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When it comes to understanding frame geometry, there is one important fundamental in design that we have to first take note of: the true importance of frame geometry is to ensure that there is a good fitting frame available to every cyclist out there. Handling and performance come second.

It is understood that a cyclist’s body posture lean angle varies according to his/her capability, and this leads to the varying placement of contact points (pedals, saddle, and handlebar) when comparing among multiple cyclists of the same body build. 

How does the positioning of contact points affect your posture?

The older way of casually identifying bike sizes by their seat tube length (STL) or top tube length (TTL) is usually inaccurate in getting a good fit. Many times, you will hear cyclists claiming they ride a 545mm top tube or 52cm seat tube frame and go ahead to make an order.

Only after the bike is built up, they find the handlebar placement is either too low or too high relative to the seating position, or the saddle cannot yield a comfortable supportive seating as the positioning is restricted by a setback seatpost. These mentioned encounters are very common among bike buyers, regardless they are beginners or veterans.

The reason for this is because seat tube or top tube lengths do not accurately represent the true sizing of a frame as there is no accurate reference as to where the contact points are placed relative to each other. Knowing this issue faced by buyers, in recent years, bike manufacturers have started to include a much easier identification using STACK and REACH when stating bike sizes, which accurately represent ‘true’ sizing of road bike frames. 

How to differentiate bike frame designs that would fit you best to gain maximum performance from it?

By using the center of the bottom bracket as a fixed reference point and measuring the distance from that point to the top of the head tube, STACK indicates the vertical height while REACH indicates the horizontal distance. With this data, we can safely locate the relative position of the handlebar when comparing geometries between different frames. 

In addition to that, to cater to masses of various capabilities, in recent years, many manufacturers have incorporated multi-position flip-flop saddle rail clamps onto frame-specific seatposts, similar to ones found on higher-end bikes the likes of Specialized’s Venge or Giant’s TCR. Simply by setting up the clamps in different positions, we can replicate multiple seat tube angles from the same seatpost and optimize the saddle position to achieve the best fit. 

Now, how does one tell which bike is suitable for purchase simply by looking at STACK and REACH?

Generally, endurance/sportive frames have slightly taller head-tube compared to race/aero frames, with a higher STACK difference commonly in the range of 20-30mm. This allows cyclists with more upright posture to be able to reach the handlebars comfortably throughout their cycling journey. When “cyclist with upright posture” is being mentioned here, it is a relative comparison between cyclists of different capability categories. It does not mean the rider is a weaker cyclist, but rather one who does not pedal heavy gears like a racing cyclist. 

This endurance category of cyclists takes up the largest portion of the road cycling population and global sales for endurance bikes cover a larger portion of the market as compared to race bikes. So don’t you worry about being slow when you show up at a ride with an endurance bike, but rather be proud of yourself for being able to finish longer distances with better average times and most importantly, pain-free!

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