Frame Geometry Comparison

A version of this article was originally
published in Triathlete Magazine

I’ve been told that I am a Medium Elite T-Class or a 54cm Cervélo, but I don’t know what frame dimensions I should be paying attention to and how they affect the bike fit. I’m 5’10” with a 31” inseam. Thanks in advance for the help.

Dennis, via the internet

Dear Dennis,

Before discussing the geometry of the bikes in question, let’s focus on bike fit as your riding position should determine your frame geometry and not the other way around.

Fitting and Fitters: While most runners would not buy shoes without knowing how they fit, one of the most common mistakes many athletes make when choosing a new bike is this very thing. They choose the bike based upon what they read, the way it looks, or what their friends say before taking into account how the bike fits their individual needs. This forces the rider to try to adapt to the bike, when, if you want to perform to your potential, your bike should adapt to your needs. The most reliable way to find a bike that fits you well is to be professionally fit first and then use this information to help you find the best frame/bike matches.

While claims of “expert fitters” abound, the reality is that fit quality can vary immensely from fitter to fitter and you owe it to yourself to be very selective. Be sure to interview your fitter, make sure they have a firm understanding of triathlon, aero positioning and biomechanics, and do not be afraid to travel to get the job done right. You get what you pay for and a comprehensive fitting should take a few hours. Your fitter should inspire confidence with their knowledge and fitting protocols. If a fitting is below your expectations or incomplete, do not hesitate to try other fitters until you find what you are looking for.

How does the fit on a Cervélo P3 Carbon and an Elite T-Class compare?

Despite what some Web sites claim, bikes cannot be consistently sized by body height, inseam length or other measurements. Without understanding your riding position and technique better, I cannot tell you what size frame you should ride or whether a given frame will fit you well. This being said, I can tell you what to look for in a well fit frame and how that relates to your riding position.

From a positioning perspective, there are three interrelated dimensions on a frame that you should be aware of:

#1 and #2) Seat Tube Angle and Top Tube Length have to be considered together. Seat tube angle plays a role in determining where your seat can be placed while top tube length directly impacts the reach from the saddle to the handlebars on the bike. While a saddle can often be slid fore or aft on its rails to achieve a different seat angle, remember that doing so directly impact the effective top tube length/reach of the bike. Pushing a saddle forward from the initial seat tube angle shortens the reach (effectively reducing top tube length), while pushing it back lengthens the reach. When considering any frame, make sure that your reach at your ideal seat position can be achieved with a properly sized aerobar and a realistic length stem (while there are exceptions, 9cm to 11cm is a common range on a triathlon bike for most average size riders).

One degree of change in seat angle results in about 1cm of effective top tube length/reach change. So, while a Medium Elite T-Class and 54cm Cervélo P3 Carbon both have 53cm top tubes, because the Elite is based on a 76.5 degree seat angle and the Cervélo is based on 78 degrees, at a given seat position, the P3 Carbon effectively has about 1.5cm more reach than the T-Class – making it fit effectively a full size longer than the Elite.

#3) Head Tube Length determines how high or low a handlebar can be positioned, thus it is very important to positioning. However, head tube length is arguably the most ignored fit dimension on the bike; some manufacturers did not even list the dimension as recently as a year ago. If your head tube is too long, you will not be able to get your handlebar low enough. If your head tube length is too short for your ideal riding position, you will not be able to position your handlebar high enough without exceeding the fork manufacturer’s safety specifications for headset spacers and/or using a very high rise stem. This can create torsional flex and potential instability in the front of the bike and can jeopardize the fork’s strength. Ideally, you want a head tube length that allows adjustment room up or down and that is compatible with the aerobars you plan to use, all of which fit differently.

While the 54cm P3 Carbon’s head tube length of 10.5cm and the Medium T-Class at 11cm initially appear very close, the dimensions on their own are misleading. Cervélo uses internal headset bearings that are built into the frame while Elite uses external bearings that are pressed into and extend beyond the frame. External bearings extend the effective head tube length by 2-3cm above the measured length, thus the Elite’s handlebars can safely go 2.5cm-3.5cm taller than the Cervélo – a big difference in how the two models fit and sometimes the difference between one fitting well and the other not. As with top tube length and seat tube angle, you must know how long a head tube you need in integrated and non-integrated designs if you want to know if a frame fits you well.

While many of the dimensions on the T-Class and P3 Carbon initially appear similar, the reality is that these two frames fit quite different. The rider that fits well on one may not fit so well on the other. To eliminate the guesswork, get professionally fit first and allow that information to help you explore how each frame really fits you. If you find that neither works well, remember there are custom builders out there (including Elite, Guru and Serotta) that can build you a compromise free custom tri bike that matches your riding position and needs ideally.

Train hard and train smart!

Ian

Originally published September 2006/Copyright © 2006