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Cutting the steerer on a carbon fork the other day I noticed that the steerer has two distinct layers, black on the outside and light grey on the inside. Fibreglass? Why?
Direction of fibres? (at a guess - outside mostly longitudinal to resist bending, inside perhaps more circumferential to resist crushing).
Could also be a weave vs UD tape layer.
Fiberglass is much 'tougher'. For parts designed to bend it's a better material than carbon which tends to become more brittle the higher modulus you go, it's just not as sexy.
Tiller extensions (and tillers) on dinghies take a similar amount of stick (pun intended) and are usually either made from fiberglass with a black resin, or a carbon outer layer to hide the fact, or a mix of carbon and fiberglass layers. It's not cheaper, it's just that 100% carbon tends to break a bit too quickly.
Same for tennis rackets and ice hockey sticks. Mostly glass fibre in the middle with a pretty a layer of carbon on the outside because people like it.
a layer of fibreglass on the inside face of a steerer makes sense it's going to be in contact with the plug (probably a knurled aluminium surface) so you are less likely to get anodic reaction plus it'll put up with a bit of abrasion on assembly...
Odd thing is I don't think my carbon steerer has a Fibreglass layer, meh nevermind...
How would you know?
We had an argument recently in the class over the use of carbon tiller, they're banned because they're better and aluminium, but expensive and fragile. The manufacturer had been supplying suspicious looking black composite extensions and had to come to the AGM to explain that they were in fact glass with black resin and black roll-wrap, he could supply them in any colour we wanted, but people wanted black because it was practically indistinguishable from carbon.
Ah yes you do sometimes have a sacrificial fibreglass layer. It'd be < 0.5mm thick though, so not that consistent with what the OP describes seeing.
Glassfibre can be tougher, but I'm not sure that would offer massive benefits somewhere like a steerer tube where you'd need to use an awful lot of it. If you call it Kevlar it still sounds sexy.
I can post a pic next week. I was quite impressed as to me blending different materials shows attention to design.