Interesting tech article on Velonews about Hincapie's steerer failure
Spacers and stress
Regarding the George Hincapie spill at Paris-Roubaix, I always wondered why that happened and then something similar happened to my brother recently. His alloy steerer tube broke in half flush to the top of the headset. He was later told that a threadless stem should be placed onto steerer with none or very few spacers over it. The rep told him that adding spacers might create a stress point on the steerer and was perhaps reason for the failure. My brother is fine and is getting a free replacement fork, but you see the issue here. If people buy bikes and are not informed, there is a potential hazard out there.
If what my brother was told is true, the information is not posted on the product itself in the form of a sticker or mentioned as a safety concern on the manufacturer's web site or manuals included with any of the forks I own.
I think most all new bikes come with a good 25 to 40mm of spacers over headset. I adjust my stem height at times during the year. Is this a safety issue?
We could venture other guesses as to the cause of steerer breakage relating to handlebar torque, the star nut position relative to the handlebars, etc., but in my brother's case he had about 40mm of spacers over stem (to allow for later adjustments) and no spacers under stem.
I decided to ask the manufacturer instead of trying to answer this one myself.
Answer from Trek:
The number and placement of spacers does have an effect on the fatigue life of an aluminum steerer tube. Running less than 5mm of spacers under the stem can concentrate or "point load" a great deal of stress in one spot on the steerer. Therefore, Bontrager recommends a minimum of 5mm, and a maximum of 40mm of spacers between the stem and the headset to maximize the fatigue life of the steerer.