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Reprinted with permission from the people at Rivendell Bicycle. Shown in Rivendell Reader #37, Winter 2006.

by John Schubert.

Most people, when they find out about trail, say “aha!” and get stuck on it. Trail matters, but if bike control is the goal, skill and judgement outrank trail by a factor of infinity. John here does a good job of explaining it — the best I’ve read, ever; but it’s still not that easy to grasp. Dive in, and don’t get stuck. –Grant

Ever wonder why a seven-year old who’s too klutzy to make his own bed can ride a bike no-hands?

There is a simple and elegent reason: he can steer –and, therefore, balance– with his butt. So can you.

For that to happen, the bike has to be built so that it naturally steers in the direction it’s leaned in. You move your butt sideways, the bike steers that way. You move your but the other way, and the bike steers that other way.

(A quick recap here: you balance a bike with minute steering corrections. As the bike starts to go out from underneath you, to your left, you steer slightly to the left. As it starts to go to the right, you steer slightly to the right. As we grow more skilled on our bikes, these balancing corrections become tiny.)

The factor that makes the bike steer in the direction that you lean is built into the steering geometry of the bike. It can be isolated and measured. It’s called trail.

Trail is your friend, in the same way that oxygen is your friend. If you have too much or too little, you’re in deep trouble.

So this month, we’re going to look at this dimension: what it is, how it interacts with other frame dimensions, and how they all come together to make your bike easy to balance and steer.

At first glance, you might suppose that this steers-as-it-leans property would be strictly a function of gravity –i.e., that when you lean the bike sideways, the steering would seek the lowest position by turning slightly in that direction.

But that’s not how it works. In fact, the effect of gravity is the other way around: a bike that isn’t leaning at all lowers its center of gravity the most when the steering is rotated 60 degrees. As you lean the bike sideways, the steering angle which produces the lowest center of gravity decreases from this 60-degree point. (We know this thanks to the three-dimensional trigonometry prowess of chemist David E. H. Hones, duly noted in the references below.)

Here’s how it does work:

Because you have a tilted steering axis, the contact patch is behind the axis (at ground level). When the bike rolls forward, the forward motion causes the wheel to move into line, trailing behind the steering axis. Lean slightly, and the trail steers–slightly–into the lean.

One cool part of this is that it makes the bike self-correcting over bumps. If a bump knocks the front wheel slightly to one side, the force acting on the contact patch brings the wheel back into line.

You can see trail in the accompanying illustration: it’s the distance by which the contact patch trails the steering axis (at ground level). [click on images to enlarge]

Trail is the magic dimension in bike design. It’s also one of the least talked about, because it doeesn’t change much. Different frame designers use different rules of thumb, but for road bikes, trail amost always hovers between 2 and 2 1/2 inches. bikes within this narrow range all have benign characteristics.

note also that fork rake and trail are in opposition to one another. If you take a bicycle and increase the fork rake, you’ll decrease the trail. Looking at the drawing makes the reason why obvious: fork rake moves the front wheel forward, towards the steering axis. Reducing moves the fork rake moves the wheel backwards, to where it trails the steering axis by a greater distance. (Wireheads can see this in the accompanying equation: trail is redcued by a trig function of fork rake. Since that particular trig equation normally comes out to about 0.95, we who round off to the nearest inch or tow can just say “Trail is reduced by fork rake. Approximately.”

Are there reasons to go outside this narrow range of trail, and have less than two or more than two and a half inces? Yes, in certain instances. These reasons are tied into some of the other factors that influence frame design.

When you turn the handlebars from side to side, the front wheel does more than just change direction. It actually moves the side some, because of the angle at which it is attached to the frame, and also because of the fork rake. The wheel will sometimes flop to one side of its own accord, to seek a lower center of gravity. This property has an inelegant name: wheel flop. At slow speeds, wheel flop is a stronger force than the self-centering force of trail. (That’s why it’s harder to balance a bike at walking speed than at higher speeds.)

On a mountain bike climbing trails at slow speed, wheel flop can be a big problem. Some mountain bike steering geometries are difficult to control at slow speeds. Today’s mountain bikes have reduced that problem by moving away from the exceptionally slack head angles (68 to 69 degrees) of early mountain bikes. Those head angles, combined with prodigious amoungs of fork rake, produced way too much wheel flop.

Even on today’s mountain bikes, though, the rules for trail are a bit different from the rules for road bike trail. A typical rigid fork mountain bike has closer to three inches of trail. Why? It needs a slack head angle (70.5 to 71 degrees). it needs minimal fork rake (less than two inches), to keep wheel flop from being a slow-speed headache.

So if you designed a road bike with three inches of trail, you probably wouldn’t like it. But on a mountain bike, it’s part of a package of frame dimensions that, overall, produce good handling.

Trail may also be modified in frame designs for tall or short riders.

If you’re much taller than average and you buy your bike from a high-end builder, you may find that it has a steeper head tube angle than bikes for mere six-footers. This steeper head angle generates less trail. The fork rake may be reduced some to partially make up for this, but in general you can expect less trail. Once again, this becomes a package which produces the best possible handling. Such a bike will typically have a long top tube to accomodate the rider’s long body. The steep head angle and lessened fork rake help keep the wheelbase from getting too long. (If the builder puts a not-so-steep head tube angle and normal rake on a bike that already has a long top tube, the wheelbase gets longer–too long, in many people’s opinion–and the weight distribution is biased too much to the rear.)

For a short rider, whose small frame will have a short top tube, shallow head angles are often used, in part to reduce or eliminate front wheel/toe clip overlap. These shallow head angels may lead the builder to favor mountain bike-style steering geometry.

Twice in my 18 years of writing road tests, I’ve come across road bikes that had too little trail, presumably because the product managers didn’t understand trail. Since bikes with slacker head angles have more fork rake, and since the head angle is difficult to observe or measure accurately, the fork rake might be construed as the only relevant dimension. (”Make that bike look more like a touring bike. Add another inch of fork rake.”)

These two bikes, each with maybe 1 1/2 inches of trail, were certainly ridable bikes, and most people wouldn’t notice the difference. But if you go around a nice high-speed bend in the road on a normal bike, and then try again with the trail-deprived bike, you’d sure notice. The normal bike makes it easy for you to pick a line and hold it around the corner. The trail-challenged bike never picks a line.. it feels somehow aimless, and you find yourself making minute steering corrections all through the corner.

However, in this world where nothing is absolutely true all the time, the same trail dimension that makes me scorn those ill-designed single bikes is absolutely perfect on a tandem. Why? Trail makes the bike steer in the direction that it’s leaned in, and on a tandem, you have an independent leaning being in the back seat. I’ve ridden tandems mistakenly built with single-bike steering geometry, and the captain is always fighting the naldlebars to steer the bike. But I have nothing b ut praise for a tandem with an inch more rake–hence an inch less of trail–than a single bike. Once againn, it’s the geometry that adds up to the best-handling package, given all the factors that have to come into play. Besides, I don’t ride my tandems no-hands anyway.

Suppose you had so much fork rake that you reduced tha trail to zero, or less than zero? Thousands of kids manage that trick by jumping their bikes. (”Wow — did you see me come off that four-foot high loading dock? I landed hard!”) Land hard, and the front fork is bent forward. Aside from the risk of riding on a bent fork (which will break, sooner or later), you do increase your fork rake and decrease the bike’s stability. When the bike has negative trail, it can be ridden by an expert, but it isn’t any fun. Instead of naturally balancing the bike, the negative trail steering geometry tries to throw the rider. The negative trail doesn’t reinforce your balancing corrections–it fights against them, and often initiates a violent shimmy for additional terror.

Trail, then is the cornerstone of why a bike feels so benign, corners so crisply, and feels secure at 40mph (a laughable concept for a 20-pound bike, no?). As I’ve stated above, it works in combination with other measurements, and for every rule there’s an important exception. When you cahnge the geometry to improve a bike’s handling in one area, you’ll probably worsen it in another. Fortunately, though, most of us are privileged to wn and ride well-designed bikes that handle well under a wide variety of circumstances. For that, we thank the magic of trail–and those other dimensions too.

References:

Banton, Drew & Miller, Cripin, The Geometry of Handling (Bicycling Magazine, July 1980)
Delong, Fred, Delong’s guide to Bicycles & Bicycling (Chilton, 1978),pp70-74
Forester, John, Effective Cycling (MIT Press, 6th Edition, 1993), pp 30-34
Jones, David E. H., The Stability of the Bicycle (Physics Today magazine, April 1970) pp34-40

John Shubert is a technical editor of Adventure Cyclist, where this article originally appeared.