Effect of lower headset stack height on angles

[rpenmanparker]

While looking for a new headset I noticed that the lower bearing stack height can vary slightly among models and brands. The variance is lower in integrated headsets which have hardly any lower stack height to begin with, but can still be as much as 1-2 mm. More in non-integrated headsets. Unlike the upper stack height which just effects the handlebar height, lower stack height effects the height of the front of the frame and the angles of the head and seat tubes. In fact I once had a 27" frameset converted to 700C by substituting a 700C fork with added stack height in the form of a brazed-on shim at the top of the fork crown. So my question is: when choosing a head set does one have to match the lower stack height to the frame designer's intended amount in order to have the proper angles and resulting handling characteristics? Or is the variance negligible? Thanks for any light you can shed on this.

 

[alfeng]

While looking for a new headset I noticed that the lower bearing stack height can vary slightly among models and brands. The variance is lower in integrated headsets which have hardly any lower stack height to begin with, but can still be as much as 1-2 mm. More in non-integrated headsets. Unlike the upper stack height which just effects the handlebar height, lower stack height effects the height of the front of the frame and the angles of the head and seat tubes. In fact I once had a 27" frameset converted to 700C by substituting a 700C fork with added stack height in the form of a brazed-on shim at the top of the fork crown. So my question is: when choosing a head set does one have to match the lower stack height to the frame designer's intended amount in order to have the proper angles and resulting handling characteristics? Or is the variance negligible? Thanks for any light you can shed on this.

Based on 'my' observation -- and, therefore in my opinion -- the variance is negligible ...

Without actually going back and measuring, I would say that about a 1" difference (yes. 2.54cm ... maybe it was more, maybe it was less) in the equivalent difference to a LOWER-or-GREATER stack height would result in about a 1º difference in the head tube angle ...

The actual difference was made in using a fork with a different shoulder-to-dropout distance but which has approximately the same offset as the other fork ... for example, THAT difference between a ROAD fork and a CX fork is probably about 1" (again, maybe it is more, maybe it is less).

So, interpolate the angular difference, accordingly.

 

[alienator]

Ok. First you gotta remember that changes in lower headset stack height change the distance to the ground along the headset axis AND the handlebar height. So, assuming the head tube angle is 73° and you increase the lower headset stackheight by 1 cm, the effective angle will increase by approximately 500 quadrillionths of a degree. I think that's small enough to be considered negligible.