Bikebuster articles

Geometry and position

Af Per Bausager

Oversize and sloping frames, which in the latter years has been the standard for road bikes, has not made it easier to choose the right size frame ? on the contrary.

Where, in the good old days, it was simple to set up rules for what to choose, today it is like crossing a jungle in order to find the right solution, unless you, by chance, is a specialist in riding position and geometry.

In the era of steel frames it was easy to count on center-center sizing for the frame height, because all tubes had the same diameter and you knew that the difference between center-center and center-top measurements was 15-20 mm. With the introduction of oversize frames this difference was not only increased ( mostly 35-50 mm), but also because of the many different diameters and profiles, it was almost impossible to set up defacto rules.

Looking at the market today unfortunately leads the conclusion that the frame manufactures haven't made it any easier. The majority has just continued producing modern oversized frames in accordance to their old steel frame blueprints, which has pushed things in the complete opposite direction of the trend among riders.

Now among elite riders there is a significant trend to ride lower and longer frames than earlier. A lower frame gives the possibility to achieve an even bigger difference between handlebar and saddle and when this is combined with a longer frame the riding position will be more straightened for the upper body, with an important aerodynamic extra value. As an example of this, Alessandro Petacchi, who is 189 cm tall, rides a frame which center-center measurement is only 54 cm. For a Pinarello Dogma this means a center-top measurement of approximately 58 cm. The length of Petacchis frame is 58,5 cm and he even uses a 14 cm. Stem . As a comparison he used to ride a 59cm (center-center)

This example shows with great clarity the problems that are discussed above. Most manufacturers hangs on to their old geometries and this means that if Petacchi was a recreational rider, and wasn't just able to order his frames from Pinarello in custom geometry, he would, in order to keep his current frame height, get a frame which is 3,5 cm short than the one he is actually riding, which obviously is not a valid solution.

As a recreational rider you could easily end up choosing between a too high or too short bike even though I realize that the fewest recreational riders will have the need for such a straightened riding position. Also it should be mentioned, that recreational riders, who have started riding in an adult age, does not have the ability to copy such an aerodynamic riding style as this, since biomechanics hardly allows achieving a satisfying power output with such a closed angle between thighs and spinal cord as need for this position. This ability is normally only found among riders who have been in competitive cycling since their childhood.

Sloping frames does not decrease the problem with geometry, in contrary to common beliefs. Even though it is true that it gives more flexibility regarding saddle height, the opposite is also true regarding length. Sloping is an invention of marketing departments with the purpose of reducing the number of varieties beneficial mainly to manufactures and retailers, but, except for the price, for sure not a benefit for the consumers.A sloping frame is typically available in 4-5 different sizes, and, even worse, only in 4-5 different lengths. This has to been seen in relation to the 12-15 different heights and lengths that Italian frame manufactures used to offer. I have to add that hardcore manufactures like Gios, Wilier, Pinarello, Colnago and De Rosa still, in their high end craft, can offer the entire range ? even including custom sized frames.
But back on track with the sloping frames. When choosing a frame or bike of this type, it's of uttermost importance that you pay more attention to the length of the frame than it's height, which is the opposite of what most people do. If you already have an older frame with a traditional geometry, which has a good fit, the right thing to do, is to measure the length of the frame center-center, and use this measurement as the decisive factor for choosing a new sloping or oversize frame. As it is, there are only few possibilities for regulating the length of the stem, especially if aesthetics are taken into consideration ? since e.g. an XL frame for a man 190 cm tall will look deranged with anything shorter than a 12 cm stem, so only the 12, 13 and 14 cm are available for selection. That, as previously mentioned, only 4-5 frame sizes are available doesn't minimize the problems.

The conclusion on the problems discussed here is that, when choosing frame/bike, first and foremost one should put attention to the length of the top tube, measured from center of the seat tube to center of the head tube, and then the frame height measured from center of the bottom bracket to the top of the seat tube.