There is probably an obvious answer to the . . . .but

[MikeW-71]

Ah, you are used to the old Quill stems that could be so easily adjusted. They have almost died out now and new road bikes and MTB's use the Aheadset system that you have on the Trek.

I don't know exactly why, my gut feeling is that it makes the front end stiffer to help handling. I could be wrong.

Manufacturers will normally leave a good amount of the steerer tube so that the bars are relatively high and it is then easy to lower the bars by removing the stem and taking a spacer out. But you can't raise the bars by adding a spacer if it is at max height already, since the steerer tube has been cut to fit. If yo need the bars higher, the best way is to flip the stem over so it points up instead of down and that is usually enough.

 

[Dogtrousers]

One reason is because it suits manufacturers because they only need supply one standard fork, rather than having to cut steerer tubes for each frame size.

Modern headsets are plug ugly compared with a lovely graceful quill stem.

 

[rebelpeter]

Thanks for all ur help ill have a go at it when i get the trek out next, cant really grasp about the spacers though but as someone said should become obvious when i have a go. It.
Meanwhile thanks for all your help . . Much appreciated . .