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Bike Improvement in Ranst specialises in adjusting your bike for the best comfort and performance. To do this, we rely on scientific insights and our own tests, which can often lead to surprising conclusions. Bike-Improvement also organises lectures for cycling clubs, cycling teams and athletes.


There is a lot to consider when adjusting your bike. Determining the right crank length is a discussion which is often conducted in the cycling world. On the basis of research and tests by Bike Improvement, we concluded that the smaller the crank length, the better.


In the past, a calculation formula was used to determine the correct crank length, including, for example, the formula based on the inside leg length. This formula takes the inside leg length x 2.16 = crank length in mm. For someone with an inside leg length of 80 cm, the correct crank length would then be 172.8 mm. According to the formula, this person would have to ride with a crank length of 172.5 mm. To go to even more extremes: someone with an inside leg length of 90 cm would then have to have a crank length of 90 cm x 2.16 = 194.4 mm. But this proposition does not quite hold true.

Power arm long

machtarm lang

Power poor short

machtarm kort
Welke cranklengte is het best?

Take, for example, a rider using a crank length of 175 mm, where the position is adjusted to a crank length of 170 mm. At the lowest point, the crank is 5 mm shorter. In order to keep exactly the same knee angle, the saddle must be placed 5 mm higher to compensate for the shorter crank.

The shorter crank length causes the body’s hip angle to become much less sharp and less pressure is exerted on the stomach and lungs. There is also better blood flow to the legs, which in turn leads to less lactic acid production.

Because the knee has to bend 1 cm less when the pedal is at the top, you can immediately apply more force. For example, bending all the way through the knees and then coming up straight. Notice that this is difficult in the beginning and becomes much easier as you get straighter and more upright.

The often stated remark: "But if you have longer cranks, you can use more power" is only partly true. A crank is a lever and one can only apply more force as the lever gets longer.

However, at one revolution with a crank length of 175 mm, you make a journey of π x diameter = 3.14 x 17.5 = 54.95 cm. Of that complete revolution, the advantage of the crank length of 175 mm is only 6.4 cm compared to a crank length of 170 mm. Compared to a crank length of 172.5 cm, the advantage is only 4 centimetres. This means that in the first case you only benefit from a longer crank at one revolution by 11.7% and in the second case by only 7%.


On the other hand, in the dead centre, when the bottom bracket is at the top, you can reach the dead centre faster with a shorter crank. This is a greater advantage than the gains that can be made from longer cranks. By riding with shorter cranks, you also ride much more smoothly, have less load on the knees (because the knee is less bent) and you experience less load in the lower back.

A study by Jim Martin, PhD in motion physiology at the University of Utah, also indicates that improvements occur with shorter cranks, but only up to a crank length of 145 mm. Only then does a loss of power occur. Furthermore, his research shows that the body uses more energy with a longer crank than with a shorter crank, mainly because the body has to make more effort to reach the 'dead' moment at the top of the pedal circle with the longer crank.

The main point that this study reveals is that there are actually no brands that produce cranks shorter than 160 mm. Even those are very hard to find. As far as Bike Improvement has been able to verify at this moment, only PowerCranks are available as shorter models. But even with a crank length of 170 mm to 165 mm, the increase in power is already very large, while the other advantages mentioned are also available.

Remarkably, since this year Campagnolo has been offering a shorter crank length of 165 mm. Shimano and SRAM already had this. Fabian Cancellara also switched from a crank length of 177.5 mm to a crank length of 172.5 mm a few years ago.

Riders have been using short cranks (160 mm) on the track for some time now, partly because they prevent them from hitting the track with their pedals when cornering. This in no way affected their speed. Unconsciously they are using the advantages of shorter cranks.

For the sake of completeness it must be said that pedalling frequency also influences crank length. The slower the pedalling frequency, the more you benefit from longer cranks. This theory is only valid when you have a pedalling frequency of less than 75 revolutions per minute.

Another study by Guido Kaandorp (Faculty of Science, Free University of Amsterdam) shows that a crank length of 140 mm does not cause a loss of power compared to longer cranks of 170 to 180 mm. On the contrary, the short crank allows the gear to rotate more smoothly. The hip angle, i.e. the angle between the torso and the thigh when the crank arm is completely at the top, increases with shorter cranks and that has the greatest impact.

tabellen 2

In the tables above you can see that the lower the pedalling frequency, the greater the crank length may be. This is because the MCF value should be as small as possible. The power referred to is the power provided by one leg. In the example above, the pedalling frequency is less than 70 rpm.


According to Bike Improvement, it is recommended that newcomers ride with a crank length of 165 mm, especially because they are even more limited by their lower gears. For the junior category, a crank length of 170 mm is recommended, but those who are shorter than 170 cm should remain faithful to a crank length of 165 mm. A crank length of 172.5 mm is recommended for fully grown riders who are taller than 190 cm.


Mistakes are also often made when determining frame size. For example, a bicycle dealer will calculate your frame size based on your inside leg length. This is not quite correct, there is more involved in determining the best frame size. Bike-Improvement often notices that the frame sizes recommended by bicycle dealers are either too big or too small.



Incorrect calculation based on inside leg length
Your torso and arm length appear to be a better basis for calculation of frame size than your inside leg length. This is because all bicycles nowadays are 'sloping'. This means that the seatpost is longer and even a clearance of more than 20 centimetres is not unheard of. This has reduced the importance of frame height.

On the other hand, the top tube length is fixed, and you can only vary the reach and stem. With a stem of 60 to 140 mm and a reach of 68 to 115 mm, the variable length is only 12.7 centimetres. Finally, neither is the length of the head tube variable.

Is that important?
Yes, it is. The length of the top tube and the height of the head tube are important for an aerodynamic sitting position. The importance of a short head tube should not be underestimated, especially for competition and cyclosportive riders, but this also applies to people with long legs, short torso and long arms. They too should look for a smaller frame with a short head tube. Finally, the same applies to people with short arms, a long torso and long legs. For them, a longer ball head tube is important.

The moral of this story
Do not just visit a bicycle dealer, rather you should have a professional fitting carried out before you buy. This is the only way to purchase a suitable bike.

Time trial adjustment
During the time trial set-up Bike Improvement measures the athlete completely, after which the bike is fully tuned to the UCI standards for riders and triathletes. The athlete takes part in the time trial or triathlon bike, and a video recording is made. The video recording is analysed via the Bike-Improvement’s sport analyser software. Based on this analysis, the position is adjusted in consultation with the athlete. New analyses follow each time until the rider has found the perfect position. This working method takes some time, but produces extremely good results.

Bike-Improvement’s recommendations

Bike-Improvement recommends that athletes use a Cobb Fifty-Five saddle or an ISM saddle. These saddles offer several advantages:

- With a classic saddle you are not quite on the tip of your saddle, which results in a painful and uncomfortable time trial posture. These saddles provide extra seating comfort.

- The Cobb Fifty-Five saddle has the added advantage of allowing you to partially circumvent the UCI rule because the saddle sits about five centimetres forward.

- The TT saddles have the advantage that, thanks to their design, they use the muscles in the pelvic floor more, so you generate more power than with a classic saddle. In the beginning it sometimes feels uncomfortable because you can feel your buttocks sitting well, but that feeling actually disappears very quickly.

Bepaling van de framemaat


During two days in Flanders Bike Valley, Bike Improvement conducted tests to find out what is important for the most aerodynamic position where you generate the most power. This test covered an ordinary bike as well as aerobic bikes and was done with different kinds of footwear, clothing, extenders and helmets. Below is a schematic representation of the results.

Price of aerometry test with Bioracer system: € 99
Price of aerometry test in wind tunnel at Flanders Bike Valley in Beringen: € 685


Aerodynamic Resistance

aerodynamic resistance
wind tunnel
conducted measurements

Above are the different possibilities we have used to carry out the tests properly

Fundamentels of aerodynamics

Fundamentels of aerodynamics

This is resistance that you experience from the headwind.


Rolling resistance - The whirlwind behind the rider is also a braking factor that must be excluded as much as possible.

Drag Coëfficiënt Cd

Drag Coëfficiënt Cd

Reduction of aerodynamic drag by reducing CdS

Reduction of aerodynamic drag by reducing CdS
aerodynamic drag

This is an example of how you can still drive as fast with less power if you meet a number of conditions. Clothing, aerobars, on top of the handlebars, in the brackets or other extenders with different hand positions.


The wind tunnel test started with measuring the power of a person riding at 50 km/h in normal clothing and with their hands on the brake levers. This requires an output of 609 watts.

positie tom dumoulin

This is the position Tom Dumoulin and Victor Campenaerts take on their TT bikes. This position is completely uncomfortable, but can be maintained for up to two hours after many hours of training. It offers advantages, but is not recommended for triathlon.

grafiek 8

Result for the placement of the extenders in or out.

grafiek 7

The used extenders of profile design. The upper extender is not allowed from UCI. It is the most aerodynamic extender and can be used for triathlon.

grafiek 4
grafiek 5

Different types of Aero shoe covers and stockings.

grafiek 6

Different types of extenders as shown below.

grafiek 15

Above you will find the test results of different helmets. The Lazer Victor helmet is clearly the most aerodynamic bike helmet of the selection. However, this does not mean that this bike helmet is equally good for everyone. Ultimately, it depends on how someone sits on the bike in relation to his or her body size and flexibility.

tabellen potitie test


At Bike-Improvement the expert team tries to improve the comfort and performance of both recreational and performance-oriented riders. Bike-Improvement has worked with many athletes in the past.


"One of these riders is Juul Maes who won the Prince of Novices in 2017. Later in 2017 he finished his season as champion of the track in Limburg. He continued his series of victories in 2018, including the Flemish Championship."

Juul Maes

" Olympic athlete Elke Vanhoof was also supported by Bike-Improvement in the run-up to the Olympic Games 2020. Elke knocked on Bike-Improvement's door at the beginning of 2017 for support after which extensive testing was done to determine her ideal crank length, gearing and steering placement. These tests were done using Myontec Mbody EMG trousers, which use sensors to measure how much the quadriceps and hamstrings are stressed. "

Elke Vanhoof


Then contact Bike Improvement in Ranst today.

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