Making Sense Of Bicycle Geometry Part Two: Trail

Part One served as an introduction to bicycle geometry. It isn’t absolutely necessary to read, but it does present many of the terms and concepts you need to understand what goes on with a bike.

 

Trail is, for many, the defining characteristic that determines how a bike handles. It generally gets little appreciation or coverage.

Trail is the length of the patch on the ground between the steering axis (head angle) and the tire contact patch; basically, how far the tire on the ground “trails” behind the steering axis. As trail is part of the steering equation, it makes sense that elements related to how the bike steers have an outsize importance in terms of what you sense while riding. Differing trail numbers can result in a bike that feels nervous or sleepy, a bike that’s easy to turn or hard.

If you’ve ever ridden a beach cruiser, you’ve probably had a dramatic sense of what trail is about. Straddle a cruiser and go. For the first few pedal strokes, the front end feels floppy, lurching a bit left and right, steering a bit more than the rider intends (this is known as oversteer), and as you start to get up to a decent cadence, the bike feels as if it is hard to prevent from wandering right and left. Cornering at low speed, it feels like the front wheel is falling into the corner. But once you are riding at a comfortable cadence, the bike is ok. Then, take the bike down a hill where the rider spins out the low gear, the bike will seem to want to go straight, steering less than the rider intends (this is known as understeer).

What you’re feeling is the result of a very long trail dimension. The thing about long trail is that the bike is harder to control at low speeds, is ok to control within a certain speed range, and then, as the bike goes faster, is great in a straight line, but becomes harder to turn. However, as cruisers are typically single-speed setups and low geared at that, most riders just get used to the low speed issue and are fine thereafter. Rarely is someone on a cruiser going 40mph and having to turn hard.

Conversely short trail bikes feel easy to control and precise to corner on at low speeds, but beyond a certain speed, the bike will start to feel as it takes countless micro adjustments to hold in a straight line. Custom brevet bikes—this is a super-small niche where people are racing super long distances typically with a decent sized handlebar bag carrying extra gear–are often made with low trail. The extra mass above the front wheel slows down the steering, so the low trail and greater mass work to balance each other somewhat. Touring bikes, especially when designed to have bags hanging off the fork are usually short trail as well—with long trail and weight on the fork, holding the bike in a straight line while climbing in a low gear can be a constant fight. The classic Belgian road bike, like an old steel Eddy Merckx, also was designed with lowish trail, to better handle on rough roads and pave, where the riding and racing speeds were lower. They’re easier to hold in a straight line when bouncing over rough pavement. These bikes generally are ridden with larger 700c tires set at lower pressures than a typical road racing bike, so they are damping the more reactive steering created by the lower trail.

Trail is composed of three things. Those three things are: head angle, fork offset, and wheel radius as measured to the ground.

head angle

Most bikes in the road racing realm have a head angle between 71- and 74-degrees. Here’s the tricky thing: for a given head angle, the greater the fork offset, generally, the less the trail. It almost feels backwards, as if it should go the other way round. But here’s a short way to see how trail works. Take a 73-degree head angle with a 700c x 23mm tire. Install a fork with a 43mm offset, and the trail is 58mm. Take the same head angle and tire, and a 45mm offset and you have 56mm of trail. Basically, the longer fork offset moves the contact patch forward, closer to the steering axis: the tire “trails” behind the steering axis by a smaller distance, thus creating lower trail.

Another way to think of it is to look at two nearly identical bikes; one has a fork with longer, more curved blades, one with shorter, straighter blades. Looking at the two, which one would be better on cobblestones? Most would choose the one with longer, more curved blades. Assuming the same head angle, that’s the one with lower trail.

Now, to see how wheel radius works in the equation, take that 73-degree head and 43mm offset, and up the tire size to 25mm, and the trail goes up to 59mm. Put a smaller tire, a 20mm tire, and the trail drops to 57mm.

Here’s a calculator to play with. http://www.yojimg.net/bike/web_tools/trailcalc.php

fork offset (aka rake)

Designers have been fiddling around with trail for years. Much of the work came from trial-and-error. It was figured out in an era when bikes were made out of steel. It was easy to play with trail because making forks with different offsets, or rakes, was easy. It also was a bit of a mystery to the public, as bike designers didn’t necessarily want to give up their secrets, and many times they might not have known exactly what was happening until afterwards. Because of the years of experience, and relatively known terrain, road bikes have had geometry fairly dialed in for a long time.

The middle of the trail range for a road bike seems to be 56mm; with most designers pegging 55-60mm as the sweet spot..   This middle range is good because not only does it not steer fast or slow, but that the steering characteristics are largely the same regardless of speed—as Tom Kellogg says, “whether it’s 10mph or 50mph, it handles the same.” This is unlike long and short trail bikes, where the steering sensations can change based on speed. Long trail bikes are harder to control going slow (oversteer), but easier fast (understeer). Short trail bikes are the reverse.

If you’re doing most of your riding on the slow end of the spectrum, a low trail bike can make sense, as it will handle well for most of your riding—it will be easy to hold in a straight line, easy to turn. You just need to realize that it might feel a bit too twitchy, feeling as if you have to frequently correct for left and right movement, at high speeds. But, once over the initial surprise or difficulty, the attention needed will probably be there and almost become part of the scenery. If you’re riding fast and mostly straight lines, high trail can make sense, as it will favor those riding conditions. You just need to realize that it might feel like you’re having a harder time holding a straight line when riding slowly. Here, too, it might come across as surprising or frustrating on the first ride, but eventually becomes normal. Humans are very adaptable, part of that is subconsciously adjusting to various inputs; as such, riders can adapt very well to a wide range of trail values.

Don’t necessarily take accepted wisdom, or even data-proven assessments as being the final word. How a bike should feel can be quite personal. 2008 Tour de France champ Carlos Sastre quietly swapped out the stock fork on his Cervélo to get less trail. And 2017 Paris-Roubaix champ Greg Van Avermaet took a BMC endurance bike, the GranFondo RBX, and had a SLR01 fork installed on it to increase trail. In both cases, the riders chose front-end handling that is the reverse of what is expected. In both cases, the results indicate it worked well enough for them.

There are some limitations to the magic of that magical 55-60mm trail range. For one thing, with the widespread saturation of carbon-fiber forks, there are actually fewer options in terms of fork offset.  This is largely the result of economics; molds are expensive to make. Generally, for road bikes, the entirety of 700c road racing bikes, there are two offsets that account for virtually all forks: 43 and 45mm. And many manufacturers stick with a single offset for their entire size run. Carbon-fiber cyclocross forks are pretty much limited to offsets of 47 and 50mm, but mostly 47mm.

Further, bikes for shorter people often have longer trail numbers. Here, it’s largely the toe overlap issue. Some companies try to minimize trail, some don’t. Go through geometry charts, and you can often find that the smallest size 700c wheel bikes have trail numbers in the 70-80mm range.

One might think that for slower disciplines like cyclocross and gravel riding, bikes would come with shorter trail. They typically don’t. Trail is usually longer on those bikes. The reason is toe overlap with bigger tires and fenders, as described in the previous article.

Tire selection for bikes designed for bigger tires, like ‘cross, gravel, and touring rigs, has the potential to make noticeable changes in trail. Take a cyclocross ride: putting skinny road tires on your ‘cross rig will change how the bike handles. Start with a 54cm cx frame with a 71.5 degree head angle and a fork with 47mm of rake. A 33mm ‘cross tire will have a generous 67mm of trail, though the actual trail might be a good bit lower thanks to the sag that comes with low tire pressure. Install 23mm tires, and the trail drops to 64mm. Take a gravel bike with a 72-degree head and 50mm of offset. Put on the max tire allowed per clearance limits, 40mm, and the trail is 63mm. Swap in a 23mm road tire, and you get 57mm of trail. If this same gravel ride is set up with 650b wheels and you install a 23mm road tire, the trail drops to 51mm.

According to Damon Rinard of Cannondale, who had a fork built so he could quickly change offsets anywhere between 32 and 80 mm without changing the fork or even the brake position, the small differences, like 2mm, no one will notice. Even slightly bigger differences, most will have a hard time noticing. No one could find an offset that wasn’t easily rideable, though riders had ranges they preferred. Riding hands off the bars might not be easy, or possible in some cases.

Interestingly, while the 55-60mm trail range seems to be widely agreed upon across the industry, a few are reconsidering. BMC, for one, is making their top road bikes with 63mm of trail for most sizes. According to Stefan Christ, their head of engineering, they pushed out the trail because of the effects of both deep-dish wheels and a super-stiff front end. It also might be worth noting that they are also outliers for specifying a fork with 41.6mm of offset.

And that brings up the important matter of how frame construction, materials, and componentry figure into trail. They are quite important. Discussing it deserves its own entry. And that will be in Part Three.

I want to thank Edwin Bull of Van Dessel, Brad DeVaney of Litespeed, Steve Fairchild of Fuji, Tom Kellogg of Spectrum Cycles, Damon Rinard of Cannondale, and Scott Warren for their time and insights.  And Bikecad.ca for the drawings.

 




most of the dimensions you should think of

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