For those who have already found their dream bike, the crucial question now arises: Which frame size is the right one? For many riders, several sizes are an option. We explain how geometry and riding behaviour change with size and help you make the right decision.
CONTENTS:
The test subjects for the size consultation
Laurin is pretty sure of himself and is happy to take part in our size experiment. As test editor of FREERIDE magazine, he is an advocate of short, handy bikes that can be effortlessly pulled onto the rear wheel or fired over the next jump with plenty of airtime. What's more, at 178 centimetres, Laurin is exactly the same height as me.
With our body size, depending on the manufacturer, the question often arises as to whether we request frame size M or L for the test. Laurin tends to favour M for the reasons mentioned above, while we at BIKE normally order all test bikes in size L. For our requirements, pedalling capability naturally also plays a major role and, accordingly, a propulsion-oriented riding position. Just as I often accuse Laurin of riding bikes that are too small, I have to put up with being accused of riding too big.
"At the latest since the reach and thus the length of modern bikes has increased drastically, bikes in size L have become almost unrideable," claims Laurin.
Even though the increase in reach and wheelbase is undeniable, this is less true for the riding position. Due to the now very steep seat angle and the short stems, the length is lost as soon as you sit in the saddle. The bikes are therefore short when seated, but long when standing downhill. The right frame size therefore also depends on the intended use.
Before we measure our two identical Canyon Spectral bikes in sizes M and L in the lab and take them out on the trails, we also have ourselves thoroughly checked. In the Munich bike lab, we are first measured from head to toe. The fitting experts have almost all the latest geometry data from bike manufacturers in their database and recommend the right bike and size based on our body measurements and requirements profile.
"Due to the much finer size gradation of road bikes, many road cyclists in particular use our advice," says Uli Plaumann from Radlabor.
However, mountain bikers are also increasingly interested in getting size advice independently of the recommendation of the supplier's website or the retailer.
Size jump: factors for choosing M or L
Reach, seat tube length and wheelbase change the most with a size increase (here from frame size M to L) and affect not only the riding position but also the handling.
The right frame size is not just a question of body size
Despite identical body sizes, arm, leg and torso lengths can differ significantly. The right frame size therefore also depends on the proportions of the rider.
In most cases, humans are built quite symmetrically. For example, the arm span usually corresponds to the height. But what is true for the average person can look completely different in individual cases. Despite being the same height, Laurin's and my proportions differ significantly, resulting in different sitting and riding positions on the same bike. Laurin needs a slightly lower seat height due to his shorter legs and a shorter seat length due to his shorter arms. So if you look purely at body height, you are ignoring important information in the search for the right frame size.
The websites of many bike manufacturers still rely on height as the only point of reference. However, the accuracy rate increases with each additional measurement of body proportions. In addition to body height, the inside leg length should therefore be used as an indicator for the optimum seat height (inside leg length x 0.885). The seat tube length of the desired bike and the height of the saddle (around 50 millimetres) can then be used to narrow down the length of the seat post or the maximum possible stroke of the telescopic seat post. It becomes even more precise when the upper body is taken into account.
Adjusting screws
If the frame size is not completely off, there is still hope. As long as the correct seat height can be set and the standover height of the frame offers enough room for manoeuvre, these adjustments can still be made.
1st cockpit
The cockpit allows the most room for manoeuvre. Stem length, tilt, spacer height and handlebar height or offset can compensate for a lot. But even here there are limits, such as a stem that is too long, which has a negative effect on handling.
2. saddle / rear seat
Thanks to the saddle frame, the saddle can be moved horizontally by a few centimetres and thus adjusted to the leg length and the position in relation to the bottom bracket. This influences the seat length and the knee angle when pedalling.
3. seat post
There are many options not only for the length of the seatpost, but also for the stroke of the dropper post. The trend is towards shorter seat tubes to allow riders with a low seat height sufficient drop.
4th crank
The crank length should also match the leg length. 175 millimetres is common. Short or tall riders in particular should adjust the length for efficient power transmission.
Short or long: a few millimetres decide the handling
Just a few millimetres can have a significant influence on the handling of a bike and also determine the comfort of a long time in the saddle. Personal riding style preferences should not be neglected either.
According to the size recommendation (based on height and inside leg length) in the Canyon webshop, both Laurin and I would have to go for a Spectral in size M. The advice that you can also choose an L frame would only come from a height of 180 centimetres. You are then also given the tip to choose the frame size that suits your riding style better. M for more manoeuvrability and L for greater stability. Other manufacturers, such as Specialized, even recommend up to three possible frame sizes based solely on body size, thus placing even more emphasis on personal preferences.
However, this is only possible in conjunction with very short seat tubes, which allow a corresponding amount of room for manoeuvre in terms of seat height. For riders with short legs, the saddle extension may otherwise be too small to be able to ride a telescopic seatpost with a standard stroke. Depending on the shape of the seat tube, the insertion depth of the seat post may also be limited. The available extension can be optimally utilised with a telescopic seatpost with adjustable stroke (e.g. from Canyon, eThirteen or Vecnum). If you have no restrictions in terms of seat height when choosing between different frame sizes, you can confidently make your decision based on the length of the frame. Racers tend to favour bikes with a longer reach and wheelbase for a smoother ride. Those who like it more playful and prefer to sit more upright are better off choosing a smaller size.
Statement from Peter Nilges, BIKE Test Manager:
When in doubt, I prefer to go for the larger frame. Due to my
body proportions with long arms and legs, the riding position is often too upright and unsportsmanlike, even on some L-bikes.
Downhill, long bikes can also handle more speed.
Influence of frame size on riding behaviour
Even the slight difference of just a few millimetres is noticeable on the trail. We explain how the size jump from M to L affects the riding behaviour.
The good news up front: If you are in the grey area between two frame sizes, you can get down the mountain with both bikes and somehow get back up again. You quickly get used to the differences. However, only one of the frame sizes would be ideal for both testers (without the aforementioned Adjusting screws to turn). I would definitely go for a size L, both in terms of riding position and handling. Laurin also found the riding position too squat due to the very steep seat angle of almost 77 degrees on the size M and sits better on the L frame.
Modern geometries like to pinch the rider with slack steering angles on the one hand and steep seat angles on the other, which leads to a particularly upright position. Downhill, however, Laurin tends to favour the Spectral in size M. Surprisingly, the M Spectral is much easier on the rear wheel and simply feels easier to handle. The chainstay length and bottom bracket height are identical, only the reach and wheelbase shift the body's centre of gravity slightly further forwards. However, the influence of these few centimetres is clearly noticeable. Thanks to the slightly higher front of the L-bike and the increased length, you can support yourself better on the handlebars when riding downhill.
Statement by Laurin Lehner, FREERIDE test editor:
In addition to your body size, your riding style also plays a role. If you are looking for a lively, manoeuvrable bike and want to play on the trail instead of riding for time, I would advise you to go for a smaller frame.
Interview with Uli Plaumann, sports scientist at Radlabor
BIKE: How much demand is there for size advice from mountain bikers?
Uli Plaumann: Here at the Bike lab size advice is the most popular service alongside bike fitting. However, most customers are racing cyclists. But there are also more and more bikers who want an independent recommendation in addition to a dealer and online tool. After the body measurement, the customer can then try out the geometry recommended by us directly on the adjustable ergometer.
Are body height and inside leg length sufficient to determine the optimum frame size?
Anything that is added to your height makes the recommendation more accurate. But upper body and arm length are also important
information in the search for the optimum bike size. However, it is not always possible to measure these values accurately at home.
How many people deviate from the average in terms of body proportions?
Our measurements show that it is actually already noticeable when a person corresponds to the standard in all measurements.
Is there a rule of thumb for choosing a larger or smaller frame when in doubt?
No, you can't answer that like that. However, it is often easier to adapt a large frame to a smaller person than the other way round. Of course, there are limits if, for example, you cannot set the correct seat height at all or are already sitting on the top tube when standing.
What data are your size recommendations based on? Who says what is optimal for the measured body lengths?
Our size recommendations are based on over 15 years of experience. We can draw on a large number of values measured in the laboratory, have access to studies and, of course, keep an eye on the many online sizing tools. As a lot has changed in MTB geometries, we naturally have to constantly revalidate our recommendations.
