This might be the commonest question we get when a new cyclist is looking for their first real road bike. I’m happy to be asked because choosing the right frame size and geometry is important to achieving an excellent and comfortable bike fit. Most cyclists can be fit across two sizes depending on their personal biometrics, flexibility and core stability.
Here in Canada frame size is given in centimeters. It is generally related to the length of the seat tube. Either C-C (center to center) or C-T (center to top). That was more straight forwards when bikes were all straight tubes! It’s still loosely true but now that we use bent tubes and exotic materials like carbon, it hasn’t made things easier! Fortunately every modern road bike has geometry specifications published by the manufacturer.
I find the most important measurements to be the:
- Effective Top Tube length (c-c). Here we do away with the curves and measure the actual horizontal length from the seat tube center to the head tube center. This sets up the reach on the bike and we want to be close to ideal so then we can dial the length in with a stem of 90-120mm which gives the bike predictable handling.
- Head tube length. Manufacturers have recognised that, for many different reasons, not everyone can tolerate an aggressive, low, racing position. Enter the new category of relaxed road frame geometry with a lengthened head tube. The not so flexible rider can sit up a little taller and scan the road ahead more easily with a big increase in comfort. And a comfortable bike is the only one you will keep riding.
Personal Biometrics There are lots of online guides and calculators such as https://www.ebicycles.com/bicycle-tools/frame-sizer They are based on your height and leg length and will get you in the ball park. Or you will see estimate charts like the following. I find they work well for people of even proportions, torso to leg length. What they do not do is predict for the long leg/short torso or conversely short leg/long torso. I said most people can ride across a couple of sizes. If you are the former you will probably want to go with the smaller frame option.
An estimator will also struggle at the extremes of adult sizes. If you are under 152cm (5 feet) tall or over 198cm (6’6″) do extra homework to help with your selection. And if you happen to be blessed with long arms in relation to your height then we love you! It gives more wiggle room to establishing your comfortable reach point.
This is but a part of fitting but it needs to be right for the rest of the fit to flow to power and comfort.
I welcome comments, feedback and questions.
Stay safe this season and keep the rubber side down.
Matt Powell is an experienced sports physiotherapist, bike fitter and cyclist. He is the owner of West 4th Physiotherapy Clinic in beautiful Vancouver, BC.
West 4th Physio believes healthy communities are ones that move. We financially support local clubs and initiatives that promote active lifestyles. Our current sponsorships include: