Take a moment to picture your favorite bike. Imagine, as well, that you have a metric tape measure in your hands. It’s easy to measure the seat tube length—you run the tape measure from the center of the bottom bracket to the center of the top tube/seat tube junction. It’s easy to measure the chainstay length by running the tape from the BB to the center of the quick release. Top tube is easy enough on an older bike with a horizontal (rather than sloping) top tube—center of the head tube/top tube to the center of the top tube/seat tube, right? And if it slopes, just run the tape from the center of the head tube/top tube parallel to the ground out to the seatpost.
With the help of a friend holding a string run between the two skewers, it’s possible to run the tape measure at a right angle to the string down to the center of the BB and get a rough measure of the BB drop, at least within a millimeter or two. I wouldn’t want to be called upon to print that measurement for public consumption, but on those occasions when I wondered if there’d been a misprint in a catalog, it was a handy reality check.
Trail is unusual in that it takes two different dimensions—head tube angle and fork rake—and quantifies their interplay. And though it describes a relationship, that relationship is a real, measurable number. In that regard, it’s rather like IQ, though without the ugly overtones.
A quick recap
Head tube angle is the acute angle drawn between the ground and a line that passes through the center of the head tube. The steeper (more upright) this is, the quicker the bike’s handling is. You can revisit my post on HTA here.
Fork rake is the distance between the fork’s steerer and the center of the front wheel’s axle. The more rake a fork has, the quicker the bike’s handling is. You can revisit my post on rake here.
What it is
The concept behind trail really isn’t all that difficult to digest. It is simply a measure of how far removed the steering axis is from the tire’s contact patch. Taking the image above, the steering axis (head tube angle) is the red line. The location of tire’s contact patch is determined by the dropouts (the yellow line) and the difference between those two points is trail (the blue line).
If that sounds like it is leaving fork rake out, it isn’t; fork rake is accounted for in the location of the tire’s contact patch. More fork rake moves the contact patch forward toward the steering axis and less fork rake moves it away from the steering axis.
The smaller the number, the more immediate the steering is. Imagine a bike with a 90-degree head tube angle and no fork rake. The red and yellow lines would be identical, a trail of 0. The bike would be so reactive that it would a challenge to ride comfortably, if at all—kind of the bicycle equivalent to a bucking bronco.
The inverse would be a bike with a very slack head tube angle (think chopper) and negative fork rake. That bike wouldn’t turn without you first issuing a letter of intent. The bigger the number, the calmer the handling will be, and at a certain point that number grows so large that the bike really won’t even turn.
Think of the range as Mini Cooper vs. school bus, with everything else in between. Road race bikes have the shortest trail. Next come the endurance bikes and track bikes, then gravel bikes and cross country mountain bikes, then trail bikes, enduro and topping out at downhill bikes.
What makes it complicated
Even if I remembered all that I learned in trigonometry, I doubt very much that I could explain that equation to you. Lucky for me, we don’t need to run through that math. For those who are interested in chasing this particular rabbit down its hole, Tr is trail; Rw is wheel radius; Ah is head tube angle; Of is fork offset. There is a pretty terrific trail calculator here, but in my next installment I’ll provide some matrices that will help illustrate the relationship of HTA to rake.
That’s not all
While I’ve described trail as the interplay of head tube angle and fork rake, there is, in fact, a third variable that can change trail: Wheel circumference. That means if you swap out a set of wheels with 40mm-wide tires on your gravel bike for another set with 28mm road tires, the trail will change; as wheel circumference goes down (small tires and wheels) trail will shrink, making the steering quicker. Bigger tires slow handling.
This effect is pretty easy to picture. Imagine being able to swap the 700C wheels on your road bike with 20-inch BMX wheels. This is a thought experiment, so we we are going to pretend that you won’t strike a pedal on the ground with each pedal stroke. With those smaller wheels, the bike will turn much more quickly. Similarly, if you could somehow put 50-inch wheels on your bike you’d find that the change would anesthetize the handling. This effect is magnified with larger tires because when someone mounts a notably larger tire on a bike that increase in tire weight compounds the increase in trail and the bike ends up feeling much lazier.
Trail is the only number routinely left out of bike geometry charts and while we can debate whether BB drop is more or less important than trail, no one can argue that trail isn’t one of the two most important dimensions of a bike’s handling. Weirder still, if manufacturers included trail in their geo charts, they wouldn’t actually need to include either HTA or fork rake. Trail renders those two numbers irrelevant. It really is the bottom line on steering geometry.
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