Trail is the distance from the center of the contact point of the front wheel with the riding surface to the intersection of the steering axis (head tube) with the surface. The trail is a function of the head angle, the fork rake, and the tire diameter. Trail has a major effect on the handling of a bicycle. More trail increases the bicycle's tendency to steer straight ahead. A bicycle with a largish trail dimension will be very stable, and easy to ride "no hands". A bicycle with a smaller trail dimension will be more maneuverable and responsive.
You can see that the front wheel of my Trek 311 contacts the ground slightly behind the point at which the steering axis intersects the ground. On my Trek the trail is about 45 mm.
There is no dispute in the bicycle industry that bikes need some positive amount of trail to be ridable. Most current manufactures and custom frame builders seem to agree that the right amount of trail is somewhere between 50 and 63 mm, with 57 mm often being considered the ideal compromise between stability and agility (see Craig Calfee's article Geometry of Bike Handling).
So what about my Trek pictured above with 45 mm of trail? That falls outside of what current manufactures consider to be ideal. Does that make the Trek unridable?
Here's where we get into the low trail debate. Some believe that a bike that carries some load on the front and has relatively wide tires handles better with a lower trail geometry. Jan Heine writes a lot about this in Bicycle Quarterly. He also wrote an interesting series of blog posts in which he discusses how he personally discovered the benefits of low trail geometry for the kinds of riding he does (see Part 4 for the discussion on low trail geometry). While the low trail evangelists are beginning to form a sizable following, it's still far from being accepted in the mainstream cycling industry. For a test, walk into to your local bike shop and ask them to show you some bikes with low trail and you'll see what I mean. "Huh? Low what?"
Why does any of this matter? For randonneuring, I find a front handlebar bag like the one you see on my Trek pictured above to be the ideal way to carry what I need. With a front bag I can get at food, clothing or anything else I might need while I'm riding. It also gives me a nice place to carry my cue sheet so that I can always see it, and the wind resistance is no greater than if I carried an equal sized bag on the back of the bike. And there's no risk of accidentally leaving it open and blissfully leaving a trail of cliff bars and arm warmers.
My first two randonneuring bikes were typical modern designs with trail somewhere in the mid to upper 50s. Decidedly high trail. On my Gunnar I never carried a load in the front mostly because there was no practical way to do it. The carbon fiber fork had no attachments for a front rack and the STI levers had cables protruding right where the bag would sit. With my Surly Cross Check I was able to mount a front bag and that's where I carried my food and gear through a countless brevets and training rides. On the Surly I never thought the handling was a problem. The bike was more nimble when the front bag was empty, but with loads up to 12 lbs, it was still "good enough" to carry me through long brevets without incident.
My experience with the Surly made me skeptical about the benefits of low trail design for front loads. The Surly's handling wasn't perfect, but it really wasn't that bad either. How much better could the handling be by reducing the trail by a few lousy millimeters? I was a skeptic, but I was also curious. And ultimately that curiosity is part of what lead me to buy the Trek. One of the interesting design features of many of the old Treks before 1985 was they had relatively low trail. So now that I have my Trek set up with a front rack and bag, I'm finally able to get some first hand experience with low trail and front loads.
Without a front load, I liked the way the Trek handled from the beginning. The handling is stable and predictable, yet if falls into corners easily. It feels rock solid at high speeds and is easy to maneuver at low speeds. The steering may not be snappy enough to make a good criterium racing bike, but for the long distance riding that I usually do there's nothing about the handling to not like.
The biggest surprise to me when I added the front rack and experimented with carrying some weight on the front is that the handling didn't seem to change much at all. It still falls into a turn easily and handles mid-turn adjustments well. The bike holds a straight line without any effort and even rides no handed almost as easily with a front load as without. One negative thing I've noticed is a slight tendency to shimmy. The only time it's gotten extreme was when the bag was loaded with a lot of weight, like close to ten lbs. With reasonable loads like what I typically carry on a brevet (under 5 lbs), shimmy has been barely perceptible. Also, the front load is noticeable when I get out of the saddle for climbing or sprinting. The front bag tends to throw the front of the bike a little more as the bike rocks from side to side.
I'll need to spend more time on this bike before I can really make up my mind as to whether a few millimeters of trail really makes a difference in the way a bike handles a front load. The Trek definitely handles differently than my old Surly, but so far the differences are both good (easier handling) and bad (tendency to shimmy).
So, the experiment continues.