axe of the Bottom Bracket Shell (BB); therefore the location of the BB will be used as reference point for the pedals.
-A- Saddle Height (SH):;
distance from the saddle center to the center of the Bottom Bracket.;
Figure 1 show the 4 dimensions used to locate the Saddle Seated Position.;
-B- Distance between Saddle and Handlebar:;
distance between handlebar and saddle measured from the front tip of the saddle to a tangent point in the back of the handlebar in the stem area;
-C- Saddle Back-Set:;
Horizontal distance from the front tip of the saddle to the vertical line crossing the BB axle.
-D- Handlebar Down-set ;
Vertical distance, from the top of the handlebar close to the stem to the upper side of the front tip of the saddle
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FIG.1
Dimension A -Saddle Height-sets the extension of the leg while pedaling.
Bernard Hinault, suggests in his manual to use this formula to calculate dimension A:
A=0,885*H ,   with H as the crotch height.
The value of dimension A calculated with this formula should not be rigidly applied, because it depends also on personal preferences as well the personal shoe, pedal set up.
Dimension C, saddle Back-set, sets the pedaling style.
Dimension C varies from cyclist to cyclist, also depending on the bicycle use. Normally it is smaller for track or time trial bicycle (5~3; cm), but it tends to be bigger (6~8 cm) for endurance or tourism bicycles.
A common way to find the Saddle Back Set is to run a plumb line from the knee cap while holding the pedal crank horizontal (see figure below). The plumb line is supposed to cross the pedal axle.
The picture on the left show what has been described above. Also in this case it is necessary not to apply rigidly these rules. It should be considered that if it is desired to get a more rapid pedaling, then it is necessary to advance the kneecap.
On the other hand a set up with the kneecap kept back is more adapted for enduro applications.
Basically it is up to the cyclist to choose this dimension according to personal preferences.
Most of the times, and for sure for track, time trial and Triathlon applications, advanced positions are clearly preferred.
The drawings below are from the book of Dr. Zeno Zani. They show some techniques to identify the right Saddle Height.
Especially it is suggested that when the leg is at the Bottom Dead Center, to keep an angle of 150~155 degrees between the femur and the tibia.
The figures on the right suggest some angles to prefer when choosing the bicycle dimensions.
It is important not to forget that these are average values. In real life these dimensions are subjective to personal preferences and sizes such as the shoe length, the set up of the pedal cleats and the pedal crank length.
Leg at the Pedal Bottom Dead Center
The picture on the right describes the minimum value suggested for the angle between femur and tibia while pedaling (35~40 deg). Bigger values of this angle increase the kneecap load. Higher kneecap loads are bad for the femur/knee cap cartilage.
The figure on the left shows the method of Dr. Haushalter to set the handlebar saddle distance (dimension B) and Handlebar down-set (dimension D):
you want to sit on the saddle and while holding the handlebar and setting yourself as you would be going at max speed, you want to have an angle at your elbow of 145 deg. Your leg need to have an angle of 90 deg and the kneecap area should be pretty close to your elbow. This technique is a little bit radical, to smooth out this position according to personal preferences it is not a bad idea.
A final table to summarize the most frequent choices for dimension A,B,C and D.
Cyclist Height (cm)          155--162      160-170       168-174         172-180        178-188         188-195

Cyclist Height ('.")         
5'.2"-5'.4"     5'.4"-5'.7"      5'.7"-5'.9"        5'.9"-5'.11"      5'.11"-6'2"     6".2"-6'.5"
Crotch Height  H(cm)         70-75          75-78          79-82            83-86             87-90             90-96
Dimension       A(cm)      
62-66           66-69         70-72,5        73,5-76           77-79,5       79,5-84,5
Dimension       B(cm)      
42-45           46-50            50-52           52-56             56-58           58-61
Dimension C -Group1(cm); 0-1                2-3              3-4             4-5                 6-7                 8-9
Dimension C -Group2 (cm)  1                  3-4              4-5              5-7                7-9                9-11
Dimension     D(cm)           3-4               5-8              6-8              9-10             10-13             12-13
Group1:Criterium-Road Races-Triathlon
Group2:Tourism Riding-Endurance Races
To help choosing the Saddle Seated Position, Lineup Co. builds a prototype of Multiposition bicycle.
On this bicycle it is possible to change the seat tube position, allowing testing ride several dimension combinations. After some trial and error it is possible to choose the dimensions A-B-C-D according to the position that best fit personal preferences and applications.
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