Understanding stack and reach is critical in comparing and finding the ideal frame size but there still seems to be a cloud of mystery around what those measurements are and how they are determined. While it is true that you don’t necessarily need to know the stack and height of your handlebars to be dialed in on your bike, but knowing those numbers can be critical in matching your position on a new bike, choosing the optimal frame size, or understanding how changes in components will alter your position.
What is stack and reach? Frame stack is the vertical distance from the bottom bracket to top of the headtube. Handlebar stack is the vertical distance from the bottom bracket to the center of the handlebar. Frame reach is the horizontal distance from the bottom bracket to the center of the headtube. Handlebar reach is the horizontal distance from the center of the bottom bracket to the center of the handlebar. We can also determine saddle stack and reach in the same manner. Essentially this is establishing an X/Y grid on the bike where the axis is at the bottom bracket. This grid method gives us a very precise manner in determining where the contact points in space are going to be.
Why is this better? It gives us a better understanding of how the bike we will positioned to your body. For example, seat angle and saddle fore aft can affect reach drastically. Stem length can equally adjust reach. It is possible to run a 56cm and 58cm frame with the exact same reach but the weight distribution of the rider (essentially where the bike will sit underneath them) will be in distinctly two different places. One may be more optimal for how that athlete is going to be using the bike and without comparing stack and reach measurements we won’t have an ideal picture of what those differences will be between the two different frame sizes.
How do we physically measure stack and reach? The cheapest way is to put your bike in the corner of a room with the rear tire against one wall and the bike leaning against the other. You can then measure from the wall that your rear tire is touching to the bottom bracket and then to the center of the headtube. Subtract the measurement headtube measurement from the bottom bracket measurement and you will get frame reach. You can then measure from the floor to the center of the bottom bracket, and the floor to the top of the headtube. Subtract the floor to bb measurement from the floor to top of headtube measurement and you get frame stack. A similar process works for saddle and handlebar positions as well.
In the fit studio we use a tool from Purely Custom to make the measurements quick and easy. These tools are expensive, but are invaluable for the numerous bike fits we do weekly at the fit studio.
Cervelo created a cool graphic showing the value of stack and reach measurements. They graphed out the relative headtube stack and reach position of numerous bikes and brands to show how they change through a size range. Any one brand isn’t necessarily better than another, but it is important to know what changes when going from one size to the next for any given brand.