I get questions periodically about trail and what it really means in terms of a bicycle’s handling. There are a number of factors that go into determining how a bike handles and trail is one of the most important numbers in determining what a bike’s character is.
So first, a definition. Trail is a simple way to describe some very fancy math. It is the distance, measured in centimeters, between the intersection of the steering axis with the ground and the intersection of a vertical line drawn from the front wheel’s axis to the ground. What that distance is between those two points is determined by head tube angle and fork rake. Here’s a graphic from Cannondale showing those three lines:
So here, D is head tube angle; F is fork rake and I is trail. The forward point of that line is determined by the frame’s head tube angle. The rear point is determined by the fork rake. That said, a fork with loads of rake can actually reverse those two points and actually result in negative trail. Measuring trail accurately in the real world is unbelievably difficult unless you have a frame jig and even then it can be hard. That’s why trail numbers are generated via an equation using math I’m grateful not to have needed since high school, involving stuff like cosins.
Some years ago Bill McGann, these days of Bikeraceinfo.com and McGann Publishing, shared with me a table his scientist father worked out of all the likely permutations of head angle and trail. That chart, the handy-dandy Trail-o-Matic, is reproduced in a somewhat abbreviated form below.
Product managers I’ve talked to will often refer to a sweet spot in trail. Back in the days of steel, depending on just whom you’re talking to, the answer tended to be in the range of 5.6cm to 5.9cm of trail. With today’s carbon fiber forks with tapered steerer and ultra-stiff carbon fiber frames, that increased stiffness means handling is more precise and bikes can be designed with less trail. I see a lot of road bikes in the range of 5.2cm to 5.4cm of trail today. Gravel bikes tend to have significantly more trail, usually a centimeter or more, when built by a custom builder, but many production gravel bikes will have trail closer to 6cm than 7cm.
Let’s back up a second. I’m guessing you’ve ridden a cruiser at some point in your life. You may recall from the experience that at ordinary cruiser-type speeds you don’t countersteer, you steer. The average cruiser has boatloads of trail and when you get one above about 16 mph, the handling gets rather shaky. It’s the classic high-speed wobble you find in some road bikes, just replicated at a not-so-high speed. As one builder once told me, “A cruiser isn’t going to turn until you tell it to, and even then, it’ll take a second to get around to it.”
So on one end of the spectrum, you’ve got road bikes with low trail. On the other end, you’ve got cruisers. In between, you have gravel bikes which can cover the entire range in between.
Here’s what I find so fascinating about trail: there are multiple ways to achieve any given trail. If you notice in that chart above, it is possible to achieve 5.8cm of trail with several different combinations of head tube angle and fork rake.
Possible combinations for 5.8cm of trail:
- 74-degree head tube with 3.8cm of fork rake
- 73.5˚ HTA with 4.1cm of rake
- 73˚ HTA with 4.4cm of rake (this yields 5.79 which is below the threshold of perception)
- 72.5˚ HTA with 4.7cm of rake (also 5.79)
- 72˚ HTA with 5.0cm of rake (also 5.79)
This matrix of possibility is handy for builders because it means that they can select the intended trail for a frameset, and if they are spec’ing a carbon fiber fork, like one of Enve’s, which are available in multiple, but not limitless, rake options, it’s possible to select the fork rake and then work backward from trail to determine the head tube angle. For instance, the Enve gravel fork comes in one rake: 50mm. That may seem limiting, and that would be true in the case of a production builder that uses a different head tube angle for each size of frame. It means that each bike will handle differently. But for a custom builder, a fork offered in a single rake isn’t a problem. If the frameset’s desired trail is 6.5, the builder simply sets the jig up for a head tube angle of 71 degrees. The resulting trail is 6.42cm, which is less than a millimeter in difference from the target trail of 6.5cm.
Things get interesting when you start considering a custome steel frame and fork. Now you can ask the question of why someone might want a frame with a 73.5˚ HTA paired with a 4.1cm-rake fork instead of one with a 72˚ HTA and 5cm of rake. The shortest, easiest answer is this: With a steeper head tube angle, the steering will be more neutral. As head angle decreases (gets slacker) wheel flop increases, which is the bike’s tendency to oversteer, once a turn is initiated. Think about a chopper at rest; the front wheel is turned either to one side or the other because the weight exerted on the front wheel makes it want to turn. However, with a gravel bike, a slacker head tube angle with more fork rake is desirable. Why? Because on a gravel bike you are likely to run a bigger tire, a slacker HTA combined with more fork rake results in a longer front center, pushing the front wheel farther from the BB and, hopefully, putting space between the toe of the rider’s shoe and the rear tire.
Today, road bike geometry has narrowed to a great degree on the production end, but with some unusual outliers (like the Trek Domane). As I wrote earlier, trail tends to run in a fairly narrow range on current carbon fiber road bikes, and that owes to the fact that with the stiffness that is possible due to carbon fiber, in both the frame and the fork, make it possible to design around sharper handling and not have that design lost in frame and fork flex. I can recall some of the early carbon fiber forks being flexible enough that it was hard to have a sense of what the front wheel was doing when taking a high-speed turn. It was adventurous, but not fun.
So, while it is still really helpful to see both fork rake and HTA when look at a bike’s geometry, it is with gravel bikes that knowing the bike’s HTA, fork rake and resulting trail can tell you much about how the bike will handle, and whether there will be room between your size 21 trail clogs and that 50mm tire. The two biggest takeaways should be: More trail means calmer steering, which will be helpful on bumpy surfaces, and it is also a helpful way to prevent toe overlap.