Lots of headset spacers!
New bike owners, chances are your stem is set up high on your fork steerer tube with a bunch of spacers underneath. It's just the way bikes come from the bike shop. If you're new to riding, getting to know your bike takes time and after a while you'll want to set it up your bike so that it's just how you like it.
Adjusting the height of the stem is similar to adjusting the angle of the steering wheel column in your car and makes a huge difference to how your bicycle will feel and handle.
This adjustment should only be made once you're really comfortable with your saddle height and position. If your bum hurts when riding it could be because too much of your weight is on the saddle and not enough of your weight is transferred over the handlebars. Lowering the stem lowers your bars and helps to adjust that balance. More of your weight on the front gives your front wheel more traction in corners and having your bars lower will also aid in climbing (front wheel is less likely to lift up on steep climbs).
Headset Adjustment - Threadless Type
Threadless headsets come on most modern bikes. The bolt in the top cap will put pressure on the stem, which presses everything together.
NOTE: The cap and bolt at the top of the stem do not secure the stem onto the steering column. The bolts on the side of the stem keep the stem from moving once the top cap bolt is tightened.
1. Begin by removing the adjusting bolt in the center of the steering column. Next, remove the top cap.
2. Loosen stem bolts that secure stem to the steering column.
3. Wiggle the stem side to side to loosen and remove stem.
4. Remove a/some spacers and install the stem back onto the steerer tube.
5. Align stem straight to wheel, put those spacers on top of the stem and secure the top bolt and cap. You want to do it up nice but NOT TOO TIGHT.
6. Tighten stem bolts.
7. Hit the front brake and check for play (or a knocking sensation) by pulling back and forth on the bike. If there's play you'll need to tighten the top cap some more.
8. Check alignment of stem and tighten stem binder bolts fully.
Image aboveshows steering tube and stem. There is usually a 3mm gap between stem and top of column. You need this gap to be no more than 5mm. Too little a gap and you might not get enough clamping force when tightening the top cap.
Spacers on top of stem.
The spacers on top of your stem might 'look' a little silly, or even dangerous. But ignore that, ride the bike and feel the difference. If you love the stem lower you can make the move to cut the fork steerer tube and remove the spacers above your stem. Your local bike shop would charge you a small fee, or you can DIY.