Stack and Reach

published in Triathlete Magazine

Dear Tech Support,

I’ve been told that “Stack and Reach” are important numbers to know when it comes to bike sizing. What is “Stack and Reach” and how can it help me size a bike properly?

Rich, VA

Rich,

A bike’s stack and reach dimensions, in conjunction with the stack and reach needs of your riding position, can be helpful numbers to know when considering bike options as they provide a lot of valuable information about a bike’s actual geometry and fit in just two numbers.

“Stack” is the vertical distance, in centimeters, from the center of the frame’s bottom bracket/crank to the top middle point of the head tube (where the fork passes through the frame). “Reach” is the horizontal distance from the center of the bottom bracket/crank to the top middle point of the head tube. For the mathematically inclined, stack and reach is a Cartesian coordinate system with the origin being the center of the bottom bracket and the X,Y being the top/center of the frame’s head tube. For example, a 51cm Cervelo S2 has a stack (Y) of 52.2cm (top of head tube is 52.2cm above the bottom bracket) and a reach (X) of 39.4cm (middle of head tube is 39.4cm in front of the bottom bracket).

Why are a frame’s stack and reach potentially helpful numbers? If you know the stack and reach requirement of your riding position (acquired by working with a bike fitter who uses a sizing cycle that will provide the measurements or measuring these distances on an existing bike that is properly set-up for you and fits you well), you can look at the stack and reach dimensions on a frame sizing chart and know whether the bike will fit you well before you buy it.

The potential benefits of a stack and reach based system compared to traditional bike size listing (Medium, 54cm…) is multi-fold:

1. It standardizes bike geometry/sizing between brands and models as it transcends the limitations of looking at just a single size related variable (top tube length, seat tube length…) on a bike. Stack and reach dimensions consider a number of key variables in relation to each other and thus offer significantly more insight into how the frame actually fits than any other two measurements on the bike.
2. It stresses the importance of rider first bike selection by placing the most important part of the bike selection process – the rider’s position – first. In order to use stack and reach well, you need to know the stack and reach coordinates of your riding position too.
3. It is not unheard of for bikes from two manufacturers that are both called the same size (Medium or 54cm, for example) to actually fit up to 2cm (a full size different) than each other. Stack and reach shows how the frame actually fits/sizes and thus allows it to be compared to other bikes on the market accurately and relatively easily.
4. It distinguishes between sometimes confusing and inter-related frame dimensions. For example, it can be easy to confuse seat tube angle (the angle a frames’ seat tube is actually built to) and seat angle (where the rider’s seat should actually be placed in space in relation to the bottom bracket). Stack and reach virtually eliminate this confusion by simply showing how the frame actually fits.

Stack and reach is a significantly improved standard than the traditional sizing methods that are based on effective seat tube length and other outdated frame dimensions that are virtually irrelevant when it comes to how the bike actually fits. Stack and reach is not without challenges though, and it is important that you understand what the dimensions do not provide.

1. Frame stack and reach dimensions do not take into account handlebar shape or stem length/angle and spacers. The size cycle based stack and reach template we use in our fittings can take this into account and there are other systems available that do as well. Regardless, you must know what handlebar your stack and reach coordinates were based, and how those dimensions relate to the dimensions on the handlebar that either comes with the bike or that you will be using, as these can vary significantly and directly affect fit.
2. Stack and reach does not include headset type. There can be up to 3cm of effective stack difference between internal and external headset frames. If you are considering a frame with an external bearing headset (some custom and specialty brands) make sure to add in the headset you will be using to the listed stack dimensions.
3. Stack and reach does not consider potential seat angle limitations on the frame. For example, if you ride a 75 degree seat angle and are considering a frame based on a 73 seat tube angle, make sure that there are seatpost options for the frame that will allow the seat to be set to 75 degrees. This can be a problem, especially with frames that use integrated seat masts or a proprietary seatpost design. Even if the stack and reach dimensions look fine on paper, you need to be sure that any frame you are considering offers a configuration that will allow your saddle to be set to your riding position.

Stack and reach is a step in the right direction and a notable improvement over the “systems” the bike industry has traditionally used. However, the best sizing representation I have seen comes from a manufacturer. Argon 18 on their E-114 TT/Tri bike that shows the factory recommended set-up ranges for key positioning items (saddle height, setback, handlebar drop and reach) for each size.  I hope that Argon 18 considers making this sizing recommendation method more visible on their site and expanding it to include all their models (road and tri). I also hope other manufacturers adopt a similar system as the bike industry could make manufacturer recommended sizing ranges much more clear to consumers and bike shops.

Whether you are using stack and reach or some other frame sizing system, make sure you get a professional fit with a qualified fitter who also understands bike geometry before you settle on a bike. A rider first fitting places the needs of the rider in front of the needs of the bike and guarantees that you will not be guessing when choosing what is likely your biggest equipment investment – your bike. Once you know your positioning coordinates, your fitter can help you narrow your options to include only those bikes that have stack and reach that are appropriate to your needs before exploring other important variables like handling, ride quality, cost and durability to find the best match for you.

Ride hard and smart.

Ian

Ian Buchanan is co-owner of Fit Werx. Fit Werx has locations in Waitsfield, VT and Peabody, MA and offers cycling and triathlon products, specialty bicycle fitting and analysis services, consultation, and technology research. Fit Werx can be reached in VT at (802)496-7570, in MA at (978)532-7348 or through the Web at www.fitwerx.com.