Limits on steerer tube extension
As I finish measuring, fitting, etc. on my project bike and contemplate putting sawblade to steerer tube, it occurs to me that there is likely a rule of thumb for determining the maximum amount of steerer tube that can extend above the headtube top race (excluding the stack height of the stem itself). The most likely variables to consider should be steerer size (1" or 1-1/8") and steerer material (Cr-Mo, Al, carbon). For example, LBS recommended AGAINST carbon steerer tubes in 1" size. Anything else?
My situation is well within my comfort level as I only need 20-25mm additional stack for ideal handlebar placement and have a 1" Cr-Mo steerer tube. If I needed more than 75mm I'd start to worry a bit. I was simply wondering if there are any general rules that might exist. Any first-hand experience with failures?