My personal rule, which is conservative, is the stack below the stem shouldn't be taller than the diameter so I prefer to stay below 25mm. Others say up to 40mm is fine and it probably is, but beyond that may be pushing a good thing too far.
It becomes a question of bending moments at the place where the steerer exits the top of the headset. The total moment is based on the straight line distance from the handle bar to the top of the headset, so those with long extensions should be more conservative than those with only 90-100mm extensions.
How far you can fudge also depends on your weight, strength and how and where you ride. If you're heavy or a hard climber or sprinter, who tends to pull on and work the bars a lot, stay conservative, but of you're a light flat ground spinning type rider the stresses will be lower giving you more room to play with.
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FB
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Last edited by FBinNY; 03-31-11 at 08:58 AM.