Understanding Trail

Understanding Trail

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.

3.8 3.9 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 5.0
70 8.33 8.22 8.12 8.01 7.91 7.80 7.69 7.59 7.48 7.37 7.27 7.16 7.05
70.5 8.01 7.91 7.80 7.69 7.59 7.48 7.37 7.27 7.16 7.05 6.95 6.84 6.73
71 7.69 7.59 7.48 7.37 7.27 7.16 7.05 6.95 6.84 6.73 6.63 6.52 6.42
71.5 7.37 7.26 7.16 7.05 6.95 6.84 6.74 6.63 6.53 6.42 6.31 6.21 6.10
72 7.05 6.95 6.84 6.74 6.63 6.53 6.42 6.31 6.21 6.10 6.00 5.90 5.79
72.5 6.74 6.63 6.53 6.42 6.31 6.21 6.10 6.00 5.89 5.79 5.68 5.58 5.48
73 6.42 6.32 6.21 6.11 6.00 5.90 5.79 5.69 5.58 5.48 5.38 5.28 5.17
73.5 6.11 6.00 5.90 5.80 5.69 5.59 5.48 5.38 5.27 5.17 5.07 4.96 4.86
74 5.80 5.69 5.59 5.48 5.38 5.27 5.17 5.07 4.96 4.86 4.75 4.65 4.54
74.5 5.49 5.38 5.28 5.17 5.07 4.97 4.86 4.76 4.66 4.55 4.45 4.34 4.24
75 5.18 5.07 4.97 4.87 4.76 4.66 4.56 4.45 4.35 4.24 4.14 4.04 3.93
75.5 4.87 4.76 4.66 4.56 4.45 4.35 4.25 4.14 4.04 3.94 3.84 3.73 3.63
76 4.56 4.46 4.35 4.25 4.15 4.05 3.94 3.84 3.74 3.63 3.53 3.43 3.32
3.8 3.9 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 5.0

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.

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12 comments

  1. Mr. Wilson

    This is a very simplified description. For example, a 73.5 deg HTA with 45mm of fork rake on a typical racing bike and descending a road with rough pavement could get very exciting in a bad way. I’m not sure you can separate trail from head tube angle, BB drop,chain stay length, frame compliance,tire size, weight distribution. There are numerous factors involved in how a bike rides/turns and you just need to ride a lot of different bikes before you can look at a geo chart and begin to get an idea about how a particular design may feel.
    Patrick, can you also state tire size that was used in computing the chart offered ?
    Apologies if I muddied the water too much.


    1. Author
      Padraig

      Yes, this is a very simplified description. That’s the point. Talking about BB drop, wheelbase, tire size, etc., is really going much deeper than this post is meant to address. I’ve written posts on the entirety of a bike’s geometry. Here, my purpose was just to discuss trail and how fork rake works with head tube angle to establish trail. That’s all.

  2. TomInAlbany

    So, will a bike with more trail be more difficult to ride no-handed and a bike with a lot less trail feel too twitchy to ride no-handed? Is the sweet spot the place where the bike is responsive but the rider can sit up and feed him/herself?


    1. Author
      Padraig

      When it comes to riding no-handed, factors other than just trail begin to affect the result, though, in broad strokes, yes, very little trail and loads of trail can make a bike more difficult to ride no-handed. The interesting wrinkle is that the faster you go, the easier it is to ride a low-trail bike no handed, and the slower you go, the easier it is to ride a high-trail bike.


    1. Author
      Padraig

      Pneumatic trail has to do with side-slip of tires and the distance (trail) between the tire is and where the geometry says it should be. It’s not a factor that plays a part in actual bike design, as in the geometry matrix.

  3. Mark V

    A couple things:

    1) Wheel diameter directly affects trail number. Increasing the outer diameter of the wheel reduces trail. When the discussion was limited to performance road bikes with 20-23 tyres, we could assume that wheel diameter was constant, and thus we could ignore it (like the chart in the post). But when 700Cx33, 700Cx50, 650Bx42, and 650Bx55 are all on the table when we are talking about gravel bikes, it becomes a relevant parameter again.

    2) Trail dimension on road bike geometry has mostly been in a narrow range for a few decades, but only if you are talking about average size frames (~ 55 to 58cm). For a number of reasons, sizes on either side of the bell curve have tended to deviate from that trail range in mainstream production. One of these reasons is the whole industry moved to spec’ing 43mm rake off-the-shelf alloy forks in the ’80s (and later carbon forks in the ’90s). That put the rake dimension well outside that 5.6-5.9mm range on some sizes. There is a rather recent trend by several manufacturers to use 3 or more fork rake options to make the trail more uniform throughout the size range. Cervelo comes to mind, as their new road bikes like the R5 now have a fork rakes of 57, 51, and 45mm so that the trail stays a uniform 57mm.

    Another reason why trail varied throughout the size range on bikes in the 1980s thru the ’00s is that trail just didn’t seem to be as important to some builders back then. In my memory, a lot of Italian builders used the same rake on all their road bike sizes even when they made their own steel forks. So arguably, trail as a design goal a fairly recent preoccupation, at least for the industry as a whole.

    I am not convinced that current frame designers have a concrete idea of what the sweet spot for trail might be for a gravel bike. For one thing, the range of tyre sizes obliterates the idea of a magic number to the tenth of a millimeter. For another, there seems to be a bit of competing philosophies in gravel bike design. If CX bikes provided the first gen of “gravel” frames, then there are designers who are massaging geometry with a gran fondo/randonneur perspective while others, particularly those makers that were primarily mtb brands, who seem to be reinterpreting the current “progressive geometry” mtb trend into gravel bikes. As a member of the former, I would like to see multiple rake options (50-65mm) for gravel forks so that trail could actually be better addressed on gravel bikes.

    1. Mark V

      one correction: I said that increasing wheel diameter decreases trail, which is wrong, it actually increases trail.

    2. TomInAlbany

      Interesting discussion. I’m not a builder and never will be. I am an engineer, though, and I like explanations!

    3. Mark V

      Another correction: “That put the TRAIL dimension well outside that 5.6-5.9CM range on some sizes.”

      In my original comment, i said the rake dimension was outside the 5.6-5.9mm range. What I meant to say was that when the rake is constant throughout the size range while the head angle varies, the trail dimension will vary beyond the supposed sweet spot of 5.6-5.9cm range for frame sizes on either end of the spectrum.

  4. Fuzz

    Wheel flop is something that really bugs me about most endurance bike geometries, as it makes the bike feel loose and unstable when climbing or standing at low speeds. Most all mountain MB geometries have the same issue, since they have also gravitated to slacker head angles. When I asked Lennard Zinn about this years ago https://www.velonews.com/2017/02/technical-faq/technical-faq-three-bikes-handle-differently_430274, it was interesting to read that he does more of a race geometry (steeper head angle) for his older clients, to reduce the wheel flop. And it was interesting to read in the past few years that Cannondale and Specialized have moved their new endurance geometries closer to a true race bike.

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