The triple tree give you something to hold on to, keep the forks attached and inline, and they allow you to steer your motorcycle. Your triple tree really are a pivotal part of your motorcycle. If you've ever looked closer at your motorcycle's steering you'll know the triple tree are simple assemblies essentially made up of three parts: the top and bottom triple clamps and the steering stem. Together, these perform several important functions when it comes to how, and if, your motorcycle performs – and that's why they're an important part of your chassis.
Of course you find the triple tree at the headstock. The triple tree connects the front wheel and axle to the frame, and is a simple hole. But it leans slightly backwards towards the rider giving it rake and has two accurately machined areas to fit bearings in. Whatever type of bearings, one thing is certain, the hole inside the bearing will be snug fit on the steering stem.
The steering stem slips through the bearings from underneath. It's taller than the headstock so there's a bit exposed below the headstock that the bottom triple clamp presses onto and a threaded section above it. A nut here squeezes the bearings together and prevents the steering stem moving up and down. Normally this nut looks like the lock-ring on preload adjusters. It has to be tight enough to take the slack out the bearings, but not tight enough to pinch otherwise the steering won't move freely. The stem is essentially a rigid part of the chassis, except it can rotate in one plane. The triple clamps can now be attached (the bottom one presses on and the top one is held down by the stem nut). Two lugs on the bottom triple clamp hit the frame and provide steering locks – approx +/- 20 degree.
The three most important things about the triple tree are weight, offset and rigidity. Weight is important because the triple tree move with the steering, so the lighter it is the easier to steer. This is why most triple trees are made from aluminum or other exotic light materials to reduce weight further.
However, the triple tree have to handle large forces in several directions under braking and cornering, so they're normally quite big, but heavily pocketed. They only need a little movement to give you feel though. While many properties affect this, the size of the bottom triple clamp has a pronounced effect and is described by the number of bolts used to tighten it.
Finally there's offset. IF the holes for the forks and steering stem were drilled so the centers all lined up there would be no off-set – and this would give far too much trail. So the fork holes are drilled 25 to 34mm in front of the stem hole to reduce the trail accordingly. More offset equals less trail. So even though the triple tree is a simple component they control how the motorcycle feels, behaves and turns, and they provide somewhere to connect the handlebars.