In the current issue of VeloNews, technical writer Lennard Zinn helps you work through the steps necessary to find that perfect position. As mentioned in the article, Alan Hills of Hills-Scientific.com in Boulder, Colorado, has shared a handy program he designed to choose the stems for his bikes. As we promised, we have included Hills’s executable calculator below.
(Editor’s Note, October 2010: The calculator page referred to in this 2003 articles appears to be lost, the victim of our site’s two platform changes since then. But we found a stem calculator site that seems just as useful.)
This executable function is to make it possible for you to predict the reach and rise of the top of your handlebar from the steering tube at the top of your headset with stems of different lengths, angles, and clamp heights.
Note the following, especially noting the method for determining stem angle (and whether it is positive or negative):
Reach is the horizontal distance from center of steerer (at the center of the stem steerer clamp) to bar center.Rise is the vertical height from base of stem steerer clamp (headset top if no spacers under stem) to bar center.Angles are in degrees.Head tube angles are positive.Stem angles are relative to 90 degrees from the steering tube (i.e., a 90-degree stem would have a stem angle of zero).Stem angles are positive if greater than a right angle and negative if less than a right angle.All lengths are in millimeters.If you are using spacers under the stem, add their total thickness to the Rise
Calculating stem length and angle for your bike
To use this executable Web calculator yourself, you first have to figure out where you want your bar to be. You will need to know the head angle of your bike and the angle of your existing stem.
If you already know about where you want it, this may be as simple as acting on the feeling that you want to have your hands, say, 2cm (20mm) further out and a centimeter (10mm) higher than on your current setup. Then you can plug in the specs of your current stem into the formula to see what vertical rise and horizontal reach from the center of the steering tube at the base of the stem clamp you have. You can then add 20mm to the reach and 10mm to the rise and compare those numbers to the rise and reach obtained by plugging different available stem options into the formula.
If you have a quill stem and threaded fork and want to recreate the same position with a threadless fork and stem, you can do that with this executable Web calculator, too. You first need to know the stack heights of your old headset and of your new, threadless one. Subtract the latter from the former to find out how much higher the top of your old headset is. Then, in the calculation for Rise, replace the “stem clamp length/2″ term with the distance from the top of the headset to the center of the stem barrel’s intersection with the quill added to the headset stack-height difference. Running these numbers through the formulas, you will come out with the vertical rise you are looking for above the top of your new threadless headset, as well as the horizontal extension from the center of the steering tube at the top of the new headset. Then you can try various specs for a threadless stem with which to replace your quill stem until you find one that gives you the same reach and rise.
If you do not have a starting point from an existing bike, you will need to figure out where to begin. The stem length and angle that will fit you best are obviously dependent on the top tube length and the height of your head tube relative to your saddle, but there is not room here to go into how to determine your frame dimensions and stem length. If you are considering buying a new bike and want to know the dimensions to start with, there are online and print resources as well as your local bike shop. Online, there is a quick bike-dimension calculator on the “Fit” page of www.zinncycles.com that gives seat tube, top tube, and stem lengths. Other online options include www.wobblenaught.com, which offers a far more in-depth fitting, while www.bikefitting.com provides a calculation of seat tube length only online and a listing of where you can find a shop near you equipped with the extensive Bikefitting.com Analysis System. Print fitting options include Appendix C of either ” Zinn & the Art of Mountain Bike Maintenance” or “Zinn & the Art of Road Bike Maintenance.”