I need help understanding the realtionship between spacer arrangements and the safety of carbon steerer tubes

[bmxbandit]

Over the past few years I've seen a trend for manufacturers to recommend very specific spacer arrangements for forks with a carbon steerer, to the point where all the major brands we sell at the shop now have a detailed recommendation for correct assembly. They all now specify either no spacers above the stem or a max of 5mm, some also specify there must be minimum of 5mm underneath too.

I'm struggling to understand why, for example, an arrangement of say 30mm under and 10mm over is significantly different to 35mm under and 5mm over. Also, why would there need to be a 5mm spacer under the stem, particularly where the headset topcap/cover is already 5-10mm tall itself.

I have heard the argument from manufacturers that the compression bung is supporting the bolts and therefor requires this precise set up, however the bungs in question are only deep enough to support one of the stem bolts. We also have one brand which still uses the older and heavier full length bungs which supports pretty much all of the exposed steerer, but they still give the same 5mm over/under advice!

From what I can understand in terms of the loads involved the stem position is the major factor ie the higher the stem, the more the leverage. So why would a slammed stem with say 10mm over be a safety issue when it's ok to run 40mm spacers with nothing on top?

[Treelegs]

Probably a legal department issue. A shop is only allowed to setup a bike in configurations which have been tested by the Designer/manufacturer.

This way if a bike is damaged while in any other configuration than those specified warranty/insurance claims could be contested as "improper use".