Distance from saddle to the bottom bracket: We call this the setback
about 120 mm
How de we determine this distance?- The difference in length between upper and lower legs.- Inseam- The position of the foot on the pedal.- The measurement is done when the cyclist is riding under power. We always notice a big difference in saddle position when the cyclist is riding at higher speed. The person shifts more to the front side of the saddle. Usually this leads to saddle pain and other discomfort.
When the frame becomes longer and higher, we see that the seat tube angle is kept smaller, so the person sits more backwards.Thus, we see with the largest frames that the seat tube angle is approximately 72°. The seat angle of the EMX-5 is even 71,4°. That means that the person sits far behind the bracket. Some people like that, but it implies that you strongly pull on your handlebars to accelerate quickly and usually you stand on the pedals.They also put a lot of pressure on the lower back and knees. In this position you definitely need to have a strong and stable back. People always have the tendency to slide to the tip of the saddle when they want to go fast.
What is the correct geometry for a frame?Let’s have a very critical look at everything.I made a study of the measurements of the last 18 months and put everything in a table to clarify everything.I have looked at the difference in height in the upper and lower leg in relation to the height of the person.We have also subdivided the people according to their length and made a scale where the length increases per 5 cm.
What do we notice in this schematic representation?The opposite of what almost all frame manufacturers claim is that large people have longer upper legs than smaller people. The median is almost everywhere at 7 cm that the upper leg is longer than the lower leg.However, the median for people over 190 cm is 4 cm and the biggest difference is 8 cm, which I found only in one person.You can of course deduce from this study that the largest frame size must have a seat tube angle that is equal to the seat tube angle of the smallest frames. There are not many frame manufacturers that build frames where the seat tube angle does not vary. For example Canyon, Dedacciai. The seat tube angle of these frames are 73,5°. With this angle you can put any person on the bike.
What should you pay attention to when you purchase a new bicycle / frame?When you look at the geometry table below of a Dedacciai Gladiatore frame, we first look at the length of the top tube A. (The most important length for determining the size of your frame) The reason is very simple. The top tube has a fixed length and you can’t take a piece out or extend it. (The seat tube, on the other hand, can do that by moving the seat post).We measure this from center top tube / center head tube to center horizontal seat post dixit seat tube.The second most important length is the size of the headtube C. This length determines how straight or deep you sit on the bike depending on the length of your arms. If you have very short arms in relation to your trunk length, you will have to choose a frame with a higher headtube. For people who compete, this is as important as the length of the (horizontal) top tube.In the recent year’s there has been juggling with reach and stack. It is claimed that it is easier to make a comparison with different types of geometries from the different manufacturers.But… because of the different seat tube angles, you can also be wrong. For example, you see that with a certain brand with a larger frame size (let’s say from Medium to Large), the reach has become 5mm longer. However, with further study you notice that the seat tube angle has decreased by half a degree, but that the length of the top tube has increased by 2cm. That means that the saddle is put more backwards and therefore you sit less comfortable on the bike. This only applies if a constant seat tube angle is used.
Geometry of the frame.The geometry of the frame is an important item to determine your bike position.Each frame manufacturer uses a different approach for the determination of the seat tube angle.We notice that the seat tube angle of small frames is around 74° (some have even 75°)
In this picture we can see that the foot is horizontal.With this foot settling, the saddle will have to stand more to the front to create an optimal bike position. (Distance X1)
In this picture we see that the foot is tilted and the knee is more forward. The saddle must be moved more backwards. (DistanceX2)
If we now compare both pictures, we see that the distance X1 is longer than distance X2.So the tilt of the foot has a major impact on the saddle setback.If we have 2 identical persons with the same inseam, the same length of upper and lower legs, those people will probably not have the same setback. The reason is that both could have a different foot tilt. This is called ‘ankling’.
CONCLUSION!It is time for the manufacturers to, once again, carry out a serious study and adjust the geometry to the current modern bicycle position determination.Another note: when you look at the geometry of MTB frames, you notice that frames from small to large have the same seat tube angle. (American background apart from European influence)In Europe, people still think that they have to sit very far behind the bracket to be able to pull hard on the handlebars to get the big gear right. (Consequence, overloading the back and knees)
Above you see a table with the geometry data of a well-known and very popular brand.When you look at the reach of the frame, you see that the reach remains almost the same for the 3 smallest sizes. This means that only the seat tube angle changes and that the person who will ride with this frame will sit more backwards. (Poor sitting position) The saddle will have to be positioned further forward. On the other hand, the length of the top tube is almost 2cm longer each time. If you look further, then you see that between the largest and the smallest frame the reach is only 22mm longer!
As of 2017 we can determine the setback using the Mbody EMG device.The setback determination measures the muscle strength and balance between quadriceps and hamstrings.In this way we ensure that the two most important muscle groups are nicely balanced.The saddle setback is the most important adjustment, after the cleat position and shoes, for the best power transmission.