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A recent question posed to us on our YouTube channel got us to thinking...

We've begun a series of technical videos to discuss our new-for-2014 road geometry, and it will all be available on a Geometry and Fit playlist on our Orbea YouTube channel.

The other day, we received a good question from a viewer who asked about our viewpoint on seat tube angles. He stated that he rode a small-sized bike with a 74.3 degree seat angle. Here's how we responded:

Thanks for your question. We’ll answer it by first making the statement that a properly designed and sized road bike allows a small range of adjustment for the rider in the familiar places – stem length, handlebar height, saddle fore/aft, saddle height, etc. To that point, we believe that 73-73.5° seat tube angles are optimized as a baseline for proper fit for the majority of road riders. That said, people are different and everyone fits differently on a bike depending on riding style, preferences, injury history, flexibility, and general athleticism. Our goal is to create bikes that fit the greatest number of riders while ensuring the ability to obtain the best possible fit.

Your 74.3° seat tube angle isn’t wrong if you can ride comfortably and can pedal efficiently. And our stance on seat tube angle isn’t necessarily revolutionary. However, we strive to maintain a very small angular range between our smallest and largest sizes. The reason for this is to provide the same ability for fore/aft saddle adjustment for the smallest and largest riders.

All too often, small road bikes are made with seat tube angles that are steeper than the 73-73.5° range that we feel is best. It is not difficult to find small road bikes with seat angles at 75-76°. What could be the reason for this?

Perhaps the manufacturer builds the entire size range with one fork rake throughout. This leads to a normalization of head tube angles to maintain a consistent “Trail” measurement throughout the lineup. Trail is synonymous with caster and is what defines the steering characteristics of the bike. This approach can lead to problems at the small end of the size range. To be honest, we’ve created problems for ourselves in this way in the past. However, we’re always learning and trying to get better all the time!

As a primer on fit for small riders, we need to start with some basic information. Some of it may seem trivial or obvious, but a complete understanding of the dynamic geometrical relationships involved in bike design takes into account many factors. Read on.

Smaller riders require a shorter reach to the handlebars. Therefore, smaller riders require a frame with a shorter reach (we have a video that defines this concept). As the reach shortens, and with a given head angle, the front center shortens. This is the distance from the hub center to the bottom bracket center. This measurement is important because if it is too short, you could experience toe overlap. Toe overlap is not desirable for obvious reasons if you’ve experienced it while turning at low speeds in a parking lot. Nevertheless, a typical 700c front wheel requires a minimum working front center for safety and rider comfort.

The typical workaround for this one-rake-small-sizes-front-center dilemma is to build around a minimum front center for the few smallest sizes. With a given head tube angle, this means that more than one size bike frame can have the same or similar reach in relation to the bottom bracket. And how to “create” a size progression at this point? Simply steepen the seat tube angle to reduce the top tube length. This is an easy way to create a size run, but perhaps not the best way. The danger is that some smaller riders may be forced into a too-forward position or have to incur the cost or trouble of finding an extreme setback seatpost to allow a proper adjustment range. When moving backwards to accomodate an ideal position over the bottom bracket, then the effective reach to the handlebars stretches out once again. For some, this creates a "properly" sized bike that is actually too long. Sure, the stem can be sized down to bring the bars back to the rider, but are these kneejerk reactions the right way to get fit on a bike? Perhaps not. For some, it may present no problems at all. Again, bike fit is based on many variables.

Why build with one fork only? Cost is the issue, as well as the historical view of bike geometry and sizing focused on seat tube and top tube lengths.

The undercurrent of our new geometry is stack and reach. What we’ve done is create our new-for-2014 road bikes with sizing that ensures that the smallest frame offers the smallest fit, and they go up from there in a logical and nearly linear progression. We’re building bikes with multiple fork rakes to strike that balance of consistent handling qualities across the size range, as well as to enjoy the ability to manipulate the head tube angles to suit a particular size, all the while protecting a minimum front center for safety. We also strive to keep our seat angles within a very narrow angular range. Once again, our goal is to create bikes that fit the greatest number of riders while ensuring the ability to obtain the best possible fit.