Spectrum: Anyone who has not just chosen a folding or compact bike will eventually have to ask themselves the question of bike size. In the following, we explain what needs to be taken into account.
It was already clear what type of bike you wanted before you made the purchase decision. Now you have decided on a model, selected the equipment and specified the color - if there are options. But there is still one hurdle to overcome: choosing the right frame size.
That doesn't sound that difficult at first - especially not when you can try it out in the bike shop. After all, there is relatively clear information about how you should ideally be positioned on the bike. Perhaps the most important of these is seat height, which is the distance between the center of the bottom bracket spindle and the surface of the saddle. To determine this precisely, the formula "Inseam length x 0,885" has proven itself. With an inseam length of 87 cm, you get a seat height of 77 cm - fits.
However, this is only a rough guide, because the optimal seat height can vary depending on the type of bike and riding style. Cyclists usually sit rather high in order to achieve the greatest possible leg extension - this way they protect their knees when pedaling powerfully. However, sitting too high can cause lower back problems and may increase pressure in the seating area.
Adjusting the saddle height according to the formula is also not always exact because it ignores components and accessories. With soft saddles, the surface gives way when you sit on it and you are positioned lower; pedals of different heights or longer pedal cranks also affect the seat height. With everyday bikes, the footwear also makes itself felt – with thin summer sandals you have to stretch your legs a centimeter more than with winter boots.
However: the average cyclist hardly notices this centimetre; a millimeter-precise height adjustment is only necessary in the sporty area anyway. It's even easier for e-bikers: Because of the motor support, they can afford not to sit perfectly. Then the saddle likes to move down a bit to make it easier to stop - and since you don't have to use as much force when starting, you still don't have to worry about your knees.
The right bike size: A question of the area of use
Back to the initial question: how about the frame size? This can be specified in centimetres, which usually means the length of the seat tube - and this is very practical because there is a formula for this too. “Seat height x 0,66 = frame height in cm” is this; for our 87 inside leg length, a seat tube of 57,5 cm should be optimal. This is possible because there is enough room to adjust the saddle up or down. But be careful: All of this only applies to conventional frame geometries. Models with a sloping top tube have significantly shorter seat tubes; so this formula doesn't get you anywhere. City bikes that are equipped with a suspension seat post also have a rather short seat tube. A very special exception are modern mountain bikes.
There is a clever formula for all-terrain bikes: "seat height x 0,226 = frame height in inches". Again related to our 87 cm inside leg length, this results in a frame height of almost 20 inches, i.e. about 50 cm. However, this result should be treated with caution, since a lot has happened in MTB geometries in recent years. The seat tubes are getting shorter and shorter in order to be able to lower the saddle further off-road. Many modern bikes, even in the largest frame size, no longer have seat tubes that are longer than 49 or 50 cm.
So what to do? Of course, you can read up on or familiarize yourself with the sometimes extremely complex topic of geometries. But that requires a lot of time, which not everyone can or wants to invest. Another option is to look at the manufacturer's website. Size recommendations are usually given here for each wheel. It is worth consulting these, especially if the provider uses the Anglo-Saxon sizes S, M, L, XL and so on. These are not very meaningful on their own, as the exact sizes can vary from one brand to another. The advantage: Within the range of a manufacturer, the customer can assume that all bikes designated as “M”, for example, will fit him – regardless of frame shape and seat tube length.
The right bike size: There is no formula that can replace a test ride
Anyway, the size charts. Of course, they are structured differently from manufacturer to manufacturer, specify different ranges for body size and determine the frame size in different ways. The table is therefore always only valid for the respective brand. To make matters worse, most providers have good reason to shy away from binding information and, for example, assign body size to two frame sizes in 10 cm increments. The decision as to which of these fits better can hardly be made in front of the screen.
Why is that? The correct seat height can certainly be achieved with different frame sizes - but depending on the frame size, the head tube length and the length of the frame front end are also different. If the customer chooses the smaller frame, the handlebars may not be positioned high enough; if he chooses the larger one, he may sit too stretched out. In any case, the seat geometry can vary from manufacturer to manufacturer. Let's take two similarly designed trekking bikes with rigid forks from Centurion and Stevens: The seat tube length is the same, but on the Stevens you sit 5 mm more stretched out and 34 mm more upright. Upright or bent, compact or stretched? The frame size alone gives no information about this.
In the case of a trekking bike, such structural differences can be compensated for if necessary with particularly short and/or steep handlebar stems; in the sporty segment, on the other hand, it is less simple. The reason for this is that the length of the stem has a significant influence on the steering behavior - if a frame that is too short is compensated for by an extra-long stem, sluggish steering is to be expected. Trail mountain bikes, for example, combine long top tubes with ultra-short stems, making them playful and agile to steer.
The dimensions "stack" and "reach" were introduced a few years ago so that cyclists can assess at a glance whether a frame or wheel is suitable for them. They indicate the vertical or horizontal distance from the center of the bottom bracket to the top edge of the head tube. Different types of bicycles can also be compared well with these measurements, since the stack (height) also includes the fork height (important for current crossers and disc racing bikes that have a lot of tire clearance, and of course for bikes with a suspension fork) and the reach (front end). ) is independent of top tube length and steering angle.
In order to know the individually optimal values and with their help to select a well-fitting frame, you have to be quite deeply involved in the matter. Whether a bike "fits" or not, often only becomes apparent when you try it out in a bike shop. And there you can also explore the limits of adjustability of saddle and handlebar height, which ultimately decide whether the purchase decision is made.