Frequently Asked Questions

Which are units used by SizeMyBike?

All measurements are in millimeters (1cm = 10 mm).

About body measurements?

All measurements need to be in millimeters and should be done bare footed.

Sternum, measure the distance from the floor to the top of the sternum indicated.

Inseam (inside leg), the way to measure the inseam is to put a book between your legs and gently rise it upwards until you can feel it against the bone. Next, measure the distance from the floor to the top side of the book. The distance between your feet should be about 200 mm.

Foot, it is suggested to measure both feet and use the average measurement.

Arm, measure the distance from the middle of your clenched fist to the clavicle. It is suggested to measure both distances and use the average measurement.

Shoulder, make the measurement between the extremities of two clavicles.

How to use the SizeMyBike results?

You must respect the following order:

1) Choose the right frame size and the right crank length. The seat tube length is given in mm (center to top).

2) Adjust the saddle (height and setback). The saddle is adjusted to produce maximum power at a minimal aerobic cost. The saddle position is identical for recreational or elite cyclists.

3) Choose and adjust the stem. It is very important in terms of comfort, aerodynamics and metabolism. Two positions were computed by SizeMyBike: comfort and performance. Indeed, recreative riders may prefer a shorter reach and higher placed handlebars.

What is the top-tube length?

Following most of the bicycle makers, SizeMyBike computes the top-tube length in a horizontal plane (center to center). For a sloping geometry, this length is shorter than the real length.

What is the definition of the tubes length?

Seat-tube length is given center to top. Top-tube length is given center to center.

How to improve your position?

SizeMyBike provides a good starting point. To get a position really perfect, you will need to make minor adjustments. A methodology can be:

Step 0 – adjusting carefully your cleats is essential: the cleat should be positioned so the first metatarsal head lies directly over the pedal axle. If the outside of your knees hurts, point your toes a little more outward.

Step 1 – seat position: the seat can be moved forward or backward. With crank horizontal, a plumb line dropped from the bottom of your kneecap would lead to the axe of the pedal.

Step 2 – hop on the saddle: ideally, at the bottom of a pedal stroke you want to feel a slight bend in your knee. Aim to come within 80% or 90% of full extension, not 100%.

Step 3 – stem length: with crank parallel to the diagonal tube, put hands on the handlebar (hands position: on the drops). The elbow must touch lightly the knee.

How to adjust the saddle height?

SizeMyBike computes an average saddle height.

You have to take into account the specificity of yours pedals and yours shoes. The saddle height is computed assuming pedal height equal to 10 mm and sole thickness equal to 10 mm.

Keep in mind that differences in the pedal height can be up to 15 mm. The thickness of the sole varies between 7 and 21 mm. You must also take into account the crank length if they differ to the SizeMyBike’s ones. A correct saddle height is achieved if the leg extended fully with heel resting on back of pedal in the 6-o’clock position and if the knee should be able to bend 25 to 30 degrees.

Having the saddle too high may result in patella tendinitis, iliotibial band friction syndrome, trochanteric tendonitis, lower back pain, kneecap pain, perineal numbness.

Having the saddle too low may result in Achilles tendonitis and Plantar fasciitis.

What is the impact of seat tube angle variation?

Seat tube angles vary from about 71 to 75 degrees. Frames with less vertical seat tube angles are more comfortable because in-plane (vertical) stiffness decreases. The steeper angle makes the bike more maneuverable whereas the shallower angle is suitable for more stability. Generally, the seat tube angle varies with the frame size (shallower angle with taller sizes). Naturally, seat tube angle impacts the saddle setback. The seat rearward will take weight off the handlebars and put it on the seat: the position may be more comfortable.

How to achieve a smooth pedal stroke?

Each muscle involved in the pedal-stroke must have periods of relaxation. The key is learning when the hamstrings are required to produce power in an efficient stroke and when they should be relaxed. The pedal stoke is divided into 3 parts: downward stroke, back stroke and up stroke.

The hamstring muscles should be very active during backstroke, because only knee flexion provides power in this range. Relaxation during another range of the pedal stroke in prevents fatigue and enables powerful backstroke contractions.

During upstroke, the cyclist must lift the knee and not the heel or the foot. If he thinks of lifting his knee powerfully, the foot and pedal will follow without contractions to bend the knee. Moreover, up stroke must be a diagonally upward/forward movement, instead of an upward and backward movement. It activates contraction of the hip-flexor muscles and relaxation of the hamstrings.

During down ward stroke, the power application begins early, at 12 o’clock, and is directed downward diagonally toward 4 o’clock. This activates the quadriceps optimally whereas the hamstrings contract moderately.

Principles of SizeMyBike computation?

The SizeMyBike algorithm is based on biomechanics. The model computes a bicycle geometry which will enable you to have the 3 contact points correctly positioned. The contact points are the pedals, the saddle and the handlebar. Saddles and pedals are positioned in order to maximize the transmitted power and to minimize the metabolic costs. The optimum is reached when the 2 following conditions are satisfied. The knee should be able to bend 25° to 30° with the pedal in the 6-o’clock position (i.e maximum extension of the leg). Moreover, the axis of pedal and the bony protrusion just below your kneecap must be aligned vertically when the pedal is horizontal. The handlebar is positioned in order to reach a compromise on the following points: aerodynamics, breathing and comfort. With hands on brake hoods, this compromise is reached for trunk angle varying between 30 and 40 degrees according to whether one privileges performance or comfort.


Road bicycle fit. MR Silberman, Clin. J. Sport Med., 2005, 15:271-276

Cycling biomechanics: road and mountain. Broker JP, In Burke ER, ed. High Tech Cycling. Champaign, IL: Human Kinetics; 2003.

The Lance Armstrong Performance Program. Armstrong L, Carmichael C, Emmaus: Rodale Press; 2000

Physiological and biomechanical factors associated with elite endurance performace. E Coyle et al.., Med Sci Sports Exerc, 1991, 23(1): 93-107

The biomechanics, etiology, and treatment of cycling injuries. WH Sanner et al., Journal of American Podiatric Medical Association, 2000, 90(7), 354-376

Some effects of knee angle and foot placement in bicycle ergometer. K. Mandroukas, Journal of Sport Medecine and Physical Fitness, 1990, 30:155-9

The biomechanics of cycling. RJ Gregor, Exercice and Sport Science Review, 1991

Multivariable Optimization of Cycling Biomechanics. H Gonzales, J. Biomechanics, 1999, Vol. 22, 1151-1161

Injury prevention for cyclists: a biomechanical approach. PR Francis, Science of cycling, 1986, pp. 145-184

Dynamic optimization analysis for equipment setup problems in endurance cycling. S. Kautz, J. Biomechanics, 1995, 28, 1391-1401

The influence of body posture on maximal performance in cycling. E. Welbergen, Eur. J. Appl. Phys., 1990, Vol 61, 138-142

Effect of body posture during cycling on heart rate. I. Faria, Al. J. Sport Medecine, 1978, Vol 18

Biomechanical study of competitive cycling. A. Dal Monte, Biomechanics III, 434-439

What is the right handlebars width?

Drop handlebars should be about the same width as your shoulders.

How to adjust the stem?

Distance between elbows and knees (at maximal height) should be small (1 inch). A good handlebar position is achieved with trunk angle between 30/40 degrees and trunk/shoulder angle between 80/90 degrees with hands on brake hoods. Moreover, distance between the elbows and knees (at maximal height) should be small (1 inch).

What is the correct crank length?

With the right crank length, your comfortable cadence is about 90/100 rpm’s. If you like turning big gears at lower cadences, go with longer crank-arms. If you prefer to spin at a higher cadence, go with shorter crank-arms. Moreover, shorter cranks reduce the dead spot at the top of the cycle. If you change your crank-arm length on your current set up, don’t forget to adjust your seat height.

How to adjust the cleats?

It is fundamental. The cleat should be positioned so the first metatarsal head lies directly over the pedal axle. If the outside of your knees hurts, point your toes a little more outward.