The TOUR test procedure is designed to evaluate road bike frames in size 57 - the best-selling size in this country. Because important evaluation categories such as weight, stiffness and comfort are size-dependent, the scores cannot be easily transferred to smaller and larger frames. In order to be able to assess other frame sizes, we requested each test frame in three sizes: the smallest and largest available size as well as the size that comes closest to the standard size 57.
STR value for road bike frames: What's behind it?
Using the carbon frame test as an example, we explain the measurement of a road bike frame: We cannot give the small and large frames final marks. However, you will find the stiffness values in the steering head and bottom bracket in the diagram in the test letters for all frame sizes - important for your purchase decision. Further key information on the different sizes can be found in the respective test reports. There you will find the STR value for each measured frame size. It sets the actual height of a frame ("stack") in relation to the actual length ("reach"). The STR value therefore indicates whether a frame is rather short and comfortable or long and therefore sporty, regardless of the nominal frame size, seat tube length and seat angle. The frames tested range from size 44 to 64 (manufacturer's specifications), the STR factors from an extremely sporty 1.32 (Trek, size 50) to a relaxed 1.58 (Cannondale, size 63).
It is noticeable that, with a few exceptions, small frames tend to be sportier than large ones. This is due to the fact that large frames grow more in height than in length (proportional growth would make more sense). The limiting factors here are the constant wheel size and the only marginal increase in wheelbase. This could lead to the conclusion that smaller people are forced into a stretched seating position and taller people into a more upright position - even if they might want to sit completely differently. But don't panic: The STR value only refers to the frame and does not take into account that stems are available in lengths of 70 to 150 millimetres, which allow a lot of leeway for position adjustment. Incidentally, it also looks better if a small bike is ridden with a shorter stem and a large bike with a longer stem.
Angles and dimensions for road bikes: these values are crucial
Steering angle / fork pre-bend
together determine the caster - decisive for the steering behaviour. Typical: steering angles around 73° and caster between 55 and 60 mm. The steeper the steering angle and the shorter the caster, the more nervous the steering behaviour
Wheelbase
short = agile driving behaviour
long = good directional stability
Chain stays
Short chainstays mean higher load on the rear wheel
Bottom bracket drop / BB drop
The bottom bracket drop (vertical distance from bottom bracket centre to axle centre) influences ground clearance and rideable crank length
What characterises small road bike frames (under 52 cm)
- have flat steering angles and consequently slower steering behaviour
- benefit from small wheels that allow more lively handling and better balance
- have (too) steep seat angles
What characterises large racing bike frames (over 60 cm) from
- are comparatively short and high
have comparatively short wheelbases
tend to have slacker seat angles
have too little ground clearance for long cranks
How to determine your body proportions, which determine your road bike size
Your stride length divided by your height results in a factor that describes your body proportions. Example: If you are 178 cm tall, your stride length of 81 cm (factor 0.455) is exactly in the centre of the spectrum. Standard frames are then a good starting point. Long-legged riders tend to choose a more comfortable frame, sitting giants a sportier one. Note: In the measurement sheet, the factor for body proportion is shown as a number between -5 (sitting giant) and +5 (long-legged), unlike in the diagram above.
Choosing the right road bike frame
This allows you to find the right road bike frame based on your body proportions and desired riding position:
Measure stride length
Stand in front of a wall with your feet at pedal distance. Press the back of a book into your crotch and measure the distance from the spine of the book to the floor. There is a tried and tested formula for the correct seat height for road bikes (the distance between the bottom bracket and the top of the saddle): Seat height = 0.885 x crotch length.
Calculate suitable frame height (stack)
Calculate the stack using the formula: 0.69 x stride length (cm) Add 2 centimetres (small frames) or 4 centimetres (large frames) for a comfort geometry.
Divide your stride length by your height (in cm)
Use the chart above to check whether you have average proportions or whether you are more of a sitting giant or a long-legged rider. Use the following matrix to find the frame with the geometry that suits you best (STR factor) among the frames with the right height for you:
- STR factor < 1.45 = racing seating position
- STR factor 1.45-1.55 = sporty seating position
- STR factor > 1.55 = comfortable seating position
Downloads:
Frame geometry: Wheel measurement (PDF)
