Originally Posted by

**soma5**
A stem that makes a 90° angle with the steerer tube (also called a 0° stem) actually rises at about 17° from the horizontal. That's because the head tube angle is right around 73° (73+17 = 90). An 80° stem rises at 10° less, or 7°. If t is used to represent the angle that the stem makes with the horizontal and r represents the extension of the stem, then the forward reach is given by r·cos(t) and the vertical reach is given by r·sin(t). So if you have an 83° stem, you'd subtract 73 from 83 to get 10, which is your value of t. Just subtract 73 from your stem angle to get t. If you measure your stem angle in terms of deviation from 90° (for example, I see stem angles quoted as ±6°) then your value of t is given by 17±6. That means if you flip the stem up, you'll get t = 23° and if you flip it down, you'll have t = 11°. If your actual head tube angle is not 73, then replace 73 in the above discussion with whatever that angle is and replace 17 in the above discussion with 90 minus whatever your head tube angle is. Clear as mud?

-soma5