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Bike Sizing Guide - Road Bikes.Choosing the correct bike should be similar to buying a new pair of shoes. The choice may be for general use, or a specific function. Style, brand, colour, price, availability and durability are all key considerations, but no one would buy a pair of shoes if they didn’t fit. The same should be said of bicycles, but far too frequently cyclists buy bikes that are the wrong size for them. Fortunately, the inherent adjustability of bikes means that you may be able to get away with a bike one size away from the ideal, but any more than that and it is unlikely you would ever be comfortable.
Bike Frame Geometry
Frame SizeSo how do you make sure a bike is the right size for you? By far the most important consideration is the frame size. Although there are many dimensions used to define a frame, ultimately there are only three that are important, the seat tube angle and the horizontal and vertical distances between the crank centre line and the top of the head tube. The traditional way to define the top of the head tube was to measure along the seat tube from the centre of the bottom bracket, then horizontally forwards along the top tube. This worked very well when frames had horizontal top tubes but can get confusing with ‘compact’ frames with sloping top tubes.
Conventional road bikes have seat tube angles within a relatively small range (72.5°-74°), so if only considering this type of bike, you could argue that there only two key dimensions, hence some manufacturers have started to use ‘stack’ and ‘reach’ to define the relative position of crank and top of head tube. This ignores the seat tube angle altogether and although people would like to suggest it is ‘better’ geometry, it tends to be similar, just measured differently.
So, just as we have started to ignore seat-tube angle, it is worth considering why everybody has used essentially the same angles here for so many years. In simple terms, we lean forwards on our bikes to maximise our aerodynamic and biomechanical efficiency and the approx. 73° seat tube angle gives the best compromise of these factors along with balance and weight on our hands and arms. Fit Guidelines shows how to achieve the correct saddle height, fore-aft location and suitable upper body positions. The frame size should be chosen to allow the hands to fall naturally to the handlebars whilst adopting these postures. This may need some adjustment of the stem length and height, but this is a relatively cheap component and easily worth changing to ensure comfort.
What can go wrong?The main reasons an ‘off the peg’ bike might not be an ideal fit usually fall into one of three categories.
Body ProportionsWith help from Leonardo De Vinci’s Vitruvian man we can begin to understand why body proportions can be so important. As a rule, our span is as wide as we are tall, so comparing both measurements will imply if you tend to have long or short arms for your height. The ratio of inseam to height is the most critical metric though. Men will typically have an inseam 46.5-47.5% of their height and although it may not sound like a big difference, less than 45% or more than 48% is noticeable short or long legged respectively. Contrary to some popular opinion, women do have longer legs, with 47.5% to 49.0% the usual range. So knowing this simple information, how might it affect our choice of bike.
Narrow / broad shoulders
Narrow / Broad hips.
Loose hamstrings are very useful for cyclists, permitting more knee extension and more aggressive torso angles. This only becomes an issue from a sizing perspective if combined with relatively long legs and as we often see with women, a plantar-flexed (toe down) ankle posture. These all combine to move the saddle up and away from the handlebars giving the ‘Long Legs, scenario above.
Tight hamstrings and lower back will inhibit knee extension and the ability to lean forwards. This will give a upright torso, compromising the reach to the handlebars.
Design and Manufacturing Implications.Small bike wheel / foot overlap.
You may notice that smaller bikes tend to have more upright seat tubes. This is driven by our desire to commonise wheel diameters to 700c. The more upright seat-tube prevents any clash between the front wheel and the feet when turning sharply. This forwards saddle position can lead to excessive weight on the hands so seat-posts with more set-back and shorter stems may be required.
Crank length selection.
There seems to be an increasing trend for some manufacturers to regard 172.5mm cranks as the new ‘standard’, fitting them to the majority of the bikes in the their ranges, including the relatively small or medium 52cm, 53cm and 54cm sizes. One might imagine that there may also be a financial incentive to do so as manufacturing cost and complexity is reduced. Cranks that are too long will inhibit the riders ability to 'spin' and can often lead to knee and lower back troubles.
So What is Your Size?
This table is a summary of the geometry of a large selection of roadbikes on sale in the UK today. It is by no means exhaustive, but does cover bikes marketed as both 'race' and 'Sportive' oriented. One can see that there is considerable overlap between sizes and variety within one size category.|
*Unlike all the other dimensions, the crank length is the generally acknowledged recommendation based on the inseam measurement, not the items typically supplied by the manufacturers.
It is not intended that this table is used to define your correct size, but to be able to compare the geometries of any bike you may be interested in purchasing against the 'norms' within that size range. By coupling this with knowledge of your body proportions, flexibility and intended riding style, a better judgement may be made.|
If still bewildered by the choice of bike size, the best approach would be to visit as many bike shops as possible , compare their advice and sit on a few bicycles. Preferably you will be able to pedal the bike on a stationary trainer and the saddle should be adjusted to somewhere close to what you need. Alternatively, see Bike Sizing Options for the telephone consultation or sizing jig options available from BikeDynamics Ltd