February 17th 2014

The effect of pedestaling on time trial bike position

To "pedestal" an aerobar means to elevate the bars above the basebar using risers instead of using headset spacers:


David Millar, 2010 Giro d'Italia.

You might do this for a few reasons:


So, assuming you currently have a bike that uses headset spacers, what would be the effect on your position if you were to replace, say, a 10mm headset spacer with a 10mm armrest riser?

First, let us consider the effect of removing the headset spacer. The crucial insight is that removing a 10mm spacer will not lower the stack height—ie. the vertical distance relative to the bottom bracket—by 10mm.

This is because spacers are not oriented perpendicular to the ground; they are stacked along the steerer tube at the head tube angle of the frame. We will assume a head tube angle of 72.5° degrees.

We can calculate that removing a 10mm spacer will reduce the stack height by:

sin(72.5°) × 10mm = 9.54mm

... and by the same argument it will also increase the effective reach—ie. the horizontal distance relative to the bottom bracket—by:

cos(72.5°) × 10mm = 3.01mm

Next, let us consider the impact of adding the riser. These are oriented perpendicular to the ground, so its addition makes no further change to the reach. However, we can now calculate the overall change in stack height:

Δ stack = -(sin(72.5°) × 10mm) + 10mm = 0.46mm

We can then repeat the calculation for any length of replacement:

Replacement (mm) Δ stack (mm) Δ reach (mm)
10 +0.46 +3.01
20 +0.93 +6.01
30 +1.39 +9.02
40 +1.85 +12.0
50 +2.31 +15.0
60 +2.78 +18.0

From this table, I can discover that if I were to replace 30mm of headset spacers with 30mm of risers it would:

  1. Increase my effective stack height by 1.39mm (likely neglible).
  2. Increase my reach by 9.02mm. This might require me to use a shorter stem to get the same position. If that was undesirable—for example, if was already using an extremely short stem—I might be forced to abandon the idea altogether.

Three further considerations must be noted:

  1. The resultant low height of the base bar could be quite drastic and prevent you breathing properly whilst climbing.
  2. Having the steerer tube cut down after to removing headset spacers will reduce the resale value of your bike.
  3. You might need to re-cable your brakes as you may have changed the distance the cables must span.

(It may seem odd to provide results for up to 60mm of headset spacers when such a large number of spacers would—at the very least—void one's warranty. However, I suspect such setups are transiently common within the confines of a fitting studio.)

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