Here's how to make sense of them...
Seat tube angle
A bike's seat tube angle – often called 'seat angle' – is simply the angle between the horizontal and a line that starts at the bottom bracket and goes up through the seat tube.
Like stack and reach, seat tube angle refers solely to the frame. It doesn't take into account a seat post's layback (the position of the clamp in relation to the center of the seat post) or whether the saddle is pushed forward or backward on its rails. These factors, along with the amount of seat post extending out of the frame, meaning that saddle position can differ significantly between bikes with exactly the same seat tube angle.
What are stack and reach and why are they important?
The seat tube angle influences but does not define saddle setback, which is the horizontal distance between the front of the saddle and the center of the bottom bracket.
Stack and reach don't take seat tube angle into account, which is one of the reasons why we cautioned that these measurements, although useful, don't give the whole story. Stack and reach tell you about the position of the top of the head tube in relation to the bottom bracket. Two bikes could have an identical stack and reach measurements but fit quite differently because of different seat tube angles.
All other things being equal, a steeper seat tube angle moves the saddle forwards – putting you more directly over the bottom bracket – and a shallower seat tube angle moves it back, shifting more of your weight from the handlebar to the saddle.
A steep seat tube angle allows racers (and anyone else who wants to ride fast) to achieve a flat-backed position for aerodynamic efficiency with a more open hip angle than would otherwise be the case. Too tight a hip angle can be uncomfortable and restrict both breathing and power.
Head tube angle
Head tube angle – often shortened to 'head angle' – is measured from the horizontal to a line running down the centre of the fork steerer tube.
Mountain bikes with front suspension usually have head tube angles in the 62-73° range while road bike head tube angles tend to be above 70°, commonly between 72° and 74°.
All other things being equal, a slacker head tube angle (a smaller number) will move the front wheel further forward and lengthen the wheelbase (the horizontal distance between the front and rear hubs) and the front-center (the horizontal distance from the center of the bottom bracket to the front hub). The alternative is for a bike designer to fix the wheelbase at a set measurement and then a slacker head tube angle will shorten the top tube.
Smaller sized road bikes sometimes have slacker head tube angles in order to reduce the amount of wheel/toe overlap and avoid problems when steering.
At the risk of insulting your intelligence, the fact that the head tube is angled rather than perpendicular means that adding headset spacers moves your handlebar horizontally as well as vertically.
Say your road bike has a 73° head tube angle and you add 25mm of headset spacers. As well as lifting your handlebar vertically about 24mm, this will move it about 7mm horizontally towards the saddle.
The Head tube angle also influences the trail. This is the distance that the center of the front tire's contact patch trails behind the point where the bike's steering axis (the fork steerer tube) intersects with the ground. It sounds more complicated than it is!
The trail is a function of the head tube angle, fork offset/rake (the distance the front hub is offset from the steering axis), and wheel/tire size. Steepening the head tube angle decreases trail, all other things being equal, as does increase the fork offset and decreasing the wheel/tire size.
Many bike brands list trails, but not mechanical trail – the perpendicular distance between the steering axis and the front tire's contact patch – which would be more informative.
As a rule, more trail results in more stable steering and a greater ability to hold a line on rougher terrain.
Less trail makes for a bike that steers more sharply, but too little can result in a twitchy feel.
Head tube angle and trail alone don't determine how a bike steers. There are many other factors involved, including stem length, handlebar type/dimensions, wheelbase, your weight distribution, and the tires used (particularly important at the moment with brands speccing ever-larger tires on road frames). We'll cover these in future features. Head tube angle and trail are helpful, though, is giving you an indication of a bike's character and how it is likely to handle.