Choosing the correct size road bike – Geometry Explained

Choosing the correct size road bike can be a challenge to say the least. There is a huge variety of sizes on the market due to new aesthetically pleasing (and structurally improving) tube shapes and modern styling which makes it all the more confusing. To add to that, not all bikes are measured the same way, so it is crucial to consider the overall geometry of the bike before buying.

Frames can be grouped into 3 main categories; traditional, semi-compact and compact.

Traditional frames (sometimes classed as 50/50 geometry) use a horizontal toptube. This tube will be similar in length to the seat-tube. A formula can be used to help select the correct size frame:

  • inside leg measurement (cm ) x 0.69

It is very important to select the correct traditional frame size because the bike has minimal stand-over height. You want to be able to clear the top tube by an inch or two when standing over the frame.

Our Winter/Audax and the 7005 CX are the traditional bikes in our range.

Ribble Cycles Winter Audax Bike


Semi-Compact frames
have a slight slope to the toptube. This achieves a touch more stand-over clearance and also allows more seatpillar to show. Unfortunately there isn’t a clear cut calculation to work out the correct size because the degree of slope of the toptube can vary, however, to give a rough estimate the following formula is often used:

  • inside leg measurement (cm) x 0.64

The correct size is normally determined by a combination of your height and inside leg measurement. If unsure about the size it is always best to ask the experts as they will know about the specific frame in greater detail.

Our semi-compact frames include the Ultralite Racing, 7005SL, Reynolds 525 Steel, Azzurro, Gran Fondo, Sportive 365, Alloy Sportive bikes and the HF83.

Ribble Cycles HF83


Compact frames
are the last style on the market which are characterised by an even steeper toptube slope. Again, the toptube slope can vary massively so a special formula won’t be accurate. A specific height guide will apply to each model. The compact design allows for smaller triangles to be used which increases the strength /stiffness of the frame, improves the manoeuvrability , handling and also shaves off a bit of weight. Another great feature about compact frames is the ability to incorporate taller headtubes compared to traditional frames. Additional height gives a more upright riding position for comfort, however still maintains a stiff and responsive ride without compromising the stand-over clearance.

The Evo-Pro Carbon, Sportive Racing and R872 frames have compact geometry.

Ribble Cycles Sportive Racing


How do I choose the correct toptube and headtube Length?

The length of the toptube is the most important measurement to consider when choosing a bike. Frame geometries will always give an effective toptube length (C measurement in the image below) which is the horizontal distance from the centre of the headtube to the centre of the seatpillar. This is a much more accurate number to use because it eliminates the differing toptube slopes that frames have.

It is also important to consider is the length of the headtube (F measurement). Generally, the shorter the headtube, the more race orientated and aerodynamic the riding position will be. Taller headtubes relax the riding position and achieves a more upright setup. This is well suited to distance events, training rides and leisure riding; basically it’s less competition focused!

Ribble Cycles Geometry

There is a good chart below which gives recommendations for the rider’s height:

Rider Height Effective Toptube Estimate (cm)
5’0″ – 5’3″ 49 – 51 (XS)
5’3″ – 5’6″ 52 – 53 (S)
5’6″ – 5’9″ 54 – 55 (M)
5’9″ – 6’0″ 56 – 58 (L)
6’0″ – 6’3″ 58 – 60 (XL)
6’3″ – 6’6″ 60 – 63 (XXL)

All that’s left to do is set the saddle height, adjust the bar and saddle angle so it is comfortable and to your own preference.

If you then wished to optimize the fit yet further to iron out all the little imperfections a bike fit will be in order. A professional bike fitter will adjust and tweak your setup to further increase your efficiency, eliminate the chance of injury and in the long run, make you ride faster!

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3 thoughts on “Choosing the correct size road bike – Geometry Explained”

  1. Hi, I’m interested in buying the Ribble Sportive Racing from the Ribble site. However, I’m quite confused about sizing.

    The chart size recommendation on this post conflicts with the information given on your frame size help page http://www.ribblecycles.co.uk/text/framesize.html

    I’m 5’11 and have a 33″ inseam ( 83.82cm ). According to the SLOPING GEOMETRY formula on this page ( 83.82 x 0 .64 = 53.6448cm ), I should be getting a 53cm sized frame. While the chart on this page suggests I go with a 56cm.

    The 53cm sized frame’s top tube is 558mm, matches more closely my current 56cm sized frame with a 560mm top tube. But now I’m not sure which to get.

    So which is right? and which do you reccommend?

    Thanks!

    ——————————————————————–

    “SLOPING GEOMETRY (Sloping Top Tube)
    Use the formula: Inside leg measurement (cm) x 0.64. The resultant size and round up or down to the nearest frame size option. If you are in between sizes we recommend you go for the smaller size.”

  2. Hi
    I am looking to buy my wife your clearance 2016 Evo pro carbon ultegra. What is the top tube length of the XS and S sizes? She is 5’4″ tall and presently rides a frame with top tube 54cm with 8cm stem and she is too stretched out.

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